BIOL205:week 5, lecture 13
→ The rare recombinants observed by Lederberg
and Tatum in the mixing of two auxotrophic
strains was due to rare spontaneous formation of
Hfr in the an F+ strain which could then donate
bacterial DNAto the F- strain.
Crossovers integrate parts of the transferred
Originoftransfer:always cuts insame place
homologous recombinationcanoccur Linear transmission of the HFR genes from a fixed point
(Elie Wollman and Francois Jacob 1957)
Q. Can we determine the order of those genes on the E. coli chromosome?
Tracking time of marker entry generates a chromosome map
ex– distance betweenlac
and azi~ 8 mins
Use time instead ofmu
Tracking time of marker entry generates a
– purple: azi
– blue: ton
– orange: lac
– green: gal Hfr strain
Canintegrate anywhere one. colichromosome
**pullinfrom2 to 1 The key elements in these results are
1. Each donor allele first appears in the F− recipients at a specific time after mating began.
2. The donor alleles appear in a specific sequence.
3. Later donor alleles are present in fewer recipient cells.
• the Hfr chromosome, originally circular, unwinds a copy of itself that is transferred to the F−
cell in a linear fashion, with the F factor entering last
• this explains why Hfr donors rarely convert the F- exconjugates.
• remainder of F plasmid is the last to go in
Two types of DNAtransfer can take place during conjugation
• the use of different Hfr strains and interrupted matings allowed researchers to figure out the the
E. coli chromomosome is circular
• because different Hfr strains can integrate at various regions of the E.
coli chromosome this lead to mapping the entire E.coli chromosome
• mapped in time units, strain E. coli K12 was shown to be 100 minutes
long Proportions of the genetic and physical maps are similar but not identical
Transposon mutagenesis can be used to map a mutation in the genome sequence
→ “jumping genes”: mobile genes
discovered by Barbara McClintock
- pieces of DNAthat encode enzymes that allow for hopping in
and out of genes
- geneticists: let transposons hop around and look for mutations
- then assume that where mutation is,