1. Describe 2 of the experiments we discussed in class that lead to discoveries on the structure and function of DNA. [6 Marks, C]

Two key experiments that illuminated the structure and function of DNA are:<br /><br />1. **Hershey-Chase Experiment (1952):** This experiment definitively proved that DNA, not protein, is the genetic material. Alfred Hershey and Martha Chase used bacteriophages (viruses that infect bacteria) labeled with radioactive isotopes. One set of phages had their protein coats labeled with radioactive sulfur (35S), while another set had their DNA labeled with radioactive phosphorus (32P). They allowed the phages to infect bacteria, then used a blender to separate the phage ghosts (empty protein shells) from the infected bacteria. They found that the radioactive phosphorus (32P), representing DNA, was inside the bacteria, while the radioactive sulfur (35S), representing protein, remained outside. This demonstrated that DNA was injected into the bacteria to direct the formation of new viruses, confirming its role as the carrier of genetic information.<br /><br />2. **Meselson-Stahl Experiment (1958):** This experiment demonstrated the semi-conservative nature of DNA replication. Matthew Meselson and Franklin Stahl grew bacteria in a medium containing heavy nitrogen (15N), allowing the bacteria to incorporate it into their DNA. They then transferred the bacteria to a medium containing normal nitrogen (14N) and allowed them to replicate for several generations. After each generation, they extracted the DNA and centrifuged it in a density gradient. The results showed a distinct band of heavy DNA (15N) in the first generation, followed by a hybrid band (intermediate density containing both 14N and 15N) in the second generation, and subsequently, both hybrid and light DNA (14N) bands in later generations. This pattern confirmed the semi-conservative model, where each new DNA molecule consists of one original (parental) strand and one newly synthesized strand.<br />