3.1.5.2

Question 1

What is the name of the model used for DNA replication and why?

Answer 1

Semi conservative replication
each new DNA molecule has one original strand and one newly synthesised strand

Question 2

Describe how DNA is replicated

Answer 2

1. DNA helicase enzyme breaks hydrogen bonds between nitrogenous bases causing the strands to separate and the helix to unwind
2. Free floating nucleotides are attracted to complementary bases on each strand of DNA
3. DNA polymerase enzyme forms phosphodiester bonds between nucleotides by condensation reactions
   Two identical DNA molecules are formed

Question 3

Describe the Meselson and Stahl experiment

Answer 3

1. Bacteria grown on substrate containing N15 (heavier nitrogen isotope) for 14 generations- incorporate N15 into DNA
2. DNA extracted and centrifuged- 100% of DNA indicated lower density
3. Bacteria then grown in N14 substrate
4. DNA extracted and centrifuged - DNA formed a single band, higher than parental generation - shows DNA molecules contained N15 and N14 isotopes
5. Bacteria continued to grow
6. DNA extracted from second generation - 2 distinct bands formed
7. Process repeated - proportion of bacteria inheriting heavier strand decreased with each generation