In chapter 3 we learned that genes have three main activities. One is to carry information, and we have spent most of the intervening chapters examining how cells decode this information through transcription and translation. Another activity of genes is to participate in replication. The next two chapters will examine this process in detail. In this chapter we will also consider DNA damage and repair. | Molecular Biology Fourth Edition Chapter 20 DNA Replication I: Basic Mechanism and Enyzmology Lecture PowerPoint to accompany Robert F. Weaver Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. General Features of DNA Replication Double helical model for DNA includes the concept that 2 strands are complementary Each strand can serve as template for making its own partner Semiconservative model for DNA replication is correct Half-discontinuous (short pieces later stitched together) Requires DNA primers Usually bidirectional 20- Three Hypotheses of Replication Three methods of DNA replication were considered: Semiconservative Conservative Dispersive 20- DNA replicates in a semiconservative manner When parental strands separate Each strand serves as template Makes a new, complementary strand Semiconservative Replication 20- Semidiscontinuous Replication DNA replication in E. coli is semidiscontinuous One strand is replicated . | Molecular Biology Fourth Edition Chapter 20 DNA Replication I: Basic Mechanism and Enyzmology Lecture PowerPoint to accompany Robert F. Weaver Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. General Features of DNA Replication Double helical model for DNA includes the concept that 2 strands are complementary Each strand can serve as template for making its own partner Semiconservative model for DNA replication is correct Half-discontinuous (short pieces later stitched together) Requires DNA primers Usually bidirectional 20- Three Hypotheses of Replication Three methods of DNA replication were considered: Semiconservative Conservative Dispersive 20- DNA replicates in a semiconservative manner When parental strands separate Each strand serves as template Makes a new, complementary strand Semiconservative Replication 20- Semidiscontinuous Replication DNA replication in E. coli is semidiscontinuous One strand is replicated continuously in the direction of the movement of the replicating fork The other strand is replicated discontinuously as 1-2 kb Okazaki fragments in the opposite direction This allows both strands to be replicated in the 5’ 3’-direction 20- DNA Replication Models 20- Priming DNA Synthesis Okazaki fragments in E. coli are initiated with RNA primers 10-12 nt long Intact primers are difficult to detect in wild-type cells because of enzymes that attack RNAs 20- Bidirectional Replication The replication structure resembles the Greek letter, DNA replication begins with the creation of a “bubble” – a small region where parental strands have separated and progeny DNA has been synthesized As the bubble expands, replicating DNA begins to take on the shape 20- Replicating Forks In DNA replication, replicating forks represent sites of DNA replication Is the process: Unidirectional – one fork moving away from the other which remains fixed at the origin of replication Bidirectional –
