Telomeres and DNA Replication

Telomeres and the End Replication Problem

  • Background on DNA Replication:

    • Involves the unwinding of DNA and the synthesis of new strands.
    • Leading and lagging strands are synthesized in opposite directions due to the antiparallel nature of DNA.

  • Removing Primers and Linking Fragments:

    • 5' to 3' Synthesis:
    • RNA primers are added by primase to provide a starting point for DNA polymerase.
    • DNA polymerase I replaces RNA primers with DNA nucleotides.
    • Nick sealing occurs with DNA ligase, linking the Okazaki fragments on the lagging strand.
Linear Chromosomes and Replication Issues
  • End Replication Problem:
    • Linear genomes face challenges during replication due to the nature of RNA primers.
    • The terminal primer sits approximately 70-100 nucleotides from the end of the chromosome, leading to unreplicated ends.
    • Result:
    • Each cell division results in progressively shorter chromosomes, risking genetic material loss over generations if not addressed.
Role of Telomeres

  • Structure and Function:

    • Telomeres: repetitive nucleotide sequences at the ends of chromosomes (in humans, the sequence is TTAGGG).
    • Protect chromosome integrity and prevent degradation during cell division.

  • Hayflick Limit:

    • The maximum number of divisions a somatic cell can undergo before cell division stops due to telomere shortening.
    • Progressive shortening correlates with aging and limits cell replication.
Telomerase: The Lengthening Enzyme

  • What is Telomerase?

    • An enzyme that extends the telomeres of chromosomes.
    • Contains a ribonucleoprotein component, hTERT, essential for its activity.

  • Mechanism of Action:

    1. Telomerase Binding:
    • The G-rich end of the telomere pairs with the RNA template of telomerase.
    1. Nucleotide Addition:
    • Nucleotides are added to the 3' end of the telomere.
    • This process can lead to several repeats being added, countering telomere shortening.
    1. Filling Gaps:
    • After telomerase acts, DNA polymerase fills in the gaps left by RNA primers.
Research Findings
  • Significance of Telomerase in Aging:
    • Genetically modified mice lacking TERT age faster than normal mice.
    • Reintroducing TERT can reverse aging effects in cells.
  • Cancer Connection:
    • Many cancer cells overexpress telomerase, allowing unchecked cell proliferation and longevity.
Conclusion
  • Telomeres play a crucial role in maintaining chromosome integrity and lifespan of cells through their repetitive sequences.
  • Telomerase has potential therapeutic implications but also raises concerns regarding cancer biology.
Important Diagrams to Review:
  • Diagrams illustrating DNA replication mechanisms.
  • Telomerase function and its interactions with telomeres.
  • The cellular impact of telomere shortening and the Hayflick limit.