Bacteria in the family tree of living organisms

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Bacteria in the family tree of living organisms
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Main players of bacterial genetics
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The E. coli chromosome has been
completely sequenced
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Replica plating J. Lederbergs invention
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A mixture of bacterial cells containing
prototrophs, leu- auxotrophs and Lac- mutants is
plated in complete medium. A total of 10 colonies
grow in this plate. The colonies are replica
plated in the following 3 plates and the number
of colonies growing in each plate are Complete
medium 10 Minimal medium
6 Minimal medium with
leucine 10 Lactose minimal medium
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• How many of the original colonies are auxotrophic
  mutants?
• 1
• 2
• 3
• 4

d. is correct, 10-6 4
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A mixture of bacterial cells containing
prototrophs, leu- auxotrophs and Lac- mutants is
plated in complete medium. A total of 10 colonies
grow in this plate. The colonies are replica
plated in the following 3 plates and the number
of colonies growing in each plate are Complete
medium 10 Minimal medium
6 Minimal medium with
leucine 10 Lactose minimal medium
2

• How many of the prototrophs are Lac-?
• 1
• 2
• 3
• 4

d. Is correct, 6-2 4
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Hfr bacteria
F plasmid can integrate in several
different locations of the bacterial chromosome
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Mapping by interrupted mating experiments E.
Wollman and F. Jacob
Hfr strs thr azir tonr lac gal F- strr
thr- azis tons lac- gal-
Sample taken after 50
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• In an interrupted mating experiment an Hfr E.
  coli prototrophic strain transfers three of its
  genes at the following times to an auxotrophic F-
  strain
• Gene from Hfr Minutes
• Ala 5
• Leu 10
• Phe 15
• The order of the genes on the bacterial
  chromosome starting from the origin are
• Ala Leu Phe
• Leu Ala Phe
• Phe Leu Ala
• Phe Ala Leu

a. Is correct
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• Assuming that the Hfr strain ala leu phe is
  sensitive to streptomycin and the F- ala- leu-
  phe- is resistant to this antibiotic, what plate
  composition would you use to select for ala
  exconjugants?
• Complete medium streptomycin
• Minimal medium leu ala, no streptomycin
• Minimal medium leu phe streptomycin
• Minimal medium leu ala phe streptomycin

c. Is correct, you do not want to add alanine to
the medium since you are testing for transfer of
ala allele.
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In another interrupted mating experiment a Lac
bacterial strain with the ability to form a pilus
conjugates with a F- strain. The resulting
exconjugants become Lac and have now the ability
to form a pilus. What type of donor is the
original Lac strain?

• F
• F
• Hfr
• F-

b. Is correct, the exconjugant carries now
a chromosomal gene from the donor (Lac) and has
gained the ability to be a donor
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Lytic Cycle
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Generalized transduction
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The following genotypes are involved in a
co-transduction experiment Donor x y z
Recipient x- y- z- (gene order arbitrary, x al
left end) x is the selectable marker The
following progeny is obtained Genotype
Frequency x y z 2000 x y z-
25 x y- z 700 x y- z-
300
The distance between x and y is shorter than the
distance between x and z True False
False, distance between x and z is shorter, since
their cotransduction freq is higher than that of
x and y.
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The following genotypes are involved in a
co-transduction experiment Donor x y z
Recipient x- y- z- (gene order arbitrary, x at
left end) x is the selectable marker The
following progeny is obtained Genotype
Frequency x y z 2000 x y z-
25 x y- z 700 x y- z-
300

• What is the order of these genes?
• x, y, z
• x, z, y
• Unable to tell

b is correct, since z is closer to x (reference
or selectable marker) than y is closer to x