Chapter 25 DNA Metabolism 1. How does a DNA molecule replicate with high fidelity? 2. How are DNA lesions(damages) repaired to maintain the integrity of genetic information? 3. How do dNA molecules recombine rearrange )? High accuracy, multitude of participants
Chapter 25 DNA Metabolism 1. How does a DNA molecule replicate with high fidelity? 2. How are DNA lesions (damages) repaired to maintain the integrity of genetic information? 3. How do DNA molecules recombine (rearrange)? High accuracy, multitude of participants
1. The deduced double helix structure of DNA revealed the possible ways for its replication(1953) Each DNA strand was proposed to act as the template(complement) of the other The way a DNA molecule replicates was hypothesized to be semiconservative: each of the newly synthesized DNA duplexes consists of one strand from the parent DNa and one strand of newly synthesized Watson and Crick, 1953).(the conservative replication would generate two daughter DNA molecules with one consisting of two new and one of two old strands
1. The deduced double helix structure of DNA revealed the possible ways for its replication (1953) • Each DNA strand was proposed to act as the template (complement) of the other. • The way a DNA molecule replicates was hypothesized to be semiconservative: each of the newly synthesized DNA duplexes consists of one strand from the parent DNA and one strand of newly synthesized (Watson and Crick, 1953). (the conservative replicationwould generate two daughter DNA molecules with one consisting of two new and one of two old strands.)
The hypothesis of semiconservative replication proposed by Watson and Crick in1953 GG New strandI Old strand 于TA Old New New Old
New strand Old strand The hypothesis of semiconservative replication proposed by Watson and Crick in 1953
2. DNA replication was proved to be semiconservative by the Meselson-Stahl experiment using E coli cells (1957) 5N (the Heavy isotope) and 4N (the Light isotope) was used (as NH4CI)to label the DNa to distinguish the old and newly synthesized DNa molecules in cells Three types of DNa molecules containing various proportions of 5N and 4N(H-H, H-L, L-L)were separated by centrifugation to equilibrium in a cesium chloride(CscI) density gradient(only two types would be expected if replication is conservative)
2. DNA replication was proved to be semiconservative by the Meselson-Stahl experiment using E. coli cells (1957) • 15N (the Heavy isotope) and 14N (the Light isotope) was used (as NH4Cl) to label the DNA to distinguish the old and newly synthesized DNA molecules in cells; • Three types of DNA molecules containing various proportions of 15N and 14N (H-H, H-L, L-L) were separated by centrifugation to equilibrium in a cesium chloride (CsCl) density gradient (only two types would be expected if replication is conservative)
DNA extracted and centrifu to equilibrium in CsCl R density gradient radioisotope labeling and density gradient centrifugation clearly (a) He eavy DNA (ON)- distinguishes Original parent molecule replications of semiconservative Hybrid DNA from conservative (b)(15N14N) daughter molecules Light DNA(N)- (e)Hybrid dNA daughter molecules
Radioisotope labeling and density gradient centrifugation clearly distinguishes replications of semiconservative from conservative