Table 3.

The main stages of research that led to the discovery of the DNA double helix (1953).

Period	Discoveries
1866	Ernst Heinrich Haeckel (Potsdam 1834–Jena 1919), in 1866, proposed that the nucleus contained the factors responsible for the transmission of hereditary traits.
1869	Johann Friedrich Miescher (Basel 1844–Davos 1895) identified in 1869, inside the nuclei of human white blood cells, ‘nuclein’, the molecule now known as DNA
1870s	Walther Flemming (Schwerin 1843–Kiel 1905) in the 1870s described the morphology of a fibrous structure within the nucleus of cells, named ‘chromatin’, now known as chromosomes. By observing this chromatin, Flemming correctly worked out how chromosomes separate during cell division, also known as mitosis.
since 1879	Ludwig Karl Martin Leonhard Albrecht Kossel (Rostock 1853–Heidelberg 1927) made great progress in understanding the basic building blocks of nuclein. Kossel isolated the five nucleotide bases that are the building blocks of DNA and RNA: adenine, cytosine, guanine, thymine, and uracil. In 1881, Kossel identified nuclein as a nucleic acid and provided its present chemical name, deoxyribonucleic acid (DNA).
since the 1880s	Theodor Boveri (Bamberg 1862–Würzburg 1915) first presented the idea that the genetic material passed down from parent to child is within the chromosomes.
since 1905	Walter Stanborough Sutton (Utica 1877–Kansas City 1916) expanded on Theodor’s observation. He found it was possible to distinguish individual chromosomes undergoing meiosis in the testes of the grasshopper and identified the sex chromosome.
since 1905	Phoebus Aaron Theodor Levene (Sagor 1869–New York 1940), and his student J. A. Mandel, described a linear complex with a phosphoric acid and a base forming a subunit they called a mononucleotide, with two or more mononucleotides bound together to form what they called a polyphosphoric acid, or polynucleotide.
since 1910	Thomas H. Morgan (Lexington 1866–Pasadena 1945) and his students (Alfred Sturtevant, Calvin Bridges, Hermann Muller, and others), provided the proof for the chromosomal theory of heredity, genetic linkage, and chromosomal crossing over and non-disjunction.
1928	Frederick Griffith (Prescott 1879–London 1941) performed important studies on Bacterial transformation.
1944	Oswald Avery (Halifax 1877–Nashville1955). In 1944, he and his colleagues Maclyn McCarty and Colin MacLeod reported that the transforming substance, the genetic material of the cell, was DNA.
1950	Erwin Chargaff (Černivci 1905–New York 2002) found that in DNA, the ratios of adenine (A) to thymine (T) and guanine (G) to cytosine (C) are equal.
1951–1952	Maurice Hugh Frederick Wilkins (Pongaroa 1916–London 2004) and Rosalind Franklin (Kensington 1920–Chelsea 1958), at King’s College London, used X-ray diffraction to study the structure of DNA in solution. They found that DNA could take two forms: crystalline or A form, and paracrystalline or B form, as «big helix with several chains, phosphates on outside, phosphate–phosphate interhelical bonds, disrupted by water».
1953	J. D. Watson, F. H. C. Crick. Molecular structure of nucleic acids. A Structure for Deoxyribose Nucleic Acid. Nature 4356 April 25: 737-38, 1953 M. H. F. Wilkins, A. R. Stokes, H. R. Wilson. Molecular Structure of Deoxypentose Nucleic Acids. Nature 4356 April 25: 738-40, 1953