Download Binary Fission • Bacteria do not undergo mitosis or meiosis • Instead

Bacterial Reproduction | OAT
But it is not the
bacterium’s
chromosome
o The plasmid is separate
from the chromosome
and its presence is not
critical for survival
Binary Fission
• Bacteria do not undergo mitosis
or meiosis
• Instead, they undergo a process
known as binary fission
• In the last chapter, we discussed
DNA replication in prokaryotes:
(a)
(a)
(b)
Prokaryotic DNA Replication
Prokaryotic DNA is round. In (a), the origin of
replication is shown up top. The origin hosts two
replisomes which will (b) work in opposite
directions around the circle to replicate the DNA.
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In binary fission, this method of
DNA replication will be used
After the DNA has been
replicated, the cell splits into two
daughter cells
o Each daughter gets one
copy of the DNA
So, binary fission produces two
genetically identical daughters
Conjugation
• Though binary fission is the only
means of bacterial cell division,
there are 3 ways that bacteria can
undergo genetic recombination
o In other words, there are
3 ways that bacteria can
share their DNA
• The first of these is called
conjugation
• Conjugation requires two
bacteria, one of which must have
a plasmid
o A plasmid is a small ring
of bacterial DNA
(b)
Bacteria With and Without Plasmids
(a) Some bacteria have plasmids (shown by the
arrow), while (b) other don’t. For conjugation,
we will need one bacterium with a plasmid and
one bacterium without a plasmid.
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The plasmid must be a
conjugative plasmid, or a
plasmid that contains the gene
for the sex pilus
The sex pilus is a tube that
extends from one bacterium and
permeates another
o It allows the transfer of
genetic information
Conjugation
With an assembled sex pilus, the plasmidcontaining cell can replicate its plasmid and
transmit the copy to the other cell.
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© 2017 J Co Review, Inc., Accessed by Guest on 06-15-2017
Bacterial Reproduction | OAT
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Once the sex pilus is constructed,
the plasmid-containing cell can
duplicate its plasmid
o And that copy can be
transmitted to the other
cell
Two common conjugative
plasmids are the F plasmid and
the R plasmid
o A bacterium containing
the F or R plasmids is F+
or R+ respectively
o A bacterium lacking
those plasmids is F- or RThe F plasmid is capable of
linking up with the bacterial
chromosome
o If this happens before the
sex pilus forms, the entire
bacterial chromosome
may be copied and
transmitted
o This has made it
increasingly difficult for
humans to combat certain
infections with
conventional antibiotics
Transformation
• Transformation is another means
of genetic recombination
whereby bacteria take up naked
DNA from their environment
• Suppose, for instance, a cell
bursts open during lysis
o Its DNA will be released
into the environment
• Another cell can uptake that
naked DNA via transformation
Transformation
Transformation is the uptake of naked DNA by a
bacterial cell.
(a)
Transduction
• Remember when we discussed
bacteriophages, we showed that
they latch onto their host and
inject their viral genetic code
(b)
The F Plasmid
(a) The F plasmid can open up and integrate
itself with the bacterial chromosome. If this
happens before the sex pilus forms, then (b), the
entire bacterial chromosome can be replicated
and transmitted through the sex pilus.
•
The R plasmid is known to be
used by bacteria to share
resistances to different antibiotics
Bacteriophages
Remember that bacteriophages digest a hole in
the bacterium, and then inject their viral code.
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© 2017 J Co Review, Inc., Accessed by Guest on 06-15-2017
Bacterial Reproduction | OAT
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Sometimes there may be a fluke
in which the virus actually takes
DNA from the bacterium
o i.e. the plasmid in the
previous illustration
might be transmitted up
into the virus’ capsid
Subsequently, when that virus
attacks another bacterium, it will
inject the bacterial DNA
o Thus, the virus facilitates
genetic recombination
Transduction
(a) A bacteriophage latches onto a bacterial host,
but instead of injecting its viral code, it takes up
DNA from the bacterium. (b) The bacteriophage
departs, carrying the bacterial DNA. (c) The
bacteriophage latches onto another bacterium,
and injects the bacterial DNA.
•
Transduction is commonly
emulated in the laboratory with
modified viruses called vectors
o The vectors are modified
so that they pose no
health threat to the
researcher
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© 2017 J Co Review, Inc., Accessed by Guest on 06-15-2017