DNA

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DNA

• Replication of the genetic code

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DNA recipe book

• Instructions for ALL proteins are encoded by DNA
• DNA resides in the nucleus
• To pass on instructions for life, need to
  replicate DNA prior to reproduction

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How do we know DNA is the genetic code of life?

• Some late 19th century observations of dividing
  cells gave us some clues

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Observations

• Late 1800s Walther Flemming sees threads
  moving changing during cell division
• Threads appear paired prior to cell division
• Paired threads separate just prior to division
• Named the thread separating process Mitosis

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Observations on thread

• Thread differs between species
• Roundworms 4 Peas 14 Humans 46
• Thread identical between individuals within a
  species
• All roundworms 4
• Same between cells within an individual
• Threads were named chromosomes which consist of
  both DNA protein

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Is it DNA or proteins that are important?

• Chromosomes consist of both, so how did
  scientists identify which one holds instructions
  for reproduction of cells?

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DNAs discovery

• 1952 Hershey Chase find that bacteriophage
  virus infects and reprograms bacteria to make
  more virus
• Consists only of external protein coat and
  internal DNA
• Inserts its DNA into bacteria, protein coat
  remains outside
• A perfect model!

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Label DNA protein separately

• Radioactive Sulfur incorporates into proteins
  only. Why?
• Heavy bacterial cells settle, while lighter phage
  particles remain in solution. (wheres the
  radioactivity?)

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Label DNA protein separately

• Radioactive Phosphorous incorporated into DNA.
  Why?
• Heavy bacterial cells settle, while lighter phage
  particles remain in solution. (wheres the
  radioactivity?)

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DNA encodes instructions for replication!
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DNAs structure

• Whats it look like? Does its structure suggest
  how replication is accomplished?

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Monomers of DNA Nucleotides

• Repeated phosphate, sugar, base motif of ALL
  nucleotides
• Phosphate-sugar backbone
• Base only difference between nucleotides

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Nitrogenous Bases

• Purines G, A 2 nitrogenous rings
• Pyrimidines C, T 1 nitrogenous ring

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Who discovered the structure?

• J. Watson F. Crick deduced double-stranded,
  helical structure from Rosalind Franklins X-ray
  crystallographic image of a DNA molecule.

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Chemical structure
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Conclusions

• Molecule is of uniform width
• Amounts of A T are identical same for C G
• H-bonds hold bases of neighboring strands
  together
• suggests precise complimentarity between
  nucleotides
• Adenine always pairs with Thymine Cytosine
  always pairs with Guanine

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Extensions

• Sequence possibilities are limitless (variation
  in sequence could account for the diversity of
  life.)
• Those threads (chromosomes) we saw separating
  with dividing cells must be DNA molecules

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Structure also suggests mechanism of replication

• Pull strands apart now each strand serves as
  template for a new strand
• Semiconservative model ½ parent molecule is
  conserved in each daughter molecule

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Replication

• Begins _at_ multiple replication centers
• Helicase unwinds and separates DNA strands
  (bubble)
• DNA polymerase adds bases opposite the template
  (parent strand)

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Structure determines direction

• Strands are anti-parallel
• Each has a 5 and a 3 end
• Refers to Carbon atom in sugar ring (i.d.
  purposes)
• DNA polymerase can only add nucleotides to the 3
  end of a strand

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Consequences of polarity

• One strand is continuously replicated
• The other is replicated in fragments (Okazaki
  fragments)
• DNA ligase joins these fragments to complete the
  new molecule
• Other polymerases proofread edit