DNA: Carries, Replicates and Recombines Information

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• DNA Carries, Replicates and Recombines
  Information
• DNA is the genetic material
• Feulgen staining (1923) revealed that DNA
  resides in the chromosomes.
• Bacterial transformation experiments by Griffith
  1928 revealed that a transforming principle
  changes a rough strain to a smooth strain in
  Streptococcus pneumonia.
• Avery, MacLeod and McCarty 1944, using
  biochemical analysis revealed that DNA is the
  transforming principle in Griffith experiments.
• Using 32P to label DNA and 35S to label
  protein, Hershey and Chase in 1952 showed that
  its the core DNA and not the protein capsule of
  T2 phage that is responsible for the lytic
  function and replication of the phage.

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• The Watson and Crick Model of DNA
• The double helix model of DNA proposed by Watson
  and Crick depends on the following findings
• Rosalind Franklin and Maurice Wilkins in 1952
  reported that the crosswise pattern of X-ray
  diffraction of DNA fibers indicates a helical
  structure for DNA
• Erwin Chargaff by analyzing the base
  composition of DNA extracted from different
  organisms that A T and G C and that (A
  G)/(CT) 1.
• Hence the Watson Crick model is
• DNA is a right handed double helix, with sugar
  and phosphate being the core and the nitrogenous
  bases forming the steps of the ladder (G is
  complementary to C and T complementary to A
• Hydrogen bonds 3 between G C and 2 between A
  T.
• 3.4 Angstrom between two nucleotides in a single
  strand and 34 Angstrom every complete turn of the
  helix
• The two strands of the helix are 20 Angstrom
  apart
• major groove and a minor groove alternate

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• RNA differs from DNA in
• ribose instead of deoxyribose
• uracil instead of thymine
• single strand instead of double strand. The
  single strand forms double strand areas by
  twisting upon itself using the same rules of base
  complementarity (A complementary to U and G
  complementary to C)
• RNA is transcribed off DNA and functions as a
  mediator in gene expression protein synthesis
• Some viruses have RNA as their genetic material
  but during infection the RNA has to revert back
  to its complementary DNA (cDNA) to cause
  successful infection.

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• DNA Replication
• Semiconservative as proposed by Watson and Crick
  in their 1953 paper on DNA structure then proved
  by Meselson and Stahl in 1958
• DNA replication in prokaryotes includes
  1) Single
  origin of replication
  2) Initiator
  protein recognizes and binds at the origin of
  replication and attracts a DNA helicase to bind
  3)
  DNA helicase catalyzes the unwinding of the
  double helix 4) Primase synthesize a
  short RNA primer to initiate polymerization by
  DNA polymerase III
  5) Elongation in the 5 ---gt 3
  direction in both the leading strand (continuous
  synthesis) and the lagging strand (discontinuous
  synthesis) at each replication fork
  6) DNA
  polymerase I digests the RNA primer and at the
  same time fills in the gap with DNA synthesis in
  the 5 to 3 direction.

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• 7) DNA ligase catalyzes the formation of the
  final phosphoester bond and close the nick in the
  DNA strand.
• 8) The primary DNA repair enzyme is DNA
  polymerase I but DNA polymersae II can serve as
  an alternate repair polymerase and can replicate
  DNA under circumstances in which the template is
  damaged.
• 9) DNA topoisomerases relax the supercoils formed
  beyond the replication fork by introducing a cut
  in the double helix. The cut strands rotate to
  unwind and then rejoin (Phosphoester bond) by a
  DNA ligase.
• In eukaryotes, the mechanism of DNA replication
  is similar except that there are multiple origins
  of replications and there are five DNA
  polymerases involved, ?, ?,?,? and ?. DNA
  polymerase ? is in charge of chromosomal
  replication (lagging leading strands), ? for
  the lagging strand and ? for repair of nuclear
  DNA.

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