Chapter 12 DNA & RNA

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Chapter 12 DNA & RNA Powered By Docstoc
					Chapter 12

DNA & RNA
             1
          12-1 DNA
• How do genes work?
• What are they made of?
• How do they determine
  characteristics of organisms?
• In the middle of the 1900s, questions
  like these were on the minds of
  biologists everywhere.
                                     2
Griffith and Transformation
• 1928: British scientist Frederick
  Griffith was trying to figure out how
  bacteria make people sick.

• Griffith wanted to learn how certain
  types of bacteria produce a serious
  lung infection known as
  pneumonia.
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Frederick Griffith




                     4
       What did he do?
• isolated two slightly different strains of
  pneumonia bacteria from mice
• Both strains grew very well, but only one of
  the strains caused pneumonia.
• disease-causing strain of bacteria = smooth
  colonies
• harmless strain = colonies with rough edges


                                            5
 Griffith's Experiments
• injected mice with the S bacteria 
  pneumonia and died.
• mice injected with R bacteria  totally
  healthy.
• Griffith wondered if the disease-causing
  bacteria might produce a poison.


                                         6
• Heat killed S bacteria into mice  Healthy
  mice!
  – The mice survived, suggesting that the cause of
    pneumonia was not a chemical poison released
    by the disease-causing bacteria.
• Heat killed S bacteria + R bacteria in mice 
  DEAD mice!
  – He found their lungs filled with the deadly S
    bacteria.
  – Some factor from the dead bacteria had
    “transformed” the harmless bacteria into
    disease-causing ones.
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            Transformation
• process in which one strain of bacteria is
  changed by a gene or genes from another
  strain of bacteria
• Griffith hypothesized since the ability to cause
  disease was inherited by the transformed
  bacteria's offspring, the transforming factor
  might be a gene (genetic material)


                                                9
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• Although Griffith made a great discovery,
  he did not follow through and figure out
  what actually transformed the bacteria.

• Avery came along and did that.




                                              11
            Avery and DNA
• 1944, a group of scientists led by Canadian
  biologist Oswald Avery at the Rockefeller
  Institute in New York decided to repeat
  Griffith's work.

• to determine which molecule was the genetic
  material responsible for transformation.


                                                12
• Oswald Avery




                 13
          Avery’s experiment
• Made an extract from the heat killed bacteria
• Used enzymes that destroy the lipids,
  carbohydrates, proteins, and RNA.
  – Transformation still occurred.
• Used DNA destroying enzyme.
  – Transformation did NOT occur.
  DNA must be the genetic material.
Avery and other scientists discovered that the
 nucleic acid DNA stores and transmits the
 genetic information from one generation of an
 organism to the next.                            14
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• Scientists are skeptical.

• Scientists don’t always believe things
  with proof from only one experiment…

• So other scientists set out to prove
  what the genetic/hereditary
  information of an organism is.

                                         16
      Hershey & Chase
• Alfred Hershey and Martha
  Chase
• 1952, American scientists
• Studied viruses (non living
  particles)

                                17
Martha Chase and Alfred Hershey




                              18
 The Hershey-Chase Experiment
• Used bacteriophages (virus that infects
  bacteria)
• composed of a DNA or RNA core and a
  protein coat
• When a bacteriophage attacks a
  bacterium, it injects its genetic
  information into the bacterium.
• Those genes take over the cell, producing
  many new viruses.
                                          19
Bacteriophage




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   Nucleic Acid or protein??
• They wanted to find out which part of the
  phage (DNA or protein), produced new
  phages.
• Grew bacteriophages with radioactive
  markers
  – phosphorus-32 (32P) only in DNA
  – sulfur-35 (35S) only in Protein


                                              23
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• mixed the marked viruses with bacteria

• waited a few minutes for the viruses to
  inject their genetic material.

• separated the viruses from the bacteria
  and tested the bacteria for radioactivity.

• Nearly all the radioactivity in the bacteria
  was from phosphorus (32P), the marker
  found in DNA.                              25
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• Hershey and Chase
  concluded that the genetic
  material of the
  bacteriophage was DNA,
  not protein.

                          27
The
Components
and Structure
of DNA



           28
           DNA components
• DNA is a long molecule made up of units
  called nucleotides.
  – Nucleotides are made up of three basic
    components:
    • 5-carbon sugar (deoxyribose)
    • a phosphate group
    • a nitrogenous base


• 4 kind of nitrogen bases found in DNA
    • adenine (A), guanine (G) = double ringed (purines)
    • cytosine (C), thymine (T) = single ringed (pyrimidines)
                                                         29
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            DNA structure
• backbone of a DNA chain = sugar and
  phosphate groups of each nucleotide.
• The nitrogenous bases stick out sideways
  from the chain.
• The nucleotides can be joined together in
  any order
  – any sequence of bases is possible.
  – With 4 bases – millions of different
    combinations are possible
                                           31
DNA




      32
            Erwin Chargaff
• American biochemist, 1940s
• discovered that the percentages of
  guanine [G] and cytosine [C] bases are
  almost equal in any sample of DNA
  – Later found that [A] = [T]


• Scientists had NO idea what this was…


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Chargaff's Rules




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          Franklin & Wilkins
• British scientists, 1952
• used a technique called X-ray diffraction to get
  information about the structure of the DNA
  molecule
• X-shaped pattern shows that the strands in
  DNA are twisted around each other like the coils
  of a spring
  – a shape known as a helix
  – the X suggests that there are two strands in the
    structure
  – Other clues suggest that the nitrogenous bases are
    near the center of the molecule
                                                     35
  Franklin     Wilkins




X-shaped DNA
(from X-ray
Diffraction
                         36
             Watson & Crick
• Francis Crick, a British physicist &
  James Watson, an American biologist
• trying to understand the structure of DNA
  – by building three-dimensional models of the
    molecule
• 1953, they are shown a picture of Franklin’s
  x-ray and immediately knew the structure
• Watson and Crick's model of DNA was a
  double helix, in which two strands were
                                             37
  wound around each other.
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              Double Helix
• Twisted ladder or spiral staircase
• realized that the double helix accounted for
  many of the features in Franklin's X-ray
  pattern
• did not explain what forces held the two
  strands together.
• discovered that hydrogen bonds could form
  between certain nitrogenous bases and
  provide just enough force to hold the two
  strands together                            42
43
                 Base Pairing
• hydrogen bonds can form only between
  certain base pairs
• adenine (A) with thymine (T)
• guanine (G) with cytosine (C)




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