DNA Replication Fork – AP Biology

Helicase

Breaks hydrogen bonds between nitrogenous bases to separate the parent strands. Forms the replication fork.

DNA Polymerase III

Synthesizes new DNA strands in the 5' to 3' direction. Requires an existing 3'-OH group to start.

Primase (RNA Polymerase)

Synthesizes short RNA primers complementary to the template. Provides the 3'-OH for DNA Pol III.

DNA Ligase

Catalyzes phosphodiester bonds to seal the nicks between Okazaki fragments on the lagging strand.

Animation Control

Leading vs Lagging

Notice how the Leading Strand (Top) is synthesized continuously towards the fork.

The Lagging Strand (Bottom) must be synthesized in chunks away from the fork (Okazaki fragments) because DNA Polymerase can only build 5' to 3'.