Genetic transfer and recombination
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Transcript of 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