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James D. Watson

James D. Watson

James Dewey Watson (born April 6, 1928) is one of the discoverers of the structure of the DNA molecule. Born in Chicago, he has been fascinated by birds since he was a child due to the influence of his father. At the age of 12, he starred on the Quiz Kids, a popular radio show that challenged precocious youngsters to answer difficult questions. Thanks to the liberal policy of Robert Hutchins, he enrolled at the age of 15 at the University of Chicago. After reading Erwin Schrodinger's book What Is Life? in 1946, he changed his direction from ornithology to genetics. Watson earned a B.Sc. in Zoology in 1947. He, Francis Crick (both of the Cavendish Laboratory, Cambridge) and Maurice Wilkins (of King's College London) were awarded the 1962 Nobel Prize for Physiology or Medicine, for their determination of the structure of DNA. The contribution made by Rosalind Franklin to the discovery has been subsequently recognised in the scientific literature about the period with biographies by Anne Sayre and more recently Brenda Maddox, plus her colleague Sir Aaron Klug's Darwin Lecture in 2003, which is published by CUP in "DNA Changing Science and Society".

The Phage Group

Upon arriving at Indiana University at Bloomington to start graduate studies, Watson was attracted to the work of Salvador Luria. Luria eventually shared a Nobel prize for his work on the Luria-Delbruck experiment, which concerned the nature of genetic mutations. Luria was part of a distributed group of researchers who were making use of the viruses that infect bacteria in order to explore genetics. Luria and Max Delbrück were among the leaders of this new "Phage Group", an important movement of geneticists from experimental systems such as Drosophila towards microbial genetics. Early in 1948 Watson began his Ph.D. research in Luria's laboratory and that spring he got to meet Delbrück in Luria's apartment and again that summer during Watson's first trip to the Cold Spring Harbor Laboratory. The Phage Group was the intellectual medium within which Watson became a working scientist. Importantly, the members of the Phage Group had a sense that they were on the path to discovering the physical nature of the gene. In 1949 Watson took a course with Felix Haurowitz that included the conventional view of that time: that proteins were genes and able to replicate themselves. The other major molecular component of chromosomes, DNA, was thought by many to be a "stupid tetranucleotide", serving only a structural role to support the proteins. However, even at this early time, Watson, under the influence of the Phage Group, was aware of the work of Oswald Avery which suggested that DNA was the genetic molecule. Watson's research project involved using X-rays to inactivate bacterial viruses ("phage"). He gained his Ph.D. in Zoology at Indiana University in 1950. Watson then went to Europe for postdoctoral research, first heading to the laboratory of biochemist Herman Kalckar in Copenhagen who was interested in nucleic acids and had developed an interest in phage as an experimental system. Watson's time in Copenhagen had one favorable consequence. He was able to do some experiments with Ole Maaloe (a member of the Phage Group) that were consistent with DNA being the genetic molecule. Watson had learned about these kinds of experiments the previous summer at Cold Spring Harbor. The experiments involved radioactive phosphate as a tracer and attempted to determine what molecular components of phage particles actually infect the target bacteria during viral infection. Watson never developed a constructive interaction with Kalckar, but he did accompany Kalckar to a meeting in Italy where Watson saw Maurice Wilkins talk about his X-ray diffraction data for DNA. Watson was now certain that DNA had a definite molecular structure that could be solved. In 1951 the highly regarded Nobel Prize winning chemist Linus Pauling published his model of the protein alpha helix, a result that grew out of Pauling's relentless efforts in X-ray crystallography and molecular model building. Watson now had the desire to learn to perform X-ray diffraction experiments so that he could work to determine the structure of DNA. That summer, Luria met John Kendrew and arranged for a new postdoctoral research project for Watson in England.

The Structure of DNA

In October 1951, Watson started at the Cavendish Laboratory, the physics department of the University of Cambridge, where he met Francis Crick. Watson and Crick started an intense intellectual collaboration that in less than a year and a half resulted in their discovery of the structure of DNA. They had unique qualifications to bring to bear on the problem. Crick soon solved the mathematical equations that govern helical diffraction theory; Watson knew all of the key DNA results of the Phage Group. In April 1952, Watson's PhD research advisor, Luria, was to speak at a meeting in England. However, Luria was not allowed to travel due to cold war hysteria over his marxist leanings. Watson used Luria's speaking slot to talk about his own work with radioactive DNA and the results of others in the Phage Group that indicated the genetic material of phages was DNA. It has been recorded that during this meeting Watson was passing on to others prior discoveries by local DNA researcher Maurice Wilkins such as the calculated width of the B-form molecule as determined by X-ray diffraction studies. By 1952 estimates from X-ray data and electron microscopy agreed that the diameter of DNA was about 2 nanometers. Watson and Crick benefitted from two travel-related strokes of luck in 1952. First, Erwin Chargaff visited England in 1952 and rubbed Watson's and Crick's noses in the fact that they knew almost nothing about nucleotide biochemistry: they soon repaired their deficiency. And second, Linus Pauling did NOT visit England. His planned visit was cancelled for political reasons and he never gained access to the King's College X-ray diffraction data for DNA until it was published in 1953, along with the Watson-Crick model. With his extensive expertise, Pauling might very well have deduced the structure of DNA a year before Watson and Crick if only he had had access to this information. It was also in 1952 that the final details of the chemical structure of the DNA backbone was determined by biochemists like Alexander Todd. During 1952, Crick and Watson had been asked not to work on making molecular models of the structure of DNA. Instead, Watson's official assignment was to perform X-ray diffraction experiments on tobacco mosaic virus. Tobacco mosaic virus was the first virus to be identified (1886) and purified (1935). Since electron microscopy revealed that virus crystals form inside infected plants, it made sense to isolate this virus for study by X-ray crystallography. Early X-ray diffraction images for tobacco mosaic virus had been collected before World War II. By 1954, Watson had deduced from his X-ray diffraction images that the tobacco mosaic virus had a helical structure. But despite his official assignment, the lure of solving the puzzle of DNA structure continued to tantalize Watson; with his friend Crick, he continued to work on this topic without official sanction. Linus Pauling had made use of molecular models to solve the structure of the protein alpha helix. Could Watson and Crick similarly solve the structure of DNA? Did Watson + Crick = Pauling? Not quite. Pauling had personally attained what was possibly the world's greatest understanding of chemistry. Neither Watson nor Crick knew much chemistry. But local X-ray crystallography expert Rosalind Franklin who had already done extensive work on DNA was within easy reach in London for consultations and other key chemical knowledge they needed would drop into their laps in 1952. Building upon (some might say stealing) the unpublished X-ray diffraction research of Franklin and Wilkins, together Watson and Crick deduced the double helix structure of DNA which they published in the journal Nature on April 25, 1953. Watson's key contribution was in discovering the nucleotide base pairs that are the key to the structure and function of DNA. This key discovery was made in the Pauling "tradition", by playing with molecular models of the four nucleobases. After he realized that A:T and C:G pairs are structurally similar it was immediately clear that such structural pairing accounted for a key biochemical fact of DNA, the so-called Chargaff ratios, experimentally determined ratios of the amounts of the four nucleotide subunits of DNA: the amount of guanine is equal to cytosine and the amount of adenine is equal to thymine. Watson's exercise in molecular modeling was facilitated by Jerry Donohue who explained to Watson and Crick the correct structures of the four bases. This allowed Watson to visually line up the complementary base pairs that could be held together by hydrogen bonds. Franklin's key contribution was when she told Watson and Crick that the phosphate backbones of DNA should be on the outside of the molecule. When Watson and Crick finally accepted this fact, the bases had to go to the inside of the DNA structure where they would have to interact chemically. Watson discovered the nature of that interaction. For their efforts, Watson, Crick, and Wilkins were awarded the Nobel Prize in Physiology or Medicine in 1962 for their discovery of DNA structure. Franklin's apparent exclusion from this Nobel Prize was due to her death in 1958 before it was awarded; unfortunately for Franklin, the Nobel Prize is not awarded posthumously. Some in the molecular biology community believe that since Franklin died early, and Wilkins was much less of a publicity-seeker, that Watson and Crick have in the popular mind overshadowed Wilkins and Franklin to an undeserved degree. In 1968 Watson wrote The Double Helix, one of the Modern Library's 100 best non-fiction books. The account is the sometimes painful story of not only the discovery of the structure of DNA, but the personalities, conflicts and controversy surrounding their work. Watson's original title was to have been "Honest Jim", in part to raise the ethical questions of sneaking around behind Franklin's back to gain access to her X-ray diffraction data before they were published. Watson seems to have never been particularly bothered by the way things turned out. If all that mattered was beating Pauling to the structure of DNA, then Franklin's cautious approach to analysis of the X-ray data was simply an obstacle that Watson needed to run around. Wilkins and others were there at the right time to help Watson and Crick do so. Also in 1968, Watson became the director of the CSHL (Cold Spring Harbor Laboratory) and made the CSHL his permanent residence in 1974. The Double Helix changed the way the public viewed scientists and the way they work. In the same way, Watson's first textbook, The Molecular Biology of the Gene set a new standard for textbooks, particularly through the use of concept heads - brief declarative subheadings. Its style has been emulated by almost all succeeding textbooks. His next great success was Molecular Biology of the Cell although here his role was more of coordinator of an outstanding group of scientist-writers. His third textbook was Recombinant DNA which used the ways in which genetic engineering has brought us so much new information about how organisms function. All the textbooks are still in print.

