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Prokaryotic DNA Replication Overview

Aug 5, 2024

Lecture Notes: Replication in Prokaryotes (E. coli)

Introduction

  • Speaker: Dr. Trupti from YouTube channel Enjoy Biochemistry
  • Topic: Replication in Prokaryotes (E. coli)
  • Series: Molecular Biology

Flow of Genetic Information

  1. Replication: Parental DNA is copied to form daughter DNA molecules with identical nucleotide sequences.
  2. Transcription: Genetic information in DNA is transferred to RNA.
  3. Translation: Genetic message in RNA is translated into a polypeptide on ribosomes.

DNA Replication

  • Definition: Synthesis of an identical duplicate copy of DNA from an existing DNA molecule.
  • Occurrence: Happens once during the cell cycle, essential for transmission of genomes.
  • Model: Semi-conservative (one parental strand and one newly synthesized strand).

Salient Features of Replication

  • Template Usage: Each DNA strand serves as a template for a new complementary strand.
  • Direction: New DNA is synthesized in the 5' to 3' direction.
  • Origin: Begins at a specific site called ORIC (245 base pairs).
  • Replication Fork: Y-shaped structure formed at the origin, moves bidirectionally.
  • Replication Bubble: Formed by the joining of two replication forks.

Leading and Lagging Strands

  • Leading Strand:
    • Replication is continuous in 5' to 3' direction.
    • Occurs in the direction of the replication fork movement.
  • Lagging Strand:
    • Replication is discontinuous (short stretches called Okazaki fragments).
    • Also in the 5' to 3' direction but opposite to the direction of the replication fork.

Factors Required for Replication

  1. Parental DNA as a template.
  2. Nucleotides: Ribonucleotides (for RNA primer synthesis) and deoxyribonucleotides (for DNA synthesis).
  3. Enzymes and Proteins:
  • DNA A
  • Helicases (DNA B and DNA C)
  • Topoisomerases (1 and 2)
  • Single-stranded binding proteins
  • Primase (DNAG)
  • DNA Polymerases (1 and 3)
  • DNA Ligase

Enzymes and Their Functions

  • DNA Polymerase:
    • Role: Catalyzes replication but requires an RNA primer.
    • DNA Polymerase 3: Main enzyme for chain elongation, has 3' to 5' exonuclease (proofreading) activity.
    • DNA Polymerase 1: Involved in gap filling, excises primers, also has proofreading activity.

Replication Process Stages

  1. Initiation:
  • Directed by a DNA sequence called the replicator (ORIC).
  • Involves DNA A protein, DNA B (helicase), DNA C (helicase loader), single-stranded binding proteins, and topoisomerases.
  1. Elongation:
  • Leading Strand: Continuous synthesis by DNA Polymerase 3.
  • Lagging Strand: Discontinuous synthesis (Okazaki fragments), DNA Polymerase 1 excises primers and fills gaps, DNA Ligase joins ends.
  1. Termination:
  • Occurs when replication forks meet termination sequences, stopping replication.

Summary

  • Replication: Semi-conservative, bidirectional, accurate (proofreading mechanism).
  • Key Enzymes: DNA Polymerase 3 (chain elongation), DNA Polymerase 1 (gap filling, proofreading), Primase (RNA primer synthesis), DNA Ligase (final linkage).
  • Stages: Initiation, elongation, termination.

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