DNA

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Topic 3.4 DNA Replication
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Beyond the syllabus The discovery of the double
helix

• Erwin Chargaff (1951)
• Rule of Base pairing
• Rosalind Franklin Maurice Wilkins (1953)
• X-ray diffraction pattern of DNA
• James Watson Francis Crick (1953)
• Molecular structure of DNA

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Chargaffs Results Similar amount of A-T and C-G
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• Franklins Results 2 periodicities for DNA 3.4 Å
  and 34 Å.

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The discovery of the double helix Watson
Crick Model
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The discovery of the double helix Watson
Crick Model

• DNA is composed of 2 chains of nucleotides that
  form a double helix shape.
• The two strands are antiparallel.
• Every complete turn of the helix measured 3.4 nm
• The backbone of the DNA molecule is composed of
  alternating phosphate groups and sugars.
• The complimentary nitrogenous bases form hydrogen
  bonds between the strands.
• A is complimentary to T and G is complimentary to
  C.

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3.4.1 DNA replication

1. Unwound and separate the DNA helix Helicase
2. New dNTPS are joined to the template by hydrogen
   bonds.
3. Complementary Base Pairing ensures identical
   copies of DNA. The parent strands act as a
   template for the new (complementary strands)
4. Adenine pairs only with Thymine (A-T)
5. Cytosine pairs only with Guanine (C-G)
6. DNA polymerase enzyme links the phosphate of the
   new nucleotide to the sugar of the nucleotide
   before it by covalent bond.

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3.4.2 DNA replication Complementary Base Pairing
Complementary Base Pairing ensures the new DNA
molecule is identical to the original no
mistakes are made so the base-sequence of
nucleotides is conserved
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3.4.3 State that DNA replication is
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Meselson Stahl Experiment
DNA Replication is semiconservative