Lecture_19_DNA_Replication Flashcards

DNA Structure and Replication

Overview

• The lecture focuses on the processes of DNA replication, exploring the mechanisms involved in making a copy of DNA and the challenges inherent to the replication process.

Design Challenge in DNA Replication

• Problem Addressed: How to make a copy of DNA.

  • Questions to consider:

    1. Where do we begin the replication process?

    2. What are the initial steps required?

Initiation of DNA Replication

• Starting Point: Replication begins at the origin of replication, a sequence element on the DNA that signals the recruitment of replication machinery, specifically, DNA polymerase.

Replication Types

• Types of Chromosomes:

  • Circular Chromosome: Common in prokaryotes.

  • Linear Chromosome: Found in eukaryotes, requiring more complex replication mechanisms.

  • Illustrated with replication complexes and the terms 'ori' (origin) and 'ter' (termination).

Unwinding the DNA

• A replication fork forms as DNA unwinds, separating the two strands to serve as templates for new strand synthesis.

• Enzymes Involved:

  • Helicase: Unwinds the DNA double helix.

  • Single-Strand Binding Proteins (SSBPs): Stabilize single strands of DNA to prevent them from reannealing.

Steps in DNA Synthesis

1. Unwinding DNA: Using helicase.

2. Laying Down an RNA Primer: This is mediated by the enzyme primase, which allows for starting the DNA synthesis.

3. Polymerizing DNA: Using DNA polymerase III, which extends the new DNA strand by adding complementary nucleotides.

Nucleotide Addition

• The DNA strands grow in the 5' to 3' direction, necessitating a preexisting 3' OH end to which new nucleotides can be added.

  • New nucleotides are added according to base pairing rules and through the formation of covalent and hydrogen bonds.

Leading and Lagging Strands

• Leading Strand: Synthesized continuously in the direction of the replication fork, allowing for straightforward addition of nucleotides.

• Lagging Strand: Synthesized in short segments (Okazaki fragments) due to the antiparallel nature of DNA strands, requiring multiple primers.

  • This complexity leads to challenges, especially as enzymes such as helicase must move in opposite directions, creating potential issues.

Proofreading and Error Correction

• DNA Polymerase Proofreading Function:

  • Corrects errors during DNA synthesis by moving into the exonuclease site, fixing about 99% of the mistakes made during synthesis.

Challenges in Replication

• Primase and Helicase Coordination: They must both move efficiently on their respective strands, ensuring a successful replication process.

• Ending Replication: Challenges exist particularly for linear chromosomes, where there's a risk of losing telomeric DNA (the protective ends of chromosomes) with each replication cycle.

Telomeres and Chromosome Integrity

• Each human chromosome can lose 50-200 base pairs of telomeric DNA after replication, contributing to cellular aging and eventual cell death.

• Solutions to telomere shortening include mechanisms like telomerase, which can replenish telomeres, allowing for continued cellular replication without loss of critical genetic information.

Conclusion

• Understanding the intricate processes of DNA replication, including the right enzymes and mechanisms, demonstrates the incredible precision and efficiency required to maintain genetic integrity throughout cell division.