Lecture Notes: Telomeres and the Cell Cycle

Overview

• Chromosomes have various structures: centromeres, telomeres, and genes.
• Focus of this lecture: Telomeres.

Hypothetical Scenario

• Skin cell placed in a petri dish and stimulated to undergo mitosis.
• Question: Does the cell keep dividing indefinitely?
  • Initial thought: Nutrients could be a limiting factor.
  • Clarification: Even with unlimited nutrients, cells stop dividing after a certain number of generations.

Key Concept: Telomeres

• Definition: Repeating DNA sequences located at the ends of chromosomes.
• Location: At the ends of chromatin (uncoiled) or chromatids (supercoiled).
• Function: Limit the number of times a cell can undergo mitotic cell cycle.

Mitotic Cell Cycle Recap

1. Interphase: G1, S, G2 phases, where cells prepare for division.
2. M Phase: Mitotic division and cytokinesis.
3. Result: Two newly divided cells, genetically identical.

Role of Telomeres in the Cell Cycle

• As cells replicate, telomeres shorten.
• Observation: Chromatin becomes shorter with each generation.

DNA Replication and Telomeres

• DNA replication is imperfect, resulting in loss of telomeres.
• DNA polymerase limitations cause incomplete replication.
• Consequence: Gradual loss of telomeres with each cell cycle.

Consequences of Telomere Shortening

• Initial Generations: Telomeres shorten, but genes remain intact.
• Later Generations: Complete loss of telomeres leads to loss of adjacent genes.
• Example: Gene responsible for ATP production can be lost, disrupting cell function.
• Result: Cell death due to loss of essential genes.

Visualization

• Telomeres and genes visualized in chromatin and chromatid forms.
• Telomere shortening leads to eventual gene loss and cell death.

Summary

• Telomeres protect genes from being lost during cell division.
• Progressive telomere shortening limits the number of cell divisions.
• Once telomeres are exhausted, essential genes are lost, leading to cell death.

Conclusion

• Telomeres serve as a biological clock limiting cell division.
• Essential for understanding cell aging and potential medical implications.