Genome Project

In 1988, Watson's achievement and success led to his appointment as the Head of the Human Genome Project at the National Institutes of Health, a position he held until 1992. Watson left the Genome Project after conflicts with the new NIH Director, Bernardine Healy. Watson was opposed to Healy's attempts to commercialize genes by granting patents on genes and ownership rights based on the identification of gene sequences. Watson left due to the legal technicality of it not being acceptable for the head of the Genome Project to at the same time have a job like the one Watson still held at Cold Spring Harbor Laboratory. Like his late colleague, Francis Crick, Watson is an outspoken atheist, known for his frank opinions on politics, religion, and the role of science in society. He has been considered to hold a number of controversial views. He is for instance a strong proponent of genetically modified crops, holding that the benefits far outweigh any plausible environmental dangers, and that many of the arguments against GM crops are unscientific or irrational. His views on these matters are covered in some depth in his book DNA: The Secret of Life (2003), particularly in chapter 6. He has also repeatedly said in public lectures "that if the gene (for homosexuality) were discovered and a woman decided not to give birth to a child that may have a tendency to become homosexual, she should be able to abort the fetus." [http://www.warroom.com/fadinggene.htm] In 1994, Watson gave up his position of director and became president of the CSHL for ten years. Currently, Watson gives public speeches and serves as chancellor of the Cold Spring Harbor Laboratory in Cold Spring Harbor, New York.

References

# "The properties of x-ray inactivated bacteriophage. I. Inactivation by direct effect." by J. D. Watson in Journal of Bacteriology (1950) volume 60 page 697-718. The [http://www.pubmedcentral.gov/picrender.fcgi?artid=385941&blobtype=pdf full text] of this article is available for download in PDF format. # See Chapter 2 of The Eighth Day of Creation: Makers of the Revolution in Biology by Horace Freeland Judson published by Cold Spring Harbor Laboratory Press (1996) ISBN 0879694785. # "The structure of tobacco mosaic virus. I. X-ray evidence of a helical arrangement of sub-units around the longitudinal axis" by J. D. Watson in Biochim Biophys Acta. (1954) volume 13 pages 10-19. # [http://www.nature.com/genomics/human/watson-crick/ Molecular structure of Nucleic Acids] by James D. Watson and Francis H. Crick. Nature 171, 737–738 (1953).

External links

The British Library: "Beautiful Minds" exhibition: http://www.bl.uk/onlinegallery/features/beautifulminds/homepage.html and listen to Francis Crick on: http://www.bl.uk/onlinegallery/features/beautifulminds/sounds.html#compton
- [http://www.cshl.edu/gradschool/jdw_.html James D. Watson, Ph.D. - Cold Spring Harbor Laboratory]
- [http://www.kcl.ac.uk/depsta/ppro/dna/scientists.html The King's College London team]
- [http://www.bbc.co.uk/bbcfour/audiointerviews/profilepages/crickwatson1.shtml Audio of Francis Crick and James Watson talking on the BBC in 1962, 1972, and 1974]
- [http://nobelprize.org/medicine/laureates/1962/watson-bio.html Nobel biography]
- [http://encarta.msn.com/encyclopedia_761560789/Watson_James_Dewey.html MSN Encarta biography] for "Watson, James Dewey". Watson, James Dewey Watson Watson, James Dewey Watson, James, D. Watson, James D. Watson, James D. ko:제임스 왓슨 ja:ジェームズ・ワトソン

April 6

April 6 is the 96th day of the year in the Gregorian calendar (97th in leap years). There are 269 days remaining.

