The History, Structure, Function, and Applications of DNA

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Unit 7

• The History, Structure, Function, and
  Applications of DNA

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The History of DNA

• It took a lot of different scientists a long time
  to figure out that DNA is the molecule
  controlling inheritance of genetic traits

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• Soon after chromosomes were discovered,
  scientists were able to grind them up and learn
  that they were about 50 protein and 50 nucleic
  acid - which is DNA.

DNA
Protein
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• Frederick Griffith
• 1928
• Experimented with Streptococcus pneumoniae, a
  bacterium that causes the lungs to fill up with
  fluid.
• identified two strains
• Smooth (S) strain Streptococcus
• Rough (R) strain Streptococcus

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• S strain bacteria appear smooth under the
  microscope because they have a slimy mucus
  coating outside their cell walls.
• This makes them much harder to cough up or for
  the immune system cells to attack.

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• R strain bacteria appear rough under the
  microscope because they dont have the mucus
  coating.

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• Injected S strain into mice
• mice died
• conclusion S strain is lethal
• Injected R strain into mice
• mice survived
• conclusion R strain harmless
• Prediction The bacteria must have the genetic
  ability to make mucus to be lethal.

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• Injected mice with boiled, heat-killed S strain
• prediction mice would survive because the
  bacteria were dead.
• observation mice survived
• conclusion Bacteria must be smooth, alive, and
  reproducing to cause the mice to die.

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• Injected mice with a mixture of dead S and living
  R bacteria
• prediction mice would survive
• observation mice died
• conclusion A new question - what happened?

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• Examined blood samples from the mice that died
  after injection with mixture of dead S and living
  R bacteria
• observation found living S bacteria
• conclusion living R are able to absorb a
  transforming factor from the dead S and transform
  themselves into S bacteria
• conclusion this transformation is passed to new
  bacteria as they divide, so it must be genetic
  material.

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• Avery and Macleod
• 1944
• continued Griffiths experiment
• concluded that the transforming factor was
  probably DNA, but their evidence was not widely
  accepted by other scientists.

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• Hershey and Chase
• 1952
• experimented with bacteria and viruses that
  infect bacteria called bacteriophages
• knew that bacteriophages are made off only two
  things, DNA and protein

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• Hershey and Chase experiment
• prediction bacteriophages must inject their
  genetic material into their host bacteria cells
  in order to reproduce.
• This genetic material must be either the protein
  or the DNA

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• Experimental Group 1.
• grow bacteriophages and feed them radioactive
  sulfur
• this will produces bacteriophages with
  radioactive protein only, since DNA contains no
  sulfur
• Allow the bacteriophages with radioactive protein
  to infect host bacteria
• observation the radioactive protein did not get
  inside the host bacteria
• conclusion the protein is not the genetic
  material

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• Experimental Group 2.
• grow bacteriophages and feed them radioactive
  phosphorus
• this will produces bacteriophages with
  radioactive DNA only, since protein contains no
  phosphorus
• Allow the bacteriophages with radioactive DNA to
  infect host bacteria
• observation the radioactive DNA was injected
  inside the host bacteria
• conclusion the DNA is the genetic material

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• Hershey and Chase proved without question that
  DNA is the genetic material, not protein.

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• Edwin Chargaff
• 1950s
• analyzed the DNA of different animals to figure
  out if the proportions of adenine, thymine,
  guanine, and cytosine in their DNA could be used
  to tell them apart.

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• Chargaffs observations
• the amount of adenine in any animals DNA is
  always equal to the amount of thymine
• the amount of guanine in any animals DNA is
  always equal to the amount of cytosine
• This is called Chargaffs Rule
• amount of A amount of T
• amount G amount of C

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• Rosalind Franklin and Maurice Wilkins
• used x-ray crystallography to photograph DNA
  crystals
• produced a diffraction pattern
• were able to measure the width and distance
  between repeats in the DNA molecule
• concluded that the DNA molecule was a certain
  width with regular repeats

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• James Watson and Francis Crick
• Feb 28,1953
• figured out the double helix shape of the DNA
  molecule

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• Watson and Crick were the first to put all the
  pieces together
• from Chargaff they learned that DNA was made of
  two purines (adenine and guanine) and two
  pyrimidines (thymine and cytosine)
• Chargaffs rules meant that their was a
  relationship between A and T, C and G

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• From Franklin and Wilkins they learned that DNA
  was long, skinny, and the same width all the way
  down

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• The Double Helix Model
• DNA is made of two parallel strands
• The two strands are held together on the inside
  by hydrogen bonds between A and T and hydrogen
  bonds between G and C
• The two strands twist together in a spiral or
  helix
• The outside of each strand is made up of
  alternating deoxyribose and phosphate groups.

