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

– 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).

Saif Salah Aldin Hasan

•Mechanism of DNA Transfer in Bacteria•Transformation

(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

Conjugation

Transformation

Transduction

DNA Transfer in Bacteria

transformation

transduction

conjugation

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

The mechanism of bacterial transformation

Shifaa Dawood Najim

•Transduction •Generalized Transduction

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.

Generalized transduction

Generalized

transduction

Shahd Ismaeel Abd

•Specialized Transduction

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

Shahd Mohamad Salim

• Conjugation• Transposons

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

Thank

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