文档介绍：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 ic information?
3. How do DNA molecules bine (rearrange)?
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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.)
Old strand
New 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)
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.
Radioisotope labeling
and density gradient
centrifugation clearly
distinguishes
replications of
semiconservative
from conservative.
The Meselson-
Stahl experiment:
DNA molecules
duplicate
semiconservatively
in E. coli cells.
15N-15N
0 generation
1 generation
2 generations
3 generations
4 generations
0 and 2 mixed
0 and 4 mixed
15N-14N
15N-15N
14N-14N
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3. A variety of simple questions were asked about DNA replication
Are the two parental pletely unwound before replication begins?
Does replication begin at random sites or at unique sites?
Does DNA replication proceed in one direction or both directions