Читать книгу Cell Biology - Stephen R. Bolsover - Страница 94

In prokaryotes the synthesis of a new DNA molecule is catalyzed by the enzyme DNA polymerase III. Its substrates are the four deoxyribonucleoside triphosphates, dGTP, dATP, dTTP, and dCTP. DNA polymerase III catalyzes the formation of a phosphodiester bond (Figure 3.3 on page 37) between the 3′ hydroxyl of the sugar residue on the most recently added nucleotide and the 5′ phosphate of the incoming nucleotide. The elongation of the new DNA molecule takes place in the 5′ to 3′ direction (Figure 4.2a). The base sequence of a newly synthesized DNA strand is dictated by the base sequence of its parental strand. If the sequence of the template strand is 3′ CATCGA 5′, then that of the daughter strand is 5′ GTAGCT 3′. In eukaryotes, DNA replication is performed by three isoforms, DNA polymerases α, δ, and ε, but the mechanism is much the same.

DNA polymerase III can only add a nucleotide to a free 3′‐hydroxyl group and therefore synthesizes DNA in the 5′ to 3′ direction. The template strand is read in the 3′ to 5′ direction. However, the two strands of the double helix are antiparallel. They cannot be synthesized in the same direction because only one has a free 3′‐hydroxyl group, the other has a free 5′‐phosphate group. No DNA polymerase has been found that can synthesize DNA in the 3′ to 5′ direction, that is, by attaching a nucleotide to a 5′ phosphate, so the synthesis of the two daughter strands must differ. One strand, the leading strand, is synthesized continuously while the other, the lagging strand, is synthesized discontinuously. DNA polymerase III can synthesize both daughter strands, but must make the lagging strand as a series of short 5′ to 3′ sections (Figure 4.1). These fragments of DNA, called Okazaki fragments after Reiji Okazaki who discovered them in 1968, are then joined together by DNA ligase.

Figure 4.2. DNA polymerase III can correct its own mistakes.