Semi-Conservative Replication (AQA A Level Biology): Revision Note

Semi-conservative replication

• Before a cell divides, it copies its DNA to ensure each daughter cell receives a full copy of the parental DNA

• DNA is copied by semi-conservative replication

  • Each new DNA molecule has one original strand and one newly synthesised strand

• This process ensures genetic continuity between generations of cells

• It’s essential for:

  • Replacing body cells with identical ones

  • Growth and development

• DNA replication occurs during the S phase of interphase in the cell cycle

Steps in semi-conservative replication

1. Initially, the enzyme helicase unwinds the DNA double helix by breaking the hydrogen bonds between the base pairs on the two antiparallel polynucleotide DNA strands

2. This forms two single polynucleotide DNA strands, which act as a template for the formation of a new strand

3. The new strand is made from free nucleotides that are attracted to the exposed DNA bases by base pairing

4. The new nucleotides are then joined together by DNA polymerase, which catalyses condensation reactions to form a new strand

5. The original strand and the new strand join together through hydrogen bonding between base pairs to form the new DNA molecule

DNA Polymerase

• Free nucleotides in the nucleus exist as nucleoside triphosphates (activated nucleotides)

  • The extra phosphate groups activate the nucleotides and provide energy for DNA replication

• Bases of activated nucleotides pair with complementary bases on the template strands

• The enzyme DNA polymerase synthesises new DNA strands from the two template strands by:

  • Catalysing condensation reactions to form the sugar-phosphate backbone

  • Removing two phosphate groups from each nucleotide to release energy which is used to form phosphodiester bonds