Genetic transfer and recombination

1. Genetic transfer and recombination • Learning Outcomes • To compare the mechanism of genetic recombination in bacteria • To describe the function of plasmids and transposons

2. 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

3. 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

4. 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

7. 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

9. 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.

10. 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

11. 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

12. 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

13. Genetic transformation in bacteria

14. 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

15. Generalized transduction

16. Specialized Transduction

17. 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

18. 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

19. 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 +

22. 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

24. 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.

25. 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