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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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