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Gamete

Gamete

Gametes (in Greek: γαμέτες) —also known as sex cells, or spores—are the specialized germ cells that come together during fertilization (conception) in organisms that reproduce sexually. The creation of gametes is called gametogenesis, in which gametocytes divide by meiosis into various gametes. In those species that produce two morphologically distinct types of gametes, and in which a particular individual produces only one type, "females" of the species produce the larger gamete called an ovum (or egg) and "males" produce the smaller gamete termed (in animals) a spermatozoon (or sperm cell). The equivalent "male" structure in higher plants is called a pollen grain. Organs that produce gametes are called gonads in animals, and archegonia or antheridia in plants. Gametes are haploid cells; that is, they contain one complete set of chromosomes (the actual number varies from species to species). When two gametes unite (typically in animals, involving a sperm and an egg), they form a zygote—a cell having two complete sets of chromosomes and therefore diploid. The zygote cell receives one set of chromosomes from each of the two gametes involved in the union. After fusion of the two gamete nuclei, and after multiple cell divisions and cellular differentiation, a zygote develops, first into an embryo, and ultimately into a mature individual capable of producing gametes. Gametes from a mature diploid individual will be produced in the gonadal tissue through meiosis—a process of cellular division that reduces the number of sets of chromosomes from two to one (i.e., produces haploid gametes). The diploid somatic cells of an individual will contain one copy of the chromosome set from the sperm and one copy of the chromosome set from the egg—that is, the cells of the offspring will have genes expressing characteristics of both the father and the mother. A gamete's chromosomes are not exact duplicates of either of the sets of chromosomes carried in the somatic cells of the individual that produced the gametes. They can be hybrids produced through crossover (a form of genetic recombination) of chromosomes, something that takes place in meiosis. This hybridization has a random element, and the chromosomes tend to be a little different in every gamete that an individual produces. This recombination and the fact that the two chromosome sets ultimately have come from either a grandmother or a grandfather on each parental side account for the genetic dissimilarity of siblings.

Gamete size and evolution

Isogamy occurs when gametes from both sexes are the same size. Anisogamy or heterogamy is the condition wherein females and males produce gametes of different sizes. Isogamy is considered to be the ancestral condition, the evolution of gametes of unequal size is a current area of evolutionary research.

Reference


- Randerson, J.P., and Hurst, L.D. 2001. The uncertain evolution of the sexes. Trends in Ecology & Evolution 16(10):571-579

External link


- [http://www.thedoctorslounge.net/fertilounge/articles/gametogenesis/index.htm Gametogenesis (spermatogenesis, oogenesis) & fertilization] Category:Classical genetics Category:Germ cells Category:Evolution Category:Reproductive system ja:配偶子

Greek language

Greek (Greek Ελληνικά, IPA – "Hellenic") is an Indo-European language with a documented history of 3,500 years. Today, it is spoken by 15 million people in Greece, Cyprus, the former Yugoslavia, particularly The Former Yugoslav Republic of Macedonia, Bulgaria, Albania and Turkey. There are also many Greek emigrant communities around the world, such as those in Melbourne, Australia which is the third-largest Greek-populated city in the world, after Athens and Thessaloniki. Greek has been written in the Greek alphabet, the first true alphabet, since the 9th century B.C. and before that, in Linear B and the Cypriot syllabaries. Greek literature has a long and rich tradition.

History

This article does not cover the reconstructed history of Greek prior to the use of writing. For more information, see main article on Proto-Greek language. Greek has been spoken in the Balkan Peninsula since the 2nd millennium BC. The earliest evidence of this is found in the Linear B tablets dating from 1500 BC. The later Greek alphabet (q.v.) is unrelated to Linear B, and was derived from the Phoenician alphabet (abjad); with minor modifications, it is still used today. Greek is conventionally divided into the following periods:
- Mycenean Greek: the language of the Mycenean civilisation. It is recorded in the Linear B script on tablets dating from the 16th century BC onwards.
- Classical Greek (also known as Ancient Greek): In its various dialects was the language of the Archaic and Classical periods of Greek civilisation. It was widely known throughout the Roman empire. Classical Greek fell into disuse in western Europe in the Middle Ages, but remained known in the Byzantine world, and was reintroduced to the rest of Europe with the Fall of Constantinople and Greek migration to Italy.
- Hellenistic Greek (also known as Koine Greek): The fusion of various ancient Greek dialects with Attic (the dialect of Athens) resulted in the creation of the first common Greek dialect, which gradually turned into one of the world's first international languages. Koine Greek can be initially traced within the armies and conquered territories of Alexander the Great, but after the Hellenistic colonisation of the known world, it was spoken from Egypt to the fringes of India. After the Roman conquest of Greece, an unofficial diglossy of Greek and Latin was established in the city of Rome and Koine Greek became a first or second language in the Roman Empire. Through Koine Greek it is also traced the origin of Christianity, as the Apostles used it to preach in Greece and the Greek-speaking world. It is also known as the Alexandrian dialect, Post-Classical Greek or even New Testament Greek (after its most famous work of literature).
- Medieval Greek: The continuation of Hellenistic Greek during medieval Greek history as the official and vernacular (if not the literary nor the ecclesiastic) language of the Byzantine Empire, and continued to be used until, and after the fall of that Empire in the 15th century. Also known as Byzantine Greek.
- Modern Greek: Stemming independently from Koine Greek, Modern Greek usages can be traced in the late Byzantine period (as early as 11th century). Two main forms of the language have been in use since the end of the medieval Greek period: Dhimotikí (Δημοτική), the Demotic (vernacular) language, and Katharévousa (Καθαρεύουσα), an imitation of classical Greek, which was used for literary, juridic, and scientific purposes during the 19th and early 20th centuries. Demotic Greek is now the official language of the modern Greek state, and the most widely spoken by Greeks today. It has been claimed that an "educated" speaker of the modern language can understand an ancient text, but this is surely as much a function of education as of the similarity of the languages. Still, Koinē , the version of Greek used to write the New Testament and the Septuagint, is relatively easy to understand for modern speakers. Greek words have been widely borrowed into the European languages: astronomy, democracy, philosophy, thespian, etc. Moreover, Greek words and word elements continue to be productive as a basis for coinages: anthropology, photography, isomer, biomechanics etc. and form, with Latin words, the foundation of international scientific and technical vocabulary. See English words of Greek origin, and List of Greek words with English derivatives.

Classification

Greek is an independent branch of the Indo-European language family. The ancient languages which were probably most closely related to it, Ancient Macedonian language (which may be regarded as a dialect of Greek) and Phrygian, are not well enough documented to permit detailed comparison. Among living languages, Armenian seems to be the most closely related to it.

Geographic distribution

Modern Greek is spoken by about 15 million people mainly in Greece and Cyprus. There are also Greek-speaking populations in Georgia, Ukraine, Egypt, Turkey, Albania, Former Yugoslav Republic of Macedonia and Southern Italy. The language is spoken also in many other countries where Greeks have settled, including Armenia, Australia, Austria, Belgium, Bulgaria, Canada, Denmark, France, Germany, Netherlands, Sweden, United Kingdom, and the United States.

Official status

Greek is the official language of Greece where it is spoken by about 99.5% of the population. It is also, alongside Turkish, the official language of Cyprus. Due to the membership of Greece and Cyprus, Greek is one of the 20 official languages of the European Union.

