Molecular Biology Chapter on Lagging Strand Synthesis

Lagging Strand Synthesis

• RNA/DNA duplex is recognized by repair enzymes which remove RNA and replace it with DNA using polymerase δ.
• DNA ligase fills nicks in the phosphate backbone, crucial for lagging strand synthesis.
• Lagging strand synthesis is slower than leading strand synthesis due to additional steps involved.

The Replication Fork

• Two leading and two lagging strands formed at one origin of replication create two complete replication forks.
• Key proteins at the replication fork include:
  • DNA Helicase
  • Single-Stranded Binding (SSB) Proteins
  • Sliding Clamp
  • Clamp Loading Protein
  • DNA Primase
  • DNA Polymerase
  • Topoisomerase I and II (not depicted in some images)

Telomere Replication

• Telomeres consist of repetitive sequences (~2500 repeats) at the ends of linear chromosomes, essential for replication and stability.
• DNA polymerase cannot replicate the very end of DNA strands, leading to potential chromosome shortening with each division.
• Telomerase, a reverse transcriptase and its unique RNA component, extends the upper strand overhang by hybridizing to the DNA overhang.
• Multiple repetitions by telomerase prevent loss of essential sequences during replication.

Telomeres in Somatic Cells

• Most somatic cells lack telomerase activity, resulting in telomere shortening with each division.
• This shortening signals apoptosis when telomeres are absent.
• Limited telomerase activity restricts somatic cell divisions to about 50 times.

DNA Repair Importance

• Approximately 100-200 mutations occur per round of DNA replication, emphasizing the need for accurate DNA replication and error correction.
• DNA damage, which can lead to mutations if uncorrected, occurs frequently compared to rare mutations.
• DNA damage can be repaired, while mutations are permanent and can contribute to genetic diseases.

Difficulty with DNA Replication

• DNA strands are antiparallel (one 5' to 3' and the other 3' to 5'), which complicates replication as it can only proceed in one direction.
• DNA Primase synthesizes short RNA primers needed for the initiation of DNA synthesis, providing a 3' hydroxyl group for polymerase extension.

Leading and Lagging Strand Synthesis Mechanics

• Leading strand synthesis progresses smoothly while lagging strand synthesis involves multiple RNA primers creating short DNA segments (Okazaki fragments).
• DNA Primase synthesizes RNA primers on the lagging strand, allowing DNA polymerase to extend the sequence in fragments away from the replication fork, which is counterintuitive to the overall unwinding process.