5 Apr 2012


To qualify as genetic material, DNA has to be able to replicate, or make a copy of, itself. This process of DNA replication occurs during the middle

of the cell cycle. What we already know about its structure is enough to explain how DNA replicates. (See Figure -9.)

To make a copy, you need an original, sometimes called a template. Because DNA is a double helix, it has templates built into it. To begin the process, the double helix unwinds. As with all metabolic activities, enzymes are needed for this process. Once the double-stranded molecule is untwisted, it begins to unzip, just like a zipper. Recall that the nitrogen base pairs A-T and G-C are connected by weak hydrogen bonds. However, at the correct moment, through the activity of an enzyme, these bonds begin to break apart. As the hydrogen bonds break, each strand of the DNA molecule becomes separate. Many free nucleotides float around in the cell. Specific enzymes match up these free nucleotides with the existing nucleotides in each DNA strand. Wherever a T is located on a strand, an A pairs to it; wherever a C is located, a G joins up, and so on. One by one, new nucleotides are joined together to make a new strand opposite each old strand. What determines the linear sequence of nucleotides in the new strands? The sequence of bases in the old strands. When replication is complete, two double-stranded DNA molecules are formed. Each molecule is made up of one old strand joined to a newly synthesized strand. How do the two new DNA molecules compare to the original one? They are identical. DNA replication has occurred. (See Figure -10.)

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