tailieunhanh - Biochemistry, 4th Edition P91

Biochemistry, 4th Edition P91. Continuing Garrett and Grisham's innovative conceptual and organizing framework, "Essential Questions," BIOCHEMISTRY guides students through course concepts in a way that reveals the beauty and usefulness of biochemistry in the everyday world. Streamlined for increased clarity and readability, this edition also includes new photos and illustrations that show the subject matter consistently throughout the text. New end-of-chapter problems, MCAT practice questions, and the unparalleled text/media integration with the power of CengageNOW round out this exceptional package, giving you the tools you need to both master course concepts and develop critical problem-solving skills you can draw upon. | How Is DNA Replicated 863 replication involves two replication forks that move in opposite directions. Bidirectional replication predicts that if radioactively labeled nucleotides are provided as substrates for new DNA synthesis both replication forks will become radioactively labeled. The experiment illustrated in Figure confirms this prediction. A template is something whose edge is shaped in a particular way so that it can serve as a guide in making a similar object with a corresponding contour. Replication Requires Unwinding of the DNA Helix Semiconservative replication depends on unwinding the DNA double helix to expose single-stranded templates to polymerase action. For a double helix to unwind it must either rotate about its axis while the ends of its strands are held fixed or positive supercoils must be introduced one for each turn of the helix unwound see Chapter 11 . If the chromosome is circular as in E. coli only the latter alternative is possible. Because DNA replication in E. coli proceeds at a rate approaching 1000 nucleotides per second and there are about 10 bp per helical turn the chromosome would accumulate 100 positive supercoils per second In effect the DNA would become too tightly supercoiled to allow unwinding of the strands. DNA gyrase a Type II topoisomerase acts to overcome the torsional stress imposed upon unwinding DNA gyrase introduces negative supercoils at the expense of ATP hydrolysis. The unwinding reaction is driven by helicases see also Chapter 16 a class of proteins that catalyze the ATP-dependent unwinding of DNA double helices. Unlike topoisomerases that alter the linking number of dsDNA through phosphodiester bond breakage and reunion see Chapter 11 helicases simply disrupt the hydrogen bonds that hold the two strands of duplex DNA together. A helicase molecule requires a single-stranded region for binding. It then moves along the single strand unwinding the double-stranded DNA in an ATP-dependent process. SSB .