DNA replication occurs during mitosis. DNA is replicated during the synthesis phase.

DNA replication occurs in 3 steps: unwinding, complementary base pairing and joining. Firstly, the helix unwinds and the two strands unzip breaking the hydrogen bonds. The enzyme, DNA helicase (shown by green play-doh), causes unwinding. Secondly, the nucleotides move into place and form H-bond with other partners on the strand. This is facilitated by the DNA polymerase (blue play-doh). Lastly, the nucleotides on the new strand form covalent bonds. The DNA ligase (orange play-doh) is a specific enzyme that facilitates the DNA strands joining together “glues together”. The leading strand is continuous as the DNA unzips, but fragments form on the lagging strand. The process is different on the “leading” strand because it is synthesized in the 5′ (phosphate) to 3′ (sugar) direction whereas the “lagging” strand is synthesized in the 3′ to 5′ direction.

To model the complementary base pairing and joining of adjacent nucleotides we detached some of the pairings to show it was unzipping using by the DNA helicase (green play-doh). Then the new strand was shown pairing with the complementary base on the leading and lagging strands. The model was helpful with visualizing the process of unwinding and unzipping, however, it does not indicate which direction the enzymes read.

*Note: Adenine=yellow (double), Thymine=blue, Guanine=purple (double), Cytosine=green

-We mixed up the pairings, we had A-G and T-C, should be A-T and G-C