One strand of DNA is degraded and the other strand is protected by a costing of single-stranded DNA-binding protein and RecA protein (required for recombination)Single strand then invades the chromosome of the recipient cell, pairing with the complementary strand of DNA and replacing the equivalent strandReplace recipient strand is then degradedIf alleles are different between host and donor, double helix DNA will be heteroduplex (heterozygous DNA)Transformation is measured by change in phenotype (a- to a+, b- to b+ or a-b-to a+b+)DNA molecules taken up by competent cells is usually only 0.2 to 0.5 percent of the complete chromosomeTherefore, unless two genes are quite close together, they will never be present on the same molecule of transforming DNA
If two genes are closely linked they may be carried on a single molecule of transforming DNAThe frequency with which two genetic markers are cotransformed (two genes transformed on one strand) can thus be used to estimate how far apart they are on the host chromosomeConjugationIn 1946, Joshua Lederberg and Edward Tatum discovered that E.coli cells transfer genes by conjugationDuring conjugation, DNA is transferred from a donor cell to a recipient cell through a specialized intercellular conjugation channel which forms betwennthemIn direct contact during conjugationDonor cells have cell-surface appendages called F pili (synthesis of F pili controlled by F factor)Those that contain F factor are able to transfer genes to other bacteriaF pili of donor cell make contact with a recipient cell that lacks an F factor and attach to the cell, so that the two cells can be pulled into close contactF pili only involved in establishing cell contact not DNA transferConjugation channel forms between the cells, and DNA is transferred from the donor to the recipient through this channelF factor can exist in two states:oAutonomous: replicates independently of bacterial chrosomoneoIntegrated: covalently inserted into bacterial chromosome and replicates like any other segment of that chromosome (episomes)Donor cell carrying autonomous F factor called F+ cell and recipient cell lacking F called called F- cellWhen a F+ cell conjugates with a F- recipient only F factor is transferred (Both cells become F+ and recipient acquire an F factor)F factor can integrate into the bacterial chromosome by site-specific recombination eventsCell that carries integrated F factor is called an Hfr cellIn integrated state, the F factor mediates the transfer of the chromosome from the Hfc cell to to a recipient F-Usually cells separate before chromosome transfer is complete (rarely will entire chromosome be transferred)Mechanism that transfers DNA from a donor cell to a recipient cell during conjugation appears to be the same if just the F factor is being transferred as in F+ X F- mating, or if bacterial chromosome is being transferred as in Hfr X F- matingTransfer initiated at oriT (one of three sites on the F factor)
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- Fall '13
- DNA, RNA molecules