Читать книгу Snyder and Champness Molecular Genetics of Bacteria - Tina M. Henkin - Страница 58

Once the precursors of DNA replication are synthesized, they must be polymerized into long double-stranded DNA molecules. A very large complex of many enzymes assembles on the DNA and moves along the DNA, separating the strands and making a complementary copy of each of the strands. Thus, two strands of DNA enter the complex and four strands emerge on the other side, forming a branched structure. Each of the emerging branches contains one old “conserved” strand and one new strand, hence the name semiconservation replication. This branched structure where replication is occurring is called the replication fork. In this section, we discuss what is happening in the replication fork, including the mechanisms used to overcome obstacles to the progression of the replication fork on the chromosome.

Figure 1.5 The pathways for synthesis of deoxynucleotides from ribonucleotides. Some of the enzymes referred to in the text are identified. THF, tetrahydrofolate; DHF, dihydrofolate.

Figure 1.6 Features of DNA. (A) Polymerization of the deoxynucleotides during DNA synthesis. The β and γ phosphates of each deoxynucleoside triphosphate are cleaved off to provide energy for the polymerization reaction. (B) The strands of DNA are antiparallel. (C) A single base can be flipped out from the double helix, which could be important in recombination and repair.