Phonology

This section generally describes the post-Classic phonology of the Greek language. :All phonetic transcriptions in this section use the International Phonetic Alphabet

Vowel sounds

Greek has 5 vowel sounds, all phonemic:

Spore

:This article is about biological spores. For the video game, see Spore (game). :S'pore is also a common abbreviation for Singapore The term spore has several different meanings in biology. Categorization by function:
- Diaspores are dispersal units of fungi, as well as mosses, ferns, fern allies, and a few other plants.
- resting stage in the life cycle of some bacteria (see endospore) and loosely applied to some animal resting stages
- Chlamydospores are thick-walled resting spores in fungi.
- Zygospores are thick-walled resting spores (hypnozygotes) of zygomycetous fungi which are produced by sexual gametocystogamy and can give rise to a conidiophore ("zygosporangium") with asexual conidiospores. Categorization by origin during life cycle:
- Meiospore is a product of meiosis (the critical cytogenetic stage of sexual reproduction), meaning it is haploid and will give rise to a haploid daughter cell(s) or a haploid individual. An example is the parent of gametophytes of the higher vascular plants (angiosperms and gymnosperms)—the microspores (give rise to pollen) and megaspores (give rise to ovules) found in flowers and cones; these plants accomplish dispersal by means of seeds.
- Mitospore (conidium, conidiospore) is an asexually produced propagule, the result of mitosis. Most fungi produce mitospores. Mitosporic fungi are also known as anamophic fungi (compare teleomorph or deuteromycetes). Categorization by motility - spores can be differentiated by whether they can move or not:
- Zoospore can move by means of one or more flagellum. It can be found in some algae.
- Aplanospore cannot move, but could potentially grow flagella.
- Autospore cannot move and does not have the potential to ever develop any flagella.
- Ballistospore is actively discharged from fungal fruit body (mushroom).
- Statismospore is not actively discharged from fungal fruit body (see puffball). Spores can be formed sexually or asexually, and therefore many different kinds of spores exist. In common parlance, the difference between "spore" and "gamete" (both together called gonites) is that a spore will germinate and develop into a Thallus (tissue) of some sort, whereas a gamete needs to combine with another gamete before developing further. However, the terms are somewhat interchangeable when referring to gametes, as indicated by the technical terminology given in the second definition above. A chief difference between spores and seeds as dispersal units is that spores have very little stored food resources compared with seeds, and thus require more favorable conditions in order to successfully germinate. In their favor, spores are very hardy and require much less energy to produce. The strategy employed in producing spores, is to reach all the favorable locations by producing and dispersing very large numbers. Spore came from a Greek word meaning seed. However, seeds (of seed plants) are not the same as spores, but are the fusion of gametes.

Diaspores

In the case of spore-shedding vascular plants such as ferns, wind distribution of very light spores provides great capacity for dispersal. Also, spores are less subject to animal predation than seeds because they contain almost no food reserve, however they are more subject to fungal and bacterial predation. Their chief advantage is that, of all forms of progeny, spores require the least energy and materials to produce. Vascular plant spores are always haploid and vascular plants are either homosporous or heterosporous. Plants that are homosporous produce spores of the same size and type. Heterosporous plants, such as spikemosses, quillworts, and some aquatic ferns produce spores of two different sizes: the larger spore in effect functioning as a "female" spore and the smaller functioning as a "male". Under high magnification, spores can be categorized as either monolete spores or trilete spores. In monolete spores, there is a single line on the spore indicating the axis on which the mother spore was split into four along a vertical axis. In trilete spores, all four spores share a common origin and are in contact with each other, so when they separate each spore shows three lines radiating from a center. Category:Botany Category:Biological reproduction Category:Germ cells ja:胞子

Germ cell

A germ cell is a kind of cell that is part of the germline, and is involved in the reproduction of organisms. There are different kinds of germ cells, which include gametogonia, gametocytes, and gametes. By a narrower definition, the term germ cell can also just refer to gametes, which are produced by meiosis of the aforementioned germ cells, but this definition is less precise.

Links


- Epigenetics
- [http://www.epigenome-noe.net/ Epigeneome NoE] Category:Germ cells ja:生殖細胞

Fertilization

Fertilisation (also known as conception, fecundation and syngamy) is fusion of gametes to form a new organism. In animals, the process involves a sperm fusing with an ovum, which eventually leads to the development of an embryo. Depending on the animal species, the process can occur within the body of the female in internal fertilisation, or outside in the case of external fertilisation. The entire process of development of new individuals is called procreation, the act of species reproduction.

Fertilisation in plants

After the female part of the flower is pollinated, pollen grains attempt to travel into the ovary by creating a path called a "pollen tube." The pollen tube does not directly reach the ovary in a straight line. It travels near the skin of the style and curls to the bottom of the ovary, then near the receptacle, it breaks through the ovule and reaches the ovum to fertilise it. This is the point when fertilisation actually occurs. After being fertilised, the ovary starts to swell and becomes a fruit. With multi-seeded fruits, multiple grains of pollen are necessary for syngamy with each ovule. The process is easy to visualize if one looks at maize silk, which is the female flower of corn. Pollen from the tassel (the male flower) falls on the sticky external portion of the silk, then pollen tubes grow down the silk to the attached ovule. The dried silk remains inside the husk of the ear as the seeds mature; if one carefully removes the husk, the floral structures may be shown. The development of the flesh of the fruit is proportional to the percentage of fertilised ovules. For example, with watermelon, about a thousand grains of pollen must be delivered and spread evenly on the three lobes of the stigma to make a normal sized and shaped fruit.

Fertilisation in mammals

All mammals rely on internal fertilisation through copulation. To deliver the sperm to the female, the male inserts his sexual organ, the penis, into the opening of the vagina, the passage into the female's other sexual organs. (This process is a part of copulation.) Once the male ejaculates, a large number of sperm cells swim toward the ovum. The capacitated spermatozoon and the oocyte meet and interact in the ampulla of the fallopian tube. In mammals, binding of the spermatozoon to the zona pellucida, an extracellular layer surrounding the oocyte, initiates the acrosome reaction. This process releases the enzyme hyaluronidase, which digests the matrix of hyaluronic acid in the vestments surrounding the oocyte. Fusion between the sperm and oocyte plasma membranes follows, allowing the entry of the sperm nucleus, mitochondria, centriole and flagellum into the oocyte. Once the ovum fuses with a single sperm cell, its cell membrane changes, preventing fusion with other sperm. This process ultimately leads to the formation of a diploid cell called a zygote. When the zygote reaches the uterus and implants in the endometrium, the female is said to be pregnant. If fertilisation takes place, the sperm usually meet the ovum in the fallopian tube, requiring the sperm cells to swim from the upper vagina through the cervix and across the length of the uterus before reaching the fallopian tube—a considerable distance compared to the size of the sperm cell.

Human fertilization

There have been some discrepancies regarding the terminology of "conception" (fertilisation) within the abortion debate. In a statement by the American Association of Pro-Life Obstetricians & Gynecologists (AAPLOG), regarding the controversial morning-after pill, AAPLOG claims: :"[Again,] one must be careful of the terminology. Many now speak of "conception" as that moment when the human blastocyst, the early ball of approximately 100 cells, implants in the mother’s uterus (womb). The time from actual fertilisation (sperm and egg unite in the Fallopian Tube) until implantation, a period of about 7-10 days, is ignored, even though no genetic change occurs in the cells during this time period. Many family planning specialists who have supported the terminology change can thus rationalize that the destruction of the human embryo between fertilisation and implantation should be labeled "contraception", rather than "early abortion".

Fertilization and Genetic Recombination

The variations that result from meiosis is enhanced by fertilization. Each person has genes for the same traits, but again, each gene's specific instructions can vary. Therefore, the gametes produced by one person are expected to be genetically different from the gametes produced by another person. When gametes first fuse at fertilization, the chromosomes donated by the parents are combined, and, in humans, this means that (2²³)², or 70,368,744,000,000, chromosomally different zygotes are possible, even assuming no chromosomal crossover. If crossover occurs once, then (4²³)², or 4,951,760,200,000,000,000,000,000,000, genetically different zygotes are possible for every couple.

