Transcript Document

Those that
‘do’
meiosis
Those that do not
not
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Four ways by which bacterial DNA can be transferred from cell to cell
Non-obligatory processes…
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Working with microorganisms:
methods of growing bacteria in the laboratory
Clonal, micro to macro,
… see a single molecular
event’s consequences
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Bacterial colonies on staining medium
Red colonies contains wild-type
bacteria able to use lactose an
energy source (lac+)
The unstained cells are mutants
unable to use lactose (lac-)
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Model organism Escherichia coli
Antonie van Leeuwenhoek 1632-1723
First to see bacterial cells and
appreciate their small size
“there are more living in the scum on the teeth in
a man’s mouth then there are men in the whole kingdom”
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Model organism Escherichia coli
Named after its discoverer
Theodore Escherich (1857-1911)
pili
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Joshua Lederberg
1925 – 2008
Edward Lawrie Tatum
1909-1975
In certain bacterium
there
was a type of sexual cycle
including a crossing over
like process
Nobel Prize
in Physiology
or Medicine in 1958
I.
II.
III.
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Bacterial conjugation
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Physical contact between bacterial cells is required for genetic recombination
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Lederberg and Tatum’s demonstration of genetic recombination
between bacterial cells
B+
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A+
Lederberg and Tatum’s demonstration of genetic recombination
between bacterial cells
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F+
F-
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F+
+
FF
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Formation of high frequency recombinant strain (Hfr)
F+
Hfr
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Bacterial conjugation and recombination
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Interrupted-mating conjugation experiments
Francois Jacob
1920 (age 92)
Elie Wollman (1917-2008)
1965 Nobel Prize in Medicine
with Jacques Monod and André Lwoff
Originated the idea that control of enzyme levels
in all18 cells occurs through feedback on transcription
Interrupted-mating conjugation experiments
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Interrupted-mating conjugation experiments
The point
21 O is now known to be the site at which the F plasmid is inserted
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Time of entry mapping is not based on recombinant frequency
the units are minutes, not RF
The Hfr chromosome, originally circular, unwinds and is transferred
to F- cell in a linear fashion, with the F factor entering last
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Insertion of the F factor into the E. coli chromosome by crossing over
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“One ring to rule them all, One ring to find them,
One ring to bring then all and in the darkness to bind them”
J. R. R. Tolkien, The Lord of the Ring
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Fine scale mapping by recombinant frequency
Incomplete genome
Complete genome
Merozygote
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To keep the circle intact there must be
an even number of crossovers
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Only one of the reciprocal products survives
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Don’t
“LAST-IN MAPPING”
Second technique
for mapping - at
high resolution
galactose
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Transferred fragment leu+
of Hfr chromosome
arg+
met+
leu-
arg-
met-
F- chromosome
To examine the recombination of these genes must select for “trihybrids”
exconjugants that have received all three donor markers
To do this, we must first select stable exconjugants bearing
the last donor allele, which in this case is leu+
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First select leu+ exconjugants and then isolate and test
a large sample of these to see which of the other markers were integrated
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4m.u
4%
9m.u
9%
4m.u
9m.u
87%
Rarely recovered
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F’ - duction
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START HERE
F’
F-
F’-duction
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