Molecular Genetics

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Molecular Genetics
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DNA Review!

• Has shape of helix or corkscrew
• Is about 2 nm in diameter
• 2m of it in a nucleus!!
• Makes a complete helical turn ever 3.4 nm
• An awesome interactive review...
• http//learn.genetics.utah.edu/content/begin/tour/
  

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Three main components

• deoxyribose sugar
• a phosphate group
• nitrogenous base
• adenine
• guanine
• thymine
• cytosine

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Turn up the Base(s)!
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• DNA is composed of many nucleotides held together
  by phosphodiester bonds (therefore it is a
  polymer)
• Sugar Phosphate Sugar - Phosphate

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Structure of deoxyribose
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The Double Helix

• DNA consists of two antiparallel strands of
  nucleotides
• Bases of one strand are paired with bases in the
  other strand
• Nitrogenous base pairs are arranged above each
  other, perpendicular to the axis of the molecule
• A Purine always bonded to a pyrimidine
• Adenine with Thymine
• Guanine with Cytosine
• Termed complimentary base pairing
• Fundamental to the storage and transfer of
  genetic information

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• Purines A G
• Pyrimidines T C (also U in RNA)
• bases are bonded together by hydrogen bonds
• right-handed helix (clockwise turn) makes
  complete turn every 10 nucleotides

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Two strands of DNA run antiparallel

• One strand runs in the 5 to 3 direction while
  the other strand runs in the 3 to 5.
• The 3 end terminates with the hydroxyl group of
  the deoxyribose sugar.
• The 5 end terminates with a phosphate group

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Finding the code...

• 5 - ATGCCGTTA - 3
• 3 - TACGGCAAT - 5
• By convention, only the 5 to 3 strand is
  written since the complementary strand can easily
  be deduced ...
• Try this one...
• 5 - TGGACGCTT - 3
• 3 - ACCTGCGAA - 5

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Homework...

• Page 216 1-3, 5, 6

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DNA Replication

• During cell division in eukaryotic cells,
  replicated genetic material in nucleus divided
  equally between two daughter nuclei via mitosis
• Followed by cytokinesis in which cell is split
  into two new cells
• Mitotic cell division essential for growth of
  tissues during embryonic development and
  childhood, tissue regeneration, repair of damaged
  tissue, growth

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• Hydrogen bonds between complimentary bases can
  break, allowing DNA helix to unzip
• Each strand then acts as a template to build the
  complimentary strand
• Results in two identical DNA molecules (one for
  each daughter cell)

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The Details...

• DNA cannot be simply pulled apart due to hydrogen
  bonding
• Two parent DNA strands must be unravelled and
  kept separate.
• Specific enzymes work together to expose the DNA
  template strands.
• DNA helicase breaks hydrogen bonds between
  complementary base pairs, resulting in
  unwinding
• DNA Gyrase enzyme relieves tension of unwinding
  helix.
• Two individual strands are kept apart by
  single-stranded binding proteins (SSBs)
• SSBs bind to exposed DNA single strands and block
  hydrogen bonding. (Prevent Annealing)

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• Replication begins in two directions from
  origin(s) as a region of DNA is unwound
• Complimentary strands are built as soon as an
  area of DNA has been unwound
• As two strands of DNA are disrupted, junction
  where they are still joined is called replication
  fork
• DNA replication proceeds toward direction of
  replication fork on one strand and away from the
  fork on the other strand
• When two replication forks are quite near each
  other, a replication bubble forms

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Building the Complementary Strands

• In prokaryotes
• DNA polymerase I, II, III are the three enzymes
  known to function in replication and repair
• In eukaryotes
• Several different types of DNA polymerase are at
  work
• enzyme that builds the complementary strand using
  the template strand as a guide in prokaryotes is
  DNA polymerase III

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DNA polymerase III adds complementary nucleotides
in the 5 to 3 direction, using RNA primers as
starting points
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The Leading StrandDNA polymerase
III...

• Functions only under certain conditions
• It synthesizes DNA in the 5 to 3 direction,
  therefore adding free deoxyribonucleoside
  triphosphates to a 3 end of an elongating strand
• Requires an initial starting 3 end to commence
  elongation
• An RNA primer of 10 60 base pairs of DNA are
  annealed to the template strand since DNA
  polymerase III cannot initiate a new
  complementary strand by itself
• Primer is synthesized by enzyme primase

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• RNA primer marks initiation sequence
• DNA polymerase III can start elongation by adding
  free deoxyribonucleotide triphosphates to the
  growing complementary strand
• Free bases in nucleoplasm used by DNA polymerase
  III to build complementary strands
• DNA polymerase III uses energy derived from
  breaking the bond between the first and second
  phosphate to drive dehydration synthesis
  (condensation rxn) that adds complementary
  nucleotide to elongating strand
• Extra two phosphates recycled by cell

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• Since DNA always synthesized in 5 to 3
  direction, and the templates strands run
  antiparallel, only one strand is able to be built
  continuously.
• This is the strand which uses the 3 to 5
  template strand and is called the leading strand
  and is build towards the replication fork.

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The Lagging Strand

• The other strand is synthesized discontinually in
  short fragments and is called the lagging strand
• Okazaki fragments are short fragments of DNA that
  are a result of the synthesis of the lagging
  strand during DNA replication (eukaryotes 100-200
  nucleotides in length, 1000-2000 in prokaryotic)
• DNA ligase joins the Okazaki fragments into one
  strand by the creation of a phosphodiester bond
• The two double stranded DNA molecules are
  produced and twist into a helix automatically

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Ensuring Quality Control of New DNA Strands DNA
Repair

• As complentary strands are built, DNA polymerase
  III and DNA polymerase I proofread newly
  synthesized strands
• When mistakes occur, either enzyme can function
  as exonuclease which backtracks past the
  nucleotide on the end of the strand that is
  incorrectly paired to a nucleotide, excises it,
  and continues adding nucleotides to the
  complimentary strand
•  

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Summary Page 222...
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Watch these!

• http//www.youtube.com/watch?vteV62zrm2P0feature
  related
• http//www.youtube.com/watch?v4jtmOZaIvS0feature
  related

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Homework...

• Page 223 1,2