DNA Replication and Telomeres: Key Concepts

Origin of DNA Replication

  • Fork: Site where replication ends
  • DNA Replication: Process of copying DNA

What Does DNA Do?

  • Genetic Code: Determines physical characteristics (e.g., hair color, allergies)
  • Amino Acids: DNA codes for 20 amino acids, the building blocks of life.

What Is DNA Replication?

  • Definition: DNA replication is the process of making an exact copy of DNA within a cell.
  • Purpose: Essential for cell division and growth.
  • Cell Cycle Phase: Replication occurs during the S phase of Interphase.

Mechanism of DNA Replication

  • Semi-Conservative Replication: Each new DNA molecule contains one old strand and one new strand.
    • Involves template strands for synthesis.

DNA Polymerases in Prokaryotes

  • Vary in subunit composition but have a common catalytic subunit.
  • Structure resembles a human hand:
    • Palm: DNA template is threaded through
    • Thumb & Fingers: Wrap around DNA.

Process of DNA Replication

  • Replication bubble: Area where DNA is unwound
  • Replication fork: Y-shaped region where DNA strands are separated.
  • Role of Helicase: Unwinds the double helix, breaking hydrogen bonds between base pairs.
Role of Proteins in DNA Replication
  • Single Strand Binding Proteins (SSB): Prevent re-annealing of strands.
  • Gyrase (Topoisomerase): Relieves tension from unwinding by making microcuts in DNA.

Primase and RNA Primers

  • Primase: RNA polymerase that synthesizes the RNA primer.
  • Function: Marks the starting point for DNA polymerases to initiate DNA synthesis.

Direction of DNA Synthesis

  • DNA polymerases synthesize DNA only from 5’ to 3’ direction.
    • Leading strands synthesized continuously.
    • Lagging strands synthesized in fragments (Okazaki fragments) due to antiparallel nature of DNA strands.

Removal of RNA Primers

  • DNA Polymerase I: Replaces RNA primers with DNA.
  • DNA Ligase: Seals gaps in the sugar-phosphate backbone, connecting Okazaki fragments.

Proofreading Mechanisms

  • Mismatch Stability: Complementary pairs are more stable than mismatched pairs.
  • DNA Polymerase Active Site Configurations: Decreases error rate to 1 in 1 million through the induced fit mechanism.
  • Proofreading Function: DNA polymerases can identify and remove incorrectly matched nucleotides.

Bacterial DNA Replication

  • Rapid division (E. coli can divide in 20-30 minutes).
  • DNA replication is tightly coordinated with cell division.

Eukaryotic DNA Replication

  • More complex than prokaryotic replication due to:
    • Presence of linear chromosomes
    • Nucleosome packaging (histones)
    • Regulation of the cell cycle
  • Bi-directional replication from multiple origins to ensure prompt DNA synthesis.

Telomeres

  • Definition: Repetitive DNA sequences at the ends of chromosomes (e.g., TTAGGG in humans).
  • Functions:
    • Protect chromosomes from damage and fusion.
    • Regulate cell division by shortening with each cell division.

Telomeres and Aging

  • Shortening of telomeres is linked to aging; once critically short, cells become senescent and cannot divide.
  • Average telomere length decreases from about 8,000 base pairs in youth to less than 4,500 base pairs with age.

Telomerase

  • Definition: Enzyme that maintains telomere length by adding TTAGGG repeats to chromosome ends.
  • Activation: Primarily in germ cells and some stem cells; most somatic cells lack active telomerase.

Telomerase and Cancer

  • Telomerase activity is often elevated in cancer cells; current research focuses on potential cancer therapies targeting telomerase activity.

Conclusion

  • Increase in telomerase activity can extend cellular lifespan and division potential, establishing a direct relationship between telomeres, cellular aging, and cancer biology.