Events

- 648 BC - Earliest solar eclipse recorded by the Ancient Greeks.
- 402 - Stilicho stymies the Visigoths under Alaric in the Battle of Pollentia
- 1320 - The Scots reaffirm their independence by signing the Declaration of Arbroath.
- 1327 - The poet Petrarch first saw his idealized love Laura in the church of Saint Claire in Avignon.
- 1652 - Dutch sailor Jan van Riebeeck establishes a resupply camp at the Cape of Good Hope, which will eventually develop into Cape Town.
- 1782 - Rama I succeeds King Taksin of Thailand, who was overthrown in a coup d'état.
- 1808 - John Jacob Astor incorporates the American Fur Company.
- 1830 - The Church of Jesus Christ of Latter-day Saints is formed by Joseph Smith, Jr. at Fayette, New York.
- 1832 - Indian Wars: Black Hawk War begins - The Sauk warrior Black Hawk enters into war with the United States.
- 1841 - John Tyler is inaugurated as the 10th President of the United States.
- 1862 - American Civil War: Battle of Shiloh begins - In Tennessee, forces under Union General Ulysses S. Grant meet Confederate troops led by General Albert Sidney Johnston at Shiloh.
- 1865 - American Civil War: Battle of Sayler's Creek - Confederate General Robert E. Lee's Army of Northern Virginia fights its last major battle while in retreat from Richmond, Virginia.
- 1869 - Celluloid is patented.
- 1886 - Vancouver, British Columbia is incorporated as a city.
- 1895 - Oscar Wilde is arrested after losing a libel case against the John Sholto Douglas, 9th Marquess of Queensberry.
- 1896 - In Athens, the opening of the first modern Olympic Games after 1,500 years after being banned by Roman Emperor Theodosius I.
- 1893 - Salt Lake Temple of the Church of Jesus Christ of Latter-Day Saints dedicated by Wilford Woodruff.
- 1903 - The Kishinev pogrom in Kishinev (Bessarabia) began, forcing tens of thousands of Jews to later seek refuge in Israel and the west.
- 1909 - Robert Peary allegedly reaches the North Pole.
- 1911 - Dedë Gjon Luli Dedvukaj, Leader of the Malësori Albanians raises the Albanian flag in the town of Tuzi, Montenegro for the first time after Gjergj Kastrioti (Skenderbeg).
- 1917 - World War I: United States declares war on Germany (see [http://en.wikisource.org/wiki/Woodrow_Wilson_declares_war_on_Germany Wilson's address to Congress]).
- 1926 - Walter Varney Airlines makes first commercial flight from Pasco, Washington, to Elko, Nevada. Varney is the root company of United Airlines.
- 1930 - Gandhi raised a lump of mud and salt (some say just a pinch, some say just a grain) and declared, "With this, I am shaking the foundations of the British Empire." Thus he started Salt Satyagraha.
- 1930 - Hostess Twinkies are invented.
- 1930 - Will Rogers starts broadcasting The Will Rogers Program on radio.
- 1931 - Little Orphan Annie debuts on the Blue Network of NBC.
- 1936 - Tupelo-Gainesville Outbreak: Another tornado from the same storm system as the Tupelo tornado hits Gainesville, Georgia, killing 203.
- 1941 - World War II: Operation Castigo begins - Germany invades Kingdom of Yugoslavia and Greece.
- 1965 - Early Bird, the first communications satellite to be placed in synchronous orbit, is launched.
- 1968 - In London, United Kingdom, Massiel wins the thirteenth Eurovision Song Contest for Spain singing "La, la, la."
- 1970 - Four California Highway Patrol officers die in one of the worst cop killings in the CHP's history; this is known as the Newhall Incident.
- 1972 - Vietnam War: Easter Offensive - The first day of clear weather in three days allows American forces to start sustained air strikes and naval bombardments.
- 1973 - Launch of Pioneer 11 spacecraft.
- 1974 - The California Jam Rock concert begins.
- 1974 - In Brighton, United Kingdom, ABBA wins the nineteenth Eurovision Song Contest for Sweden singing "Waterloo."
- 1984 - Members of Cameroon's Republican Guard from country's northern region attack various government buildings in an unsuccessful attempt to overthrow the government headed by Paul Biya.
- 1987 - Sugar Ray Leonard takes the middleweight boxing title from Marvin Hagler.
- 1993 - Russian nuclear accident at Tomsk 7.
- 1994 - The Rwandan Genocide begins when the aircraft carrying Rwandan president Juvénal Habyarimana and Burundian president Cyprien Ntaryamira is shot down by extremists.
- 1998 - Pakistan tests medium-range missiles capable of hitting India.
- 1998 - The Dow Jones Industrial Average gains 49.82 to close at 9,033.23 -- its first-ever close above 9,000.
- 2001 - Miller Park opens in Milwaukee, Wisconsin.
- 2004 - Rolandas Paksas becomes the first president to be peacefully removed from the post by impeachment.

Births

- 1483 - Raphael, Italian painter and architect (d. 1520)
- 1651 - André Dacier, French classical scholar (d. 1722)
- 1664 - Arvid Horn, Swedish statesman (d. 1742)
- 1671 - Jean-Baptiste Rousseau, French poet (d. 1741)
- 1725 - Pasquale Paoli, Corsican patriot and military leader (d. 1807)
- 1812 - Alexander Herzen, Russian writer (d. 1870)
- 1815 - Robert Volkmann, German composer (d. 1883)
- 1818 - Aasmund Olavsson Vinje, Norwegian poet (d. 1870)
- 1820 - Nadar, French photographer (d. 1910)
- 1823 - Joseph Medill, Mayor of Chicago (d. 1899)
- 1826 - Gustave Moreau, French painter (d. 1898)
- 1866 - Butch Cassidy, American outlaw (d. 1909)
- 1878 - Erich Mühsam, German author (d. 1934)
- 1884 - Walter Huston, Canadian-born actor (d. 1950)
- 1890 - Anthony Fokker, Dutch designer of aircraft (d. 1939)
- 1892 - Donald Wills Douglas, Sr., American industrialist (d. 1981)
- 1892 - Lowell Thomas, American travel writer (d. 1981)
- 1902 - Veniamin Kaverin, Russian writer (d. 1989)
- 1903 - Mickey Cochrane, baseball player (d. 1962)
- 1903 - Doc Edgerton, American electrical engineer (d. 1990)
- 1911 - Feodor Felix Konrad Lynen, German biochemist, recipient of the Nobel Prize in Physiology or Medicine (d. 1979)
- 1920 - Edmond H. Fischer, Swiss-American biochemist, recipient of the Nobel Prize in Physiology or Medicine
- 1926 - Sergio Franchi, Italian-born singer and actor (d. 1990)
- 1926 - Gil Kane, Latvian-born cartoonist (d. 2000)
- 1926 - Ian Paisley, British politician
- 1927 - Gerry Mulligan, American musician (d. 1996)
- 1928 - James D. Watson, American geneticist, recipient of the Nobel Prize in Physiology or Medicine
- 1929 - André Previn, German-born composer and conductor
- 1931 - Ivan Dixon, American actor and director
- 1933 - Roy Goode, British lawyer
- 1934 - Anton Geesink, Dutch judoka
- 1937 - Merle Haggard, American musician
- 1937 - Billy Dee Williams, American actor
- 1938 - Paul Daniels, English magician
- 1938 - Roy Thinnes, American actor
- 1941 - Phil Austin, American comedian
- 1941 - Zamfir, Romanian musician
- 1942 - Barry Levinson, American film producer and director
- 1944 - Felicity Palmer, English soprano
- 1947 - John Ratzenberger, American actor
- 1949 - Horst Ludwig Störmer, German-born physicist, Nobel Prize laureate
- 1951 - Bert Blyleven, Dutch Major League Baseball player
- 1952 - Udo Dirkschneider, German singer (Accept and U.D.O.)
- 1952 - Marilu Henner, American actress
- 1954 - Thom Bray, American actor
- 1955 - Michael Rooker, American actor
- 1965 - Frank Black, American singer and songwriter (Pixies)
- 1969 - Bison Dele, American basketball player (disappeared 2002)
- 1969 - Ari Meyers, Puerto Rican actress
- 1970 - Olaf Kölzig, South African hockey player
- 1973 - Rie Miyazawa, Japanese actress and singer
- 1975 - Zach Braff, American actor
- 1976 - Candace Cameron, American actress

