This type of mutation occurs when a chromosomal section is separated from the chromosome, rotates 180 degrees, and rejoins the chromosome in an opposite orientation. This type of mutation can affect a gene at many levels. If an inversion disrupts a promoter region, the gene may not be transcribed at all. If the coding sequence is disrupted, a non-functional gene product (protein) may result.
Translocations
This type of chromosomal aberration results when one portion of a chromosome is transferred to another chromosome. This can be a very harmful event if it leads to a subsequent gain or loss of genetic material. Additionally, when a gene from one chromosome moves to another chromosome, large changes in the ability to regulate expression of the gene may occur. Some forms of leukemia result from translocations. In these cases, various genes controlling growth of white blood cells are constantly turned on, leading to an uncontrolled proliferation of these cells and the various clinical manifestations of leukemia.
Lacz mutations*
LacZ mutations are an example of particular mutations found in the LacZ gene of E.coli, which encodes the lactose hydrolyzing enzyme ß-galactosidase. There is a special compound known as X-gal that can be hydrolyzed by ß-galactosidase to release a dark blue pigment. When X-gal is added to the growth medium in petri plates, Lac+ E. coli colonies turn blue, whereas Lac– colonies with mutations in the LacZ gene are white. By screening many colonies on such plates it is possible to isolate a collection of E. coli mutants with alterations in the LacZ gene. PCR amplification of the LacZ gene from each mutant followed by DNA sequencing allows the base changes that cause the LacZ– phenotype to be determined. A very large number of different LacZ mutations can be found, but they can be categorized into three general types: missense, nonsense and frameshift .
Causes of mutations
Mutations are caused by substances that disrupt the chemical structure of DNA or the sequence of its bases. Radiation, various chemicals, and chromosome rearrangements are some of the many sources of mutation.
Mutation rates
All of us are subjected to mutagenic events throughout our lifetime. Depending upon the type of mutation, the frequency ranges from 10-2/cell division to 10-10/cell division. Our cells have numerous mechanisms to repair and/or prevent the propagation of these mutations.
Suppressor mutations*
A powerful mode of genetic analysis is to investigate the types of mutations that can reverse the phenotypic effects of a starting mutation. Say that you start with a mi- λ phage mutant that makes small plaques. After plating a large number of these mutant phages, rare revertants can be isolated by looking for phage that have restored the ability to make large plaques. These revertants could have either been mutated such that the starting mutation was reversed, or they could have acquired a new mutation that somehow compensates for the starting mutation. The possibilities are:
