Modern experimental methods for the manipulation and analysis of DNA grew out of a detailed understanding of its molecular structure and replication. Therefore, to understand these methods, one needs to know something about the molecular structure of DNA. We saw in Chapter 1 that DNA is a helix of two paired, complementary strands, each composed of an ordered string of nucleotides, each bearing one of the bases A (adenine), T (thymine), G (guanine), or cytosine (C). WatsonCrick base pairing between A and T and between G and C in the complementary strands holds the strands together. The complementary strands also hold the key to replication, because each strand can serve as a template for the synthesis of a new complementary strand. We will now take a closer look at DNA structure and at the key features of its replication.
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