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Hydrogen bonds
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• Beadle and Tatum
• 1941
• experimented with mutated molds
• discovered that a single mutated gene produces a
  single mutated enzyme
• mutated enzymes dont work properly and can cause
  disease

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• The one gene-one protein idea
• Every different protein or enzyme made in the
  cell must have its own unique gene stored in the
  genetic material.
• defective genes make defective proteins, and this
  can cause genetic diseases like
• cystic fibrosis
• sickle cell anemia

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The Structure of DNA

• DNA is a double helix molecule
• Each strand is complementary to the one across
  from it
• A only pairs across from T
• G only pairs across from G

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• DNA Replication
• DNA replication copies DNA
• occurs during the S part of the cell cycle
• occurs inside the nucleus
• Each strand acts as a template for the newly
  forming strand
• Enzymes work like machines to replicate DNA

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• DNA replication- how the enzymes do it
• 1. DNA helicase unwinds the helix and unzips the
  two strands by breaking the weak hydrogen bonds
• 2. DNA polymerases attach to each side and begin
  adding complementary nucleotides
• DNA ligases seal the phosphate to sugar bonds

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The Function of DNA

• So far we have covered
• the history of scientific research proving DNA is
  the molecule storing hereditary information
• the structure of DNA and how it copies itself
• Now we will investigate exactly how DNA codes for
  proteins.

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• DNA controls heredity by coding for how proteins
  are made
• enzymes operate cell metabolism
• examples catalase, amylase, sucrase, proteases,
  polymerases, etc.
• structural proteins build many cell parts
• example keratin builds hair
• example actin and myosin build muscle

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• DNA is like a set of recipes the cell uses to
  manufacture just about everything involving
  proteins
• Organisms inherit slightly different recipes,
  therefore their proteins are slightly different
• Protein synthesis is the manufacture of proteins
  according to recipes.

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• Protein Synthesis
• takes place in the cytoplasm
• actual work is performed by ribosomes
• Ribosomes are made of RNA

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• RNA
• made of 4 RNA nucleotides
• adenine, guanine, cytosine, and uracil
• single-stranded NOT double stranded
• contains the sugar ribose instead of deoxyribose.

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• Protein Synthesis takes place in two steps
• transcription - inside nucleus
• translation - in cytoplasm

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• Transcription
• A section of DNA containing a gene (protein
  recipe) unwinds and unzips
• RNA polymerase builds a strand of RNA
  complementary to the DNA

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• Transcription produces a strand of RNA
  complementary to the DNA called mRNA
• mRNA is a temporary copy of the gene
• mRNA can leave the nucleus through pores in the
  nuclear membrane
• mRNA can be read by the ribosomes in the
  cytoplasm to make proteins.

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• Translation
• takes place in the cytoplasm
• a ribosome attaches to the mRNA
• as the ribosomes slides along, the mRNA strand is
  read in 3-base long words called codons.
• Each codon specifies only one amino acid
• Each codon is matched to a complementary
  anticodon on a tRNA molecule
• each tRNA molecule carries the correct amino acid

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• Building a polypeptide chain
• mRNA codons are translated to tRNA anticodons
• as the tRNAs line up side by side on the
  ribosome, they deliver amino acids in sequence
• Each new amino acid attaches to the one before it
  with a peptide bond
• The chain of amino acids is called a polypeptide

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• Polypeptides are folded into proteins
• after completion, most polypeptide chains are
  moved inside the RER
• inside the RER, the polypeptide chain is folded
  into a three-dimensional shape
• The shape of a protein determines its function
• enzyme proteins must be folded to produce their
  active sites

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• The codon chart
• each codon specifies one and only one amino acid
• Examples
• UUU codes for phenylalanine (phe)
• GUG codes for valine (val)

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• Some amino acids have up to six different codons,
  while others have only one
• Serine(ser) can be specified by the codons UCU,
  UCA,UCG,UCC, AGU, and AGC
• Tryptophan has only one codon - UGG

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• Signal Codons
• RNA polymerase needs a signal to know where to
  start translating the mRNA strand, and a signal
  to stop translating
• AUG is the start codon that begins the
  polypeptide chain with Methionine (met)
• UAA, AUG, and UGA are stop codons that signal
  the ribosome to let go of the mRNA strand.

