DNA Replication Lecture Notes

Introduction

• DNA: Director for cells, codes for traits
• DNA replication is crucial for cell division (mitosis)

Where and When DNA Replication Occurs

• Location: Occurs in the nucleus (eukaryotic cells)
  • Prokaryotes lack a nucleus and replicate differently
• Timing: Occurs during interphase, not during mitosis
  • Interphase: Cell grows, carries out processes, and replicates DNA
  • DNA replication precedes mitosis and meiosis

Key Enzymes in DNA Replication

• Helicase: "Unzipping enzyme"
  • Unwinds the 2 strands of DNA
  • Hydrogen bonds holding strands are weak
• DNA Polymerase: "The builder"
  • Replicates DNA molecules to build new strands
• Primase: "The initializer"
  • Creates RNA primers for DNA polymerase to initiate replication
• Ligase: "The gluer"
  • Joins DNA fragments together

DNA Replication Process

1. Origin of Replication
   • Identified by specific DNA sequences
   • Multiple origins possible
2. Unwinding DNA
   • Helicase unwinds DNA
   • SSB proteins bind to keep strands separated
3. Primer Placement
   • Primase creates RNA primers on both strands
4. Complementary Strands
   • DNA polymerase adds DNA bases
   • Strands are anti-parallel (5’ to 3’ and 3’ to 5’)
   • DNA polymerase works in 5’ to 3’ direction

Leading vs. Lagging Strand

• Leading Strand
  • Runs 5’ to 3’ - continuous replication
• Lagging Strand
  • Runs 3’ to 5’ - requires extra primers
  • Produces Okazaki fragments
  • Ligase glues Okazaki fragments together

DNA Proofreading

• DNA polymerase has proofreading ability
  • Minimizes errors in DNA replication
  • Prevents incorrectly coded genes and proteins

Conclusion

• DNA replication results in two identical double helix DNA molecules
• Called semi-conservative replication
  • Each new DNA contains one original and one new strand

• Follow us on Twitter (@amoebasisters) and Facebook
• Stay curious!