DNA Replication Lecture Notes

Introduction

• Purpose of DNA Replication: Essential for cell replication to make more cells.
  • DNA is the genetic portion making cells what they are.
• Cell Cycle Context: DNA replication occurs in the S phase of the cell cycle.
• Cell Replication: Creates two identical cells from one, duplicating maternal and paternal chromosomes.

Fundamental Concepts

Why DNA Replication?

• To enable cell replication and the cell cycle.
• Occurs specifically in the S phase.

Semi-Conservative Model

• DNA replication is semi-conservative: involves old (parental) and new (daughter) strands.
  • Old strands separate, new strands synthesize complementary nucleotides.
  • Results in two new double-stranded DNA molecules.

Direction of Replication

• Occurs in a 5' to 3' direction.
  • Adds nucleotides by attaching phosphate groups to 3' OH groups of preceding nucleotides.

Bi-Directional Replication

• Occurs in both directions from the origin of replication, forming replication forks.
• Helicases unwind DNA, DNA Polymerases synthesize new DNA bi-directionally.

Stages of DNA Replication

Initiation

• Origin of Replication: AT-rich areas easier to break due to fewer hydrogen bonds.
  • Multiple origins in eukaryotic cells.
• Pre-Replication Protein Complex: Binds to the origin and separates the DNA strands.
• Single-Stranded Binding Proteins: Prevent re-annealing, protect from nucleases.
• Helicase: Unwinds DNA, dependent on ATP.
• Topoisomerases: Alleviate supercoiling caused by helicase.
  • Types: 1, 2, 4 (Eukaryotic vs. Prokaryotic).
  • Clinical significance: Targeted by drugs in cancer and bacterial infections.

Elongation

• Primase: Lays down RNA primers for DNA Polymerase III to build on.
• DNA Polymerase III: Synthesizes new DNA by reading 3' to 5' and synthesizing 5' to 3'.
  • Leading Strand: Continuous synthesis toward replication fork.
  • Lagging Strand: Discontinuous synthesis creating Okazaki fragments.
• Proofreading Function: Exonuclease activity to correct errors during synthesis.
• DNA Polymerase I: Removes RNA primers and fills gaps with DNA.
• Ligase: Connects Okazaki fragments on the lagging strand.
• Clinical Application: Nucleoside Reverse Transcriptase Inhibitors (NRTIs) in HIV treatment prevent DNA polymerase from adding nucleotides.

Termination

• Process Completion: Occurs when replication forks meet and polymerases fall off.
• Telomeres: End regions that shorten with each replication cycle.
  • Protected by telomerase in certain cells to prevent gene loss.
  • Clinical significance: Telomerase activity in stem cells and cancer cells.
  • Hayflick Limit: Maximum replication cycles before involving genes.

Conclusion

• DNA replication is critical for cell division and is a complex process involving various enzymes and stages.
• Understanding replication processes and their regulation is important for applications in medicine and biotechnology.