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DNA replication
James Watson and Francis Crick worked out the structure of DNA in 1953
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The semi-conservative model was
proved to be right from scientific experiments by Meselsohn and Stahl
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The model that is
universally accepted is that of the semi-conservative model as half of the parent DNA is conserved in
each new DNA molecule
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There are four further requirements required
for successful semi-conservative replication: four nucleotides (A, C, G and T), enzyme DNA
polymerase, enzyme DNA helicase and ATP to drive the process
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This causes the double helix structure
of the DNA molecule to separate into two strands and unwinds
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The two new strands will act as templates
for the new strands of DNA
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The exposed polynucleotide strands then acts as a template to which the free nucleotides bind to
the base pairs forming weak hydrogen bonds between the free nucleotides and the bases
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Also, formed
are strong chemical bonds between sugar and phosphate
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This is why DNA replication is described as semi-conservative as half of the chain is part
of the original DNA molecule and half is brand new
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They designed their experiment on three main facts
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The scientists used E
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They cultured bacteria in a N15 medium
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They then shifted the
bacteria to an N14 medium, the DNA was isolated at different times corresponding to replication
cycles 0, 1 and 2
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This rules out the
conservative replication model, which predicted that both heavy density DNA and light density DNA
would be present, but none of middle density will be present
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After two replication cycles, two bands of DNA were seen, one of middle density and one of light
density
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