Genetics Lecture No. 11: Generating Mutants
Wednesday February 13 , 2013
-In earlier research on mutations, we relied on mutants which either occurred spontaneously or were
induced (random). In the present day, we would rather generate mutants with a specific mutation in a
specific place, if possible.
-Early mutants are often morphological mutants, typically originating from spontaneous mutations. The
problem with using early mutants is that their characteristic spontaneous morphological mutations
occur too slowly and in too few individuals of the population. Thus it is necessary to utilize mutants
other than morphological ones such as: mutants in biochemical pathways, mutants defined by subtle
changes, mutants characterized by maturation time differences, etc.). By inducing mutations, we can
increase the frequency of mutants produced and create a larger variety of mutants in the process.
Inducing Mutations With EMS:
-A popular mutagen important for inducing mutations is the alkylating agent ethyl methanesulfonate
(EMS). EMS induces point mutations (base-pair substitutions) by transferring its ethyl group to a guanine
nucleotide, forming O-6-ethylguanine. This change to guanine makes it more likely to pair with thymine,
resulting ultimately in a transition mutation (GC -> AT). EMS is also easy to use due to its solubility and
its ability to be taken up by cells. After using EMS on a population, you can screen the individuals for
morphological mutants or biochemical mutants. Assays (an investigative analytic procedure for
qualitatively assessing or quantitatively measuring the presence or amount or the functional activity of a
target entity, which can be a drug or biochemical substance or a cell in an organism or organic sample)
can be used to observe: developmental mutants, mutants with different behaviour patterns, mutants
which react differently to the environment (light, salt, toxins, wind, touch, etc.), or any mutation effect
you can think of. In summary, you build a collection of mutants.
-Some problems however associated with EMS include: The production of random mutations, no
selection of mutants, often more than one mutation per mutant, single base-pair changes somewhere in
the genome, and difficulty of tracking mutations on the molecular levels.
-In a collection of mutants, if you identify mutants with the same phenotype, do they carry a mutation in
one gene or several genes? Complementation is a process in which heterozygosity for chromosomes
bearing mutant recessive alleles for two different genes produces a normal phenotype. Given two
genes, Grr and Ugh, a test cross between homozygous mutant parents (grr and ugh) produces F1
offspring (grr ugh) with mutant phenotypes (no complementation if the mutation is in the same gene;
Grr = Ugh). A test cross between homozygous mutant parents (UGH