DNA replication is initiated when the enzyme helicase creates a structure called a replication fork. This is the area in which the helicase unzips and where the two strands part from each other. This is done when the helicase unwinds a strand of the DNA and breaks the hydrogen bonds between the complementary strands of DNA. After this separation occurs, the two strands can be named the leading strand and the lagging strand (see diagram to the left). From here, a different enzyme known as DNA polymerase "catalyzes the synthesis of complementary strands of DNA" (DiscoveryEducation). The enzyme completes this replication of DNA by matching up the As to the Ts and the Cs to the Gs. DNA polymerase can only build onto a "free 3’ end" of a chain. Synthesis of the leading strand proceeds continuously in the 5’ to 3’ direction. Synthesis of the lagging strand occurs in stages because it will need to go in the opposite direction. Another enzyme called DNA ligase takes care of the lagging strand. This enzyme attaches the segments of the strand, or Okazaki fragments and creates the complete replication. It does this by using and combining the backbone, which is made of sugar and phosphate groups (DiscoveryEducation).