Lecture 35: DNA Replication

Introduction

• Review of DNA and nucleotide structure concepts.
• Focus on DNA replication, applicable to both prokaryotes and eukaryotes.
• Replication Fork: The site where DNA replication occurs, resembling a fork in the road.

Key Enzymes in DNA Replication

1. DNA Helicase

• Unwinds the DNA double helix by breaking hydrogen bonds between bases.
• Creates two single-stranded DNA templates for replication.

2. Topoisomerase

• Relieves tension in the DNA helix as it unwinds by cutting the DNA, allowing it to uncoil, then resealing it.
• Works simultaneously with helicase.

3. Single-Strand Binding Proteins

• Bind to single-stranded DNA to prevent it from re-winding.

4. Primase

• Builds a short RNA primer complementary to the DNA template, necessary for DNA polymerase to start.

5. DNA Polymerase III

• Extends RNA primers by adding DNA nucleotides, building the complementary strand.
• Sliding Clamp: A protein that holds DNA polymerase III in place.

Directionality in DNA Replication

• DNA Polymerase works 5' to 3' direction only.

Leading Strand

• Made continuously towards the replication fork.

Lagging Strand

• Made discontinuously away from the fork, creating Okazaki fragments.

Okazaki Fragments

• Short DNA sequences synthesized on the lagging strand.
• Each fragment begins with a new RNA primer.

DNA Polymerase I

• Replaces RNA primers with DNA.

DNA Ligase

• Seals gaps between DNA fragments, finalizing a continuous DNA strand.

Differences Between Prokaryotic and Eukaryotic Replication

• Eukaryotic Replication:
  • Slower due to complexity and size of chromosomes.
  • Multiple origins of replication along a chromosome.
• Unique Eukaryotic Problem: Inability to replace the RNA primer at chromosome ends.
  • Leads to chromosome shortening.

Telomeres and Telomerase

• Telomeres: Repetitive DNA sequences at chromosome ends without genes.
• Telomerase: Enzyme that adds telomere repeats to prevent net loss during replication.

Conclusion

• DNA replication in both prokaryotes and eukaryotes shares fundamental processes but differs mainly in complexity and chromosome structure.
• Understanding DNA replication is critical for grasping genetic information processing.