Читать книгу Snyder and Champness Molecular Genetics of Bacteria - Tina M. Henkin - Страница 59

The properties of the DNA polymerases, the enzymes that join the deoxynucleotides together to make the long chains, are the best guides to an understanding of the replication of DNA. These enzymes make DNA by linking one deoxynucleotide to another to generate a long chain of DNA. This process is called DNA polymerization, hence the name DNA polymerases.

Figure 1.6 shows the basic process of DNA polymerization by DNA polymerase. The DNA polymerase attaches the first phosphate (the α phosphate) of one deoxynucleoside triphosphate to the 3′ carbon of the sugar of the next deoxynucleoside triphosphate, in the process releasing the last two phosphates (the β and γ phosphates) of the first deoxynucleoside triphosphate to produce energy for the reaction. Then the α phosphate of another deoxynucleoside triphosphate is attached to the 3′ carbon of this deoxynucleotide, and the process continues until a long chain is synthesized.

DNA polymerases also need a template strand to direct the synthesis of the new strand (Figure 1.7). As mentioned in “Base Pairing” above, complementary base pairing dictates that wherever there is a T in the template strand, an A is inserted in the strand being synthesized and so forth according to the base-pairing rules. The DNA polymerase can move only in the 3′-to-5′ direction on the template strand, linking deoxynucleotides in the new strand in the 5′-to-3′ direction. When replication is completed, the product is a new double-stranded DNA with antiparallel strands, one of which is the old template strand and one of which is the newly synthesized strand.

There are two DNA polymerases that participate in normal DNA replication in Escherichia coli; they are called DNA polymerase III and DNA polymerase I (Table 1.1). DNA polymerase III is a large protein complex in which the enzyme that polymerizes nucleotides works with numerous accessory proteins. In E. coli and other bacteria, DNA polymerase III is responsible for the bulk of DNA replication on both DNA strands. As discussed below, DNA polymerase I has a number of features that are important because replication is continually reinitiated on one of the DNA strands. It also plays a role in DNA repair, as discussed in chapter 10. Table 1.1 lists many of the DNA replication proteins, the genes encoding them, and their functions.

Figure 1.7 Functions of the primer and template in DNA replication. (A) The DNA polymerase adds deoxynucleotides to the 3′ end of the primer by using the template strand to direct the selection of each base. (B) Simple illustration of 5′-to-3′ DNA synthesis. The wavy green line represents the primer.