CBG Lecture 18: Bacterial Conjugation Flashcards
what experiment showed that physical contact is needed between bacteria to allow conjugation
U tube with filter small enough to block bacteria going through
no conjugation/gene transfer happened
why is the transfer of genetic material not reciprocal
as it occurs only in one direction
one cell acts as donor, other recipient
what is donor ability imposed by
its a hereditary state imposed by a fertility factor (F)
what is F
a fertility factors
which is recipient, F+ or F-
F- is recipient
F+ is donor
how is the F factor transmitted
during physical contact or conjugaion
what does the F plasmid do
directs the synthesis of pilli which initiate contact with a recipient and draw it closer, allowing the F DNA to pass through a pore into the recipient cell
what happens during conjugation
the pilus pulls 2 bacteria together
then a bridge (essentially a pore) forms between the two cells then one stran dof plasmid DNA passes into the recipient bacterium and each single strand becomes dbl again
how do HFr cells arise
result from the intrgration of the F factor into the chromosome as opposed to the plasmid
what does HFr stand for
high frequency (indicates a high frequency of recombination)
what happens during conjugation between HFr and F=
a part of the chromosome is transferred with F and random breakage interrupts the transfer before the entire chromosome is transferred
in conjugation between HFr and F-, what happens after random breakage interrupts the transfer of chromosmoe
the chromosomal fragment can the recombine with the reipient chromosome
what is interrupted mating
crossing Hfr str(s)wildtype with F- str(r) mutant then put into kitchen blender to disrupt mating cell pairs, then plated onto a medium containing streptomycin to kill Hfr
in which strains does every cell donate chromosomal alleles during F transfer
Hfr cells
why is the frequency of recombinance for HFr cells greater than F cells
because F factor is integrated into chromosome and evey cell donates chromosomal alleles during F transfer
which type of bacterial strains are used for genetic mapping? why
Hfr strains because they transfer chromosomal markers efficiently
what is an F’ f prime cell
when the integrated F factor leaves the chromosome of an Hfr cell and moves back into the cytoplasm and in some rare cases carries a few host chromosomal genes with it
which cells are involved in mapping by interrupted conjugation
Hfr and F=
what are exoconjugants
the remaining f- cells after the Hfr cells have been killed after interrupted conjugation
define auxotrophic
cant synthesise aas themselves
what is fertility factor F
an episome
what is an episome
plasmid which can either be free in the cytoplasm or integrated into the bac chromosome
where does transfer between Hfr and F- begin
in the middle of F factor at oriT
where is oriT
origin of transfer, in the middle of F factor
does recombination take place between HFr and F- cells
not always but there must be crossing over if transfer is to be permanent
what happens if transfer between HFr and F- cells is interrupted
linear DNa is broken down F- remains F-
between HFr and F- cells does all the transferred material remain in the recipient
no only any parts which is invorporated into the bacterial chrom by corssing over
what gene is responsible for the production of the sex pilus
tra (transfer) genes
what is an insertion sequence of F plasmid
IS = insertion sequence
through these sequences the plasmid will eventually be able to insert in the chromosome by homologous recombination
what is an F’ cell
bacterial chrom with F plasmid containing a few chromosomal genes
what is Hfr cell
just bacterial chromosome with integrated F plasmid in bac chrom
what is F- cell
just bact chrom (no F plasmid)
what is F+ cell
bac chrom with F plasmid
what does plating onto streptomycin do
kill HFr cells
in 1957: Jacob and Wollman – 2 experiment, what did plating onto strp medium lacking leucine and threonine do
kill untransform F-, so only selects transformed F-
which number of crossing over events will be lowest frequency
quadruple crossover
