Replisome refers to a complex of proteins that are engaged in the elongation of the newly synthesized strand of deoxyribonucleic acid. The replisome assembles at the replication fork, the region where the new DNA stand is being made.

The replisome complex assumes three principle functions, DNA polymerase, DNA primase and DNA helicase. The helicase, a donut-shaped enzyme, initiates replication by unwinding the two parental DNA strands. The structure of the helicase was elucidated in 1999 using the technique of x-ray crystallography. It has been determined that the helicase moves along the DNA strand very quickly, 300 paired nucleotides per second. Once the DNA strands are unwound, single stranded DNA binding proteins attach to each unwound strand, preventing the strands from winding back together. Subsequently, DNA polymerase catalyzes the elongation of each strand. The polymerase is able to operate in a continuous fashion on the leading parental strand that is unwinding. On the other strand, the DNA primase molecule builds primers, which are eventually connected to the other replicated DNA strand.