See also


- In vitro fertilisation
- Superfetation
- Superfecundation

References


- Evans JP, Florman HM. 2002. The state of the union: the cell biology of fertilization. Nature Medicine. 8 Suppl S57-63. Category:Biological reproduction Category:Fertility Category:Pollination simple:Fertilization

Sexual reproduction

Sexual reproduction is a process of reproduction involving the merging of two gametes from the same (usually) species to produce a new organism. One advantage of this form of reproduction over asexual reproduction is that the DNA of the offspring is significantly different from that of the two gametes; this allows species to change more rapidly than through mutation alone. The DNA is different because each contributing organism randomly and independently donates half of their DNA to the sex cells in a process called meiosis. These cells then, through a variety of processes, depending on the particular species, meet and merge together to produce a new organism with different DNA. Sexual reproduction is the primary method of reproduction for the vast majority of visible organisms, including almost all animals and plants, though the process is often significantly different, especially for plants and trees. Bacterial conjugation, the transfer of DNA between two bacteria, is often mistakenly confused with sexual reproduction, because the mechanics are similar. The first fossilized evidence of sexually reproducing organisms is from eukaryotes of the Stenian period, about 1.2 to 1 billion years before the present time.

Sexual reproduction of protists and fungi

Many protists and fungi reproduce sexually. Although they are unicellular, at times of reproduction the "father" cell and the "mother" cell combines together. Next, their genetic information combines together into a new formation, and by cell division the offspring is born.

Reproduction in flowering plants

:Main articles: Plant sexuality, Flowering plants, Flowers In flowering plants, a stamen produces gametes called pollen grains, which attach to a pistil, in which the female gametes (ovules) are located. Here, the female gamete is fertilized and develops into a seed. The ovary, which produced the gamete then grows into a fruit, which surrounds the seed(s). Plants may either self-pollinate or cross-pollinate.

Reproduction in reptiles

Female reptiles lay eggs, fertilized by the male, in which the young gestate.

Reproduction in birds

Male and female birds both have cloacas. The female lays eggs, fertilized by the male, in which the young gestate.

Reproduction in mammals

In placental mammals, the offspring are born as young, complete animals with the entire sets of sex organs, although not functioning. After several months or years, the sex organs in the mammals start to grow and the animal becomes sexually mature. Most mammals are only fertile during certain times of the year; during those times, they are sometimes said to be “in heat.” At this point, the animal is ready to mate. Individual male and female mammals meet and carry out copulation, the beginning stage of sexual reproduction. In primates, the sexual partner for each primate is monogamously specific. For most other mammals, males and females occasionally exchange sexual partners.

The mammalian male

The male reproductive system contains two main divisions: the penis, which is inserted into the female and carries the sperm inside it, and the testes, which produce the sperm. In humans, both of these organs are outside the abdominal cavity, but they can be primarily housed within the abdomen in other animals (for instance, in dogs, the penis is internal except when mating). Having the testes outside the abdomen best facilitates temperature regulation of the sperm, which require specific temperatures to survive. Sperm are the smaller of the two gametes and are generally very short-lived, requiring males to produce them continuously from the time of sexual maturity until death. They are motile and swim by chemotaxis.

The mammalian female

The female reproductive system likewise contains two main divisions: the vagina and uterus, which act as the receptacle for the male's sperm, and the ovaries, which produce the female's ova. All of these parts are always internal. The vagina is attached to the uterus through the cervix, while the uterus is attached to the ovaries via the Fallopian tubes. At certain intervals, the ovaries release an ovum (the singular of ova), which passes through the fallopian tube into the uterus. If, in this transit, it meets with sperm, the sperm penetrate and merge with the egg, fertilizing it. The fertilization usually occurs in the oviducts, but can happen in the uterus itself. The zygote then implants itself in the wall of the uterus, where it begins the processes of embryogenesis and morphogenesis. When developed enough to survive outside the womb, the cervix dilates and contractions of the uterus propel the fetus through the birth canal, which is the vagina. The ova are larger than sperm and are generally all created by birth. They are for the most part stationary, aside from their transit to the uterus, and contain nutrients for the later zygote and embryo. Over a regular interval, a process of oogenesis matures one ovum to be sent down the Fallopian tube attached to its ovary in anticipation of fertilization. If not fertilized, this egg is flushed out of the system through menstruation in humans and great apes and reabsorbed in all other mammals in the estrus cycle.

Gestation

:Main articles: Mammalian gestation, Pregnancy Gestation, called pregnancy in humans, is the period of time during which the fetus develops, dividing via mitosis inside the female. During this time, the fetus receives all of its nutrition and oxygenated blood from the female, filtered through the placenta, which is attached to the fetus' abdomen via an umbilical cord. This drain of nutrients can be quite taxing on the female, who is required to ingest significantly higher levels of calories. In addition, certain vitamins and other nutrients are required in greater quantities than normal, often creating abnormal eating habits. The length of gestation, called the gestation period, varies greatly from species to species; it is 38 weeks in humans, 56-60 in giraffes and 16 days in hamsters.

Birth

Once the fetus is sufficiently developed, chemical signals start the process of birth, which begins with contractions of the uterus and the dilation of the cervix. The fetus then descends to the cervix, where it is pushed out into the vagina, and eventually out of the female. The newborn, which is called an infant in humans, should typically begin respiration on its own shortly after birth. Not long after, the placenta is passed as well. Most mammals eat this, as it is a good source of protein and other vital nutrients needed for caring for the young. The end of the umbilical cord attached to the young’s abdomen eventually falls off on its own.

Monotremes

Monotremes, only five species of which exist, all from Australia and New Guinea, lay eggs. They have one opening for excretion and reproduction called the cloaca. They hold the eggs internally for several weeks, providing nutrients, and then lay them and cover them like birds. After less than two weeks the young hatches and crawls into its mother’s pouch, much like marsupials, where it nurses for several weeks as it grows.

Marsupials

Marsupials reproduce in essentially the same manner, though their young are born at a far earlier stage of development than other mammals. After birth, marsupial joeys crawl into their mother’s pouch and attach to a teat, where they receive nourishment and finish developing into self-sufficient animals.

See also


- Reproduction
- Sex organ
- Flowering plants
- Reptile
- Bird
- Mammals

References

# Pang, K. "Certificate Biology: New Mastering Basic Concepts", Hong Kong, 2003. # [http://www.biolreprod.org/ Journal of Biology of Reproduction], accessed in August 2005. Category:Developmental biology Category:Biological reproduction Category:Sexuality

Gametocyte

A gametocyte is a eukaryotic germ cell that divides by mitosis into other gametocytes or by meiosis into gametes. Male gametocytes are called spermatocytes, and female gametocytes are called oocytes. The development of gametogonia to primary gametocytes and of primary gametocytes to secondary gametocytes is called gametocytogenesis. The term gametocyte is used, for example, when talking about gametocytes of four species of the genus Plasmodium which transmit malaria. Category:Germ cells

Meiosis

:For the article on the figure of speech, see meiosis (figure of speech). :Meiosis should not be confused with miosis, a different process with the same pronunciation. In biology, meiosis is the process that transforms one diploid cell into four haploid cells in eukaryotes in order to redistribute the diploid's cell's genome. Meiosis forms the basis of sexual reproduction and can only occur in eukaryotes. In meiosis, the diploid cell's genome, which is composed of ordered structures of coiled DNA called chromosomes, is replicated once and split twice, producing four sets of haploid cells each containing half of the original cell's chromosomes. These resultant haploid cells will fertilize with other haploid cells of the opposite gender to form a diploid cell again. The cyclical process of splitting by meiosis and genetic recombination through fertilization is called the life cycle. The result is that the offspring produced during germination after meiosis will have a slightly different blueprint which has instructions for the cells to work, contained in the DNA. This allows sexual reproduction to occur. Biochemically, meiosis uses many similar processes mitosis uses in order to manipulate the redistribution of chromosomes, but with a vastly different outcome.