Deaths

- 1199 - King Richard I of England (killed in battle) (b. 1157)
- 1362 - James I, Count of La Marche, French soldier (b. 1319)
- 1490 - King Matthias Corvinus of Hungary
- 1520 - Raphael, Italian painter and architect (b. 1483)
- 1528 - Albrecht Dürer, German artist (b. 1471)
- 1551 - Joachim Vadian, Swiss humanist (b. 1484)
- 1571 - John Hamilton, Scottish prelate and politician
- 1590 - Francis Walsingham, English spymaster
- 1605 - John Stow, English historian
- 1655 - David Blondel, French protestant clergyman (b. 1591)
- 1686 - Arthur Annesley, 1st Earl of Anglesey, English royalist statesman (b. 1614)
- 1707 - Willem van de Velde, the younger, Dutch painter (b. 1633)
- 1755 - Richard Rawlinson, English minister and antiquarian (b. 1690)
- 1829 - Niels Henrik Abel, Norwegian mathematician (b. 1802)
- 1862 - Albert Sidney Johnston, American Confederate general (b. 1803)
- 1883 - Benjamin Raymond, Mayor of Chicago (b. 1801)
- 1906 - Alexander Kielland, Norwegian author (b. 1849)
- 1935 - Edwin Arlington Robinson, American poet (b. 1869)
- 1961 - Jules Bordet, Belgian immunologist and microbiologist, recipient of the Nobel Prize in Physiology or Medicine (b. 1870)
- 1963 - Otto Struve, Russian-born astronomer (b. 1897)
- 1970 - Sam Sheppard, American accused murderer (b. 1923)
- 1971 - Igor Stravinsky, Russian composer (b. 1882)
- 1974 - Willem Marinus Dudok, Dutch architect (b. 1884)
- 1986 - Raimundo Orsi, Argentine-Italian footballer
- 1992 - Isaac Asimov, Russian-born author (b. 1920)
- 1994 - Juvénal Habyarimana, President of Rwanda (b. 1937)
- 1994 - Cyprien Ntaryamira, President of Burundi (b. 1956)
- 1996 - Greer Garson, Irish actress (b. 1904)
- 1998 - Wendy O. Williams, American musician (Plasmatics) (b. 1949)
- 1998 - Tammy Wynette, American musician (b. 1942)
- 2000 - Habib Bourguiba, President of Tunisia (b. 1903)
- 2003 - David Bloom, American reporter (pulmonary embolism) (b. 1963)
- 2003 - Babatunde Olatunji, Nigerian drummer (b. 1927)
- 2004 - Larisa Bogoraz, Soviet dissident (b. 1929)
- 2005 - Rainier III, Prince of Monaco (b. 1923)

Holidays and observances

- Feast day of St. Sixtus and Marcellinus of Carthage in the Roman Catholic Church.
- The start of the tax year in the United Kingdom (arising from the 11 day correction to March 25 at the adoption of the Gregorian calendar in 1752).
- Tartan Day, a day set aside for the celebration of the Scottish influence on America.
- Community of Christ Birthday, a day of importance to some members of Community of Christ as it is the anniversary of when it was officially organized on April 6 1830CE.

External links

- [http://news.bbc.co.uk/onthisday/hi/dates/stories/april/6 BBC: On This Day]
- [http://www.tnl.net/when/4/6 Today in History: April 6] ----- April 5 - April 7 - March 6 - May 6 -- listing of all days ko:4월 6일 ja:4月6日 simple:April 6 th:6 เมษายน

1928

1928 (MCMXXVIII) was a leap year starting on Sunday (link will take you to calendar).

Events

January-May

- January 6-7 - River Thames floods in London - 14 drowned
- January 7 - Moat at the Tower of London, previously drained in 1843, is completely refilled by a tidal wave
- January 12 - US murderer Ruth Snyder executed at Ossining
- January 17 - OGPU arrests Lev Trotsky in Moscow; he assumes a status of passive resistance and is exiled to Turkestan
- February - Kurume University (Japan) established
- February 11 - 1928 Winter Olympic Games open in St. Moritz, Switzerland
- February 12 - Heavy hails kill 11 in England
- February 25 - Charles Jenkins Laboratories of Washington, DC becomes the first holder of a television license from the Federal Radio Commission.
- March 12 - Malta becomes a British dominion
- March 12 - In California, the St. Francis Dam north of Los Angeles fails killing 400
- March 21 - Charles Lindbergh is presented the Congressional Medal of Honor for his first trans-Atlantic flight.
- April 10 - Pineapple Primary - Republican Party primary elections in Chicago preceded by assassinations and bombings
- April 12 - Bomb attack against the King of Italy in Milan - 17 bystanders dead
- April 22 - Earthquake destroys Corinth - 200.000 buildings destroyed
- May 15-17 - Christian X of Denmark visits Finland
- May 15 - Royal Flying Doctor Service of Australia, commenced operations
- May 15 - Release of the animated short Plane Crazy, featuring the first appearances of Mickey and Minnie Mouse.
- May 23 - Bomb attack against Italian consulate in Buenos Aires - 22 dead, 41 injured
- May 24 - Airship Italia crashes on the North Pole; one of the occupants is Italian general Umberto Nobile
- May 30 - A rescue expedition leaves for the North Pole

June-August

- June 11 - Medical doctor's strike begins in Vienna
- June 14 - Students take over the medical wing of Rosario University in Argentina
- July 6 - The world's largest hailstone falls in Potter, Nebraska.
- July 12 - Mexican aviator Emilio Carranza dies in a solo plane crash in the New Jersey Pine Barrens while returning from a goodwill flight to New York City.
- June 17 - Aviator Amelia Earhart starts her attempt to become the first woman to successfully pilot an aircraft across the Atlantic Ocean (she succeeded the next day).
- July 17 - Jose del León Toral assassinates Alvaro Obregon, president of Mexico
- June 20 - Shooting incident in Yugoslavian parliament - Punica Rasic shoots 3 opposition representatives and injures three others
- June 24 - Swedish aeroplane rescues part of Italian North Pole expedition, including Umberto Nobile. Soviet icebreaker Krasin saves the rest July 12
- July 16 - Leon Toral assassinates Álvaro Obregón, president of Mexico
- July 25 - USA recalls its troops from China
- July 27 - Tich Freeman becomes only bowler ever to take 200 first-class wickets before end of July.
- July 28 - Official opening ceremony of the 1928_Summer_Olympics in Amsterdam.
- August 16 - Murderer Carl Panzram is arrested in Washington, DC after killing about 20 people.
- August 25 - Ahmet Zogu proclaims himself King Zog I of Albania; he is crowned September 1
- August 28 - The Kellogg-Briand Pact was signed in Paris - it was the first treaty which outlawed aggressive war.

