Lecture Notes: DNA Replication

Introduction to DNA Replication

• Replication: Process of duplicating a DNA strand.
• Original DNA Template: Used as a blueprint to make new copies.

Origin of Replication

• Starting Point: Called the "origin of replication."
• Replication Bubble: Formed as DNA strands unwind, creating space.
• Replication Forks: Located on either side of the bubble, replication proceeds outward toward these forks.

DNA Replication in Bacteria vs. Humans

• Bacteria: Single origin of replication due to circular DNA.
• Humans: Multiple origins due to long DNA (up to 3 meters per cell).

Key Proteins and Enzymes

1. DNA Helicase

• Function: Binds to one strand to break base pairs (5’ to 3’ direction).
• Analogy: Like a zipper, unzipping the DNA strands.
• Hydrogen Bonds: Holds base pairs together; helicase breaks these.

2. DNA Topoisomerase

• Function: Prevents DNA from supercoiling ahead of helicase.
• Analogy: Like smoothing hair to prevent tangles.

3. Single-Strand Binding Proteins (SSBPs)

• Function: Prevents separated DNA strands from re-annealing.

4. DNA Polymerase

• Function: Adds nucleotides to 3’ end, forming covalent bonds.
• Direction: 5’ to 3’ only.

5. DNA Primase

• Function: Adds RNA primer for DNA polymerase to start.
• Analogy: Like primer in painting or makeup.

DNA Strands: Leading vs. Lagging

Leading Strand

• Characteristics: Synthesized continuously in 5’ to 3’ direction.
• Replication Direction: Towards the replication fork.

Lagging Strand

• Characteristics: Synthesized discontinuously as Okazaki fragments.
• Assembly: Fragments glued together by DNA ligase.
• Replication Direction: Away from the replication fork.

Replication Accuracy

• Error Rate: Very low, one mistake per 100 million nucleotides.
• Mechanisms Ensuring Accuracy:
  • Stable hydrogen bonding.
  • DNA polymerase specificity.
  • Proofreading ability of DNA polymerase.

Conclusion

• Review: Watch the video again for better understanding.
• Questions: Address during class or office hours.
• Further Study: Check additional resources and notes on Blackboard.