The Story of Our DNA.

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The Story of Our DNA.

• Chapter 11, Section 1 Notes

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Important Discoveries DNAs Function

• Frederick Griffith (1928) Transformation
• -Turned non-lethal bacteria into lethal
• -Hypothesized that a gene was responsible (He did
  NOT know what biomolecule genes were made of)

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Avery (1944)

• DNA is the molecule responsible for heredity
• a. Treated an extract from the heat-killed
  bacteria with enzymes to destroy everything
  except the nucleic acid.
• Resulted in an identical outcome to Griffiths
  experiment

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Hershey-Chase (1952)

• -Studied bacteriophages
• -Determined that the DNA not the protein coat was
  the source of genetic information

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Now that scientists agree that DNA carries the
information

• How is DNA organized?

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Chargraff (1950)

• Determined that nucleotides (the building
  blocks of DNA) occur in specific ratios
• - Guanine cytosine
• - Adenine thymine
• Did not know why this was true, only that every
  organism followed this rule

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Franklin (1950s)

• - Used x-ray diffraction
• - Three important pieces of information
• 1) Each strand is coiled (helix)
• 2) Two strands of in the structure
• 3) The nitrogen bases are near the center

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Watson and Crick (1953)

• - Used Franklins experiment to determine the 3D
  structure of DNA
• -The double helix explained both how DNA could
  carry information and how that information could
  be copied
• - Their model explained why AT and CG

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What we know now about the structure of DNA

• Double helix
• Made of a phosphate backbone
• Rungs are the nucleotide bases
• Bases attach in certain patterns
• C ? G
• A ? T
• One side is called the 3 side the other is
  called 5
• On the ends of the chromosomes in eukaryotes are
  structures called telomeres that are repeating
  bases that protect the DNA

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DNA Replication

• The process of duplicating DNA

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Commonalities in all organisms

• One side of the double helix gives the
  instructions for the other
• DNA replication occurs in both directions
• The site where DNA separation and duplication
  occurs is called a fork

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Prokaryotes

• More simple
• DNA is not in nucleus
• Plasmid single, circular DNA molecule
• Much less DNA than a eukaryote
• DNA replication begins at a single point

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Eukaryotes

• DNA is more complex
• DNA is in a membrane-bound nucleus
• DNA is wrapped tightly around proteins to form
  chromosomes
• More DNA than prokaryotes
• DNA replication occurs at hundreds of points

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How Replication Occurs

• Step 1 DNA is opened
• DNA Helicase reduces the strain caused by the
  unwinding of the DNA

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Step 2 New DNA is formed

• DNA Polymerase Helps the daughter strand form
  by matching base pairs
• Also proofreads the DNA and corrects mistakes
• Complimentary base pairs are attached to each
  side of the now single-sided DNA
• Each strand acts as a template
• New strands of DNA are ½ new and ½ original
• DNA has a north and south they are called
  5 and 3
• Base pairs are copied following the 3 to the 5
  direction of the parent strand

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• Leading strand the side of the DNA that is
  replicated faster follows the 3 ? 5 of the
  parent strand
• Lagging strand It is replicated more slowly
• Okazaki fragment the pieces that are formed in
  the lagging strand, because replication can only
  occur in the 3 to 5 direction as the DNA is
  unwound.

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Step 3 The pieces of DNA are connected

• DNA ligase attaches the Okazaki fragments
  together