September-December

- September 1 - Richard Byrd leaves New York for Arctic
- September 3 - Alexander Fleming discovers Penicillin
- September 15 - Tich Freeman sets all-time record for number of wickets taken in an English cricket season.
- September 16 - The 1928 Okeechobee Hurricane kills at least 2,500 people in Florida.
- October 2 Saint Josemaria Escriva, founds Opus Dei
- October 7 - Haile Selassie crowned king (not yet emperor) of Abyssinia
- October 12 - An iron lung respirator is used for the first time at Children's Hospital, Boston.
- November 3 - cartoon star Mickey Mouse appears in Steamboat Willie, an animated short produced by Walt Disney.
- November 4 - At Park Central Hotel in Manhattan, Arnold Rothstein, New York City's most notorious gambler, is shot to death over a poker game.
- November 6 - Swedes start a tradition of eating Gustavus Adolphus pastries to commemorate the old warrior king.
- November 6 - U.S. presidential election, 1928: Republican Herbert Hoover wins by a wide margin over Democrat Alfred E. Smith.
- November 10 - Hirohito was enthroned as Emperor of Japan.
- November 11 - US gambling king Arnold Rothstein is shot to death in New York City
- December 3 - In Rio de Janeiro, a seaplane sunk near Cap Arcona with Alberto Santos-Dumont on board.
- December 5 - Police disperses Sicilian gangs' meeting in Cleveland
- December 21 - U.S. Congress approves the construction of The Boulder Dam, later renamed The Hoover Dam
- December 31 - Bells of Big Ben first time in a radio

Unknown dates

- Charles King elected president of Liberia with 600,000 votes; the whole of country has only 15,000 voters.
- Chaco war
- Coca Cola enters Europe through the Amsterdam Olympics.
- Eliot Ness begins to lead the prohibition unit in Chicago, Illinois.
- The old Canaanite city of Ugarit is rediscovered.
- Turkey switches from the Arabic to the Latin-based modern Turkish alphabet.
- The right to vote extended to all women in the United Kingdom.
- Frederick Griffith conducts the Griffith experiment, indirectly proving existence of DNA.
- Motorola is founded.
- First (and last) Best Title Writing Academy Award given.
- The Episcopal Church in the United States of America ratifies a new revision of the Book of Common Prayer.
- W2XBS, RCA's first television station, is established in New York City.
- Australian farmer, Jack Trott, finds Rhizanthella gardneri in his garden.

Births

January

- January 5 - Ali Bhutto, President of Pakistan and Prime Minister of Pakistan (d. 1979)
- January 5 - Walter Mondale, U.S. Senator and Presidential candidate
- January 7 - William Peter Blatty, American writer
- January 11 - David L. Wolper, television producer
- January 16 - William Kennedy, American author
- January 17 - Jean Barraqué, French composer (d. 1973)
- January 17 - Vidal Sassoon, English cosmetologist
- January 23 - Chico Carrasquel, Venezuelan Major League Baseball player (d. 2005)
- January 23 - Jeanne Moreau, French actress
- January 24 - Desmond Morris, anthropologist and writer
- January 26 - Roger Vadim, French film director (d. 2000)
- January 30 - Hal Prince, American stage producer and director

February

- February 5 - Andrew Greeley, American Catholic priest and novelist
- February 9 - Frank Frazetta, American illustrator
- February 9 - Roger Mudd, American journalist
- February 23 - Vasili Lazarev, cosmonaut (d. 1990)
- February 26 - Fats Domino, American musician
- February 26 - Anatoli Filipchenko, cosmonaut
- February 27 - Ariel Sharon, Prime Minister of Israel

March-April

- March 4 - Alan Sillitoe, English writer
- March 6 - Gabriel García Márquez, Colombian writer, Nobel Prize laureate
- March 8 - Gerald Bull, Canadian engineer (d. 1990)
- March 10 - James Earl Ray, American assassin (d. 1998)
- March 12 - Edward Albee, American dramatist
- March 16 - Christa Ludwig, German mezzo-soprano
- March 19 - Hans Küng, Swiss theologian
- March 19 - Patrick McGoohan, Irish actor
- March 20 - Fred Rogers, American children's television host (d. 2003)
- March 24 - Byron Janis, American pianist
- March 25 - Jim Lovell, astronaut
- March 28 - Zbigniew Brzezinski, Polish-born U.S. National Security Advisor
- March 31 - Gordie Howe, Canadian hockey player
- March 31 - Lefty Frizzell, American country music performer
- April 1 - Jane Powell, American dancer, actress, and singer
- April 1 - George Grizzard, American actor
- April 2 - Serge Gainsbourg, French singer (d. 1991)
- April 4 - Maya Angelou, American poet and novelist
- April 6 - James D. Watson, American geneticist, recipient of the Nobel Prize in Physiology or Medicine
- April 7 - James Garner, American actor
- April 7 - Alan J. Pakula, American producer and director (d. 1998)
- April 8 - Eric Porter, English actor (d. 1995)
- April 9 - Tom Lehrer, American songwriter
- April 12 - Jean-François Paillard, French conductor
- April 19 - Alexis Korner, British blues musician (d. 1984)
- April 23 - Shirley Temple, American actress and politician

May-June

- May 3 - Dave Dudley, American singer (d. 2003)
- May 4 - Hosni Mubarak, President of Egypt
- May 8 - Theodore Sorenson, American lawyer and speechwriter
- May 9 - Colin Chapman, English automotive engineer (d. 1982)
- May 9 - Pancho Gonzalez, American tennis player (d. 1995)
- May 9 - Barbara Ann Scott, Canadian figure skater
- May 12 - Burt Bacharach, American composer
- May 16 - Billy Martin, baseball player and manager (d. 1989)
- May 18 - Pernell Roberts, American actor
- May 23 - Rosemary Clooney, American singer and actress (d. 2002)
- May 26 - Jack Kevorkian, American physician
- June 1 - Georgi Dobrovolski, cosmonaut (d. 1971)
- June 1 - Bob Monkhouse, English comedian and game show host (d. 2003)
- June 13 - John Forbes Nash, Jr., American mathematician, recipient of the Nobel Prize in Economics
- June 14 - Ernesto Rafael Guevara de la Serna, Argentine-born revolutionary (d. 1967)
- June 19 - Nancy Marchand, American actress (d. 2000)
- June 25 - Alexei Abrikosov, Russian physicist, Nobel Prize laureate
- June 26 - Jacob Druckman, American composer (d. 1996)

July-September

- July 5 - Warren Oates, American actor (d. 1982)
- July 10 - Moshe Greenberg, American-Israeli Bible scholar
- July 11 - Bobo Olson, American boxer (d. 2002)
- July 12 - Elias James Corey, American chemist, Nobel Prize laureate
- July 13 - Leroy Vinnegar, American musician (d. 1999)
- July 16 - Robert Sheckley, American writer
- July 25 - Keter Betts, American jazz bassist (d. 2005)
- July 26 - Stanley Kubrick, American film director (d. 1999)
- July 26 - Bernice Rubens, British novelist (d. 2004)
- August 6 - Andy Warhol, American artist (d. 1987)
- August 10 - Eddie Fisher, American singer
- August 12 - Bob Buhl, baseball player (d. 2001)
- August 15 - Nicolas Roeg, English film director
- August 18 - Marge Schott, baseball team owner (d. 2004)
- August 25 - Herbert Kroemer, German-born physicist, Nobel Prize laureate
- September 11 - William Kienzle, American author (d. 2001
- September 14 - Angus Ogilvy, husband of Princess Alexandra of Kent (d. 2004)
- September 15 - Julian Cannonball Adderley, American saxophonist
- September 19 - Adam West, American actor
- September 22 - James Lawson, American civil rights activist and minister
- September 30 - Elie Wiesel, Romanian Holocaust survivor, writer, and lecturer, recipient of the Nobel Peace Prize

