Bacterial Recombination

Conjugation Involving Hfr Bacteria

Occasionally, the F factor integrates into a random position in the bacterial chromosome. When this happens, the bacterial cell is called Hfr instead of F+. Hfr bacteria are still able to initiate conjugation with F- cells, but the outcome is completely different from conjugation involving F+ bacteria:

As mentioned above, Hfr cells are formed when the F factor integrates into the bacterial chromosome. This integration occurs at a random location.

The Hfr cell is still able to initiate conjugation with an F- cell.

When DNA transfer begins, the Hfr cell tries to transfer the entire bacterial chromosome to the F- cell. The first DNA to be transferred is chromosomal DNA, and the last DNA to be transferred will be the F factor DNA.

Transfer of the bacterial chromosome is almost never complete. Pili are fairly fragile structures, and shear forces tend to break the pilus, disrupting DNA transfer before the entire chromosome can be transferred. As a result, the F factor itself is almost never transferred to the recipient cell. This cell will remain F-. This cell will receive new DNA from the Hfr cell however, and this new DNA can undergo recombination at a high frequency with the host chromosome, because the DNA sequences will be homologous. In fact, Hfr is short for 'high frequency recombination'. This recombination can result in gene conversion events, if the transferred DNA and the corresponding region of host DNA contain different alleles of the same gene.

Mapping Genes on Bacterial Chromosomes

Bacteria, since they are usually haploid, cannot have their chromosomes mapped by the same techniques as eukaryotes (For a reminder of how this works, see the module on linkage and mapping). They can, however, be mapped by using Hfr bacterial conjugation. For example, imagine that an F- cell has mutant alleles of two genes, a and b (the F- would therefore be a-, b-). If this cell undergoes conjugation with an Hfr cell that is a+, b+ (in other words, wild type), the F- cell should undergo gene conversion to a+, b+ when both of those genes have been transferred by conjugation. By determining how long it takes the b gene to transfer after the a gene has transferred, it is possible to get a relative idea of how far apart the two genes are on a chromosome.

The experiment would be done this way: a+, b+ Hfr cells would be mixed with a-,b- F- cells. The time of mixing would be designated 'time zero'. At regular intervals, a small amount of the mixture would be removed and conjugation would be disrupted using a blender (the shear force of the blender would cause any pili to break). These bacteria would then be tested for gene conversion (for example, if the mutations rendered the F- bacteria auxotrophic, the bacteria could be tested by growing them on minimal medium, or minimal medium supplemented with the necessary nutrient required because of one or the other mutation). If the a gene was converted to wild type at 8 minutes after time zero, and the b gene was converted to wild type at 19 minutes after time zero, then the distance between the two genes would be '11 minutes' (because that was the difference in time required to transfer the b gene compared to the a gene). Bacterial map distances are always expressed in minutes, because of this technique.

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As mentioned above, Hfr cells are formed when the F factor integrates into the bacterial chromosome. This integration occurs at a random location.
The Hfr cell is still able to initiate conjugation with an F- cell.
When DNA transfer begins, the Hfr cell tries to transfer the entire bacterial chromosome to the F- cell. The first DNA to be transferred is chromosomal DNA, and the last DNA to be transferred will be the F factor DNA.
Transfer of the bacterial chromosome is almost never complete. Pili are fairly fragile structures, and shear forces tend to break the pilus, disrupting DNA transfer before the entire chromosome can be transferred. As a result, the F factor itself is almost never transferred to the recipient cell. This cell will remain F-. This cell will receive new DNA from the Hfr cell however, and this new DNA can undergo recombination at a high frequency with the host chromosome, because the DNA sequences will be homologous. In fact, Hfr is short for 'high frequency recombination'. This recombination can result in gene conversion events, if the transferred DNA and the corresponding region of host DNA contain different alleles of the same gene.