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  Section: Genetics » Sexuality and Recombination in Bacteria and Viruses
 
 
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High resolution mapping

 
     
 
Content
Sexuality and Recombination in Bacteria and Viruses
Three Methods for Transfer of Genetic Material 
Sexual conjugation in bacteria 
Culture media and mutant strains
Discovery of gene transfer
Discovery of linkage in bacteria
Donor and recipient strains
- Mechanism of chromosome transfer
- Physical structures involved in chromosome transfer
Linkage maps in bacteria
Conjugation mapping through interrupted mapping
Circular linkage map
Linkage information from transformation
Recombination after gene transfer
High resolution mapping
Linear order of genes
Replication and recombination in viruses 
Replication of bacteriophages
Lysogenic bacteria
Transduction
Recombination in viruses
Circular genetic maps in viruses


Interrupted mating technique described earlier allows only low resolution mapping. For high resolution mapping within a distance of 2 minutes, the interrupted mating is not useful, and we need to study the stable recombinants, rather than the genes transferred. For example if ade+and leu+are the two genes transferred, we can calculate leu+and leu-among ade+(if ade+ is more frequent than leu+). The proportion of (ade+ leu-)among total ade+ = {(ade+ leu+)+ (ade+ leu-)}, will give the recombination distance between ade+ and leu+.


It has been shown that 1 minute on the map is equal to 20 per cent recombination, calculated as above.

Linear order of genes
If two genes are very close (b and c) with respect to a third gene (a), then it becomes difficult to determine the gene order (a—b—c or a—c—b), because the recombination frequencies between a and b and between a and c will be similar. This can be resolved by using reciprocal crosses : abc+ (Hfr) x a+b+c (F-) and a+b+c (Hfr) x abc+ (F-). In this case prototrophs will result in equal frequencies from two reciprocal crosses if the gene order is a-b-c and they will be produced in dramatically different frequencies, if the gene order is a-c-b (Fig. 12.16).
 
Determination of gene order from relative frequencies of recombinants in reciprocal crosses (ab + x ++c).
Fig. 12.16. Determination of gene order from relative frequencies of recombinants in reciprocal crosses (ab + x ++c).

 
     
 
 
     




     
 
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