DNA Replication: HL IB Biology Revision Notes 2025

Importance of DNA Replication

• Semi-conservative replication
  • One parent strand is retained in the daughter molecule, known as the template strand.
  • New strand formed by adding complementary free nucleotides (A-T, C-G).
  • Takes place in the nucleus.
  • Essential for growth, replacement of old/damaged cells, and reproduction.

Semi-Conservative Replication

• DNA Polymerase Function
  • Links nucleotides to form a new strand using a pre-existing strand as a template.
• Crick and Watson's Hypothesis
  • Proposed semi-conservative replication.
  • Supported by Meselson and Stahl’s experiment.

Helicase & DNA Polymerase

• DNA Replication Process
  • Helicase unwinds DNA, breaking hydrogen bonds.
  • New strands formed by base-pairing using DNA polymerase.
  • DNA polymerase works in 5' to 3' direction.

Electrophoresis & PCR

Gel Electrophoresis

• Separates DNA/RNA/proteins using an electric current based on size and charge.
• DNA moves towards the anode due to negative charge.
• Smaller fragments move faster.

DNA Separation Method

• DNA collected and prepared using PCR.
• DNA fragments separated by gel electrophoresis.
• Visual output developed with radioactive or fluorescent probes.

Polymerase Chain Reaction (PCR)

• Purpose: Amplifies specific DNA/RNA fragments.
• Applications: DNA profiling, genetic engineering, COVID-19 testing.
• Requirements: Target DNA/RNA, DNA polymerase (Taq), free nucleotides, buffer solution.
• Stages: Denaturation, Annealing, Elongation.

Applications of Electrophoresis & PCR

DNA Profiling

• Used in forensic and paternity investigations.
• Involves gel electrophoresis to create unique genetic fingerprints.
• VNTRs (Variable Number Tandem Repeats) used for profiling.

Enhancing Reliability

• Increase number of VNTR markers to improve reliability.
• Important in legal contexts to ensure accuracy.

Mechanism of DNA Replication (HL)

Directionality of DNA Polymerase

• Replication occurs in 5' to 3' direction.
• DNA polymerase adds nucleotides to the 3' end.

Leading & Lagging Strands

• Antiparallel strands: Leading synthesized continuously, lagging in Okazaki fragments.
• Okazaki fragments joined by DNA ligase.

Enzymes in DNA Replication

• Helicase: Unwinds DNA, separates strands.
• Single-stranded binding proteins: Keep strands apart.
• DNA Primase: Generates RNA primer.
• DNA Polymerase I & III: Extend DNA strands, remove RNA primers.
• DNA Ligase: Joins Okazaki fragments.

Proofreading Replicated DNA

• DNA polymerase III proofreads new strands.
• Recognizes and corrects incorrect nucleotides to prevent mutations.