|1745||Maupertuis proposes an adaptationist account of organic design||Presupposes some mechanism for transmitting adaptations|
|1859||Darwin publishes The Origin of Species, vastly strengthening the adaptationist hypothesis|
|1865||Gregory Mendel publishes evidence for the discreteness
and combinatorial rules of inherited traits.
Mendel, Gregor (1963/1865). Experiments in Plant Hybridisation. Cambridge, MA: Harvard University Press.
|Traits are carried by discrete units, or genes; the results are not appreciated until 1900|
|1869||Miescher discovers "nuclein" (DNA) in the cells from pus in open wounds -- cells composed mostly of nuclear material. It became known as nucleic acid after 1874, when Miescher separated it into a protein and an acid molecule.||Suspected of exerting some function in the hereditary process|
|1920s||Nucleic acid found to be a major component of the chromosomes||Its molecular structure was thought to be simple, so it was not a good candidate for a carrier of genetic information|
|1930s||Chemical nature of nuclei acid investigated. It was thought to be a tetranucleotide composed of one unit each of adenylic, guanylic, thymidylic and cytidylic acids||The ubiquitous presence of nucleic acid in the chromosome was generally explained in purely physiological or structural terms|
|The molecular weight of nucleic acid was found to be much higher than the tetranucleotide hypothesis required, but it was still viewed as a uniform polymer, like starch, unaffected by its biological source||Hereditary information was commonly thought to reside in the chromosomal proteins, since these differ across species, between individuals, and even within an organism|
|1944||Oswald Avery identifies nucleic acids as the active
principle in bacterial transformation
Oswald Avery (1877-1955) was a bacteriologist whose research on pneumococcus bacteria made him one of the founders of immunochemistry and laid the foundation for later discoveries that launched the science of molecular genetics.
|"If the results of the present study of the transforming principle are confirmed, then nucleic acids must be regarded as possessing biological specificity the chemical basis of which is as yet undetermined."|
|1950||Erwin Chargaff shows that the four nucleotides
are not present in nucleic acids in stable proportions, and that the nucleotide
composition differs according to its biological source.
Chargaff, Erwin (1989). In Retrospect. A Commentary on Studies on the Structure of Ribonucleic Acids. Biochimica et Biophysica Acta 1000: 15-33.
|The nucleic acids are not monotonous polymers.|
|1952||Alfred Hershey and Martha Chase show that on infection of the host bacterium by a virus, at least 80% of the viral DNA enters the cell and at least 80% of the viral protein remains outside.||DNA rather than proteins carry genetic information.|
|1953||Watson and Crick determine that deoxyribonucleic
acid (DNA) is a double-strand helix of nucleotides. Each nucleotide consists
of a deoxyribose sugar molecule to which is attached a phosphate group
and one of four nitrogenous bases: two purines (adenine and guanine) and
two pyrimidines (cytosine and thymine). The nucleotides are joined together
by covalent bonds between the phosphate of one nucleotide and the sugar
of the next, forming a phosphate-sugar backbone from which the nitrogenous
bases protrude. The two strands are linked by selective hydrogen bonds:
the purine adenine bonds only with the pyrimidine thymine, and the purine
cytosine only with the pyrimidine guanine.
Watson and Crick (1953). A Structure for Deoxyribonucleic Acid. Nature
|DNA replication is possible through the complementary nature of the
two strands. The chemical complexity of the molecule is thought to be
sufficient to store the requisite information.
The precise manner in which the information in the DNA is activated to build an organism is still very poorly understood; what is firmly demonstrated is that so-called structural genes manufacture the proteins for living tissues.
|Early 1970s||Comparisons between chimpanzee and human genomes finds that they diverge
by only 1.6%--less than most sibling species, which barely differ in morphology,
and far less than that between any pair of congeneric species (King & Wilson 1975: 113)
King, M.C. and A.C. Wilson (1975). Evolution at two levels in Humans and Chimpanzees. Science 188: 107-116
|The theoretical implications are unclear; morphological and behavioral differences between the two species appeared to be unaccounted for by the genetic material (cf. Cherry et al, 1978; for an update, see Gibbons 1998).|
|Early 1970s||The discovery of regulator genes--genes that control the timing and output of structural genes||Since a regulator gene may control thousands of structural genes, and indeed other regulator genes, the logical inference is that human and chimpanzee genomes are being switched on and off in quite different ways (King & Wilson 1975)|
|1980s||McClintock discovered transposable strands
of genes in maize already in the 1940s, but her work was not fully recognized
for a generation.
McClintock, Barbara (1987). The Discovery and Characterization of Transposable Elements: The Collected Papers of Barbara McClintock. New York: Garland, 1987.
|The genome may be controlling aspects of its own mutation|
History of Genetics Timeline
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