All organisms duplicate their DNA with extraordinary accuracy before each cell division. In this section, we explore how an elaborate “replication machine” achieves this accuracy, while duplicating DNA at rates as high as 1000 nucleotides per second.
As introduced in Chapter 1, DNA templating is the mechanism the cell uses to copy the nucleotide sequence of one DNA strand into a complementary DNA sequence (Figure 5–2). This process requires the separation of the DNA helix into two template strands, and entails the recognition of each nucleotide in the DNA template strands by a free (unpolymerized) complementary nucleotide. The separation of
Figure 5–1 Germ-line cells and somatic cells carry out fundamentally different functions. In sexually reproducing organisms, the germ-line cells (red) propagate genetic information into the next generation. Somatic cells (blue), which form the body of the organism, are necessary for the survival of germ-line cells but do not themselves leave any progeny.
Figure 5–2 The DNA double helix acts as a template for its own duplication.
Because the nucleotide A will pair successfully only with T, and G only with C, each strand of DNA can serve as a template to specify the sequence of nucleotides in its complementary strand by DNA base-pairing. In this way, a double-helical DNA molecule can be copied precisely.
the DNA helix exposes the hydrogen-bond donor and acceptor groups on each DNA base for base-pairing with the appropriate incoming free nucleotide, aligning it for its enzyme-catalyzed polymerization into a new DNA chain.
The first nucleotide-polymerizing enzyme, DNA polymerase, was discovered in 1957. The free nucleotides that serve as substrates for this enzyme were found to be deoxyribonucleoside triphosphates, and their polymerization into DNA required a single-strand DNA template. Figure 5–3 and Figure 5–4 illustrate the stepwise mechanism of this reaction.
The DNA Replication Fork Is Asymmetrical
During DNA replication inside a cell, each of the two original DNA strands serves as a template for the formation of an entire new strand. Because each of the two daughters of a dividing cell inherits a new DNA double helix containing one original and one new strand (Figure 5–5), the DNA double helix is said to be replicated “semiconservatively.” How is this feat accomplished?
3˜ end of strand 5˜ end of strand
Figure 5–3 The chemistry of DNA synthesis. The addition of a deoxyribonucleotide to the 3ʹ end of a polynucleotide chain (the primer strand) is the fundamental reaction by which DNA is synthesized. As shown, base-pairing between an incoming deoxyribonucleoside triphosphate and an existing strand of DNA (the template strand) guides the formation of the new strand of DNA and causes it to have a complementary nucleotide sequence. The way in which complementary nucleotides base-pair is shown in Figure 4–4.
direction of chain growth