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DNA Structure and Replication Powerpoint Lecture

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DNA Structure and Replication Powerpoint Lecture Powered By Docstoc
					DNA: The Molecular Basis
     of Inheritance
     Building a Structural
       Model of DNA
• After most biologists became convinced that
  DNA was the genetic material of life, the next
  challenge was to determine its structure.
• Rosalind Franklin produced a picture of the
  DNA molecule by using a technique called X-
  ray crystallography Franklin produced a
  picture of the DNA molecule using this
  technique
LE 16-6




Rosalind Franklin   Franklin’s X-ray diffraction
                    photograph of DNA
• Based on the images, two other scientists
  named Watson and Crick were able to
  determine that DNA molecules took a double
  helix shape.
 LE 16-7




                                 5 end
                                                 Hydrogen bond
                                                                 3 end

 1 nm

                      3.4 nm




                                     3 end

                       0.34 nm
                                                                    5 end

Key features of DNA structure       Partial chemical structure               Space-filling model
  Sugar–phosphate          Nitrogenous
     backbone                 bases
       5 end




                           Thymine (T)




                           Adenine (A)




                           Cytosine (C)




Phosphate                                 DNA nucleotide

     Sugar (deoxyribose)
       3 end
                           Guanine (G)
• Watson and Crick built models of a double
  helix to match to the X-rays and chemistry of
  DNA
  o The side strands, or “backbones” of the DNA
    molecule are made of a sugar (deoxyribose)
    paired with a phosphate.
  o The deoxyribose backbones are joined together by
    a series of molecules called nitrogenous bases.
        Nitrogenous Bases
• There are two types of nitrogenous bases:
   o Purines
      • Much wider
      • Include adenine and guanine
   o Pyramidines
      • Much narrower
      • Include cytosine and thymine
LE 16-UN298
                How do the four bases
                combine to form DNA?

                Purine + purine: too wide




              Pyrimidine + pyrimidine: too narrow




                Purine + pyrimidine: width
                matches data from X-rays
• Watson and Crick
  reasoned that the
  pairing was more
  specific –
  o Adenine paired only
    with Thymine
  o Guanine paired only
    with Cytosine
Base Pairing to a Template
          Strand
• DNA is a double-helix molecule made of two
  intertwining strands.
• The two strands of DNA are complementary,
  meaning each has a set of bases that
  corresponds with the other.
• In DNA replication, the molecule is be
  separated into its two strands.
  o Two new strands can be made from these
    templates, duplicating the molecule.
LE 16-9_1




            The parent molecule has
            two complementary
            strands of DNA. Each base
            is paired by hydrogen
            bonding with its specific
            partner, A with T and G with
            C.
LE 16-9_2




  The parent molecule has        The first step in replication
  two complementary              is separation of the two
  strands of DNA. Each base      DNA strands.
  is paired by hydrogen
  bonding with its specific
  partner, A with T and G with
  C.
LE 16-9_3




The parent molecule has     The first step in replication   Each parental strand now
two complementary           is separation of the two        serves as a template that
strands of DNA. Each base   DNA strands.                    determines the order of
is paired by hydrogen                                       nucleotides along a new,
bonding with its specific                                   complementary strand.
partner, A with T and G
with C.
LE 16-9_4




The parent molecule has     The first step in replication   Each parental strand now    The nucleotides are
two complementary           is separation of the two        serves as a template that   connected to form the
strands of DNA. Each base   DNA strands.                    determines the order of     sugar-phosphate back-
is paired by hydrogen                                       nucleotides along a new,    bones of the new strands.
bonding with its specific                                   complementary strand.       Each “daughter” DNA
partner, A with T and G                                                                 molecule consists of one
with C.                                                                                 parental strand and one
                                                                                        new strand.
     Origins of Replication
• Replication begins at special sites called
  origins of replication.
  o The two DNA strands are separated, opening up a
    replication “bubble”
  o Each chromosome may have hundreds or even
    thousands of origins of replication
  o Replication proceeds in both directions from each
    origin, until the entire molecule is copied
LE 16-12


                             Parental (template) strand
 Origin of replication                                                            0.25 µm
                                 Daughter (new) strand




       Bubble                Replication fork




            Two daughter DNA molecules


In eukaryotes, DNA replication begins at may sites        In this micrograph, three replication
along the giant DNA molecule of each chromosome.          bubbles are visible along the DNA
                                                          of a cultured Chinese hamster cell
                                                          (TEM).
Elongating the DNA Strand
• Enzymes called DNA polymerases catalyze the
  elongation of new DNA.
• The rate of elongation is about 500 nucleotides per
  second in bacteria and 50 per second in human cells.
        Proofreading and
         Repairing DNA
• DNA polymerases also proofread newly made
  DNA, replacing any incorrect nucleotides.
• Two types of repair:
  o In mismatch repair, the enzymes replace incorrect
    bases with the correct ones.
  o In nucleotide excision repair, enzymes cut out and
    replace entire stretches of DNA that are damaged.
   Replicating the Ends of
      DNA Molecules
• DNA polymerase has one significant limitation.
• The enzyme has no way to complete one of
  the ends.
  o Every time the DNA is copied, it becomes a little
    shorter.
• Cells will divide countless times over the
  lifespan of an organism. How can DNA be
  protected, given this limitation?
• Eukaryotic chromosomal DNA molecules
  have at their ends repeating nucleotide
  sequences called telomeres.
  o Telomeres are DNA, but do not actually encode
    for any traits.
  o Telomeres do not prevent the shortening of DNA
    molecules, but they postpone it.
• Eventually, the telomeres are worn
  down and essential genes begin
  to be lost from the chromosomes.
   o This is one of the hypothesized causes
     of aging.
• An enzyme called telomerase
  catalyzes the lengthening of
  telomeres in stem cells.
   o This enzyme cannot be produced
     indefinitely due to an increasing risk of
     the cell growing uncontrollably
     (cancer)

				
DOCUMENT INFO
Description: This is a Powerpoint lecture for a unit on DNA synthesis and replication, written for an introduction to biology course.