Table 9.3: DNA Replication Enzymes and Roles

Initiator protein

Binds to the origin of replication and separates DNA strands to initiate DNA replication, opening the origin for other proteins.

DNA helicase

Unwinds DNA at the replication fork, creating and enlarging it by separating the two DNA strands to provide single-stranded templates.

Single-strand-binding proteins (SSBs)

Attach to single-stranded DNA to prevent secondary structures from forming, stabilizing unwound DNA and keeping templates accessible.

DNA gyrase

Moves ahead of the replication fork, making and resealing breaks in the DNA double helix to release torque that builds up from unwinding. Also known as DNA topoisomerase II.

DNA primase

Synthesizes a short RNA primer to provide a 3'\text{-OH} group, which is required for DNA polymerases to attach DNA nucleotides and begin synthesis.

DNA polymerase III

Elongates a new nucleotide strand from the 3' end in the 5'\to 3' direction and proofreads with 3'\to 5' exonuclease activity.

DNA polymerase I

Removes RNA primers using 5'\to 3' exonuclease activity and replaces them with DNA using 5'\to 3' polymerase activity.

DNA ligase

Joins Okazaki fragments by sealing breaks in the sugar–phosphate backbone of newly synthesized DNA, forming phosphodiester bonds.