Biology Assignment
Question 1
Griffith's experiment has been summarized for you to help you answer the questions below:
Frederick Griffith showed that a component of dead bacterial cells could confernew properties to live bacteria of the same species. The property in question was the presence of a surface polysaccharide capsule of the bacterium Streptococcus pneumoniae, the bacterium that causes pneumonia.
Two types of these bacteria are found, based on two main types of surface polysaccharide:
1) R (rough) cells have no polysaccharide capsule. These bacteria do not cause disease. They are nonvirulent.
2) S (smooth) cells have a polysaccharide capsule. These bacteria cause disease and are virulent.
The S (smooth) cells fall into several different subtypes based upon the polysaccharide capsule. These are designated SI through SXXIII.
You can isolate mutants of all these S strains that no longer make a capsule and are no longer virulent. The lack of a polysaccharide capsule makes one R strain indistinguishable from another. However, R strains are also designated R(I) through R(XXIII), depending upon their origin. An R strain derived from SIII is designated R(III).
The central experiment was:
Experiment: Inject a mixture of R(II) and heat-killed SIII into a mouse.
Results: The mouse dies of pneumonia, and virulent SIII bacteria can be isolated fromthe mouse.
Griffith et al. also did some control experiments to support experiment 1.
Negative Control: Inject R(II) or heat-killed SIII alone into a mouse.
Results: The mouse lives.
Positive Control: Inject live SIII alone into a mouse.
Results: The mouse dies of pneumonia, and only virulent SIII bacteria can be isolatedfrom the mouse.
a) For each of the control experiments listed, explain:
• What alternative models does the result of this control experiment rule out? That is, complete the sentence, "Without this control result, you could argue that the mice died in experiment 1 because...."
• What would it have meant for their model if the results of this control experiment were the reverse of those expected (the mouse dies instead of living and vice versa), assuming that the results of experiment 1 were the same as above?
b) Another potential problem with their experiments was the possibility that, at a low frequency, R(II) can mutate back to the SII from which it was derived. (Note: R(II) cannot revert to any other subtype of S.) Based on this, why was it essential that they use a mixture of R(II) and heat-killed SIII instead of a mixture of R(II) and heat-killed SII?
Question 2: Print out this page, and fill out using a pen/pencil.
DNA replication involves many different enzymatic activities. Match each enzyme/proteinlisted below with the function(s) that it has in the DNA replication process.
Enzyme/Protein Function Choose from:
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Enzyme/Protein
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Function
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Choose from:
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Topoisomerase
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a)
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3'→5'growth of new DNA strand
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Primase
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b)
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Prevent hydrogen bond formation
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DNA polymerase that only is able to correct errors
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c)
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3'→5' exonuclease activity
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Helicase to unwind DNA
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d)
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5'→3' exonuclease activity
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DNA polymerase to replace RNA with DNA
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e)
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Makes RNA primer complementary to the lagging strand
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Single stranded binding proteins
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f)
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Makes RNA primer complementary to the
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g)
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Adds nucleotides
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h)
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Separates the two DNA strands
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i)
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Maintains DNA polymerase on template
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j)
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Provides 3' hydroxyl for initiation of DNA Polymerization
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k)
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Untangles super-coiled DNA
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