Principles of Genetics and Breeding

1.1.3.2 DNA ERA

• In 1952, American geneticists Alfred Hershey and Martha Chase showed that when a type of virus called a bacteriophage infects a bacterium, it is the virus’s DNA—not protein—that enters the bacterium to cause infection.
• In the early 1950s researchers began to apply techniques of X-ray diffraction to learn about the basic structure of DNA. X-ray diffraction can determine molecular structures by measuring patterns of scattered X rays after they pass through a crystalline substance. British physical chemist Rosalind Franklin and British biophysicist Maurice Wilkins used X-ray diffraction to obtain DNA images of unprecedented clarity.
• Yet the exact three-dimensional structure of DNA remained unclear. In 1953 American biochemist James Watson and British biophysicist Francis Crick proposed a model of DNA that is still accepted today: A double helix molecule formed by two chains, each composed of alternating sugar and phosphate groups, connected by nitrogenous bases. Watson and Crick (along with Wilkins) were awarded the 1962 Nobel Prize in physiology or medicine for their discoveries.
• Watson and Crick speculated that the structure of DNA provided some obvious clues about how the molecule could replicate itself. They proposed a replication model in which each strand of DNA serves as a template for making exact copies. This model of replication, called semi-conservative replication, was demonstrated in 1958 by American molecular biologists Matthew Meselson and Franklin Stahl.
• In the late 1950s, South African geneticist Sydney Brenner and other scientists confirmed that RNA acted as an intermediary between DNA and protein production.
• In 1961 Crick and Brenner determined that groups of three nucleotides, now known as codons, code for the 20 amino acids that form the foundation of proteins.
• American biochemists Marshall Nirenberg. Heinrich Matthaei synthesized repeated nucleotide sequences that led to the production of repeated single amino acids. They identified how certain codon combinations code for a specific amino acid.
• A process developed by American geneticist Har Gobind Khorana helped scientists create a “dictionary” of codons that defined specific amino acids, thus resolving the remaining ambiguities in the genetic code. Only 12 years after the structure of DNA was deduced, the genetic code was solved.