Structure, replication and repair of DNA

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Structure, replication and repair of DNA

• Chapter 10

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• On Feb. 28, 1953 Watson and Crick discovered the
  structure of DNA
• Nearly beaten to the discovery by Rosalind
  Franklin (she would have won if she had not been
  socially isolated)
• Chemical make up had been known for 30 years, but
  not the three-dimensional structure, which would
  give a clue as to how it worked.
• Nucleotides - sugar-phosphate backbone and
  nitrogenous bases.

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Chargaffs Rules

• The amount of A amount of T
• The amount of C amount of G

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New bases always add on to the 3 end of the
molecule,
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What is a gene?

• A section of the DNA that carries information
• One gene one enzyme or one protein
• -- set of related polypeptides or enzymes

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• DNA replication is semiconservative each strand
  acts as a template for building the complimentary
  strand.
• Each new molecule contains half of the original
  DNA.

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Semiconservative replication
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• There are one or more spots on a DNA molecule
  where replication always begins.
• These are called origins of replication.
• The most studied DNA belongs to a bacterium
  called Escherichia coli.

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Bacterial DNA
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• Replication forks are the areas where the DNA
  unzips and is copied.
• Joining small molecules (like nucleotides)
  together to make large molecules is called
  polymerization, and the large molecules are
  polymers.
• DNA polymerase enzyme that joins nucleotides
• DNA always replicates by adding to the 3 end.

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• DNA polymerase needs something to add the
  nucleotides to.
• A primase copies short pieces of DNA as RNA
  primer.
• DNA polymerase then joins DNA nucleotides to the
  primer.
• When it meets the primer, it replaces it with
  DNA.
• The ends are joined by an enzyme called DNA
  ligase.

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• Eukaryotic DNA is much longer than prokaryotic
  DNA.
• Each chromosome has many origins of replication
  where DNA is copied.
• It is easy to copy the 5 to 3 or leading strand
  of DNA, but what about the 3 to 5 or lagging
  strand?

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Okazaki fragments
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• 
  
• A T G C T C G A C T A G C A A C G T A G
• T A C G A G C T G A T C G T T G C A T C

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Mutations

• A mutation is a permanent, inheritable change in
  the DNA.
• To be passed on to the next generation, this
  mutation must be present in the gametes (eggs or
  sperm).
• Mutagens are factors that speed up the rate of
  mutation formation
• Mutation rate
• Mutation hot spots

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Types of mutations

• Point mutations one to a few base changes
• Chromosomal mutations large sections of
  chromosomes are alteredPoint mutations can be
• Base substitutions
• Insertions
• Deletions

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• Chromosomal mutations can be
• deficiencies deletions of many nucleotides
• Translocations sections of DNA moved to another
  chromosome
• Inversions a section of DNA turned upside down
• Duplications large sections appear twice
• Aneuploidy

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• THEDOGSAWTHECAT
• THEDOGSAWTHYCAT
• THEDOGSAWTHECOT
• THEDOGSAWTHECAT
• A
  
• THEDOGASAWTHECAT
• THE DOG ASA WTH ECA T
• THEDOGSAWTHECAT
• THO GSA WTH ECA T
• THEDOHOWAREYOUGSAWTHECAT
  

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Why dont we see more mutations?

• Silent mutations can occur in non-coding (junk
  ) DNA.
• If we change the last codon, in many cases we get
  the same amino acid.
• We have pairs of chromosomes, so a good gene may
  cover for a bad one.
• We have about 50 enzymes that police our DNA
  looking for changes and fixing them.

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Factors that cause mutations

• Naturally occurring
• Radiation
• Chemicals
• Viruses