DNA Replication

• DNA carries the information for making all of the cell's proteins. These proÃ�­teins implement all of the functions of a living organism & decide the organism'Ã�­s characteristics. When the cell reproduces, it's pass all of this information on to the daughter cells.

• Before a cell can reproduce, it must first replicate, or make a replica of, its DNA. Where DNA replication occurs depends on whether the cells is a prokaryote or a eukaryote (see the RNA sidebar on the earlier page for more about the categories of cells). DNA replication occurs in the cytoplasm of prokaryotes & in the nucleus of eukaryotes. Irrespective of where DNA replication occurs, the basic method is the same.

• The structure of DNA lends itself basically to DNA replication. Each side of the double helix runs in opposite (anti-parallel) directions. The beauty of this structure is that it can unzip down the middle & 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 called a replication fork, which then moves down the whole length of the molecule.

Let's look at the details:

1. An enzyme called DNA gyrase makes a nick in the double helix & 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 & keep them separated
4. An enzyme complex called DNA polymerase "walks" down the DNA strands & 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 in to 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 & fingernails & bone marrow cells. Other cells go through several rounds of cell division & cease (including specialized cells, like those in your brain, muscle & heart). Finally, some cells cease dividing, but can be induced to divide to repair injury (such as skin cells & liver cells). In cells that do not constantly divide, the cues for DNA replication/cell division come in the kind of chemicals. These chemicals can come from other parts of the body (hormones) or from the environment.