bacterial genetics screencasts - horizontal transfer of chromosomal genes by HFR strains

how can conjugative plasmids mediate transfer of chromosomal genes?

via integration into the genome

what commonly mediates integration of conjugative plasmids into the E. coli genome?

IS elements

what type of DNA are IS elements common in

bacterial DNA

what are IS elements?

Insertion Sequences, DNA sequences that act as simple transposon elements

where can IS elements be found?

in other elements e.g. plasmids and at multiple chromosomal sites

what allows integration to occur at different orientations?

the orientations of the IS elements

do all HFR strains transfer the same part of the chromosome first?

no, they transfer different parts of the chromosome first

what is the direction of transfer

whether genes on the right or left of the integrated plasmid are transferred

what is direction of transfer dictated by?

orientation of the plasmid with respect to the genome

what does having a population of different HFR strains mean for chromosome transfer?

almost all of the chromosome can be transferred efficiently

what interrupts and means that entire chromosomes are not transferred?

random breakages of cell connections

is the pattern of transfer from HFR strains to recipients reproducible in an experiment?

yes

describe an experiment to show that transfer from HFR strains to recipients is reproducible

• HFR strain sensitive to streptomycin with functional forms of the genes of interest

• an F minus strain resistant to streptomycin has resistance genes that can be readily assayed

• samples are removed at specific times after mixing the strains

• which disrupts mating by agitation (interrupted mating)

• streptomycin resistant cells tested for alleles present from the donor

• these must have taken part in conjugation (ex-conjugants)

• each donor allele only appears in the recipients at a specific time after mating

• and in a specific sequence read from left to right

• the max yield of cells containing a specific donor allele is smaller for donor markers that entered later

• so we can conclude that gene transfer occurs from a fixed position on the donor chromosome which is the origin of transfer and continues in a linear fashion

what is the origin of transfer?

a fixed position on the donor chromosome where transfer originates

does transfer occur in a linear fashion?

yes

if a gene is located further from the origin of transfer, is is transferred earlier or later?

later

why is the max yield for cells with later donor markers present smaller?

the process will most likely stop before the furthermost genes are transferred

what dictates the polarity of transfer from the chromosome?

the orientation of F insertion

how can we construct a map of allele transfer?

by looking at when the alleles appear in the recipient

what provides the order of genetic markers/

the natural gradient of transfer

what are the units of distance given in for the allele map?

minutes

what is this map of the transferred alleles called?

a linkage map

what would improve the resolution of the linkage map?

• more markers that tell donor and recipient apart

• more HFR strains used

is conjugal transfer reciprocal in matings?

NO

why is conjugal transfer not reciprocal?

one acts as a donor and one as a recipient and is changed by the genetic material received form the donor

how does conjugal transfer contrast with sexual reproduction in regards to exchange of genetic material

both parents donate equally in sexual reproduction, only one is a donor in conjugal transfer