5.2 DNA Replication (Elongation)

UNIT 3: MOLECULAR GENETICS

Lesson 3: DNA Replication (Elongation)

DNA Replication Overview

• Phases of DNA Replication:

  1. Initiation

     • Unwinding of the DNA double helix.

     • Bases become exposed for new base pairing.

  2. Elongation

     • Creation of two new DNA strands using parent DNA as a template.

     • New DNA forms double helices containing one parent and one daughter strand.

  3. Termination

     • Completion of replication process, allowing two new DNA molecules to separate and re-form double helix.

     • Dismantling of replication machinery.

Origin of Replication

Formation of Replication Bubble

1. DNA strands unwind to create a replication bubble.

2. DNA replication begins at two replication forks and continues outward.

3. DNA synthesis occurs in both directions from the forks.

Elongation Phase

RNA Primers

• Necessary for the formation of new DNA strands.

• RNA Primer:

  • Small segment of RNA laid down on the parent strand by enzyme RNA Primase.

  • Defines where DNA polymerase III binds for nucleotide addition.

Function of DNA Polymerase III

• Catalyzes the addition of deoxyribonucleotides to the new strand.

• Adds nucleotides to the 3’ hydroxyl end of an existing chain, synthesizing in a 5’ to 3’ direction.

Leading Strand

• Synthesized continuously in the 5’ to 3’ direction.

• New nucleotides added by DNA polymerase III, complementary to the parent strand, from the origin towards the replication fork.

  

Lagging Strand

• Synthesized discontinuously using short segments known as Okazaki Fragments due to opposing orientation of DNA strands (anti-parallel structure).

• Each Okazaki fragment starts with a separate RNA primer laid down by RNA primase.

• DNA Polymerase III adds nucleotides 5’ to 3’, moving away from the replication fork.

  

Completion of Lagging Strand

• Exonuclease removes RNA primers, and DNA Polymerase I fills the gaps between Okazaki fragments with complementary DNA nucleotides.

• DNA Ligase: seals the Okazaki fragments by forming phosphodiester bonds.