Occurrence of meiosis in eukaryotic life cycles

mitosis mitosis mitosis Meiosis occurs in all eukaryotic life cycles involving sexual reproduction, comprising of the constant cyclical process of meiosis and fertilization. This takes place alongside normal mitotic cell division. In multicellular organisms, there is an intermediary step between the diploid and haploid transition where the organism grows. The organism will then produce the germ cells that continue in the life cycle. The rest of the cells, called somatic cells, function within the organism and will die with it. The organism phase of the life cycle can occur between the haploid to diploid transition or the diploid to haploid transition. Some species are diploid, grown from a diploid cell called the zygote. Others are haploid instead, spawned by the proliferation and differentiation of a single haploid cell called the gamete. Humans, for example, are diploid creatures. Human stem cells undergo meiosis to create haploid gametes, which are sperm cells for males or ova for females. These gametes then fertilize in the uterus of the female, producing a diploid zygote. The zygote undergoes progressive stages of mitosis and differentiation to create an embryo, the early stage of human life. There are three types of life cycles that utilise sexual reproduction, differentiated by the location of the organism stage. In the gametic life cycle, of which humans are a part, the living organism is diploid in nature. Here, we will generalize the example of human reproduction stated previously. The organism's diploid germ-line stem cells undergo meiosis to create haploid gametes, which fertilize to form the zygote. The diploid zygote undergoes repeated cellular division by mitosis to grow into the organism. Mitosis is a related process to meiosis that creates two cells that are genetically identical to the parent cell. The general principle is that mitosis creates somatic cells and meiosis creates germ cells. In the zygotic life cycle, the living organism is haploid. Two organisms of opposing gender contribute their haploid germ cells to form a diploid zygote. The zygote undergoes meiosis immediately, creating four haploid cells. These cells undergo mitosis to create the organism. Fungi and many protozoa are members of the zygotic life cycle. Finally, in the sporic life cycle, the living organism alternates between haploid and diploid states. Consequently, this cycle is also known as the alternation of generations. The diploid organism's germ-line cells undergo meiosis to produce gametes. The gametes proliferate by mitosis, growing into a haploid organism. The haploid organism's germ cells then combine with another haploid organism's cells, creating the zygote. The zygote undergoes repeated mitosis and differentiation to become the diploid organism again. The sporic life cycle can be considered a fusion of the gametic and zygotic life cycles, and indeed its diagram supports this conclusion.

Chromosomal distribution in meiosis

alternation of generations As a diploid cell is formed from the genetic recombination of two haploid cells, within a diploid cell there are normally two sets of chromosomes that code for the same information, one from the mother's haploid cell and the other from the father's. These chromosomes are called homologous chromosomes. Homologous chromosomes need not be genetically identical. For example, one particular locus (location) on one of the father's chromosomes may code for green eyes, while the same locus on the mother's chromosome may code for brown eyes. This genetic variety produced by sexual reproduction is the key to its power. Before division, the genome is replicated. Each chromosome now contains two identical sister chromatids joined together by a region of DNA called the centromere. Meiosis I, the first round of division, separates homologous chromosomes. Meiosis II, the second round of division, separates sister chromatids. There are four haploid cells produced at the conclusion of meiosis. This description suggests that two out of four gametes will contain the maternal set of chromosomes, while the other two will contain the paternal set. In reality, however, the gametes are genetically varied, containing a mix of both paternal and maternal genetic information. This is accomplished in two processes. During meiosis I, the concept of independent assortment is used to redistribute information. Homologous chromosomes will eventually part ways into separate cells. However, homologous chromosomes are oriented independently of their companions. That means that each daughter cell has a fifty-fifty chance of receiving the maternal chromosome or the paternal chromosome. At the same time during meiosis I, when the chromosomes are pairing up together for a short time before being split apart during synapsis, chromosomal crossover occurs. During this time, nonsister chromatids of homologous chromosomes may exchange segments at random locations called chiasmata. The chromosome that is subjected to crossing over is then called a recombinant chromosome. The diagram shown above summarizes the distribution of the chromosomes. Chromosomes which are the same size (one light blue and one red to show parentage) are homologous to each other. They are replicated before meiosis so that each chromosome contains two genetically identical sister chromatids (the vertical bars of the H-like structure). Crossing over occurs between nonsister chromatids of the two homologous chromosomes. Homologous chromosomes are separated in meiosis I. In this case, each daughter cell receives one recombinant mother chromosome and recombinant father chromosome. Meiosis II splits up the sister chromatids. At conclusion, four genetically varied gametes are produced.

Process

Because meiosis is a "one-way" process, it cannot be said to engage in a cell cycle that mitosis does. However, the preparatory steps that lead up to meiosis are identical in pattern and name to the interphase of the mitotic cell cycle. Interphase is divided into three phases:
- Growth 1 (G1) phase: Characterized by increasing cell size from accelerated manufacture of organelles, proteins, and other cellular matter.
- Synthesis (S) phase: The genetic material is replicated.
- Growth 2 (G2) phase: The cell continues to grow. It is immediately followed by meiosis I, which divides one diploid cell into two haploid cells by separation of homologous chromosomes, and meiosis II, which divides two haploid cells into four haploid cells by separation of sister chromatids. Meiosis I and II are both divided into prophase, metaphase, anaphase, and telophase subphases, similar in purpose to their analogous subphases in the mitotic cell cycle. Therefore, meiosis encompasses the interphase (G1, S, G2), meiosis I (prophase I, metaphase I, anaphase I, telophase I), and meiosis II (prophase II, metaphase II, anaphase II, telophase II).

Meiosis I

Prophase I

In the leptotene stage, the cell's genetic material, which is normally in a loosely arranged pile known as chromatin, condenses into visible threadlike structures. Along the thread, centromeres are visible as small beads of tightly coiled chromatin. Recall that centromeres are connection sites between sister chromatids, which are not yet distinguishable. As the chromatin becomes progressively ordered and visible, homologous chromosomes find each other and bind together. In this process, called synapsis, a protein structure called the synaptonemal complex attaches the homologous chromosomes tightly together all along their lengths. The zygotene stage sees the completion of synapsis. The combined homologous chromosomes are said to be bivalent, a reference to the two homologous chromosomes. They may also be referred to as a tetrad, a reference to the four sister chromatidsa. During this stage, one percent of DNA that wasn't replicated during S phase is replicated. The significance of this cleanup act is unclear. The pachytene stage heralds crossing over. Nonsister chromatids of homologous chromosomes exchange segments of genetic information. Because the chromosomes cannot be distinguished in the synaptonemal complex, the actual act of crossing over is not perceivable through the microscope. During the diplotene stage, the synaptonemal complex degrades. Homologous chromosomes fall apart and begin to repel each other. The chromosomes themselves uncoil a bit, allowing some transcription of DNA. They are held together by virtue of recombination nodules, betraying the sites of previous crossing over, the chiasmata. Chromosomes recondense during the diakinesis stage. Sites of crossing over entangle together, effectively overlapping, making chismata clearly visible. In general, every chromosome will have crossed over at least once. The nucleoli disappears and the nuclear membrane disintegrates into vesicles. During these stages, centrioles are migrating to the two poles of the cell. These centrioles, which were duplicated during interphase, function as microtubule coordinating centers. Centrioles sprout microtubules, essentially cellular ropes and poles, during crossing over. They invade the nuclear membrane after it disintegrates, attaching to the chromosomes at the kinetochore. The kinetochore functions as a motor, pulling the chromosome along the attached microtubule toward the originating centriole, like a train on a track. There are two kinetochores on each tetrad, one for each centrosome. Microtubulesa that attach to the kinetochores are known as kinetochore microtubules. Other microtubules will interact with microtubules from the opposite centriole. These are called nonkinetochore microtubules.

Metaphase I

As kinetochore microtubules from both centrioles attach to their respective kinetochores, the homologous chromosomes align equidistant above and below an imaginary equatorial plane, due to continuous counterbalancing forces exerted by the two kinetochores of the bivalent. Because of independent assortment, the orientation of the bivalent along the plane is random. Maternal or paternal homologues may point to either pole.

