DNA STRUCTURE AND REPLICATION by zujt6g

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									DNA STRUCTURE
     AND
 REPLICATION
HISTORY
   1869 – Friedrich Miescher – “discovered” DNA in
    nucleus
   1909 – Archibald Garrod – lack of inheritance of
    certain proteins. Proteins are genetic material
   1928 – Fredrich Griffith – Identified DNA as
    source of genetic material using bacteria
   1930’s – Oswald Avery, Colin MacLeod and
    Maclyn McCarty – Confirmed DNA
   1950 – Alfred Hershey and Martha Chase
    confirmed DNA is biochemical of heredity
Hershey and Chase
HISTORY of STRUCTURE
   1909 – Phoebus Levene – Identified ribose
    and 1929 – identified deoxyribose and
    determined nucleotide structure
   Early 1950’s
       Erwin Chargoff - Equal numbers of A & T and C
        &G
       Maurice Wilkins and Rosalind Franklin – x-rayed
        DNA to show repeating nucleotide structure
Rosalind Franklin
HISTORY of STRUCTURE
   1953 – James Watson
    and Francis Crick
    combined data to
    create a 3-D model of
    structure called the
    double helix
Watson and Crick
DNA STRUCTURE
   DNA displays anti-
    parallelism = 2 chains
    of nucleotides run
    opposite to each other
    in a head to tail
    relationship
   3’ to 5’ direction vs. 5’
    to 3’ direction
DNA STRUCTURE
VOCABULARY
   Deoxyribonucleic acid – DNA – chain of
    nucleotides
   Chromosomes – DNA wrapped around
    proteins called histones
   Nucleosomes – DNA and histones wrapped
    in bead-like structure
   Chromatin – active form of chromosomes
DNA STRUCTURE
DNA STRUCTURE
DNA STRUCTURE
   DNA = deoxyribonucleic acid
   Stores and transmits genetic information
    needed for all cell functions
   Polymer consisting of 1000’s of nucleotides
   Nucleotide =
       5 carbon sugar deoxyribose
       Phosphate group
       1 of 4 nitrogen bases
DNA STRUCTURE
DNA STRUCTURE
   Nitrogen Bases:
   Adenine
                      double ringed purines
   Guanine
   Thymine
                      single ringed pyrimidines
   Cytosine
NITROGEN BASES
DNA STRUCTURE
   Structure of DNA is double strand of
    covalently bonded nucleotides in twisted
    ladder shape = double helix
   ‘Sides’ of ladder = sugar & phosphate
    groups
   ‘Rungs’ of ladder = nitrogen bases
DNA STRUCTURE
DNA STRUCTURE
    Bases are paired together in specific manner =
    ‘Base pairing rule’
       Adenine only pairs with Thymine
       Guanine only pairs with Cytosine
   Bases held together in ‘rungs’ by weak hydrogen
    bonds
            2 hydrogen bonds between A & T
            3 hydrogen bonds between C & G
   The structure of DNA and sequence of bases
    determines its function
DNA STRUCTURE
DNA REPLICATION
   Why does DNA make copies of itself?
       To pass genetic information on to new generations of cells
   Replication begins when a helicase break the
    hydrogen bonds between bases at initiation
    sites
   Helicase is an enzyme that unwinds DNA and
    holds the strands apart during replication
DNA REPLICATION
   RNA polymerase attracts RNA nucleotides
    to build RNA primer at initiation site
   RNA primer attracts DNA polymerase
   DNA polymerase – enzyme that pairs up
    free nucleotides with exposed
    complementary bases on each strand and
    proof reads the new strand
DNA REPLICATION
   Ligase enzymes help nucleotides bond,
    forming one new strand on each original
    strand
   Called semi-conservative replication
   Original strand acts as template
   Each DNA replicates at 100’s of points
    simultaneously, then the sections rejoin
   Open portions are called replication forks
DNA REPLICATION
DNA REPLICATION
    Occurs in 5’ to 3’ direction
   Continuous replication in one direction –
    this is the leading strand.
   Replication in lagging strand occurs in
    segments called Okazaki fragments.
DNA REPLICATION



                  Okazaki
                  fragment
DNA MUTATIONS AND REPAIR
   Replication of DNA is very accurate
       Mistakes only occur 1 in 100,000 bases
       Polymerase corrects errors found
Mutation and Repair
   Dimers:
       U-V radiation causes covalent bonds to form
        between adjacent pyrimidines
       Causes kinks in chain insertion of non-
        complementary bases
       Photoreactivation = enzyme photolyase breaks
        the dimer with energy from blue light
Mutation and Repair
   Excision repair:
       Cutting of bond between deoxyribose and base
        to remove dimers
       DNA polymerase fills in correct nucleotides
   Mismatch repair:
       Enzymes proofread loops in DNA to show
        inaccurate alignment
       Mismatches occur in short repeating segments
        called microsatellites
THYMINE DIMER

								
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