DNA Replication Lecture Notes

Introduction

• DNA: Director for cells, codes for traits.
• DNA replication: Making more DNA during cell division.

Location and Timing

• Where:
  • Eukaryotic cells: Occurs in the nucleus.
  • Prokaryotic cells: No nucleus, but replication still occurs.
• When:
  • Before cell division, specifically during interphase (before mitosis or meiosis).

Key Enzymes in DNA Replication

1. Helicase (Unzipping enzyme):
   • Unzips DNA strands by breaking hydrogen bonds.
2. DNA Polymerase (Builder):
   • Replicates DNA molecules to build new strands.
   • Can only add new bases to the 3’ end of a new strand.
3. Primase (Initializer):
   • Creates RNA primers for DNA polymerase to start.
4. Ligase (Gluer):
   • Glues DNA fragments (Okazaki fragments) together.
5. SSB Proteins (Single-Stranded Binding Proteins):
   • Bind to DNA to keep strands separated.
6. Topoisomerase:
   • Prevents DNA supercoiling during replication.

Process of DNA Replication

• Origin:
  • Identified by specific DNA sequences.
• Unwinding:
  • Helicase unwinds DNA; SSB proteins keep strands apart.
• Priming:
  • Primase adds RNA primers.
• Building:
  • DNA polymerase builds new strands in the 5’ to 3’ direction.
• Leading and Lagging Strands:
  • Leading: Continuous synthesis in the 5’ to 3’ direction.
  • Lagging: Discontinuous, forms Okazaki fragments; requires multiple primers.

Final Steps

• Replacement and Sealing:
  • RNA primers replaced with DNA bases.
  • Ligase seals gaps between Okazaki fragments.
• Outcome:
  • Two identical double-helix DNA molecules (semi-conservative replication: one old, one new strand).

Error Checking

• Proofreading:
  • DNA polymerase has proofreading abilities to ensure accuracy and prevent errors.

Conclusion

• Detailed understanding of DNA replication is crucial for medical advancements.
• Further reading recommended for deeper understanding.