DNA replication occurs during the synthesis (S) phase of the cell cycle and starts at predefined DNA sequences known as replication origins. Once the origins of replication have fired, the DNA replication proteins organize into a structure called the replication fork (RF), where a group of proteins coordinate DNA replication. Two main activities happen at the fork: DNA unwinding and DNA synthesis. The RF unwinds the unreplicated DNA ahead of it through a helicase enzyme complex. As their name suggests, helicases modify the structure of the DNA helix and promote unwinding and separation of the two DNA strands. The second activity of DNA synthesis at the RF is undertaken by DNA polymerase. This enzyme links together, or polymerizes, DNA bases in the correct sequence using the template DNA strand, and it generates two copies of the genome that are later divided into daughter cells in metaphase, or M phase.