Lecture 35: DNA Replication

Introduction

• Brief review of DNA and nucleotide structure
• Focus on DNA replication in both prokaryotes and eukaryotes
• DNA replication is similar between prokaryotes and eukaryotes

DNA Replication Overview

• Part of binary fission and the eukaryotic cell cycle
• Occurs at the replication fork

Key Enzymes in DNA Replication

1. DNA Helicase

• Unwinds the DNA double helix
• Breaks hydrogen bonds between bases
• Creates two single-stranded templates of DNA

2. Topoisomerase

• Relieves tension in the DNA helix as it unwinds

3. Single-Strand Binding Proteins

• Stabilize single-stranded DNA and prevent rewinding

4. Primase

• Builds a short RNA primer complementary to the DNA

5. DNA Polymerase III (DNA Pol III)

• Extends the RNA primer, building DNA complementary to the template
• Has an associated sliding clamp protein to hold it in place
• Builds DNA in the 5' to 3' direction

Leading vs. Lagging Strand

Leading Strand

• Synthesized continuously in the 5’ to 3’ direction toward the replication fork

Lagging Strand

• Synthesized discontinuously in the 5’ to 3’ direction away from the replication fork
• Made in short segments called Okazaki fragments
• Each fragment starts with an RNA primer

Further Enzymes and Processes

DNA Polymerase I (DNA Pol I)

• Replaces RNA primers with DNA
• Uses DNA from the next fragment as a template

DNA Ligase

• Seals gaps between DNA fragments, forming a continuous strand

Differences Between Prokaryotic and Eukaryotic Replication

Eukaryotic Replication

• Slower and more complex due to chromatin and histone proteins
• Requires multiple origins of replication due to larger chromosome size
• Linear chromosomes lead to unique issues with replication at chromosome ends

Telomeres and Telomerase

Problem with Linear Chromosomes

• Inability to replace the RNA primer at the 5’ end leads to chromosome shortening

Telomeres

• Regions of repetitive DNA at chromosome ends
• Protect important genetic information from being lost

Telomerase

• Enzyme that extends telomeres
• Uses an RNA template to add repetitive DNA sequences

Conclusion

• DNA replication is critical for cell division and maintaining genetic information
• Differences exist between prokaryotic and eukaryotic replication

Study Tips

• Use visual diagrams to help understand the replication process
• Redraw figures to reinforce learning