genetic transfer in bacteria - university of...
TRANSCRIPT
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
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
– 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 bacteria4- 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).
(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 cell-
to-cell contact and a conjugative plasmid in the
donor cell
Three main processes of genetic recombination in prokaryotes
fragments of homologous DNA from a donor chromosome are
transferred to a recipient cell
1. Transformation
• Transformation : is the alteration of a bacterial cell’sgenotype by the uptake of naked, foreign DNA from thesurrounding environment.
– For example, harmless Streptococcus pneumoniaebacteria can be transformed to pneumonia-causing cells.
– This occurs when a live nonpathogenic cell takes up apiece of DNA that happened to include the allele forpathogenicity from dead, broken-open pathogenic cells.
– The foreign allele replaces the native allele in thebacterial chromosome by genetic recombination.
– The resulting cell is now recombinant with DNA takenfrom 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
(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.
The introduction of DNA into cells
by mixing the DNA and the cell
Transformed cell
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.
Concept
Transduction
of certain Bacteriophages : Lytic and lysogenic cycles
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.
Specialized transduction:
Specialized transduction occurs via atemperate (can incorporate its genomeinto 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.
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 TransductionSpecialized 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 ) andgenes of drug resistance
Direct contact between two conjugating bacteria is first
made via a pilus. The cells are then drawn together for
the actual transfer of DNA.
Bacterial conjugation (mating) is a process of genetic
transfer that involves cell-to-cell contact.
Conjugation
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 movefrom one location to another in a cell’s genome.
• Transposon movement occurs as a type ofrecombination between the transposon and anotherDNA site, a target site.– In bacteria, the target site may be within the chromosome,
from a plasmid to chromosome (or vice versa), or betweenplasmids.
• Transposons can bring multiple copies for antibioticresistance into a single R plasmid by moving genes tothat 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