DNA and Replication

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DNA and Replication
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History of DNA
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History of DNA

• Early scientists thought protein was the cells
  hereditary material because it was more complex
  than DNA
• Proteins were composed of 20 different amino
  acids in long polypeptide chains

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History of DNA

• Chromosomes are made of both DNA and protein
• Experiments on bacteriophage viruses by Hershey
  Chase proved that DNA was the cells genetic
  material

Radioactive 32P was injected into bacteria!
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Discovery of DNA Structure

• Erwin Chargraff showed the amounts of the four
  bases on DNA ( A,T,C,G)
• In a body or somatic cell
• A 30.3
• T 30.3
• G 19.5
• C 19.9

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Chargaffs Rule

• Adenine must pair with Thymine A - T
• Guanine must pair with Cytosine G - C
• The bases form weak hydrogen bonds

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Rosalind Franklin
Rosalind Franklin took diffraction x-ray
photographs of DNA crystals
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Watson and Crick

• In the 1950s, Watson Crick built the first
  model of DNA using Franklins x-rays

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DNA Structure
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DNA

• Two strands coiled called a double helix
• Sides made of a pentose sugar Deoxyribose bonded
  to phosphate (PO4) groups
• Center made of nitrogen bases bonded together by
  weak hydrogen bonds

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DNA Double Helix

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DNA

• Stands for Deoxyribonucleic acid
• Made up of subunits called nucleotides
• Nucleotide made of
• 1. Phosphate group
• 2. 5-carbon sugar
• 3. Nitrogenous base

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DNA Nucleotide

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DNA

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Antiparallel Strands

• One strand of DNA goes from 5 to 3 (sugars)
• The other strand is opposite in direction going
  3 to 5 (sugars)

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DNA Replication
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Replication Facts

• DNA has to be copied before a cell divides
• DNA is copied during interphase of mitosis
• New cells will need identical DNA strands

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Three Phases of Replication

• Initiation Unzipping DNA
• Elongation Making new DNA strand
• Termination Ends at telomeres (specific repeated
  bases) Zipping DNA back together.

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DNA Replication

• Begins at Origins of Replication
• Two strands open forming Replication Forks
  (Y-shaped region) The unzipping of the DNA
  strand is carried out by an enzyme DNA helicase.
• New strands grow at the forks

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DNA Replication

• Enzyme Helicase unwinds and separates the 2 DNA
  strands by breaking the weak hydrogen bonds
• Single-Strand Binding Proteins attach and keep
  the 2 DNA strands separated and untwisted

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DNA Replication

• Before new DNA strands can form, there must be
  RNA primers present to start the addition of new
  nucleotides
• DNA polymerase can then add the new nucleotides
• The joining of RNA primers and DNA polymerase
  creates an Okazaki fragment. Only occurs on
  lagging strand

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Leading and Lagging Strands

• Leading strand starts at the 3 end of the
  parent DNA forming the complementary strand of 5
  to 3
• Lagging strand is discontinuous (patchwork for
  DNA) requires many RNA primers for Okazaki
  fragments

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Okazaki Fragment
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DNA Replication

• DNA polymerase can only add nucleotides to the 3
  end of the DNA
• This causes the NEW strand to be built in a 5 to
  3 direction

Direction of Replication
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Finishing the New DNA Strands

• DNA Ligase zips up the lagging DNA strand
• This is done by joining the old and new strands
  of DNA with hydrogen bonds.

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Remember the Strands are Antiparallel

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Replication of Strands
Replication Fork
Point of Origin
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Proofreading New DNA

• DNA polymerase initially makes about 1 in 10,000
  base pairing errors
• Enzymes proofread and correct these mistakes
• The new error rate for DNA that has been
  proofread is 1 in 1 billion base pairing errors

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Semiconservative Model of Replication

• Idea presented by Watson Crick
• The two strands of the parental molecule
  separate, and each acts as a template for a new
  complementary strand
• New DNA consists of 1 PARENTAL (original) and 1
  NEW strand of DNA

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DNA Damage Repair

• Chemicals ultraviolet radiation damage the DNA
  in our body cells
• Cells must continuously repair DAMAGED DNA
• Excision repair occurs when any of over 50 repair
  enzymes remove damaged parts of DNA
• DNA polymerase and DNA ligase replace and bond
  the new nucleotides together

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Question

• What would be the complementary DNA strand for
  the following DNA sequence?
• DNA 5-CGTATG-3

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Answer

• DNA 5-GCGTATG-3
• DNA 3-CGCATAC-5

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DNA by the Numbers

• Each cell has about 2 m of DNA.
• The average human has 75 trillion cells.
• The average human has enough DNA to go from the
  earth to the sun more than 400 times.
• DNA has a diameter of only 0.000000002 m.