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10.1. DNA as hereditary material

DNA as hereditary material

The deoxyribonucleic acid or DNA, is a nucleic acid containing the genetic instructions necessary for the development and functioning of all living beings known and some virus , and is responsible for hereditary transmission.

The discoverer of what is now known as DNA was the British doctor Frederick Griffith, in 1928, when he was researching a vaccine to prevent pneumonia during the influenza pandemic that took place after the First World War.

He isolated two strains of pneumococcus (Streptococcus pneumoniae), the smooth strain (S), harmful, and another rough strain (R), not virulent, since the S was covered by a thick polysaccharide capsule that protected it from the body's immune system.

When he injected S bacteria into the mice, they became ill and died. Bacteria reproduced because they were protected by their capsule . By studying the remains of the mice, he recovered S.

If he killed the S bacteria with heat, and injected the bacteria into the mice, they survived.

If he injected R bacteria into the mice, the mice survived, as the bacteria died because they lacked a protective polysaccharide capsule.

Then, to better understand the process, he injected a combination of R (non-lethal) with heat-inactivated S (non-lethal) bacteria, and the mouse died. In addition, live S bacteria were found.

This mystery could not be solved by Griffith, although he deduced that some substance of bacteria S, resistant to heat, had transformed bacteria R into virulent bacteria S.

It was in 1944 when AveryMacLeod , and McCarty were able to explain it. Bacteria killed S by heating had intact DNA, which when released, could come into contact with the bacterial wall of R, penetrate it and enter the bacterial DNA, causing the bacteria to manufacture the capsule of polysaccharides , becoming virulent and transmitting that information to their descendants.

This was the first experimental proof that DNA contained hereditary information, although this discovery was not sufficiently valued due to the lack of knowledge of the structure of DNA. It was thought that this only happened in bacteria and viruses, and that higher beings should have another more complex mechanism.

From these and many other discoveries the structure of DNA became known.