B2. DNA Replication

Question 1

Why does DNA replicate?

Why? What is it called? What does it mean?

Answer 1

Why does DNA replicate?
DNA copies itself before cell division so that each new cell has the full amount of DNA. The method is called semi-conservative replication because half of the strands in each new DNA molecule are from the original DNA molecule. This means that there’s genetic continuity between generations of cells (i.e. the cells produced by cell division inherit their genes from their parent cells).

Question 2

DNA Replication (3 steps)

Answer 2

1. The enzyme DNA helicase breaks the hydrogen bonds between bases on the two polynucleotide DNA strands. This makes the helix unwind to form two single strands.
2. Each original single strand acts as a template for a new strand. Complementary base pairing means that free-floating DNA nucleotides are attracted to their complementary exposed bases on each original template strand -A with T and C with G.
3. Condensation reactions join the nucleotides of the new strand together -catalysed by the enzyme DNA polymerase. Hydrogen bonds form between the bases on the original and new strands.

Each new DNA molecule contains one strand from the original DNA molecule and one new strand.

Question 3

Evidence for semi-conservative replication

Answer 3