 Learning Outcomes  To compare the mechanism of genetic recombination in bacteria  To describe the function of plasmids and transposons

 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

 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

 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.

X Mice are injected with both Type R, non- virulent and heat-killed, Type S Streptococcus 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 cellsType R colonies Type S cellsType S colonies Mixture of Type R and Type S colonies Type R cells + DNA from Type S cells

 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

Genetic transformation in bacteria

 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

Generalized transduction

Specialized Transduction

 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

 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

 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 Hfr cell 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

 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.

 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 thatRetrotransposons 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.reverse transcriptase  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