1.5.2 DNA replication

Question 1

Why is DNA replication described as ‘semi-conservative’?

Answer 1

• Strands from original DNA molecule act as a template
• New DNA molecule contains 1 old strand and 1 new strand

Question 2

Outline the process of semi-conservative DNA replication

Answer 2

1. DNA helicase breaks H-bonds between base pairs
2. Each strand acts as a template
3. Free nucleotides from nuclear sap attach to exposed bases by complementary based pairing
4. DNA polymerase catalyses condensation reactions that join adjacent nucleotides on new strand
5. H-bonds reform

Question 3

Describe the Meselson-Stahl experiment

Answer 3

1. Bacteria were grown in a medium containing heavy isotope 15N for many generations
2. Some bacteria were moved to a medium containing light isotope 14N. Samples were extracted after 1 and 2 cycles of DNA replication
3. Centrifugation formed a pellet. Heavier DNA settled closer to the bottom of tube

Question 4

What is the enzyme DNA helicase used for?

Answer 4

It causes the two strands of DNA to seperate breaking the hydrogen bonds between the complementary bases

Question 5

What is the enzyme DNA polymerase used for?

Answer 5

DNA polymerase joins the nucleotides together forming phosphodiester bonds