Copying the Code

What role does DNA polymerase play in copying DNA?

When Watson and Crick discovered the structure of DNA, they immediately recognized one genuinely surprising aspect of the structure. Base pairing in the double helix explains how DNA can be copied, or replicated, because each base on one strand pairs with one—and only one—base on the opposite strand. Each strand of the double helix therefore has all the information needed to reconstruct the other half by the mechanism of base pairing. Because each strand can be used to make the other strand, the strands are said to be complementary.

The Replication Process Before a cell divides, it duplicates its DNA in a copying process called replication. This process, which occurs during late interphase of the cell cycle, ensures that each resulting cell has the same complete set of DNA molecules. During replication, the DNA molecule separates into two strands and then produces two new complementary strands following the rules of base pairing. Each strand of the double helix of DNA serves as a template, or model, for the new strand.

Figure 12–8 shows the process of DNA replication. The two strands of the double helix have separated, or “unzipped,” allowing two replication forks to form. As each new strand forms, new bases are added following the rules of base pairing. If the base on the old strand is adenine, then thymine is added to the newly forming strand. Likewise, guanine is always paired to cytosine. For example, a strand that has the base sequence TACGTT produces a strand with the complementary base sequence ATGCAA. The result is two DNA molecules identical to each other and to the original molecule. Note that each DNA molecule resulting from replication has one original strand and one new strand.

In Your Notebook In your own words, describe the process of DNA replication.