分子生物学Chapter 4 DNA Replication.doc

Chapter 4 DNA Replication Introduction The Watson-Crick duplex structure of DNA immediately suggested how genetic material was duplicated from one generation to the next. The realization that bacterial genomes and eukaryotic chromosomes consist of single DNA molecules millimeters to centimeters in length raised a host of structural and biochemical questions about DNA replication. How does replication begin? Which enzymes take part in DNA synthesis, and what are their functions? How does duplication of the long helical duplex occur without the strands becoming tangled? As we will see, to carry out DNA replication, all cells use the same kinds of enzymes, including DNA polymerases, which assemble deoxyribonucleotides into a new strand; helicases, which unwind duplex DNA; single-stranded DNA-binding proteins; and exonucleases. Bacteria and yeast contain three different DNA polymerases, and mammalian cells contain five. Each type of polymerase has unique functions during DNA replication and repair. Additionally, some of the enzymes that cut and repair damaged DNA also participate in genetic recombination. Another common feature of the three processes discussed in this chapter is involvement of large multisubunit complexes, each containing many enzymes and structural proteins. These molecular machines have evolved to ensure that DNA replication, repair, and recombination occur very rapidly and with exquisite precision. Autoradiographic analyses carried out in the early 1960s on whole replicating chromosomes labeled with a short pulse of the radioactive DNA precursor 3H-thymidine revealed a localized region of replication that moves along the parental DNA double helix. Because of its Y-shaped structure, this active region is called a DNA replication fork. At a replication fork the DNA of both new daughter strands is synthesized by a multienzyme complex that contains the DNA polymerase. Figure 4-1 The replication fork is the region of DNA in which there is a transition