October-December

- October 1 - George Peppard, American actor (d. 1994)
- October 8 - Bill Maynard, British actor
- October 9 - Einojuhani Rautavaara, Finnish composer
- October 27 - Kyle Rote, American football player (d. 2002)
- October 30 - Daniel Nathans, American microbiologist, recipient of the Nobel Prize in Physiology or Medicine (d. 1999)
- November 3 - Osamu Tezuka, Japanese artist (d. 1989)
- November 3 - George Yardley, American basketball player (d. 2004)
- November 10 - Ennio Morricone, Italian composer
- November 11 - Carlos Fuentes, Panamanian writer
- November 17 - Rance Howard, American actor
- November 29 - Paul Simon, U.S. Senator from Illinois (d. 2003)
- December 7 - Noam Chomsky, American linguist
- December 15 - Friedensreich Hundertwasser, Austrian artist (d. 2000)
- December 16 - Philip K. Dick, American author (d. 1982)
- December 25 - Dick Miller, American actor

Unknown date

- Sultan Azlan Muhibbudin Shah ibni Almarhum Sultan Yusuff Izzudin Shah Ghafarullahu-lahu, King of Malaysia

Deaths

- January 1 - Loie Fuller, American dancer (b. 1862)
- January 6 - Alvin Kraenzlein, American athlete (b. 1876)
- January 11 - Thomas Hardy, English writer (b. 1840)
- January 29 - Douglas Haig, British soldier (b. 1861)
- January 30 - Johannes Andreas Grib Fibiger, Danish scientist, recipient of the Nobel Prize in Physiology or Medicine (b. 1867)
- February 1 - Hughie Jennings, baseball player (b. 1869)
- February 4 - Hendrik Lorentz, Dutch physicist, Nobel Prize laureate (b. 1853)
- February 15 - Herbert Henry Asquith, Prime Minister of the United Kingdom (b. 1852)
- February 16 - Eddie Foy, American vaudevillian (b. 1856)
- April 2 - Theodore William Richards, American chemist, Nobel Prize laureate (b. 1868)
- April 5 - Roy Kilner, English cricketer (b. 1890)
- June 4 - Chang Tso-lin, Chinese warlord (b. 1873)
- June 22 - A. B. Frost, American illustrator (b. 1851)
- August 12 - Leos Janacek, Czech composer (b. 1854)
- August 30 - Wilhelm Wien, German physicist, Nobel Prize laureate (b. 1864)
- October 22 - Andrew Fisher, fifth Prime Minister of Australia (b. 1862)
- December 1 - José Eustasio Rivera, Colombian writer (b. 1888)
- December 10 - Charles Rennie Mackintosh, Scottish architect (b. 1868)
- Robert Abbe, American surgeon (b. 1851)

Nobel Prizes

- Physics - Owen Willans Richardson
- Chemistry - Adolf Otto Reinhold Windaus
- Physiology or Medicine - Charles Jules Henri Nicolle
- Literature - Sigrid Undset
- Peace - not awarded ko:1928년 ms:1928 ja:1928年 simple:1928 th:พ.ศ. 2471

DNA

:For other uses, see DNA (disambiguation). DNA (disambiguation) Deoxyribonucleic acid (DNA) is a nucleic acid that contains the genetic instructions specifying the biological development of all cellular forms of life (and most viruses). DNA is a long polymer of nucleotides and encodes the sequence of the amino acid residues in proteins using the genetic code, a triplet code of nucleotides. In complex cells (eukaryotes), such as those from plants, animals, fungi and protists, most of the DNA is located in the cell nucleus. By contrast, in simpler cells called prokaryotes (the eubacteria and archaea), DNA is not separated from the cytoplasm by a nuclear envelope. The cellular organelles known as chloroplasts and mitochondria also carry DNA. DNA is often referred to as the molecule of heredity as it is responsible for the genetic propagation of most inherited traits. These traits can range from hair colour to disease susceptibility. During cell division, DNA is replicated and can be transmitted to offspring during reproduction. Lineage studies can be done based on the facts that the DNA in mitochondria (mitochondrial DNA) only comes from the mother, and the male "Y" chromosome only comes from the father. Every person's DNA, their genome, is inherited from both parents. The mother's mitochondrial DNA together with twenty-three chromosomes from each parent combine to form the genome of a fertilized egg. As a result, with certain exceptions such as red blood cells, most human cells contain 23 pairs of chromosomes, together with mitochondrial DNA inherited from the mother.

DNA Overview

red blood cell This section presents an introductory and therefore incomplete overview of DNA.
- Genes can be loosely viewed as the organism's "cookbook" or "blueprint";
- A strand of DNA contains genes, areas that regulate genes, and areas that either have no function, or a function we do not (yet) know (also see last bullet point in this section for the difference between DNA and RNA);
- DNA is organized as two complementary strands, head-to-toe, with bonds between them that can be "unzipped" like a zipper, separating the strands;
- DNA is a chain of chemical "building blocks", called "bases", of which there are four types: these can be abbreviated A, T, C, and G. Each base can only "pair up" with one single predetermined other base: A+T, T+A, C+G and G+C are the only possible combinations; that is, an "A" on one strand of double-stranded DNA will "mate" properly only with a "T" on the other, complementary strand;
- N.B.: U occasionally replaces T, notably in PBS1 phage DNA; you can thus substitute "U" for "T" throughout this section.
- Because each strand of DNA has a directionality, the sequence order does matter: A+T is not the same as T+A, just as C+G is not the same as G+C;
- For each given base, there is just one possible complementary base, so naming the bases on the conventionally chosen side of the strand is enough to describe the entire double-strand sequence;
- The genetic information contained in a strand of DNA is determined by the sequence of bases along its length;
- The cell begins DNA replication by forcibly unzipping the DNA double strand down the middle, and then recreates the "other half" of each new single strand by drowning each half in a "soup" made of the four bases. An enzyme makes a new strand by finding the correct "base" in the soup and pairing it with the original strand. In this way, the base on the old strand dictates which base will be on the new strand, and the cell ends up with an extra copy of its DNA.
- Mutations are simply chemical imperfections in this process: a base is accidentally skipped, inserted, or incorrectly copied, or the chain is trimmed, or added to; many basic mutations can be described as combinations of these accidental "operations". Mutations can also occur through chemical damage (through mutagens), light (UV damage), or through other more complicated gene swapping events.
- DNA (for DeoxyriboNucleic Acid) differs from RNA (for RiboNucleic Acid) by having the sugar 2-deoxyribose instead of ribose in its backbone (ribose contains one extra oxygen atom compared to deoxyribose -- in other words, DNA contains deoxygenated ribose, whereas RNA contains "plain" ribose.) This is the basic chemical distinction between RNA and DNA.

