In the 50's scientists didn't agree about which molecule - proteins or DNA - carry the hereditary genetic information. This experiment was designed to answer this question. It was known that bacteriophages transfer the genetic information into the bacterial cells they infect. With this in mind scientists prepared two separate bacterial colonies and infected each one with differently modified bacteriophages. The first group was infected with phages marked with radioactive phosphorus - a structural element of the DNA; the other one with phages marked with radioactive sulfur - abundant in proteins. The cultures were left for the phages to infect bacteria and transfer their genetic information into bacterial cells. Afterwards, the radioactivity inside the cells and in the outer mixture was measured. In the trial with sulfur no radioactive sulfur was observed inside the cells, but the outside mixture was abundant in radioactive sulfur - no sulfur was transferred inside the cells. In the trial with phosphorus radioactive phosphorus were only observed inside the bacterial cells, proving that phosphorus was transported into the cells along the DNA molecule it was built in. This experiment proved that DNA is the hereditary material used to transfer the genetic information.
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