Download Genetic Transfer in Bacteria

Genetic Transfer in Bacteria
Submitted By :
Saif Raad Abdul Jabar
Saif Salah Aldin Hasan
Shifaa Dawood Najim
Shahd Ismaeel Abd
Shahd Mohamad Salim
Supervised by :
Dr : Kawkab Adris Mahmod
Saif Raad Abdul Jabar
•Structure of Bacterial Chromosome
•Plasmid
The bacterial chromosome and plasmids
A. Bacterial chromosome
• Singular circular strand of DNA
• Aggregated in a dense area- nucleiod
• Long molecule of DNA tightly coiled around protein
molecules.
B. Plasmids
– Nonessential pieces of DNA
• Often confer protection- resistance to drugs
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–
–
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Tiny, circular
Free or integrated
Duplicate and are passed on to offspring
Used in genetic engineering
Plasmid :
are circular pieces of DNA that exist outside the
main bacterial chromosome and carry their own
genes for specialized functions .
in genetic engineering , plasmids are one means
used to introduce foreign genes into a bacterial
cell
1- Fertility-(F) plasmids :
They are capable of conjugation (they contains the genes for the pili).
2- Resistance-(R) plasmids :
contain gene (s) that can build resistance against one or several
antibiotics or poisons.
3- Col-plasmids :
contain genes coding for colicines , proteins that can kill other
bacteria
4- Degradative plasmids :
able to digest unusual substances, e.g., toluene or salicylic acid.
5- Virulence plasmids :
turn a bacterium into a pathogen. (one that causes disease).
Saif Salah Aldin Hasan
•Mechanism of DNA Transfer in Bacteria
•Transformation
Three main processes of genetic recombination in prokaryotes
fragments of homologous DNA from a donor chromosome are
transferred to a recipient cell
(1) Transformation, which involves donor DNA free
in the environment
(2) Transduction, in which the donor DNA transfer
is mediated by a virus
(3) Conjugation, in which the transfer involves cellto-cell contact and a conjugative plasmid in the
donor cell
Transformation
Transduction
Conjugation
DNA Transfer in Bacteria
transformation
transduction
conjugation
1. Transformation
• Transformation : is the alteration of a bacterial cell’s
genotype by the uptake of naked, foreign DNA from the
surrounding environment.
– For example, harmless Streptococcus pneumoniae
bacteria can be transformed to pneumonia-causing cells.
– This occurs when a live nonpathogenic cell takes up a
piece of DNA that happened to include the allele for
pathogenicity from dead, broken-open pathogenic cells.
– The foreign allele replaces the native allele in the
bacterial chromosome by genetic recombination.
– The resulting cell is now recombinant with DNA taken
from two different cells.
Transformation
A number of prokaryotes have been found to be
naturally transformable, including certain
species of both gram-negative and gram-positive
Bacteria and some species of Archaea. However,
even within transformable genera, only certain
strains or species are transformable
The introduction of DNA into cells
by mixing the DNA and the cell
(a) Binding of free DNA by a
membrane-bound DNA binding
protein.
(b) Passage of one of the two strands
into the cell while nuclease activity
degrades the other strand.
(c)
The single strand in the cell is
bound by specific proteins, and
recombination with homologous
regions of the bacterial chromosome
mediated by RecA protein occurs.
Transformed cell
The mechanism of bacterial transformation
Shifaa Dawood Najim
•Transduction
•Generalized Transduction
Transduction
Concept
Transduction : involves transfer of host genes from
one bacterium to another by viruses.
In generalized transduction, defective virus
particles randomly incorporate fragments of the
cell's chromosomal DNA; virtually any gene of the
donor can be transferred, but the efficiency is low.
In specialized transduction, the DNA of a temperate
virus excises incorrectly and brings adjacent host
genes along with it; only genes close to the
integration point of the virus are transferred, but
the efficiency may be high.
Lytic and lysogenic cycles of certain Bacteriophages :
Transduction happens through either the lytic cycle and the lysogenic cycle.
If the lysogenic cycle is adopted, the phage chromosome is integrated into the
bacterial chromosome, where it can remain dormant for thousands of generations.
If the lysogen is induced (by UV light for example), the phage genome is excised from
the bacterial chromosome and initiates the lytic cycle, which culminates in lysis of
the cell and the release of phage particles.