Anaphase I

Kinetochore microtubules shorten, severing the recombination nodules and pulling homologous chromosomes apart. Since each chromosome only has one kinetochore, whole chromosomes are pulled toward opposing poles, forming two haploid sets. Each chromosome still contains a pair of sister chromatids. Nonkinetochore microtubules lengthen, pushing the centrioles further apart. The cell elongates in preparation for division down the middle.

Telophase I

The first meiotic division effectively ends when the centromeres arrive at the poles. Each daughter cell now has half the number of chromosomes but each chromosome consists of a pair of chromatids. The microtubules that make up the spindle network disappear, and a new nuclear membrane surrounds each haploid set. The chromosomes uncoil back into chromatin. Cytokinesis, the pinching of the cell membrane in animal cells or the formation of the cell wall in plant cells, occurs, competing the creation of two daughter cells. Cells enter a period of rest known as interkinesis or interphase II. No DNA replication occurs during this stage. Note that many plants skip telophase I and interphase II, going immediately into prophase II.

Meiosis II

Prophase II takes an inversely proportional time compared to telophase I. In this prophase we see the disappearance of the nucleoli and the nuclear envelope again as well as the shortening and thickening of the chromatids. Centrioles move to the polar regions and are arranged by spindle fibres. The new equatorial plane is rotated by 90 degrees when compared to meiosis I, perpendicular to the previous plane. In metaphase II, the centromeres contain two kinetochores, organizing fibers from the centrosomes on each side. This is followed by anaphase II, where the centromeres are cleaved, allowing the kinetochores to pull the sister chromatids apart. The sister chromatids by convention are now called sister chromosomes, and they are pulled toward opposing poles. The process ends with telophase II, which is similar to telophase I, marked by uncoiling, lengthening, and disappearance of the chromosomes occur as the disappearance of the microtubules. Nuclear envelopes reform; cleavage or cell wall formation eventually produces a total of four daughter cells, each with an haploid set of chromosomes. Meiosis is complete.

Significance of meiosis

Meiosis facilitates stable sexual reproduction. Without the halving of ploidy, or chromosome count, fertilization would result in zygotes that have twice the number of chromosomes than the zygotes from the previous generation. Successive generations would have an exponential increase in chromosome count, resulting in an unwieldy genome that would cripple the reproductive fitness of the species. Polyploidy, the state of having three or more sets of chromosomes, also results in developmental abnormalities or lethality. Most importantly, however, meiosis produces genetic variety in gametes that propagate to offspring. By crossing over and independent assortment, the gene pool of the species is dynamic and easily adaptable to changing environments and situations. Without genetic variation, progeny would be identical in traits to their parents, a dangerous weakness in a world where survival of the fittest is very much in effect.

Nondisjunction

Nondisjunction is the failure of a chromosome to split correctly during meiosis. This results in the production of gametes which have either more or less of the usual amount of genetic material, and is a common mechanism for trisomy or monosomy. Nondisjunction can occur in the meiosis I or meiosis II phases of cellular reproduction. This is a cause of several medical conditions in humans, including:
- Down Syndrome - trisomy of chromosome 21
- Patau Syndrome - trisomy of chromosome 13
- Edward Syndrome - trisomy of chromosome 18
- Klinefelter Syndrome - an extra X chromosome in males
- Turner Syndrome - only one X chromosome present
- Jacob syndrome- an extra Y chromosome in males

Meiosis in humans

In females, meiosis occurs in precursor cells known as oogonia that divide twice into oocytes. These stem cells stop at the diplotene stage of meiosis I and lay dormant within a protective shell of somatic cells called the follicle. Follicles begin growth at a steady pace in a process known as folliculogenesis, and a small number enter the menstrual cycle. Menstruated oocytes continue meiosis I and arrest at meiosis II until fertilization. The process of meiosis in females is called oogenesis. In males, meiosis occurs in precursor cells known as spermatogonia that divide twice to become sperm. These cells continuously divide without arrest in the seminiferous tubules of the testicles. Sperm is produced at a steady pace. The process of meiosis in males is called spermatogenesis.

See also


- Mitosis
- Ploidy
- Spermatogenesis
- Oogenesis Category:Cell biology ja:減数分裂

Female

Female is the sex of an organism, or a part of an organism, which produces egg cells. The "egg cell" (ovum) is defined as the larger gamete in a heterogamous reproduction system, while the smaller, usually motile gamete (sperm cell) is produced by the male. A female individual cannot reproduce sexually without access to the gametes of a male. Some organisms can reproduce both sexually and asexually. There is no single genetic mechanism behind sex differences in different species, and the existence of two sexes seems to have evolved multiple times independently in different evolutionary lineages. Other than the defining difference in the type of gamete produced, differences between males and females in one lineage cannot always be predicted by differences in another. The concept is not limited to animals; egg cells are produced by chytrids, diatoms, water molds, and land plants, among others. In land plants, 'female' and 'male' designate not only the egg- and sperm-producing organisms and structures, but also the structures of the sporophytes that give rise to male and female plants. A common symbol used to represent the female gender is ♀ (Unicode: U+2640), a circle with a small cross underneath. This symbol also represents the planet Venus and is a stylized representation of the goddess Venus' hand mirror.

See also


- Sex-determination system
- Woman and girl, female humans Category:Sex simple:Female

Male

Male is the sex of an organism, or a part of an organism, which produces sperm. The "sperm" is defined as the smaller, ordinarily motile gamete in a heterogamous reproduction system, while the larger gamete, the ovum, is produced by the female. A male individual cannot reproduce sexually without access to the gametes of a female. There is no single genetic mechanism behind sex differences in different species, and the existence of two sexes seems to have evolved multiple times independently in different evolutionary lineages. Other than the defining difference in the type of gamete produced, differences between males and females in one lineage cannot always be predicted by differences in another. The concept is not limited to animals; sperm cells are produced by chytrids, diatoms, and land plants, among others. In land plants, 'female' and 'male' designate not only the egg- and sperm-producing organisms and structures, but also the structures of the sporophytes that give rise to male and female plants. A common symbol used to represent the male gender is ♂ (Unicode: U+2642), a circle with an arrow pointing northeast. This is a stylized representation of the god Mars' shield and spear.

See also


- Sex-determination system
- Man and boy, male humans Category:Sex ja:オス

Pollen

), morning glory (Ipomea purpurea), hollyhock (Sildalcea malviflora), lily (Lilium auratum), primrose (Oenothera fruticosa), and castor bean (Ricinus communis).]] Pollen is a fine to coarse powder consisting of microgametophytes (pollen grains), which carry the male gametes of seed plants. Each pollen grain contains one or two generative cells (the male gametes) and a vegetative cell. The group of three cells is surrounded by a cellulose cell wall and a thick, tough outer wall made of sporopollenin. Pollen is produced in the microsporangium (anther of an angiosperm flower or male cone of a coniferous plant). Pollen grains come in a wide variety of shapes, sizes, and surface markings characteristic of the species (see photomicrograph at right). Most, but certainly not all, are spherical. Pollen grains of pines, firs, and spruces are winged. The smallest pollen grain, that of the Forget-me-not plant (Myosotis sp.), is around 6 µm (0.006  mm) in diameter. The study of pollen is called palynology and is highly useful in paleontology, archeology, and forensics. Except in the case of some submerged aquatic plants the mature pollen-grain has a double wall, a thin delicate wall of unaltered cellulose (the endospore or intine) and a tough outer cuticularized exospore or exine. The exine often bears spines or warts, or is variously sculptured, and the character of the markings is often of value for identifying genus, species, or even cultivar or individual. Germination of the microspore begins before it leaves the pollen-sac. In very few cases has anything representing prothallial development been observed; generally a small cell (the antheridial or generative cell) is cut off, leaving a larger tube-cell. The transfer of pollen grains to the female reproductive structure (pistil in angiosperms) is called pollination. This transfer can be mediated by the wind, in which case the plant is described as anemophilous (literally wind-loving). Anemophilous plants typically produce great quantities of very lightweight pollen grains, often with air-sacs, and generally have inconspicuous flowers. Entomophilous (literally insect-loving) plants produce pollen that is relatively heavy, sticky and protein-rich, for dispersal by insect pollinators attracted to their flowers. When placed on the stigma, under favorable circumstances, the pollen-grain puts forth a pollen-tube which grows down the tissue of the style to the ovary, and makes its way along the placenta, guided by projections or hairs, to the mouth of an ovule. The nucleus of the tube-cell has meanwhile passed into the tube, as does also the generative nucleus which divides to form two male- or sperm-cells. The male-cells are carried to their destination in the tip of the pollen-tube.

