Lesson 21 - DNA replication

Question 1

Describe the process of semi-Conservative replication of DNA

Answer 1

1. The enzyme, DNA helicase, causes the DNA double helix to unwind and separate into 2 strands by breaking the hydrogen bonds between complementary base pairs, exposing
   the bases.
2. Both separate strands then act as a template for DNA replication to occur.
3. Free activated DNA nucleotides (in the nucleoplasm) are attracted to and hydrogen bond to their complementary bases on the exposed template strands by complementary base pairing.
4. The newly added nucleotides are then joined together by the enzyme DNA polymerase, which catalyses the formation of phosphodiester bonds, between adjacent nucleotides.
5. This results in two identical DNA molecules being produced, each of which contains one of the original DNA strands and one newly synthesised DNA strand (semi-conservative).

Question 2

Cell division

Answer 2

● The 2 main stages of cell division:
- Nuclear division - the process by which the nucleus divides
- Cytokinesis - the process by which the whole cell divides (the cytoplasm divides into daughter cells)

Question 3

What needs to happen before a
nucleus and cell can divide?

Answer 3

DNA molecules replicated

Question 4

Why are the 2 strands of DNA said to be antiparallel?

Answer 4

• Carbon atoms in pentose sugar are
  numbered
• Carbon number 5 (5-prime) has a
  phosphate group attached and carbon number 3 (3-prime) has a hydroxyl
  group attached
• In a DNA molecule, one strand runs in the 5’ to 3’ direction and one strand runs in the 3’ to 5’ direction

Question 5

Why can nucleotides only be added in the 5’ to 3’ prime direction?

Answer 5

The enzyme DNA polymerase can only attach nucleotides to the hydroxyl (OH) group on the 3’ carbon molecule