DNA Structure and Replication

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DNA Structure and Replication

• Hershey-Chase Experiments (1952)
• Demonstrated that DNA is the genetic material
• DNA stands for Deoxyribonucleic Acid

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Hershey-Chase Experiments
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DNA Structure and Replication

• Structure of DNA
• James Watson and Francis Crick determined the
  structure of DNA in 1953
• They built models to conform to x-ray
  crystallography data collected by others.
• In this technique, X-rays are diffracted as they
  pass through aligned fibers of a purified
  molecule.
• The diffraction pattern can be used to deduce the
  three-dimensional shape of molecules.

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• DNA is a chain of nucleotides
• Each nucleotide is a complex of three subunits
• Phosphoric acid (phosphate)
• A pentose sugar (deoxyribose)
• A nitrogen-containing base
• There are two chains arranged in a double helix
• Held together by hydrogen bonds between the chains

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• Four Possible Bases
• Adenine (A) - a purine
• Guanine (G) - a purine
• Thymine (T) - a pyrimidine
• Cytosine (C) - a pyrimidine
• Complimentary Base Pairing
• Adenine (A) always pairs with Thymine (T)
• Guanine (G) always pairs with Cytosine (C)

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Overview of DNA Structure
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DNA Structure and Replication

• Semi-conservative replication
• Each daughter DNA molecule consists of one new
  chain of nucleotides and one from the parent DNA
  molecule
• The two daughter DNA molecules will be identical
  to the parent molecule

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• When a double helix replicates, each of the
  daughter molecules will have one old strand and
  one newly made strand.

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• A large team of enzymes and other proteins carry
  out DNA replication
• It takes E. coli less than an hour to copy each
  of the 5 million base pairs in its single
  chromosome and divide to form two identical
  daughter cells.
• A human cell can copy its 6 billion base pairs
  and divide into daughter cells in only a few
  hours.
• Remarkably accurate process - only one error per
  billion nucleotides.
• More than a dozen enzymes and other proteins
  participate in DNA replication.

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Origins of Replication

• Special sites where replication of a DNA molecule
  begins.
• Bacteria one origin site.
• The enzymes separate the strands, forming a
  replication bubble.
• Replication proceeds in both directions until the
  entire molecule is copied.
• Eukaryotes 100s or 1000s of origin sites per
  chromosome.
• At the origin sites, the DNA strands separate
  forming a replication bubble with replication
  forks at each end.
• The replication bubbles elongate as the DNA is
  replicated and eventually fuse.

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Replication of DNA

• Enzyme DNA helicase unwinds and unzips the
  double-stranded DNA
• New complementary DNA nucleotides fit into place
  along divided strands by complementary base
  pairing. These are positioned and joined by DNA
  polymerase
• DNA ligase connects the short replicated strands
  together
• The two double helix molecules are identical to
  each other and to the original DNA molecule

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5 end
3 end
3 end
5 end

• DNA polymerases (enzymes) catalyze the elongation
  of new DNA at a replication fork by adding
  nucleoside triphosphates.
• As each nucleotide is added, the last two
  phosphate groups are hydrolyzed and the energy
  released drives the polymerization of the
  nucleotide to the new strand.

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Overview of DNA Replication
DNA helicase
DNA polymerase
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In Summary

• The leading strand (3 to 5) is copied
  continuously into the replication fork from a
  single primer.
• The lagging strand (5 to 3) is copied away
  from the fork in short segments, each requiring
  a new primer.