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.