STAHL AND MESELSON’S WORK ON SEMICONSERVATIVE( 半保留 ) NATURE OF DNA REPLICATION

1. STAHL AND MESELSON’S WORK ON SEMICONSERVATIVE(半保留) NATURE OF DNA REPLICATION

2. CONTENT 1.INTRODUCTION 2.MESELSON-STAHL EXPERIMENT 3.DNA REPLICATION 4.REPLICATION 5.EQUATION 6.SEMICONERVATIVE REPLICATION

3. INTRODUCTION • MATTHEW STANLEY MESELSON (1930-) • Born in Denver, Colorado • Studied chemistry at the University of Chicago • Worked at the California Institute of Technology

4. FRANKLIN WILLIAM STAHL (1929-) • Born in Boston. • Received a B.A. from Harvard University in 1951. • Work in the University of Rochester after graduated.

5. PHOTOES • In 1954, Meselson met Franklin Stahl • In 1958, they proved that DNA was replicated in a semi-conservative way.

6. Meselson-Stahl experiment • The Meselson-Stahl experiment was an experiment to prove that DNA replication was semiconservative. • Semiconservative replication means that when the double stranded DNA helix was replicated, each of the two double stranded DNA helices consisted of one strand • Nitrogen is a major constituent of DNA, the genetic material of all cells. • It is commonly found in the 14N isotope, but it can also be found in the heavier 15N isotope.

7. E. coliwere grown for several generations in a medium with 15N. • The DNA of the resuling cells had a higher density (was heavier). • E. coli cells with only 15N in their DNA were put back into a 14N medium and were allowed to divide only once. • DNA was then extracted from a cell and was compared to DNA from 14N DNA and 15N DNA. • It was found to have exactly an intermediate density. This supported the idea of semiconservative replication. • The DNA was intermediate in density because it had an all 15N DNA strand and an all 14N DNA strand. The all 15N strand was one of the original strands in the original cell. The all 14N strand was a newly synthesized strand.

8. DNA REPLICATION • DNA replication or DNA synthesis is the process of copying a double-stranded DNA strand, prior to cell division • The two resulting double strands are identical, and each of them consists of one original and one newly synthesized strand. • This is called semiconservative replication. • The process of replication consists of three steps, initiation, replication and termination

9. Replication • After the helicase unwinds the DNA, single-strand binding protein is used to hold the DNA • RNA primase is then bound to the starting DNA site. • Begin of replication, an enzyme called DNA polymerase binds (凝固)to the RNA primase, which indicates the starting point for the replication. • DNA polymerase can only synthesize new DNA • The DNA polymerase can only travel on one side of the original strand without any interruption. • Since the DNA replication on the lagging strand is not continuous, a new DNA polymerase has to be added each time as the helicase unwinds more DNA. • The replicated DNA is fragmented.

10. Equation (DNA)n + dNTP ↔ (DNA)n+1 + PPi

11. SEMICONSERVATIVE REPLICATION • Semiconservative replication describes the method by which DNA is replicated in all known cells. • Conservative replication would leave the original, template DNA strands intact and would produce a copy composed of entirely new DNA base pairs. • Dispersive replication would produce two copies of the DNA, both containing a mixture of old and new DNA base pairs. • Semiconservative replication would produce two copies that each contained one of the original strands, and one entirely new copy. • The deciphering of the structure of DNA by Watson and Crick in 1953 suggested that the semiconservative model was correct  

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