DNA in practice

DNA in crime

Forensic scientists can use DNA located in blood, semen, skin, saliva, or hair left at the scene of a crime to identify a possible suspect, a process called genetic fingerprinting or DNA profiling. In DNA profiling the relative lengths of sections of repetitive DNA, such as short tandem repeats and minisatellites, are compared. DNA profiling was developed in 1984 by English geneticist Alec Jeffreys, and was first used in 1986 in the Enderby murders case in Leicestershire, England. Many jurisdictions require convicts of certain types of crimes to provide a sample of DNA for inclusion in a computerized database. This has helped investigators solve old cases where the perpetrator was unknown and only a DNA sample was obtained from the scene (particularly in rape cases between strangers). This method is one of the most reliable techniques for identifying a criminal, but is not always perfect, for example if no DNA can be retrieved, or if the scene is contaminated with the DNA of several possible suspects.

DNA in computation

Despite its biological origins, DNA plays an important role in computer science, both as a motivating research problem and as a method of computation in itself, called DNA computing. As a simple example, research on string searching algorithms, which find an occurrence of a sequence of letters inside a larger sequence of letters, was motivated by DNA research, where it is used to find specific sequences of nucleotides in a large sequence. In other applications like text editors, even simple algorithms for this problem usually suffice, but DNA sequences cause these algorithms to exhibit near-worst-case behavior due to their small number of distinct characters. Databases have also been strongly motivated by DNA research, which requires special tools for storing and manipulating DNA sequences. Databases specialized for this purpose are called genomic databases, and have a number of unique technical challenges associated with the operations of approximate matching, sequence comparison, finding repeating patterns, and homology searching. In 1994, Leonard Adleman of the University of Southern California made headlines when he discovered a way of solving the directed Hamiltonian path problem, an NP-complete problem, using tools from molecular biology, in particular DNA. The new approach, dubbed DNA computing, has practical advantages over traditional computers in power use, space use, and efficiency, due to its ability to highly parallelize the computation (see parallel computing)(there is labor worth mention involved in retrieving answers computed these computational DNA techniques.). A number of other problems, including simulation of various abstract machines, the boolean satisfiability problem, and the bounded version of the Post correspondence problem, have since been analyzed using DNA computing. Due to its compactness, DNA also has an important role in cryptography, where in particular it allows unbreakable one-time pads to be efficiently constructed and used.[http://citeseer.ist.psu.edu/gehani99dnabased.html]

Overview of molecular structure

one-time pad Although sometimes called "the molecule of heredity", pieces of DNA as people typically think of them are not single molecules. Rather, they are pairs of molecules, which entwine like vines to form a double helix (see the illustration at the right). Each vine-like molecule is a strand of DNA: a chemically linked chain of nucleotides, each of which consists of a sugar, a phosphate and one of five kinds of nucleobases ("bases"). Because DNA strands are composed of these nucleotide subunits, they are polymers. The diversity of the bases means that there are five kinds of nucleotides, which are commonly referred to by the identity of their bases. These are adenine (A), thymine (T), uracil (U), cytosine (C), and guanine (G). U is rarely found in DNA except as a result of chemical degradation of C, but in some viruses, notably PBS1 phage DNA, U completely replaces the usual T in its DNA. Similarly, RNA usually contains U in place of T, but in certain RNAs such as transfer RNA, T is always found in some positions. Thus, the only true difference between DNA and RNA is the sugar, 2-deoxyribose in DNA and ribose in RNA. In a DNA double helix, two polynucleotide strands can associate through the hydrophobic effect and pi stacking. Specificity of which strands stay associated is determined by complementary pairing. Each base forms hydrogen bonds readily to only one other -- A to T and C to G -- so that the identity of the base on one strand dictates the strength of the association; the more complementary bases exist, the stronger and longer-lasting the association. The cell's machinery is capable of melting or disassociating a DNA double helix, and using each DNA strand as a template for synthesizing a new strand which is nearly identical to the previous strand. Errors that occur in the synthesis are known as mutations. The process known as PCR (polymerase chain reaction) mimics this process in vitro in a nonliving system. Because pairing causes the nucleotide bases to face the helical axis, the sugar and phosphate groups of the nucleotides run along the outside; the two chains they form are sometimes called the "backbones" of the helix. In fact, it is chemical bonds between the phosphates and the sugars that link one nucleotide to the next in the DNA strand.

The role of the sequence

Within a gene, the sequence of nucleotides along a DNA strand defines a messenger RNA sequence which then defines a protein, that an organism is liable to manufacture or "express" at one or several points in its life using the information of the sequence. The relationship between the nucleotide sequence and the amino-acid sequence of the protein is determined by simple cellular rules of translation, known collectively as the genetic code. The genetic code is made up of three-letter 'words' (termed a codon) formed from a sequence of three nucleotides (e.g. ACT, CAG, TTT). These codons can then be translated with messenger RNA and then transfer RNA, with a codon corresponding to a particular amino acid. There are 64 possible codons (4 bases in 3 places

4^3

) that encode 20 amino acids. Most amino acids, therefore, have more than one possible codon. There are also three 'stop' or 'nonsense' codons signifying the end of the coding region, namely the UAA, UGA and UAG codons. In many species, only a small fraction of the total sequence of the genome appears to encode protein. For example, only about 1.5% of the human genome consists of protein-coding exons. The function of the rest is a matter of speculation. It is known that certain nucleotide sequences specify affinity for DNA binding proteins, which play a wide variety of vital roles, in particular through control of replication and transcription. These sequences are frequently called regulatory sequences, and researchers assume that so far they have identified only a tiny fraction of the total that exist. "Junk DNA" represents sequences that do not yet appear to contain genes or to have a function. The reasons for the presence of so much non-coding DNA in eukaryotic genomes and the extraordinary differences in genome size ("C-value") among species represent a long-standing puzzle in DNA research known as the "C-value enigma". Some DNA sequences play structural roles in chromosomes. Telomers and centromeres typically contain few (if any) protein-coding genes, but are important for the function and stability of chromosomes. Some genes code for "RNA genes" (see tRNA and rRNA). Some RNA genes code for transcripts that function as regulatory RNAs (see siRNA) that influence the function of other RNA molecules. The intron-exon structure of some genes (such as immunoglobin and protocadeherin genes) is important for allowing alternative splicing of pre-mRNA which allows several different proteins to be made from the same gene. Some non-coding DNA represents pseudogenes that can be used as raw material for the creation of new genes with new functions. Some non-coding DNA provided hot-spots for duplication of short DNA regions; such sequence duplication has been the major form of genetic change in the human lineage (see evidence from the Chimpanzee Genome Project). Exons interspersed with introns allows for "exon shuffling" and the creation of modified genes that might have new adaptive functions. Large amounts of non-coding DNA is probably adaptive in that it provides chromosomal regions where recombination between homologous portions of chromosomes can take place without disrupting the function of genes. Some biologists such as Stuart Kauffman have speculated that there must be mechanisms by which the rate of evolution of a species can be increased or decreased. Non-coding DNA provides mechanisms for gene creation, modification and recombination it is probably important for control of the rate of human evolution. Sequence also determines a DNA segment's susceptibility to cleavage by restriction enzymes, the quintessential tools of genetic engineering. The position of cleavage sites throughout an individual's genome determines one kind of an individual's "DNA fingerprint".