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Mutations

• Mutations occur when the DNA genes sequence of
  base pairs is changed by either a mistake during
  replication or by a random chemical reaction that
  damages the DNA
• Mutations can be harmful, helpful, or silent

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• Harmful mutations cause a change in the DNA that
  produces a defective protein.
• Helpful mutations are very rare, and cause a
  change in the DNA that produces a better protein
• Silent mutations cause a change in the DNA that
  has no effect on the protein

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• A point mutation occurs when a single base pair
  is removed and a different base pair is
  substituted.
• Another name for a point mutation is a
  substitution
• Sickle Cell Anemia is caused by a single mutation
  that changes the codon from GAG to GUG

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• A frame-shift mutation occurs when a base pair is
  deleted or an extra base pair is added.
• frame-shift mutations are caused by deletions or
  additions to the gene
• after a frame-shift mutation, all the codons
  downstream from the mutation will be read wrong
  by the ribosomes during translation.
• frame-shift mutations are always harmful.

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• Sentence analogy to show a frame-shift mutation
• THE CAT ATE THE RAT normal
• _HEC ATA TET HER AT deletion
• THE CAT AAT ETH ERA T addition

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• Inversion mutations occur when a section of a
  gene is cut out and re-inserted into the gene
  backwards in the same location

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• A transposition mutation occurs when a section of
  a gene is cut out and re-inserted somewhere else.
• Indian corn kernels are different colors because
  of a transposition mutation

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• A repetitive sequence mutation produces a stretch
  of DNA that repeats a series of base pairs over
  and over.
• Huntingtons Disease is caused by the repetitive
  sequence mutation CAG
• 10-15 repeats normal
• 16-35 repeats mild symptoms
• 35 repeats fatal disease

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Mutagens

• Mutagens are environmental factors or chemicals
  that cause mutations in DNA
• Mutagens that cause mutations in genes regulating
  cell division can cause cancer
• Mutagens that cause cancer are known as
  carcinogens

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• Carcinogens increase the risk of cancer
• cigarette smoke
• saccharine
• asbestos
• Ultraviolet (UV) radiation
• X-rays

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DNA Technology

• Scientists have used what they know about DNA to
  change medicine, industry, and agriculture
• Scientists can now change genetic code
• Genetic engineering is the manipulation of
  genetic code to produce combinations of genes

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• Recombinant DNA is one kind of genetic
  engineering
• DNA from different animals or plants is
  recombined to produce completely new
  combinations
• bacteria, animals or plants are given metabolic
  abilities they never had before
• This technology is very controversial

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Scientists can now grow recombinant tobacco
plants that glow in the dark because a gene from
fireflies was added to their genome.
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• Soybeans have been genetically modified so that
  they are not killed by Roundup, a popular brand
  of herbicide

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• Examples of recombinant DNA
• bacteria have been engineered to manufacture
  human insulin for diabetics

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• How recombinant DNA techniques were used to
  create insulin-producing bacteria
• 1. locate the insulin gene that you want to move
  to the bacterial cell
• 2. cut out the insulin gene using restriction
  enzymes
• 3. locate a plasmid from a bacterial cell and cut
  it with the same restriction enzymes
• 4. mix the insulin gene and the plasmid DNA

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• 5. Add DNA ligase to the mixture to bond the
  target gene into the bacterial plasmid DNA
• 6. Insert the recombinant plasmid into living
  bacteria.

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Gel Electrophoresis

• Gel electrophoresis is a way to analyze DNA by
  cutting it up into fragments, then sorting the
  fragments according to size.
• Electricity causes the fragments to sort
  themselves in a pan of special gel
• Restriction enzymes cut up DNA by breaking it at
  very specific places only.

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• DNA fingerprinting
• get two samples of DNA that you want to compare
• cut up each sample using the same restriction
  enzyme
• run each sample through gel electropheresis side
  by side
• compare the banding patterns

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Suspect 2
Crime Scene Specimen
Suspect 1