Replication

During mitosis (cell division) cells make copies of their chromosomes. The chromosomes duplicate themselves so that the cells that come from the original cell will have the same DNA. In order for this copy to be made, DNA must go through replication. Replication is the name given for the act of copying DNA strands.
To begin replication, an enzyme called DNA helicase unwinds the long DNA strand. As the strand unwinds, another enzyme, DNA polymerase, travels up from the 3' end attaching complimenting bases to the unzipped strand. Then it travels down toward the 5' end connecting complimenting bases to this side of the double helix. Another DNA polymerase continues this process by traveling up the 3' end, reaching the DNA helicase (which is still "unzipping" the the DNA strand), and then traveling down the 5' end. Eventually, this process results in two strands of DNA; the cell is ready to divide.

A Point to Ponder There is one slight problem with these strands, though. They are split in different sections. Remember, the DNA polymerase travels up one end of the unzipped strand, meets the DNA helicase, and works its way down the other end of the strand. Then another DNA polymerase travels over the path of the original enzyme and begins working where the other one could not. Thus there are fragments that have to be bonded which are called Okazaki fragments after the person who discovered them. Eventually an enzyme, DNA ligase, bonds these fragments together to complete the chains.

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