DNA replication

The structure of DNA lends itself easily to DNA replication. Each side of the double helix runs in opposite, or anti-parallel directions. The beauty of this structure is that it can "unzip" down the middle and each side can serve as a pattern or template for the other side (called semi-conservative replication). However, DNA does not unzip entirely. It unzips in a small area (replication fork), which then moves down the entire length of the molecule. Cellular proofreading and error-checking mechanisms ensure near perfect fidelity for DNA replication.

Replication steps are as follows:

1. An enzyme called DNA gyrase makes a nick in the double helix and each side separates
2. An enzyme called helicase unwinds the double-stranded DNA
3. Several small proteins called single strand binding proteins (SSB) temporarily bind to each side and keep them separated
4. An enzyme complex called DNA polymerase "walks" down the DNA strands and adds new nucleotides to each strand. The nucleotides pair with the complementary nucleotides on the existing stand (A with T, G with C).
5. A subunit of the DNA polymerase proofreads the new DNA
6. An enzyme called DNA ligase seals up the fragments into one long continuous strand
7. The new copies automatically wind up again

Different types of cells replicated their DNA at different rates. Some cells constantly divide, like those in your hair and fingernails and bone marrow cells. Other cells go through several rounds of cell division and stop (including specialized cells, like those in your brain, muscle and heart). Finally, some cells stop dividing, but can be induced to divide to repair injury (such as skin cells and liver cells). In cells that do not constantly divide, the cues for DNA replication/cell division come in the form of chemicals. These chemicals can come from other parts of the body (hormones) or from the environment.