Transcript Microbial Genetics

Microbial Genetics
Genetic transfer and
recombination
Genetic recombination
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Genetic recombination through sexual
reproduction is an important means of variation
in eukaryotes
Prokaryotes do not have an equivalent process
of sexual reproduction
However, prokaryotes do have mechanisms by
which DNA can be transferred between strains of
the same species, or even between different
species
Contributes to a population’s genetic diversity
Gene transfer
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Vertical gene transfer
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Occurs during reproduction. Transfer of genes
from an organism to its offspring
Horizontal gene transfer
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Transfer of genes from one organism to
another within the same generation
Horizontal gene transfer
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Involves one way transfer from a donor cell to a
recipient cell
A recipient cell that incorporates DNA from the
donor is called a recombinant
Genes are transferred naturally between
bacteria by three mechanisms
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Transformation: DNA is transferred as “naked” DNA
Conjugation: DNA is transferred between bacteria that
are in contact with each other
Transduction: DNA is transferred by a bacterial virus
(bacteriophage)
Homologous recombination
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DNA introduced into bacteria usually does not
have a mechanism to replicate itself
It relies on integration into the genome of the
host bacterium in order to survive and be
passed on
Incoming DNA
Host genome
Recombinant
genome
gene A
gene Z
gene C
gene A
gene B
gene C
gene A
gene Z
gene C
Griffith’s experiment, 1928
Streptococcus pneumoniae
“Naked” DNA
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When a bacterial cell
lyses, it releases its DNA
into the environment
Transformation
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DNA is transferred as
naked DNA
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DNA breaks into pieces
on cell lysis
DNA is taken up by the
recipient cell
A region of the recipient
DNA is replaced by the
donor DNA
(recombination)
Unrecombined DNA is
degraded
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Transformation occurs naturally in very few
genera of bacteria
 Bacillus, Haemophilus, Neisseria,
Acinetobacter, and some strains of
Streptococcus and Staphylococcus.
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The recipient cell must be in a physiological
state to take up DNA.
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Changes in the bacterial cell wall make it
permeable to large DNA molecules
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Some bacteria, which are not normally can be
made so in laboratory
Conjugation
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DNA transfer between two bacteria that
are in contact with one another
Contact between donor and recipient cells
is initiated by sex pili
DNA is transfer through a conjugation
bridge or open pore between donor and
recipient cell
Mediated by a plasmid, called an F-factor
(fertility factor) or a conjugative plasmid
Plasmids
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Small, circular molecules of
DNA
Replicate independently of
the chromosome
Usually dispensable for
growth, but under some
conditions provide a
selective advantage such
as antibiotic resistance or a
unique metabolic pathway
Conjugative plasmids: carry
genes for conjugation
including sex pili
Mechanism of conjugation
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Donor contacts
recipient, attaches
using sex pilus
F-factor initiates
transfer of a copy of
itself
Recipient is
converted to a new
donor cell
Importance of plasmids
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Providing a selective advantage
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Enhancing pathogenicity
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Some Pseudomonas sp. have plasmid encoded
enzymes to degrade petroleum allowing them to live
in fuel tanks or fuel spills
The E. coli strain causing infant or traveler’s diarrhea
carries plasmids for toxin production and bacterial
attachment.
Antibiotic resistance
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Many antibiotic resistance genes are carried on
plasmids which can be rapidly transferred to other
bacteria, resulting in widespread resistance to
antibiotics and strains that resistant to multiple
antibiotics such as methicillin-resistant
Staphylococcus aureus or Golden Staph
Transduction
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Mediated by a bacterial virus
(bacteriophage or phage)
DNA from the donor is transferred to the
recipient inside the phage particle
Two types of transduction
Generalized
 Specialized
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Generalized transduction
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Donor cell is infected with
a phage
Donor DNA is
incorporated into the
phage (transducing
phage)
The donor cell lyses
The transducing phage
infects the recipient cell
and injects the donor
DNA
DNA integrated into the
genome
Transposons
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Segments of DNA that can
move from one region of
DNA to another and
integrate through nonhomologous recombination
Contain information for their
own transposition
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Transposase enzyme for
cutting and resealing DNA
Short terminal repeats which
the transposase recognizes
as recombination sites
Insertion sequences are the simplest transposons
Complex transposons carry other genes e.g., antibiotic
resistance genes
Natural history of a transposon
chromosome
transposon
plasmid
Genomes of bacteria are elastic
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Bacterial genomes are often receiving
genetic information from other bacteria
through genetic transfer and
recombination
In the same way that mutations can be
beneficial, neutral or harmful, so is the
recombination of incoming DNA
New gene combinations are maintained if
they provide the organism with a selective
advantage