DNA replication

Main article: DNA replication DNA replication DNA replication or DNA synthesis is the process of copying the double-stranded DNA prior to cell division. The two resulting double strands are generally almost perfectly identical, but occasionally errors in replication can result in a less than perfect copy (see mutation), and each of them consists of one original and one newly synthesized strand. This is called semiconservative replication. The process of replication consists of three steps: initiation, replication and termination.

Mechanical properties relevant to biology

Main article: Mechanical properties of DNA.

Strands association and dissociation

The hydrogen bonds between the strands of the double helix are weak enough that they can be easily separated by enzymes. Enzymes known as helicases unwind the strands to facilitate the advance of sequence-reading enzymes such as DNA polymerase. The unwinding requires that helicases chemically cleave the phosphate backbone of one of the strands so that it can swivel around the other. The strands can also be separated by gentle heating, as used in PCR, provided they have fewer than about 10,000 base pairs (10 kilobase pairs, or 10 kbp). The intertwining of the DNA strands makes long segments difficult to separate.

Circular DNA

When the ends of a piece of double-helical DNA are joined so that it forms a circle, as in plasmid DNA, the strands are topologically knotted. This means they cannot be separated by gentle heating or by any process that does not involve breaking a strand. The task of unknotting topologically linked strands of DNA falls to enzymes known as topoisomerases. Some of these enzymes unknot circular DNA by cleaving two strands so that another double:stranded segment can pass through. Unknotting is required for the replication of circular DNA as well as for various types of recombination in linear DNA.

Great length versus tiny breadth

The narrow breadth of the double helix makes it impossible to detect by conventional electron microscopy, except by heavy staining. At the same time, the DNA found in many cells can be macroscopic in length -- approximately 5 centimetres long for strands in a human chromosome. Consequently, cells must compact or "package" DNA to carry it within them. This is one of the functions of the chromosomes, which contain spool-like proteins known as histones, around which DNA winds.

Entropic stretching behavior

When DNA is in solution, it undergoes conformational fluctuations due to the energy available in the thermal bath. For entropic reasons, more floppy states are thermally accessible than stretched out states; for this reason, a single molecule of DNA stretches similarly to a rubber band. Using optical tweezers, the entropic stretching behavior of DNA has been studied and analyzed from a polymer physics perspective, and it has been found that DNA behaves like the Kratky-Porod worm-like chain model with a persistence length of about 53 nm. Furthermore, DNA undergoes a stretching phase transition at a force of 65 pN; above this force, DNA is thought to take the form that Linus Pauling originally hypothesized, with the phosphates in the middle and bases splayed outward. This proposed structure for overstretched DNA has been called "P-form DNA," in honor of Pauling.

Different helix geometries

The DNA helix can assume one of three slightly different geometries, of which the "B" form described by James D. Watson and Francis Crick is believed to predominate in cells. It is 2 nanometres wide and extends 3.4 nanometres per 10 bp of sequence. This is also the approximate length of sequence in which the double helix makes one complete turn about its axis. This frequency of twist (known as the helical pitch) depends largely on stacking forces that each base exerts on its neighbors in the chain.

Supercoiled DNA

The B form of the DNA helix twists 360° per 10.6 bp in the absence of strain. But many molecular biological processes can induce strain. A DNA segment with excess or insufficient helical twisting is referred to, respectively, as positively or negatively "supercoiled". DNA in vivo is typically negatively supercoiled, which facilitates the unwinding of the double-helix required for RNA transcription.

Sugar pucker

There are four conformations that the ribofuranose rings in nucleotides can acquire: # C-2' endo # C-2' exo # C-3' endo # C-3' exo Ribose is usually in C-3'endo, while deoxyribose is usually in the C-2' endo sugar pucker conformation. The A and B forms differ mainly in their sugar pucker. In the A form, the C3' configuration is above the sugar ring, whilst the C2' configuration is below it. Thus, the A form is described as "C3'-endo." Likewise, in the B form, the C2' configuration is above the sugar ring, whilst C3' is below; this is called "C2'-endo." Altered sugar puckering in A-DNA results in shortening the distance between adjacent phosphates by around one angstrom. This gives 11 to 12 base pairs to each helix in the DNA strand, instead of 10.5 in B-DNA. Sugar pucker gives uniform ribbon shape to DNA, a cylindrical open core, and also a deep major groove more narrow and pronounced that grooves found in B-DNA.

Conditions for formation of A and Z helices

The two other known double-helical forms of DNA, called A and Z, differ modestly in their geometry and dimensions. The A form appears likely to occur only in dehydrated samples of DNA, such as those used in crystallographic experiments, and possibly in hybrid pairings of DNA and RNA strands. Segments of DNA that cells have methylated for regulatory purposes may adopt the Z geometry, in which the strands turn about the helical axis like a mirror image of the B form.

Table of comparison of the properties of different helical forms

Non-helical forms

Other, including non-helical, forms of DNA have been described, for example a side-by-side (SBS) configuration. Indeed, it is far from certain that the B-form double helix is the dominant form in living cells.

Direction of DNA strands

The asymmetric shape and linkage of nucleotides means that a DNA strand always has a discernible orientation or directionality. Because of this directionality, close inspection of a double helix reveals that nucleotides are heading one way along one strand (the "ascending strand"), and the other way along the other strand (the "descending strand"). This arrangement of the strands is called antiparallel.

Chemical nomenclature (5' and 3')

For reasons of chemical nomenclature, people who work with DNA refer to the asymmetric ends of ("five prime" and "three prime"). Biologists and the DNA enzymes they use, predominantly read nucleotide sequences in the "5' to 3' direction". However, because chemically produced DNA is synthesized and manipulated in the opposite or in non-directional manners, the orientation should not be assumed. In a vertically oriented double helix, the 3' strand is said to be ascending while the 5' strand is said to be descending.

Sense and antisense

As a result

James D. Watson

The Phage Group

The Structure of DNA

Genome Project

References

Further reading

External links

April 6

Events

Births

Deaths

Holidays and observances

External links

1928

Events

January-May

June-August

September-December

Unknown dates

Births

January

February

March-April

May-June

July-September

October-December

Unknown date

Deaths

Nobel Prizes

DNA

DNA Overview

DNA in practice

DNA in crime

DNA in computation

Overview of molecular structure

The role of the sequence

DNA replication

Mechanical properties relevant to biology

Strands association and dissociation

Circular DNA

Great length versus tiny breadth

Entropic stretching behavior

Different helix geometries

Supercoiled DNA

Sugar pucker

Conditions for formation of A and Z helices

Table of comparison of the properties of different helical forms

Non-helical forms

Direction of DNA strands

Chemical nomenclature (5' and 3')

Sense and antisense