DNA Replication in Cell Biology

Overview

• The lecture discusses the DNA replication process, its fundamental mechanisms, and clinical significance.
• Editor: Aldrich Christiandy

Outline

1. Fundamentals
2. Process of DNA Replication
3. Telomerase
4. Clinical Significance
5. Appendix
6. Review Questions
7. References

I. Fundamentals

A. Function of DNA Replication

• DNA replication is essential for cell division (one parent cell divides into two daughter cells).
• It occurs during the S-phase of the cell cycle.
• Converts one double-stranded DNA (dsDNA) molecule into two.

B. Semiconservative Nature

• Parental dsDNA strands serve as templates for new daughter strands.
• Results in mixed old (parental) and new (daughter) strands.

C. Directionality

• DNA polymerase adds nucleotides in a 5' to 3' direction.
• Utilizes 3' OH group of deoxyribose sugar as a starting point for nucleophilic attack.

D. Bidirectional Replication

• DNA polymerase uses templates to synthesize daughter strands along two replication forks in a 5' to 3' direction.

II. Process of DNA Replication

A. Initiation

• Begins at the origin of replication, an A-T rich area recognized by a pre-replication complex.
• Forms replication bubble and prevents strand reassociation using single-strand binding proteins (SSBP).
• Helicase unwinds DNA at replication forks, creating leading and lagging strands.
• Topoisomerases resolve supercoil tension ahead of replication forks.

B. Elongation

• RNA Primer Synthesis: Primase creates RNA primers for DNA polymerase to initiate synthesis.
• DNA Synthesis: DNA polymerase synthesizes new strands, proofreading through 3'-5' exonuclease activity.
• Okazaki Fragments: Discontinuous fragments on the lagging strand are joined by DNA ligase.

C. Termination

• DNA polymerase ends replication at telomeres.
• Telomeres shorten over time, limiting replication cycles (Hayflick Limit).

III. Telomeres & Telomerase

A. Structure

• Noncoding DNA fragments at chromosome ends, contain TTAGGG repeats.

B. Function

• Prevent gene loss during replication as they shorten with each cell division.

C. Regulation

• Telomerase extends telomeres using reverse transcription, important for cells with high replication rates.

IV. Clinical Significance

A. Helicase Defect

• BLM gene mutation causes Bloom syndrome with features like short stature and increased cancer risk.

B. Drugs Modulating Topoisomerase

• Eukaryotic Cells: Irinotecan & topotecan inhibit Topoisomerase I.
• Prokaryotic Cells: Fluoroquinolones inhibit Topoisomerase II & IV.

C. DNA Polymerase Inhibition

• Anti-retroviral drugs act as nucleoside analogues inhibiting DNA polymerase III, used in HIV treatment.

D. Telomerase in Cancer

• Cancer cells increase telomerase activity to elongate telomeres, promoting uncontrolled replication.

V. Appendix

• Figures and additional resources for understanding DNA replication.

VI. Review Questions

• Questions provided to test comprehension of DNA replication concepts.

VII. References

• Various references from textbooks and scientific articles supporting the information provided.