Genetic transfer and recombination

GENETIC TRANSFER AND RECOMBINATION

Learning OutcomesTo compare the mechanism of genetic

recombination in bacteriaTo 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.

X

• Mice are injected with both Type R, non-virulent and heat-killed, Type S Streptococcus

• DNA carrying genes fromthe virulent, heat-killed cellstransforms the non-virulentbacterial cells, making themlethal to the mice

DNA transformation: in vitro experiment

Type R cells Type R colonies

Type S cells Type S colonies

Mixture ofType R and Type S

colonies

Type R cells+ DNA fromType S cells

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

Genetic transformation in bacteria

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

Generalized transduction

Specialized Transduction

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 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

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