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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 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 Thank You