DNA/RNA Era: ~1940’s-1950

At Stanford University in 1941, George Beadle and Edward Tatum were irradiating a bread mold called Nuerospora to induce genetic mutations. They observed that certain colonies of the mutated mold couldn’t survive on the nutrients they were being currently given. So Beadle and Tatum started replacing each nutrient with another, they discovered which metabolic pathways had been lost. By doing this, they showed these failures were caused by induced mutations in single genes. Their conclusion was that each gene gives a specific code to make one particular protein for one function.

13 years after her discovery of crossing-over, Barbara McClintock made another discovery in 1944. She discovered that genetic elements from break points can move from one location on that chromosome or another chromosome.

In 1944 Oswald Avery was working with his colleagues at the Rockefeller Institute when they made a very exciting discovery. They showed that completely a colony dead, virulent, or very harmful bacteria transformed colonies of live, non-virulent strains of bacteria into the virulent form. Avery took the ‘transforming agent’ from the dead cells, purified it, and then ran various tests. The tests were composed of methods that either destroyed protein or nucleic acids. The results was that the ‘transforming agent’ was made up of deoxyribose nucleic acid, or DNA. The experiments had shown the transformation was passed from generation from generation, but Avery didn’t identify DNA as the physical basis of the gene.

Joshua Lederberg was working with the bacterium Escherichia coli using the ‘nutritional mutant’ technique from Beadle and Tatum in 1946. Joshua learned that when a few different mutant colonies were mixed, some of the bacterium regained the ability to synthesize nutrients that they had previously lost. His conclusion was that this change was due to genetic crossing between the different mutated strains. That conclusion proved that bacteria reproduced sexually, which went against the current assumption of them reproducing through cell division. The work that Lederberg did was the seed for a new era of bacterial genetics, and laid the foundation for recombinant DNA technology.

Hans Grüneberg published his book ‘Animal Genetics and Medicine’ in 1947. Hans furthered his argument for the relevance of the genetic studies of mice and other models to our comprehension of human diseases in this book. He also introduced a new concept, inherited diseases.

In 1948 the American Society of Human Genetics was founded. They said the purpose of this group was ‘to provide leadership in research”. The American Society of Human Genetics and it’s importance of representing geneticists has grown immensely with the many advances of medical genetics in the later 50’s.

The next discovery was made by Murray Barr, he discovered sex chromatin. Even with all the advances geneticists have made, they still weren’t able to tell the different mammalian chromosomes apart accurately. Barr observed dark-staining bodies inside the nuclei of neurons in female cats. Upon further inspection, Barr learned that the bodies were made of chromosomal material. He also confirmed that other female mammals have these structures, which were later named ‘sex chromatin’ or ‘Barr Bodies’. Barr’s discovery of this made it possible to diagnose individuals who had conditions where they had an extra of missing copy of the X chromosome.

The last discovery of the decade was also a big one. In 1950 Erwin Chargaff and Ernst Vischer analysed DNA from a number of different sources. From that, they discovered that the ratios of the four bases of DNA weren’t equal, but there is always the same number of A’s as T’s and also the same number of G’s and C’s. This set the groundwork for a discovery that was made 3 years later by James Watson and Francis Crick. They discovered the double helix shape of DNA.

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