| 1833 | Robert Brown describes the nucleus in cells of orchids |
| 1866 | Gregor Mendel publishes ‘Studies of plant hybridisation’ |
| 1868 | Friedrich Miescher discovers ‘nuclein’ (DNA) in pus cells |
| 1878 | Albrecht Kossel isolates ‘yeast nuclein’ (later shown to be RNA) |
| 1880 | Walther Flemming describes nuclear ‘threads’ made of ‘chromatin’ during cell division (‘mitosis’) in the salamander |
| 1882 | Flemming suggests that chromatin and nuclein are identical |
| 1885 | Kossel extracts two bases, guanine and adenine, from thymus nuclein, followed by thymine (1893), cytosine (1894) and uracil (1900) |
| 1888 | Wilhelm Waldeyer renames Flemming’s threads ‘chromosomes’ |
| 1889 | Richard Altmann renames nuclein ‘nucleic acid’ |
| 1900 | Mendel’s work is ‘rediscovered’ by Carl Correns, Hugo de Vries and Erich von Tschermak |
| 1903 | Walter Sutton formulates the ‘chromosome theory of inheritance’ |
| 1904 | William Bateson begins a pro-Mendel crusade and coins the word ‘genetics’ |
| 1909 | Wilhelm Johannsen invents the words ‘gene’, ‘genotype’ and ‘phenotype’ Phoebus Levene identifies the sugar in yeast nucleic acid (RNA) as ribose |
| 1912 | Levene proposes that nucleic acids are a small ‘tetranucleotide’, containing one of each of the four bases Max von Laue takes the first X-ray photograph of a crystal |
| 1914 | Lawrence Bragg formulates Bragg’s Law of X-ray crystallography; with his father William, develops ‘a new crystallography’ |
| 1915 | Thomas Hunt Morgan publishes The Mechanism of Mendelian Inheritance, based on mutations in the fruit fly |
| 1927 | Fred Griffith shows that dead pneumococci bacteria can ‘transform’ (change the genetic characteristics of) live pneumococci, when injected into living mice |
| 1928 | Levene and Kossel both claim that genes are made of protein, not nucleic acid |
| 1929 | Levene identifies the sugar in thymus nucleic acid (DNA) as deoxyribose
Martin Dawson, in Oswald Avery’s lab at the Rockefeller, confirms Griffith’s finding of transformation of pneumococci, also in living mice |
| 1931 | Dawson and Richard Sia achieve transformation in vitro |
| 1932 | Lionel Alloway in Avery’s lab extracts the ‘transforming principle’ responsible for transformation but cannot identify it chemically |
| 1937 | Torbjörn Caspersson deduces that DNA molecules are very long, thin cylinders, and much bigger than a ‘tetranucleotide’ |
| 1938 | Florence Bell takes X-ray photographs of DNA; she and Bill Astbury suggest that the bases in the DNA molecule are stacked ‘like a pile of pennies’ |
| 1940 | Colin MacLeod in Avery’s lab detects DNA in extracts of ‘transforming principle’ but does not follow up the observation |
| 1941 | Alfred Mirsky extracts ‘chromosin’ (DNA with associated protein) from cell nuclei |
| 1942 | Maclyn McCarty and Avery show that the ‘transforming principle’ consists of DNA, with tiny amounts of contaminating protein |
| 1944 | Erwin Schrödinger suggests in his book What is Life? that genes are ‘aperiodic crystals’
Avery, MacLeod and McCarty publish their landmark paper showing that DNA is the ‘transforming principle’ and the genetic material in pneumococci Mirsky insists that protein, not DNA, mediates transformation and is the genetic material |
| 1947 | Rollin Hotchkiss shows that DNA contains unequal amounts of the four bases, thus ruling out the hypothetical ‘tetranucleotide’
André Boivin proves that DNA also transforms other bacteria (E. coli) Masson Gulland proposes that the DNA molecule is held together by hydrogen bonding between bases Gulland’s PhD student Michael Creeth proposes that DNA consists of two straight strands of DNA, linked by hydrogen bonding between bases on opposing strands |
| 1948 | Erwin Chargaff reports that amounts of adenine and thymine are equal, as are those of cytosine and guanine, in different sources of DNA
Linus Pauling discovers the alpha-helix, crucial in shaping protein molecules |
| 1949 | Sven Furberg works out that the bases lie perpendicular to the backbone of DNA, and proposes a single-stranded, helical structure for DNA |
| 1950 | Ray Gosling at King’s takes an X-ray photograph showing a regular ‘crystalline’ appearance of DNA (the A form) |
| 1951 | January: Rosalind Franklin joins the Biophysics Unit at King’s, to work on the X-ray structure of DNA
May: Wilkins presents the crystalline DNA structure at a meeting in Naples and inspires Jim Watson to solve its structure Elwyn Beighton in Leeds takes an X-ray photograph that shows the helical features of DNA (B form). The photograph is ignored July: Wilkins presents DNA structures at a meeting in Cambridge and is told by Franklin to stop working on DNA Alec Stokes at King’s predicts the X-ray pattern of a helical molecule October: Jim Watson joins Francis Crick at the Cavendish Laboratory in Cambridge and persuades him to pursue the structure of DNA November: Wilkins meets Watson and Crick and tells them that the most likely structure contains three helical strands of DNA Watson attends a colloquium at King’s where Wilkins and Franklin present their work on DNA Bruce Fraser at King’s builds a model of DNA containing three helical strands, which Wilkins rejects December: Using data from King’s, Crick and Watson build a three-stranded model of DNA, which is fatally flawed; Wilkins breaks off their collaboration |
| 1952 | January: Franklin and Gosling characterise the A and B forms of DNA
April: John Griffith in Cambridge calculates that hydrogen bonding will attract adenine to thymine, and cytosine to guanine May: Gosling takes Photograph 51, showing the helical features of DNA (B form) July: Franklin decides that ‘crystalline’ DNA (A form) cannot be a helix, causing Wilkins to have doubts about the helical nature of DNA in general December: Pauling proposes a DNA model with three helical strands, also fatally flawed |
| 1953 | February: Watson visits King’s; Wilkins shows him Photograph 51, in which Watson sees the diagnostic features of a helical structure
March: Franklin leaves King’s to study viral structure at Birkbeck College, London Watson realises that the pairing of bases on opposing strands is the key to the structure of DNA. Using Franklin’s data and without her knowledge, he and Crick construct the double helix April: Nature publishes three papers on the double helix, by Watson and Crick; Wilkins et al; and Franklin and Gosling July: Watson and Crick publish a follow-up paper in Nature on the self-replication of DNA |
| 1958 | 16 April: Rosalind Franklin dies of ovarian cancer, aged 38 |
| 1962 | Watson, Crick and Wilkins share the Nobel Prize for Physiology or Medicine |
| 1968 | Watson publishes The Double Helix |
| 2001 | Independent scientific tribunal clears Gregor Mendel of having falsified his data |