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https://docs.google.com/document/u/3/d/1ETbBXJ3WADxbDzySv8ZoLJVHJ7bMU717w0yqElzHsfc/mobilebasic
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✓ Replication is the copying of DNA from parental DNA.
3 possible Methods
DNA REPLICATION
Dispersive MethodConservative Method Semiconservative
✓ Watson & Crick proposed
Semi-conservative model of DNA
Replication.
✓ It suggests that the parental DNA strands
act as template for the synthesis of new
complementary strands.
✓ After replication, each DNA molecule
would have one parental and one new
strand.
✓ Matthew Meselson & Franklin Stahl (1958)
proved Semi-Conservative model.
DNA REPLICATION
Semi-Conservative Method
Parental DNADaughter DNA
MATTHEW MESELSON & FRANKLIN STAHL✓ Matthew Meselson & Franklin Stahl (1958) proved
Semi-Conservative model.
✓ They prepared 2 culture media of E. coll.
✓ One preparation contains 15N: heavy isotope of N.
✓ 15N was incorporated into both strands of bacterial
DNA and the DNA became heavier.
✓ Other preparation contains 14N salts.
✓ 14N was incorporated in both strands of DNA and
became lighter.
✓ These 2 types of DNA can be separated by
centrifugation in a CsCI density gradient.
DNA REPLICATION✓ They took E. coli cells from 15N medium and
transferred to 14N medium.
✓ After one generation (i.e. after 20 minutes),
they isolated and centrifuged the DNA.
✓ Its density was intermediate (hybrid)
between 15N DNA and 14N DNA.
✓ This shows that in newly formed DNA, one
strand is old (15N type) and one strand is
new (14N type).
✓ This confirms semi-conservative replication.
✓ After two generation (i.e. after 40 min),
there was equal amounts of hybrid DNA
and light DNA.
DNA REPLICATION✓ Taylor & colleagues (1958) performed similar experiments on Vicia faba (faba beans) using
radioactive thymidine to detect distribution of newly synthesized DNA in the chromosomes.
✓ It proved that the DNA in chromosomes also replicate semi-conservatively.
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