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Genetic transfer and recombination. Learning Outcomes To compare the mechanism of genetic recombination in bacteria To describe the function of plasmids and transposons. Genetic recombination. refers to the rearrangement of DNA from separate groups of genes
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Genetic transfer and recombination • Learning Outcomes • To compare the mechanism of genetic recombination in bacteria • To describe the function of plasmids and transposons
Genetic recombination • refers to the rearrangement of DNA from separate groups of genes • usually involves DNA from different organisms • exchange between 2 DNA to form new combinations of genes in a chromosome • contributes to genetic diversity • in eukaryotes the transfer can be done as a part of the sexual cycle • in prokaryotes in several ways
In all of the mechanism, transferred involved; • donor cell- that give a portion of its total DNA • recipient cell- that received a portion of DNA from donor cell • parts of donor DNA is incorporated into the recipient DNA • when some of the donor’s DNA has been integrated into the recipient’s DNA, the resultant cell is called a recombinant
Genetic transfer in prokaryotes • Transformation • gene transfer from one bacterium to another as ‘naked’ DNA in solution • Transduction • donor DNA transfer is mediated by a virus • Conjugation • transfer involve cell to cell contact • conjugative plasmid is the donor cell
Transformation • genes are transferred from donor to recipient bacteria as naked DNA in solution • refer to Griffith’s expt • used Streptococcus pneumoniae • two strain; virulent (pathogenic)-capsule-cause pneumonia avirulent- lack capsule-no pneumonia disease • DNA from virulent can enter avirulent, changing avirulent strain genetically so that their progeny were encapsulated--become virulent
i). DNA transformation: in vivo experiment Mice are injected either with Type R, non-virulent Streptococcus or with heat-killed, virulent Type S cells. The mice are healthy.
Mice are injected with both Type R, non-virulent and heat-killed, Type S Streptococcus X • DNA carrying genes from • the virulent, heat-killed cells • transforms the non-virulent • bacterial cells, making them • lethal to the mice
DNA transformation: in vitro experiment Type R cells Type R colonies Type S cells Type S colonies Type R cells + DNA from Type S cells Mixture of Type R and Type S colonies
Mechanism of genetic transformation in bacteria • Process by which free DNA is incorporated into a recipient cell and bring about genetic change--recombinant cell • work best when the donor and recipient cells are closely related • recipient cell have to physiological state to take up the donor DNA--be competent • Competence- alterations in the cell wall that make it permeable to large NDA molecules • some bacteria are naturally competent but some have to undergo treatment to make it competent
Transduction • DNA is transferred from donor to recipient via a bacteriophage • bacterial DNA is incorporated into the bacteriophage • generalized transduction: any bacterial genes are transferred • specialized transduction: specific regions of DNA are transferred
Plasmids • self-replicating covalently closed circular DNA molecules that are usually not essential for survival • several types of plasmids • dissimilation plasmid • Code for enzymes for catabolism of certain unusual sugars and hydrocarbon • Conjugative plasmid- • F factor-carries genes for sex pili and transfer of plasmid • R factors (resistant factors) • Significant medical importance • Carry genes that confer upon their host cell resistance to antibiotics, heavy metals, or cellular toxins
Conjugation • requires contact between donor and recipient cells • mediated by plasmid (a circular DNA that replicate independently from cell chromosome) • differ from transformation; • cell to cell contact • opposite mating type; donor cell carry plasmid, recipient cell do not • gram -negative- used pili for contact (sex pili) • gram-positive cell- sticky surface molecule
Conjugation cont….. • 2 kinds; • Plasmid transfer • Chromosome transfer • Plasmid transfer • F+donor contains F plasmid • F - recipient cells do not contain F plasmid • sex pilus is formed • one strand of DNA is transferred into the recipient cell from the donor cell • F -become F +
Chromosome transfer • when F plasmid is integrated into the chromosome, an Hfrcell is formed • Hfr = high frequency recombinant • during conjugation, an Hfr cell can transfer chromosomal DNA into the recipient cell (F-) • usually the chromosome breaks before it is fully transferred • F- become recombinant F- cell
Transposons • small segments of DNA that can move from one region of a chromosome to another region of the same chromosome or to a different chromosome or DNA molecule • found in chromosomes, plasmids, viruses • simple to complex structures • can carry any type of gene, including antibiotic resistance genes • cause mutations • increase (or decrease) the amount of DNA in the genome.
Transposons: Mobile DNA • There are three distinct types: • Class II Transposons consisting only of DNA that moves directly from place to place. • Class III Transposons; also known as Miniature Inverted-repeats Transposable Elements or MITEs. • Class I-Retrotransposons that • first transcribe the DNA into RNA and then • use reverse transcriptase to make a DNA copy of the RNA to insert in a new location. • Transposons in Bacteria • insertion sequence-simplest transposon- carry a genes for transposase • complex transposon - carry other genes (code for antibiotic) in addition to transposase genes