Hay fever

Main article: Hay fever Allergy to pollen is called hay fever. Generally pollens that cause allergies are those of anemophilous, because the lightweight pollen grains are produced in great quantities for wind dispersal. Breathing air containing these pollen grains brings them into contact with the nasal passages. In the US, people often falsely blame the conspicuous entomophilous goldenrod flower for allergies. Since this pollen does not become airborne, the only way to get goldenrod pollen on the nasal passages would be to stick the flower up one's nose. The late summer and fall pollen allergies are usually caused by ragweed, a widespread anemophilous plant. Arizona was once regarded as a haven for people with pollen allergies, since ragweed does not grow in the desert. However, as suburbs grew and people began establishing irrigated lawns and gardens, ragweed gained a foothold and Arizona lost its claim of freedom from hay fever. Anemophilous spring blooming plants such as oak, birch, hickory, pecan, and early summer grasses may also induce pollen allergies. Cultivated flowers are most often entomophilous and do not cause allergies.

Miscellaneous

The "tapping panel dryness disease" of the rubber plant is caused by a virus transmitted on pollen grains. virus Pollen is sold as a nutritional supplement, marketed as "bee pollen" (even though it is of course from flowers). There is doubt amongst conventional practitioners that taking pollen has any biological effect, although may possibly cause allergic reactions in sensitive people. Many trees and flowering plants are a good source of pollen for honeybees. Bees will collect pollen from some grasses and grains when they cannot find pollens with more nutritional value, however, anemophilous plants such as grasses generally have very low real value to bees. Some windblown pollen is likely to be inadvertently collected by bees, since they bear a static charge. Ragweed and pine pollen can settle on leaves and other flowers, to add to the total quantity of pollens that are found upon analysis of gathered pollen.

External links

static
- [http://www.geo.arizona.edu/palynology/polident.html Pollen and Spore Identification Literature]
- [http://www.flmnh.ufl.edu/natsci/paleobotany/paleobotany.htm Paleobotany and Palynology at the Florida Museum of Natural History]
- [http://www.geo.arizona.edu/palynology/ Palynology at the University of Arizona]
- [http://www.shef.ac.uk/uni/academic/N-Q/palysc/palyshef.html Palynology at the University of Sheffield]
- [http://www.bio.uu.nl/~palaeo/Engels/engels.html Palynology in Utrecht, the Netherlands]
- [http://www.quackwatch.org/01QuackeryRelatedTopics/DSH/bee.html Bee Pollen, Royal Jelly, and Propolis] - A sceptical view of the benefits of taking bee pollen.
- [http://www.watchtower.org/library/g/2003/7/22a/article_01.htm Pollen - Menace or Miracle?], discussion on its purpose in biology and as an allergen Category:Botany Category: plant anatomy Category: plant morphology Category:Pollination ja:花粉

Gonad

The gonad is the organ that makes gametes. Gametes are haploid germ cells. For example, sperm and egg cells are gametes. In vernacular use, gonads is slang for testicles. The usage of such is similar to "balls." As such, in an era where more women are reaching levels of power, one can replace the vernacular saying "He's got balls" to the gender-neutral "They've got gonads". Gonads start developing as a common anlage, and only later are differentiated to male or female sex organs. The SRY gene, located on the Y chromosome and encoding the testis determining factor, decides the direction of this differentiation. Category:Reproductive system

Animal

:For the Muppet Show character, see Animal (Muppet). For the professional wrestler, see Joseph Laurinaitis.

    - Porifera (sponges)
    - Ctenophora (comb jellies)
    - Cnidaria (coral, jellyfish, anenomes)
    - Placozoa (trichoplax)
- Subregnum Bilateria (bilateral symmetry)
    - Acoelomorpha (basal)
    - Orthonectida (flatworms, echinoderms, etc.)
  - Rhombozoa (dicyemids)
  - Myxozoa (slime animals)
  - Superphylum Deuterostomia (blastopore becomes anus)
    - Chordata (vertebrates, etc.)
    - Hemichordata (acorn worms)
    - Echinodermata (starfish, urchins)
    - Chaetognatha (arrow worms)
  - Superphylum Ecdysozoa (shed exoskeleton)
    - Kinorhyncha (mud dragons)
    - Loricifera
    - Priapulida (priapulid worms)
    - Nematoda (roundworms)
    - Nematomorpha (horsehair worms)
    - Onychophora (velvet worms)
    - Tardigrada (water bears)
    - Arthropoda (insects, etc.)
  - Superphylum Platyzoa
    - Platyhelminthes (flatworms)
    - Gastrotricha (gastrotrichs)
    - Rotifera (rotifers)
    - Acanthocephala (acanthocephalans)
    - Gnathostomulida (jaw worms)
    - Micrognathozoa (limnognathia)
    - Cycliophora (pandora)
  - Superphylum Lophotrochozoa (trochophore larvae / lophophores)
    - Sipuncula (peanut worms)
    - Nemertea (ribbon worms)
    - Phoronida (horseshoe worms)
    - Ectoprocta (moss animals)
    - Entoprocta (goblet worms)
    - Brachiopoda (brachipods)
    - Mollusca (mollusks)
    - Annelida (segmented worms) Animals are a major group of organisms, classified as the kingdom Animalia or Metazoa. In general they are multicellular, capable of locomotion and responsive to their environment, and feed by consuming other organisms. Their body plan becomes fixed as they develop, usually early on in their development as embryos, although some undergo a process of metamorphosis later on. Along with sponges, gastropods, emus, dolphins and all other animals, Homo sapiens sapiens meet all the criteria above for membership in the group of organisms known as animals and they do not meet the criteria of the other groups. Some humans often consider themselves separate from animals, not on the grounds of biology, but through the use of "other contexts". Whilst self-delusion may be a unique characteristic of the human species it is not cause for exclusion from the Kingdom Animalia. The name animal comes from the Latin word animal, of which animalia is the plural, and ultimately from anima, meaning vital breath or soul.

Characteristics

Aristotle divided the living world between animals and plants, and this was followed by Carolus Linnaeus in the first hierarchical classification. Since then biologists have begun emphasizing evolutionary relationships, and so these groups have been restricted somewhat. For instance, microscopic protozoa were originally considered animals because they move, but are now treated separately. Kingdom Animalia has several characteristics that set it apart from other living things. First, animals are eukaryotic. This separates them from the Kingdom Monera. Second, animals are multicellular, which separates them from Kingdom Protista. Third, they are heterotrophic, setting them apart from Kingdom Plantae and several plant-like protists. Finally, Kingdom Animalia consists of organisms without cell walls, which makes it unique compared to Kingdom Plantae, algae, and Kingdom Fungi.

