DNA Replication Fork

DNA Replication Fork

DNA replication is the process by which a cell duplicates its genetic material before cell division. The replication fork is the Y-shaped structure where the double helix is unwound and new strands are synthesized.

Key Components

• DNA Helicase: Unwinds the double helix by breaking hydrogen bonds between base pairs, creating the replication fork.
• Single-Strand Binding Proteins (SSB): Stabilize the separated strands to prevent reannealing.
• Primase: Synthesizes short RNA primers (~10 nucleotides) to provide 3'-OH for polymerase.
• DNA Polymerase III: The main replicative polymerase, synthesizes DNA in 5'→3' direction only.
• DNA Ligase: Joins Okazaki fragments by forming phosphodiester bonds.

Leading vs Lagging Strand

Because DNA polymerase only works in the 5'→3' direction:

• Leading strand: Synthesized continuously toward the replication fork.
• Lagging strand: Synthesized discontinuously as short Okazaki fragments (100-200 bp in eukaryotes, 1000-2000 bp in prokaryotes) away from the fork.

Okazaki Fragment Processing

After synthesis, RNA primers are removed by RNase H and DNA Polymerase I (in prokaryotes) or FEN1 (in eukaryotes). The gaps are filled with DNA, and DNA ligase seals the nicks to create a continuous strand.

The Replisome

The replisome is a large molecular machine containing helicase, primase, and polymerase. In eukaryotes, the CMG complex (Cdc45-MCM-GINS) serves as the replicative helicase, while Pol α-primase initiates Okazaki fragments and Pol δ/ε extend them.