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DNA: Carries, Replicates and Recombines Information

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3.4 Angstrom between two nucleotides in a single strand and 34 ... The two strands of the helix are 20 Angstrom apart. major groove and a minor groove alternate ... – PowerPoint PPT presentation

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Title: 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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