DNA Replication

When a cell divides, each newly formed cell must have a copy of the original cell's genetic information (DNA) so it will be able to synthesize the proteins necessary to build cellular parts and carry on metabolism. DNA replication (re"pli-ka'shun) is the process that creates an exact copy of a DNA molecule. It occurs during interphase of the cell cycle.

As replication begins, hydrogen bonds break between the complementary base pairs of the double strands comprising the DNA molecule. Then the double-stranded structure unwinds and pulls apart, exposing unpaired nucleotide bases. New nucleotides pair with the exposed bases, forming hydrogen bonds. An enzyme, DNA polymerase, catalyzes this base pairing. Enzymes then knit together the new sugar-phosphate backbone. In this way, a new strand of complementary nucleotides extends along each of the old (original) strands. Two complete DNA molecules result, each with one new and one original strand (fig. 4.24). During mitosis, the two DNA molecules that form the two chromatids of each of the chromosomes separate so that one of these DNA molecules passes to each of the new cells. Clinical Application 4.3 discusses the polymerase chain reaction (PCR), a method for mass-producing, or amplifying, genes. PCR has revolutionized biomedical

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