The Double Helix The Complementary Model

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The Double HelixThe Complementary Model

• J.D. Watson
• F.H. Crick

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Evidence for Fibrous Nature of DNA
Physico-chemical analysis

• DNA is a long asymmetrical chain that consists of
  a 5-carbon sugar and phosphate backbone joined in
  a 3-5 direction by phosphodisester bonds.
• Each sugar has 1 of 4 bases attached to it
  Adenine, Guanine, Cytosine, and Thymine.

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DNA Has Two Chemical Chains X-ray Crystallography

• Two alternating forms of DNA exists.
• A crystalline form that occurs at 75 humidity
  (top) with a 2.8 A reflexion about its meridian..
• A para-crystalline form that occurs at higher
  humidity (bottom) with 3.4 A reflexion about its
  meridian.
• Density data indicate possibly two distinct
  polynucleotide chains

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Hypothesized Structure

• DNA is three dimensional.
• DNA has two chains that are coiled around a
  single axis and held together by hydrogen bonds.
• Both chains follow right handed helices.
• The phosphates and sugar groups are on the
  outside and the bases on the inside.
• A repeat distance of 34A with a reflexion of
  spacing 3.4A.

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Hypothesized Structure

• The two chains are held together by hydrogen
  bonds between the bases.
• The base pairing is specific.
• Adenine-Thymine Guanine-Cytosine
• Each chain complements the other because of the
  specific base pairing.

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Evidence supporting the Complementary Model

• X-ray pictures suggests that
• DNAs basic structure is helical.
• High concentration of atoms on the circumference
  of the helix.
• The polynucleotide chains are not distinct from
  each other.

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Evidence supporting the Complementary Model

• Titration curves of DNA suggest
• Hydrogen bond formation is characteristic of DNA
  structure
• Analytical data of the bases
• suggests
• The amount of A T and C G is very close.

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Characteristics of Genetic Material

• Must be able to self-replicate.
• Must exert a highly specific influence on the
  cell.

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DNA Replication The Complementary Model

• Of the two characteristics Watson Crick
  proposed only a practical mechanism for
  replication.
• Complementary base pairing is the backbone of
  replication.
• But how does the interior of the helix allow
  itself to be replicated?

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DNA Replication The Complementary Model

• Proposed that DNA unwinds by breaking the
  hydrogen bonds between the strands.
• The single strands would then serve as a template
  to which complementary free nucleotides would
  attach and form 2 helices where there was
  originally one.
• They were unaware of the numerous protein
  machinery (helicase, ligase, polymerases, etc.)
  that facilitated this process.

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DNA Replication Today

• DNA replication