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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Nucleotides monomers for DNA
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Phosphate Sugar (deoxyribose)
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Chargaffs Rules
The amount of A amount of T The amount of C
amount of G This is because A and T are
always paired together on opposite strands (so
are G C).
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DNA is a double helix A helix is a twisted
molecule. DNA is two helices held together by
hydrogen bonds.
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DNA strands have a directionality, like a pencil,
and they run in opposite directions antiparallel.
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Review of directionality in DNA
New bases always add on to the 3 end of the
molecule
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Bases held together by H bonds G-C held w/ 3 A-T
held w/ 2
The two DNA strands twist around an imaginary
axis that runs up the center of the molecule.
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DNA and information flow

• Central Dogma
• Genetic information is encoded into the structure
  of the DNA molecule
• To use this information, DNA is copied into RNA
  (transcription), and the RNA is used to direct
  proteins synthesis (translation).
• For a cell to reproduce, the DNA molecule has to
  be copied DNA replication.

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Semiconservative replication
Each new DNA molecule is made of one old strand
and one new one.
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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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Prokaryotic DNA has one origin where replication
starts. Eukaryotic DNA has multiple origins.
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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Correct complementary base is added to new
growing DNA strand.
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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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• 
  
• 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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3
5
3
3
5
3
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In answer to a question a piece of DNA is
flipped.
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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
• Certain chemicals, radiation

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Types of mutations

• Chromosomal mutations large sections of
  chromosomes are altered
• Point mutations one to a few base changes Point
  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

http//users.rcn.com/jkimball.ma.ultranet/BiologyP
ages/T/Translocation_8-14.gif
translocation
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Review of how aneuploidy occurs
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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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Trivia about mutations

• Mutations are rare
• There are some hot spots, but any particular
  gene has about a 1 in 10 million chance of
  mutating.
• Mutations are generally harmful
• Genes that work the best have evolved, so usually
  a change is either neutral or harmful.

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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