Download Mutations Terminology

MUTATION AND MUTAGENS
DEFINITIONS
mutation; a heritable change in the DNA
At the level of the phenotype/genotype:
recessive mutation produces recessive allele: A
a
dominant mutation produces dominant allele: b
B
forward: from the common to a new allele
reverse: a return to the original allele
somatic: a mutation in a somatic cell
suppressor: a second mutation "corrects" another
intragenic -2nd amino acid change restores folding/activity
intergenic - change at second gene locus masks error
ex. nonsense suppressor; tRNA anticodon change permits
recognition of stop codon (in competition with Release Factors)
At the DNA level:
point; a single base change; can be reverted
transition; (AT
GC changes; purine for purine)
transversion; (puríne for pyrimidine);
(AT
TA or AT
CG or GC
CG)
single base addition or deletion
At the level of mRNA translation
missense: wrong amino acid inserted (neutral if no effect on the
enzyme made, but missense mutations are often "leaky", temperature
sensitive or low enzymatic activity, etc)
nonsense: the mutation creates a stop codon
samesense: (or silent) -the same amino acid will be coded
frameshift: added or lost base changes reading frame
Detection of Mutation; Measuring Mutation Rates:
First test was devised by H. J. Muller for use in Drosophila and proved
that X-rays are mutagenic.
Relied on three X-linked "genes"
C = dominant crossover suppressor (actually an inversion)
l = recessive lethal
B = Bar eyes BB females have rod-shaped eyes, Bb are wide-bar
and bb are normal kidney shaped eyes
Protocol: wide bar females heterozygous for the 3 genes were crossed to
normal but x-irradiated males. The wide bar female progeny,
ClB
irr.
containing one X chromosome and one X were crossed to
normal males. Any female that produced daughters but no sons
irr
had a new recessive lethal on the X . The dose response was
linear until high doses masked individual mutations. The rate was
3 lethals/100 irradiated chromosomes per 1 Kr.
More current tests include the "Ames test" that detects reversion of a
known mutation in a gene in Salmonella to measure both the rate
and type of base change induced by a test substance. A series of
mutant strains, all defective in a gene required to make the amino
acid histidine can differentiate specific transition-, transversionand frameshift-inducing agents, because only revertant cells can
grow into a colony on minimal medium.
See: AMES test image
Strains can be obtained from the Salmonella Genetic Stock Center:
http://www.acs.ucalgary.ca/~kesander/
Another direct test using mammalian cells is also widely used. It is based on mouse
cancer cells that will not grow on medium containing trifluorothymidine
unless a mutation eliminates the second (functional) copy of the Thymidine
kinase gene. This test is not limited to point mutations.
See: http://www.mlamutations.com/mla/mla.html