第7-2章微生物学(赵立平).pdfVIP

Lecture 7-2 Principles of Microbial Molecular Biology Chapter 7 in BROCK BIOLOGY OF MICROORGANISMS School of Life Science and Technology, Shanghai Jiao Tong University III. DNA REPLICATION DNA复制 Shanghai Jiao Tong University 7.4 Mechanism of replication-hypothesis by Watson and Crick If the actual order of the bases on one of the pairs of chains were given, one could write down the exact order of the bases on the other one because of the specific pairing. Thus, one chain is, as it were, the complement of the order, and it is this feature which suggests how the deoxyribonucleic acid molecule might duplicate itself. Shanghai Jiao Tong University Now, our model for deoxyribonucleic acid is, in effect, a pair of templates, each of which is complementary to the other. We imagine that prior to duplication the hydrogen bonds are broken, and the two chains unwind and separate. Each chain then acts as a template for the formation onto itself of a new companion chain, so eventually we shall have two pairs of chains, where we had only one before. Moreover, the sequence of the pairs of bases will have been duplicated exactly. 1953, Nature, 171:964-967 DNA replication Shanghai Jiao Tong University Meselson and Stahl experiment 1958, PNAS, 44: 671-682 Label DNA with heavy isotope of nitrogen, N15 Method: Density gradient equilibrium sedimentation • Cesium chloride氯化铯 • Centrifugation离心 • Detection of peaks Process: Growth in N15 medium then transfer to N14 medium • Heavy DNA • Light DNA Shanghai Jiao Tong University Shanghai Jiao Tong University Heavy isotope and DNA density