Part 3: DNA Replication
Copying the Genome
DNA replication is the molecular mechanism by which a DNA molecule produces two identical copies. This process is fundamental to cell division and inheritance. The semi-conservative nature of replication, predicted by Watson and Crick and confirmed by Meselson-Stahl, ensures faithful transmission of genetic information.
The Replication Fork
Replication Fork Components
Helicase
Unwinds DNA
Unwinds DNA
Primase
Makes RNA primers
Makes RNA primers
DNA Pol III
Main polymerase
Main polymerase
Ligase
Joins fragments
Joins fragments
Key Concepts
Leading Strand
- • Synthesized 5' → 3' continuously
- • Same direction as fork movement
- • Single RNA primer needed
- • Faster synthesis
Lagging Strand
- • Synthesized 5' → 3' in fragments
- • Opposite to fork movement
- • Multiple RNA primers (Okazaki)
- • Requires ligase to join
Replication Machinery
DNA Polymerase III
Main replicative polymerase, 5'→3' synthesis, proofreading
DNA Polymerase I
Removes RNA primers, fills gaps
Helicase (DnaB)
Unwinds double helix at replication fork
Primase (DnaG)
Synthesizes short RNA primers
SSB Proteins
Stabilize single-stranded DNA
Topoisomerase
Relieves supercoiling ahead of fork
DNA Ligase
Joins Okazaki fragments
Clamp Loader
Loads sliding clamp onto DNA
Replication Fidelity
Error rate: approximately 1 in 10⁹-10¹⁰ base pairs
10⁻⁵
Base selection
10⁻²
Proofreading
10⁻³
Mismatch repair