Question

Why the mutant strains are used?

Answer 1

Answer:

Bacterial mutants are typically described by comparison to a standard, well-characterized, reference strain called the ‘wild-type’ strain. Bacterial mutants have often lost some growth property (e.g., failure to utilize a particular carbon or nitrogen source or failure to grow without a particular nutrient), or acquisition of some new growth property (e.g., ability to grow in the presence of some toxic substance). Genes can be divided into two categories based on the phenotypes of the corresponding mutants: nonessential gene products are only required under specific growth conditions, while essential gene products are required under all conditions. The genes of lactose catabolism are nonessential because they are only required for growth on medium with lactose as the sole carbon source. In contrast, the genes encoding RNA polymerase are essential because they are required for growth on all media. Null mutations in a nonessential gene will prevent growth on a medium that requires that gene product but such mutants will still grow on other media. In contrast, null mutations in an essential gene are lethal. Consequently, such mutants cannot be recovered from haploid bacteria. Nevertheless, it is possible to isolate more subtle mutations in essential genes. For example, it is possible to isolate mutations that alter a subunit of RNA polymerase that make the organism resistant to the antibiotic rifampicin. It is also possible to isolate mutations where some phenotype is observed under certain ‘nonpermissive’ conditions but not under other ‘permissive’ conditions

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