Structure

With a few exceptions, most notably the sponges (Phylum Porifera), animals have bodies differentiated into separate tissues. These include muscles, which are able to contract and control locomotion, and a nervous system, which sends and processes signals. There is also typically an internal digestive chamber, with one or two openings. Animals with this sort of organization are called metazoans, or eumetazoans when the former is used for animals in general. All animals have eukaryotic cells, surrounded by a characteristic extracellular matrix composed of collagen and elastic glycoproteins. This may be calcified to form structures like shells, bones, and spicules. During development it forms a relatively flexible framework upon which cells can move about and be reorganized, making complex structures possible. In contrast, other multicellular organisms like plants and fungi have cells held in place by cell walls, so develop by progressive growth. Also, unique to animal cells are the following intercellular junctions: tight junctions, gap junctions, and desmosomes.

Reproduction and development

Nearly all animals undergo some form of sexual reproduction. Adults are diploid or occasionally polyploid. They have a few specialized reproductive cells, which undergo meiosis to produce smaller motile spermatozoa or larger non-motile ova. These fuse to form zygotes, which develop into new individuals. Many animals are also capable of asexual reproduction. This may take place through parthenogenesis, where fertile eggs are produced without mating, or in some cases through fragmentation. A zygote initially develops into a hollow sphere, called a blastula, which undergoes rearrangement and differentiation. In sponges, blastula larvae swim to a new location and develop into a new sponge. In most other groups, the blastula undergoes more complicated rearrangement. It first invaginates to form a gastrula with a digestive chamber, and two separate germ layers - an external ectoderm and an internal endoderm. In most cases, a mesoderm also develops between them. These germ layers then differentiate to form tissues and organs. Animals grow by indirectly using the energy of sunlight. Plants use this energy to turn air into simple sugars using a process known as photosynthesis. These sugars are then used as the building blocks which allow the plant to grow. When animals eat these plants (or eat other animals which have eaten plants), the sugars produced by the plant are used by the animal. They are either used directly to help the animal grow, or broken down, releasing stored solar energy, and giving the animal the energy required for motion. This process is known as glycolysis.

Origin and fossil record

Animals are generally considered to have evolved from flagellate protozoa. Their closest living relatives are the choanoflagellates, collared flagellates that have the same structure as certain sponge cells do. Molecular studies place them in a supergroup called the opisthokonts, which also include the fungi and a few small parasitic protists. The name comes from the posterior location of the flagellum in motile cells, such as most animal sperm, whereas other eukaryotes tend to have anterior flagella. The first fossils that might represent animals appear towards the end of the Precambrian, around 600 million years ago, and are known as the Vendian biota. These are difficult to relate to later fossils, however. Some may represent precursors of modern phyla, but they may be separate groups, and it is possible they are not really animals at all. Aside from them, most animal phyla with known phyla make a more or less simultaneous appearance during the Cambrian period, about 570 million years ago. It is still disputed whether this event, called the Cambrian explosion, represents a rapid divergence between different groups or a change in conditions that made fossilization possible.

Groups of animals

The sponges (Porifera) diverged from other animals early. As mentioned, they lack the complex organization found in most other phyla. Their cells are differentiated, but not organized into distinct tissues. Sponges are sessile and typically feed by drawing in water through pores all over the body, which is supported by a skeleton typically divided into spicules. The extinct Archaeocyatha, which have fused skeletons, may represent sponges or a separate phylum. Among the eumetazoan phyla, two are radially symmetric and have digestive chambers with a single opening, which serves as both the mouth and the anus. These are the Cnidaria, which include anemones, corals, and jellyfish, and the Ctenophora or comb jellies. Both have distinct tissues, but they are not organized into organs. There are only two main germ layers, the ectoderm and endoderm, with only scattered cells between them. As such, these animals are sometimes called diploblastic. The tiny phylum Placozoa is similar, but individuals do not have a permanent digestive chamber. The remaining animals form a monophyletic group called the Bilateria. For the most part, they are bilaterally symmetric, and often have a specialized head with feeding and sensory organs. The body is triploblastic, i.e. all three germ layers are well-developed, and tissues form distinct organs. The digestive chamber has two openings, a mouth and an anus, and there is also an internal body cavity called a coelom or pseudocoelom. There are exceptions to each of these characteristics, however - for instance adult echinoderms are radially symmetric, and certain parasitic worms have extremely simplified body structures. Genetic studies have considerably changed our understanding of the relationships within the Bilateria. Most appear to belong to four major lineages: # Deuterostomes # Ecdysozoa # Platyzoa # Lophotrochozoa In addition to these, there are a few small groups of bilaterians with relatively similar structure that appear to have diverged before these major groups. These include the Acoelomorpha, Rhombozoa, and Orthonectida. The Myxozoa, single-celled parasites that were originally considered Protozoa, are now believed to have developed from the Bilateria as well.

Deuterostomes

Deuterostomes differ from the other Bilateria, called protostomes, in several ways. In both cases there is a complete digestive tract. However, in protostomes the initial opening (the archenteron) develops into the mouth, and an anus forms separately. In deuterostomes this is reversed. In most protostomes cells simply fill in the interior of the gastrula to form the mesoderm, called schizocoelous development, but in deuterostomes it forms through evagination of the endoderm, called enterocoelic pouching. Deuterostomes also have a dorsal, rather than a ventral, nerve chord and their embryos undergo different cleavage. All this suggests the deuterostomes and protostomes are separate, monophyletic lineages. The main phyla of deuterostomes are the Echinodermata and Chordata. The former are radially symmetric and exclusively marine, such as sea stars, sea urchins, and sea cucumbers. The latter are dominated by the vertebrates, animals with backbones. These include fish, amphibians, reptiles, birds, and mammals. In addition to these, the deuterostomes also include the Hemichordata or acorn worms. Although they are not especially prominent today, the important fossil graptolites may belong to this group. The Chaetognatha or arrow worms may also be deuterostomes, but this is less certain.

Ecdysozoa

The Ecdysozoa are protostomes, named after the common trait of growth by moulting or ecdysis. The largest animal phylum belongs here, the Arthropoda, including insects, spiders, crabs, and their kin. All these organisms have a body divided into repeating segments, typically with paired appendages. Two smaller phyla, the Onychophora and Tardigrada, are close relatives of the arthropods and share these traits. The ecdysozoans also include the Nematoda or roundworms, the second largest animal phylum. Roundworms are typically microscopic, and occur in nearly every environment where there is water. A number are important parasites. Smaller phyla related to them are the Nematomorpha or horsehair worms, which are visible to the unaided eye, and the Kinorhyncha, Priapulida, and Loricifera, which are all microscopic. These groups have a reduced coelom, called a pseudocoelom. The remaining two groups of protostomes are sometimes grouped together as the Spiralia, since in both embryos develop with spiral cleavage.

Platyzoa

The Platyzoa include the phylum Platyhelminthes, the flatworms. These were originally considered some of the most primitive Bilateria, but it now appears they developed from more complex ancestors. A number of parasites are included in this group, such as the flukes and tapeworms. Flatworms lack a coelom, as do their closest relatives, the microscopic Gastrotricha. The other platyzoan phyla are microscopic and pseudocoelomate. The most prominent are the Rotifera or rotifers, which are common in aqueous environments. They also include the Acanthocephala or spiny-headed worms, the Gnathostomulida, Micrognathozoa, and possibly the Cycliophora. These groups share the presence of complex jaws, from which they are called the Gnathifera.

Lophotrochozoa

The Lophotrochozoa include two of the most successful animal phyla, the Mollusca and Annelida. The former includes animals such as snails, clams, and squids, and the latter comprises the segmented worms, such as earthworms and leeches. These two groups have long been considered close relatives because of the common presence of trochophore larvae, but the annelids were considered closer to the arthropods, because they are both segmented. Now this is generally considered convergent evolution, owing to many morphological and genetic differences between the two phyla. The Lophotrochozoa also include the Nemertea or ribbon worms, the Sipuncula, and several phyla that have a fan of cilia around the mouth, called a lophophore. These were traditionally grouped together as the lophophorates, but it now appears they are paraphyletic, some closer to the Nemertea and some to the Mollusca and Annelida. They include the Brachiopoda or lamp shells, which are prominent in the fossil record, the Entoprocta, the Phoronida, and possibly the Ectoprocta or moss animals.