The lytic cycle leads to the production of new phage particles which are released by
lysis of the host.
In transduction, DNA is transferred
from cell to cell through the agency of
viruses.
Genetic transfer of host genes by viruses
can occur in two ways :
Generalized transduction
And
Specialized transduction
Generalized Transduction
• In generalized transduction, a small piece of
the host cell’s degraded DNA is packaged
within a capsid, rather than the phage
genome.
– When this phages attaches to another bacterium,
it will inject this foreign DNA into its new host.
– Some of this DNA can replace the similar gene of
the second cell.
– This type of transduction transfers bacterial genes
at random.
Generalized transduction:
host DNA derived from virtually any
portion of the host genome becomes a
part of the DNA of the mature virus
particle in place of the virus genome.
Generalized transduction
In generalized transduction, virtually any genetic marker
can be transferred from donor to recipient
During a lytic infection, the
enzymes responsible for
packaging viral DNA into the
bacteriophage sometimes
accidentally package host
DNA. This DNA cannot
replicate, it can undergo
genetic recombination with
the DNA of the new host.
Generalized transduction
Generalized
transduction
Shahd Ismaeel Abd
•Specialized Transduction
Specialized transduction:
Specialized transduction occurs via a
temperate (can incorporate its genome
into the bacterial cell) phage.
Specialized Transduction
• Specialized transduction :
- occurs via a temperate phage.
– When the prophage viral genome is cut
from the host chromosome, it sometimes
takes with it a small region of the host
bacterial DNA.
– These bacterial genes are injected along
with the phage’s genome into the next host
cell.
– Specialized transduction only transfers those genes
near the prophage site on the bacterial
chromosome.
Specialized Transduction
the DNA of lambda is inserted into the host DNA
at the site adjacent to the galactose genes
On induction, Under rare conditions, the phage
genome is excised incorrectly
A portion of host DNA is exchanged for phage
DNA, called lambda dgal ( dgal means
"defective galactose“ )
Phage synthesis is completed
Cell lyses and releases defective phage
capable of transducing galactose genes
Specialized Transduction
Specialized transduction only transfers those genes
near the prophage site on the bacterial chromosome
Genes transferred by transduction include :
genes of toxins such :
( botulinum, diphtheria , cholera ) and
genes of drug resistance
Shahd Mohamad Salim
• Conjugation
• Transposons
Conjugation
Bacterial conjugation (mating) is a process of genetic
transfer that involves cell-to-cell contact.
Direct contact between two conjugating bacteria is first
made via a pilus. The cells are then drawn together for
the actual transfer of DNA.
Conjugation
involves a donor cell, which contains a
particular type of conjugative plasmid, and a recipient cell,
which does not.
The genes that control conjugation are contained in the tra
region of the plasmid . Many genes in the tra region have to do
with the synthesis of a surface structure, the sex pilus . Only
donor cells have these pili,
The pili make specific contact with a receptor on the recipient
and then retract, pulling the two cells together. The contacts
between the donor and recipient cells then become stabilized,
probably from fusion of the outer membranes, and the DNA is
then transferred from one cell to another.
Mechanism of DNA Transfer During Conjugation
A mechanism of DNA synthesis in
certain bacteriophages, called rolling
circle replication, was presented here
to explains DNA transfer during
conjugation .
if the DNA of the donor is labeled, some
labeled DNA is transferred to the
recipient but only a single labeled strand
is transferred. Therefore, at the end of
the process, both donor and recipient
possess completely formed plasmids.
Drug resistance genes are transferred by
conjugation between bacteria .
Jumping Genes
• A transposon is a piece of DNA that can move
from one location to another in a cell’s genome.
• Transposon movement occurs as a type of
recombination between the transposon and another
DNA site, a target site.
– In bacteria, the target site may be within the chromosome,
from a plasmid to chromosome (or vice versa), or between
plasmids.
• Transposons can bring multiple copies for antibiotic
resistance into a single R plasmid by moving genes to
that location from different plasmids.
– This explains why some R plasmids convey resistance to
many antibiotics.
Mechanism for transposition
Transposons :
that carry
antibiotic
resistance genes
can randomly
“hop” into a
bacterial
chromosome
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