User:Madeleine Price Ball/Draft
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[edit] History
Although the science of genetics began with the work of Gregor Mendel in the mid-1800's, there were some theories of inheritance that preceded Mendel. A popular theory during Mendel's time was the concept of blending inheritance: the idea individuals inherit a smooth blend of traits from their parents. Mendel's work disproved this, showing that traits are composed of combinations of distinct genes rather than a continuous blend. Also popular at the time was the theory of inheritance of acquired characteristics: the belief that individuals inherit traits that have been strengthened in their parents. This theory (commonly associated with Jean-Baptiste Lamarck) is now known to be wrong, the experiences of individuals do not affect the genes they pass to their children.[1]
[edit] Mendelian and classical genetics
The modern science of genetics traces its roots to Gregor Johann Mendel, a German-Czech Augustinian monk and scientist who studied of the nature of inheritance in plants. In his paper "Versuche über Pflanzenhybriden" ("Experiments on Plant Hybridization"), presented in 1865 to the Brunn Natural History Society, Gregor Mendel traced the inheritance patterns of certain traits in pea plants and showed that they could be described mathematically.[2] Although this pattern of inheritance could only be observed for a few traits, Mendel's work suggested that statistics was a useful tool for studying inheritance.
The importance of Mendel's work was not understood until early in the 1900's, after his death, when his research was re-discovered by other scientists working on similar problems. The word genetics itself was coined in 1905 by William Bateson, a proponent of Mendel's work, in a letter to Adam Sedgwick.[3][4] (The adjective genetic, derived from the Greek word genno (γεννώ): to give birth, predates the noun and was first used in a biological sense in 1860.[5]) Bateson popularized the usage of the word genetics to describe the study of inheritance in his inaugural address to the Third International Conference on Plant Hybridization in London, England, in 1906.[6]
After the rediscovery of Mendel's work, scientists tried to discover which molecules in the cell were responsible for inheritance. In 1910 Thomas Hunt Morgan argued that genes are on chromosomes, based on observations of a sex-linked white eye mutation in fruit flies.[7] In 1913 his student Alfred Sturtevant used the phenomenon of genetic linkage to show that genes are arranged linearly on the chromosome.[8]
[edit] Molecular genetics
Although genes were known to exist on chromosomes, chromosomes are composed of both protein and DNA: scientists didn't know which of these was responsible for inheritance. In 1928, Frederick Griffith discovered of the phenomenon of transformation (see Griffith's experiment): dead bacteria could transfer genetic material to "transform" other still-living bacteria. Sixteen years later, in 1944, Oswald Theodore Avery, Colin McLeod and Maclyn McCarty identified the molecule responsible for transformation as DNA.[9] The Hershey-Chase experiment in 1952 also showed that DNA (rather than protein) was the genetic material of the viruses that infect bacteria, further evidence that DNA was the molecule responsible for inheritance.[10]
James D. Watson and Francis Crick solved the structure of DNA in 1953, using the X-ray crystallography work of Rosalind Franklin that indicated DNA had a helical structure (ie. shaped like a corkscrew).[11][12] Their double-helix model had two strands of DNA with the nucleotides pointing inwards, each matching a complementary nucleotide on the other strand to form what looks like rungs on a twisted ladder.[13] This structure showed that genetic information exists in the sequence of nucleotides on each strand of DNA. The structure also suggested a simple method for duplication: if the strands are separated, new partner strands can be reconstructed for each based on the sequences in the old strand.
Although the structure of DNA showed how inheritance worked, it was still not known how DNA influenced the behavior of cells. In the following years scientists tried to understand how DNA controls the process of protein production. It was discovered that the cell uses DNA as a template to create matching messenger RNA (a molecule with nucleotides, very similar to DNA). The nucleotide sequence of a messenger RNA is used to create an amino acid sequence in protein; this translation between nucleotide and amino acid sequences is known as the genetic code.
With this molecular understanding of inheritance, an explosion of research became possible. One important development was chain-termination DNA sequencing in 1977 by Frederick Sanger: this technology allows scientists to read the nucleotide sequence of a DNA molecule.[14] In 1983 the polymerase chain reaction was developed by Kary Banks Mullis, providing an quick way to isolate and amplify a specific section of a DNA from a mixture.[15] These and other techniques, through the pooled efforts of the Human Genome Project and parallel private effort by Celera Genomics, culminated in the sequencing of the human genome in 2003.[16]