History of classification

In Linnaeus' original scheme, the animals were one of three kingdoms, divided into the classes of Vermes, Insecta, Pisces, Amphibia, Aves, and Mammalia. Since then the last four have all been subsumed into a single phylum, the Chordata, whereas the various other forms have been separated out. The above lists represent our current understanding of the group, though there is some variation from source to source.

Usage of the word animal

In everyday usage animal refers to any member of the animal kingdom that is not a human being, and sometimes excludes insects (although including such arthropods as crabs). This confusion stems primarily from the familiarity with zoo animals, farm animals and pets, not from an analytical distinction between insects, humans and the rest of the animal kingdom.

Examples

Some well-known types of animals, listed by their common names:
- alpaca, ant, antelope, badger, bat, bear, bee, beetle, bird, bison, butterfly, cat, chicken, cockroach, coral, cow, deer, dinosaur, dog, dolphin, earthworm, elephant, elk, fish, fly, fox, frog, giraffe, goat, gorilla, hippopotamus, horse, human, iguana, jellyfish, kangaroo, lion, lizard, llama, lynx, monkey, mouse, nightingale, octopus, owl, ox, parrot, penguin, pig, quail, rabbit, rat, rhinoceros, salamander, scorpion, seahorse, shark, sheep, sloth, snake, spider, squid, starfish, tiger, turtle, urial, vole, whale, wolf, yak, zebra

See also


- Altruism in animals
- Amphibian
- Animal intelligence
- Animal locomotion
- Animal rights
- Biblical terms
  - Clean animals
  - Unclean animals
- Biology
- Biota
- Bird
- Fish
- Insect
- Mammal
- Macrofossil
- Prehistoric life
- Reptile
- Zoology
- Zoo

References

External links


- [http://www.animool.com/animals/index.jsp Animals Search Engine]
- [http://www.wikianimals.com wikianimals.com] - Documenting the animal kingdom
- [http://tolweb.org/tree?group=Animals&contgroup=Eukaryotes Tree of Life]
- [http://www.arkive.org A Multimedia Database of Various UK or Endangered Species]
- [http://freepages.genealogy.rootsweb.com/~wakefield/animals.html Animals and Birds Names] - Large table of words: animal, collective, male, female, young, & home
- [http://www273.pair.com/med/words/animal_adjectives.htm English Animal Adjectives]
- [http://www.georgetown.edu/faculty/ballc/animals/animals.html Sounds of the World's Animals] - animal sounds in many languages
- [http://www.findsounds.com/ FindSounds - Search the Web for Sounds] - sound files including animal sound files
- [http://www.australianfauna.com/ Australian Animals]
- [http://www.animalreviews.com AnimalReviews] - animals reviewed and evaluated
- [http://animals.timduru.org/ The animal photo archive] - Photos of animals
- [http://www.wildlife-photo.org Photo gallery of animals pictures from the entire world.]
- [http://www.wildlife-photo.org/birds_list.htm Birds Name Check List in Latin, English, Russian and Hebrew.]
- [http://www.wildanimalsonline.com Wild Animals Online] - an online encyclopedia of wild animals - facts, photos Category:Animals zh-min-nan:Tōng-bu̍t ko:동물 ms:Haiwan ja:動物 simple:Animal th:สัตว์

Archegonium

An archegonium (pl: archegonia) (from the Greek arche meaning "beginning" and gonos meaning "born") is a multicellular structure or organ of the gametophyte phase of certain plants producing and containing the ovum or female gamete. The archegonium has a long neck and a swollen base. Archegonia are typically located on the surface of the plant thallus, although in the horned liverworts they are embedded. They are also much-reduced and embedded in the megasporangium of gymnosperms. The term is not used for angiosperms or the gnetophytes Gnetum and Welwitschia because the comparable "structure" is reduced to just a few cells, and the function of surrounding the gamete is completely assumed by diploid cells of the megasporangium. The corresponding male organ is called the antheridium. ---- Category:Plant anatomy Category: Cryptogams category: gymnosperms

Plant


- Land plants (embryophytes)
  - Non-vascular plants (bryophytes)
    - Marchantiophyta - liverworts
    - Anthocerotophyta - hornworts
    - Bryophyta - mosses
  - Vascular plants (tracheophytes)
    - Lycopodiophyta - clubmosses
    - Equisetophyta - horsetails
    - Pteridophyta - "true" ferns
    - Psilotophyta - whisk ferns
    - Ophioglossophyta - adderstongues
    - Seed plants (spermatophytes)
      - †Pteridospermatophyta - seed ferns
      - Pinophyta - conifers
      - Cycadophyta - cycads
      - Ginkgophyta - ginkgo
      - Gnetophyta - gnetae
      - Magnoliophyta - flowering plants Magnoliophyta Plants are a major group of living things (about 300,000 species), including familiar organisms such as trees, flowers, herbs, and ferns. Aristotle divided all living things between plants, which generally do not move or have sensory organs, and animals. In Linnaeus' system, these became the Kingdoms Vegetabilia (later Plantae) and Animalia. Since then, it has become clear that the Plantae as originally defined included several unrelated groups, and the fungi and several groups of algae were removed to new kingdoms. However, these are still often considered plants in many contexts. Indeed, any attempt to match "plant" with a single taxon is doomed to fail, because plant is a vaguely defined concept unrelated to the presumed phylogenic concepts on which modern taxonomy is based.

Embryophytes

:See main article at Embryophytes Most familiar are the multicellular land plants, called embryophytes. They include the vascular plants, plants with full systems of leaves, stems, and roots. They also include a few of their close relatives, often called bryophytes, of which mosses and liverworts are the most common. All of these plants have eukaryotic cells with cell walls composed of cellulose, and most obtain their energy through photosynthesis, using light and carbon dioxide to synthesize food. About three hundred plant species do not photosynthesize but are parasites on other species of photosynthetic plants. Plants are distinguished from green algae, from which they evolved, by having specialized reproductive organs protected by non-reproductive tissues. Bryophytes first appeared during the early Palaeozoic. They can only survive where moisture is available for significant periods, although some species are desiccation tolerant. Most species of bryophyte remain small throughout their life-cycle. This involves an alternation between two generations: a haploid stage, called the gametophyte, and a diploid stage, called the sporophyte. The sporophyte is short-lived and remains dependent on its parent gametophyte. Vascular plants first appeared during the Silurian period, and by the Devonian had diversified and spread into many different land environments. They have a number of adaptations that allowed them to overcome the limitations of the bryophytes. These include a cuticle resistant to desiccation, and vascular tissues which transport water throughout the organism. In most the sporophyte acts as a separate individual, while the gametophyte remains small. Devonians (Pteridophyta) more closely allied to seed plants than they are to clubmosses (Lycopodiophyta)]] The first primitive seed plants, Pteridosperms (seed ferns) and Cordaites, both groups now extinct, appeared in the late Devonian and diversified through the Carboniferous, with further evolution through the Permian and Triassic periods. In these the gametophyte stage is completely reduced, and the sporophyte begins life inside an enclosure called a seed, which develops while on the parent plant, and with fertilisation by means of pollen grains. Whereas other vascular plants, such as ferns, reproduce by means of spores and so need moisture to develop, some seed plants can survive and reproduce in extremely arid conditions. Early seed plants are referred to as gymnosperms (naked seeds), as the seed embryo is not enclosed in a protective structure at pollination, with the pollen landing directly on the embryo. Four surviving groups remain widespread now, particularly the conifers, which are dominant trees in several biomes. The angiosperms, comprising the flowering plants, were the last major group of plants to appear, emerging from within the gymnosperms during the Jurassic and diver