DNA Structure and Function

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I bet that you can read this information but not
the one above.
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The requirements for an information system or a
code
There must be at least 2 signals The order of
the signals is important The grouping of the
signals is important
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DNA Structure and Function
Nucleotide Sugar Phosphate Nitrogen Base
Nitrogen Bases Purines Adenine and
Guanine Pyrimidines Cytosine and Thymine
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Functions of DNA

• Information to make proteins
• Copy itself (must pass information on to new
  cells

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Structure Function Relationships
Base Pairs Why do Purines only bond with
Pyrimidines? Why does only A pair with T and G
pair with C
Bonding Why is the backbone bonded together with
covalent bonds? Why are the base pairs hydrogen
bonded
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Determination of DNA Structure

• Rosalind Franklin develops new x-ray
  crystallography technique to get a clear picture
  of DNA
• In 1953 Watson and Crick
• published the structure of DNA
• using Franklins
• data without
• her knowledge

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What the X-ray Crystallography Looked Like
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DNA is obviously the information molecule It
plays both a genetic role and a hereditary role

• How much information is there in the DNA of a
  cell?
• 6 x 109 base pairs per cell
• 0.34 nm (10-9m) between base pairs
• The length of DNA/cell
• 2 m

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There is a lot of DNA in one cell!

• Amount of DNA in 1 cell is equal to 30 complete
  sets of Encylopedia Brittanica
• If the nucleus was the size of a tennis ball,
  there would be 4 miles of DNA in it.
• How do you fit 2 m of DNA in a 1x10-5 nucleus?
• Compaction Ratio
• 20m thread/.02m capsule
• 1000 x compacted
• 2m DNA/1x10-5 m nucleus
• 200,000 x compact
• ????????
• DNA is arranged into chromatin wrapped around
  proteins to pack it in

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Chromatin

• Chromatin 2 m of DNA must fit in a 1x10-5 m
  nucleus. DNA wrapped around proteins to organize
  it and allow it fit into the nucleus
• Remember it is condensed 200,000 x to fit in
  the nucleus
• It is still loosely coiled enough that enzymes
  can get into the DNA to copy it and make mRNA for
  protein synthesis
• It is the normal form of DNA during all phases of
  the cell cycle except mitosis

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Chromatin
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Chromosomes

• DNA compacted 12,000 times from chromatin
• Cannot read or copy the DNA in chromosomes it
  is too tightly wound
• Formed solely during mitosis in order to divide
  the doubled DNA in ½
• Protects the DNA when the nucleus breaks down in
  mitosis

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A Real Chromosome
DNA released from a chromosome
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What is the relationship between DNA, genes,
chromosomes, and the genome?

• A gene is a segment of DNA that codes for a
  protein
• There are thousands of genes on each chromosome
  which is a folded up piece of chromatin
• All of the genes on all of the chromosomes is the
  genome

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The Cell Cycle
Makes a few proteins to get ready for mitosis
2 new cells form
Hours to days Making proteins Normal cell
functions
Growth 1
Growth 2
G0
Makes an exact copy of DNA 3-6 hours
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DNA Replication is Semi-Conservative
A - T G - C G - C C - G A - T T - A T - A C - G
A G G C A T T C

• - T
• C
• - C
• - G
• - T
• - A
• - A
• - G

T C C G T A A G
A - G - G - C - A - T - T - C -
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Replication Problems to Overcome

• If you open all 2 m of DNA it will randomly base
  pair and tangle up
• How do you copy 6x109 base pairs in 3-6 hours

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Steps of Replication

• Enzymes open the DNA at Origins of Replication
  (there are many on each piece of DNA)
• Each origin makes 2 replication forks
• Helicase begins to unzip DNA
• Many DNA polymerase enzymes add the complementary
  nucleotides (they form covalent bonds between the
  sugar of 1 nucleotide and the phosphate of
  another)
• DNA proof-reading enzymes check for mistakes
  (this happens as base pairs are added)

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DNA Mutations change in 1 or more nucleotide
nitrogen bases in the DNA

• Errors in replication DNA polymerase makes an
  error about 1000 base pairs
• DNA proofreading enzymes attached to the
  polymerase correct the mistakes
• There are usually about 3-6 permanent
  mistakes/replication

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How can mutations occur

• A mistake in replication not picked up by the
  proofreading enzyme
• Environmental insults
• Radiation (UV, X-rays, radioactive molecules)
• Toxic chemicals in air, food, cigarette smoke

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How can mutations affect cells and organisms?

• It may not change anything in the cell at all
• If nucleotides change, repair enzymes may fix
  them
• If nucleotides are changed and repair enzymes
  dont fix them, the cell should undergo apoptosis
  (cell suicide) before it enters S phase.

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How can mutations affect cells and organisms?

• If the change isnt picked up by repair enzymes
  and the cell doesnt undergo apoptosis
• It can make a cell make a mishappen protein
• That protein can be a little or a lot mishappen
• Depending on how mishappen and how important that
  protein is to the cell will determine if the cell
  malfunctions or not
• If it malfunctions the cell may die or it could
  become cancerous

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How can mutations affect cells and organisms?

• Unless it becomes cancerous the malfunctioning
  cell will not change or affect the organism as a
  whole!
• In order for a mutation to have an effect on the
  organism as a whole it must occur in the sperm,
  egg, or embryo then every cell of the
  offsprings body will have the mutation so it
  will actually change the trait of the organism.

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Effects of DNA Mutations on Organisms (if in
sperm or egg)

• No effect
• Change is not in a gene
• Still codes for same aa
• aa change doesnt change protein folding
• Negative Effect
• Significantly changes protein shape so that a
  function cannot be performed or some body part
  is misshapen etc.
• Positive Effect
• The change in the protein shape gives the
  organism a survival advantage