DNA Replication (A-level Biology)

Why Does DNA Replicate?

• DNA replication is essential for cell division, ensuring each daughter cell has a full set of DNA.
• Occurs during the S phase of the cell cycle.
• Enables genetic continuity, passing DNA from parents to offspring.

The Process of DNA Replication

1) Double Helix Unwinding

• Unwinding of DNA: The double helix unwinds, using each strand as a template.
• Role of DNA Helicase: An enzyme that breaks hydrogen bonds between base pairs.
• Replication Fork: The site where DNA unwinds, moving in one direction.

2) Semi-Conservative Replication

• Template Strands: Original strands serve as templates for new strands.
• Parent and Daughter Strands: Each new DNA has one parent and one new strand.

3) DNA Polymerase (Condensation Reactions)

• Condensation Reactions: Needed to form new DNA strands.
• Role of DNA Polymerase: Catalyzes the joining of nucleotides and ensures accuracy.

Mechanism of DNA Polymerase

DNA Polymerase Direction

• 5' to 3' Direction: DNA polymerase adds nucleotides only in this direction.
• Leading and Lagging Strands:
  • Leading strand synthesized continuously.
  • Lagging strand synthesized in segments (Okazaki fragments).

DNA Polymerase Reads and DNA Ligase Catalyzes

• Reading and Proofreading: DNA polymerase reads and corrects errors.
• DNA Ligase Role: Catalyzes condensation to join nucleotides.

Key Concepts of DNA Replication

• Enzymes Involved: Helicase, primase, DNA polymerase, and ligase.
• Role of RNA Primers: Initiate replication, providing a starting point for DNA polymerase.
• Accuracy Assurance: Proofreading by DNA polymerase and removal of incorrect nucleotides.
• Consequences of Errors: Can lead to mutations, some of which may cause diseases.

Importance of DNA Replication

• Genetic Continuity: Essential for transferring genetic information across generations.
• Cell Division and Repair: Necessary for forming new cells and repairing damaged DNA.