DNA Replication
This animation shows DNA replication, which occurs during the S phase of the cell cycle. It is the process by which a DNA helix gets copied, generating two new daughter strands. Once DNA replication is complete, two new helices are formed, each of which contains one daughter strand and one original (parent) strand. This type of replication is known as semi-conservative.
DNA replication begins when a section of the double helix unwinds and the two strands separate, breaking the hydrogen bonds that normally hold the complimentary purine and pyrimidine bases together.
In the next step, an RNA primer (a short, single strand of RNA that is complimentary to the DNA bases) binds to the parent DNA strand and serves as the starting point for DNA synthesis. DNA polymerase, the enzyme that replicates DNA, cannot start without this primer, which has a required hydroxyl (OH) group on its 3' end.
DNA polymerase then uses the original parent strand as a template for daughter strand DNA synthesis. Basically, the polymerase brings in a base that is complimentary to the one on the parent strand and attaches it to the 3' end of the new strand.
Ultimately, the RNA primers are removed, DNA polymerase fills in the spaces where the RNA primer used to be, and a protein called DNA ligase seals the gaps between the adjacent D NA strands that are formed during replication. DNA replication is now complete, and the cell is ready to move into the G2 phase of the cell cycle.