MUTATION
Though DNA is a highly stable molecule, changes occurs in DNA structure and during replication. Mutation is defined as the sudden heritable phenotypic change in an organism. The individuals showing these changes are known as mutants. The allele producing the changed phenotype is called mutant allele.
Even though mutation is source of new variation, most of the mutations are harmful. Hugo de Vries in 1900 coined the term mutation. Seth Wright in 1791 observed a short-legged breed of sheep which cannot jump over the fence. This breed was called as Ancon breed. Morgan in 1910 reported a white-eyed mutant in Drosophila. More the 500 mutations are observed in fruit flies so far. H.J. Muller
(1927) won Nobel prize for his work in Drosophila.
FEATURES OF MUTATION
- Mutant alleles are generally recessive.
- Mutations are random.
- Most mutations have harmful effect. The frequency of beneficial mutation is about 0.1%.
- Mutations are recurrent.
- Mutations occur at very low frequencies in nature.
Mutations may occur at very low frequency and such mutations are called spontaneous mutations. The rate of spontaneous mutations vary between 10-7 and 10-+. Some genes show exceptionally high rates of spontaneous mutations: they are known as mutable genes. Some genes increase the spontaneous mutation rates of other genes and such genes are called mutator genes. Some genes suppress the mutation of other genes of the genome and are termed as antimutator genes. Some sites within the genome have very high rate of mutation and are called as hot spots.
CLASSIFICATION OF MUTATION
I. BASED ON SIZE
(A) Point Mutation A mutation which arise due to a change in the base sequence of a gene is called as gene mutation or point mutation. It usually involves a single nucleotide or nucleotide pair.
1. Same sense/ Silent mutation— change in a codon (usually at the third position) that fails to change the amino acid and hence no change in the protein. They contribute to variability in the DNA sequence of individuals of a species.
2. Nonsense mutation— Nonsense mutation changes a codon for an amino acid into a termination codon, resulting in a shortening of the protein product due to a chain-termination signal. It often produces a mutant phenotype.
3. Missense mutation— Missense mutation alters a single base and changes the amino acid sequence in the polypeptide chain. They have serious effect on the protein produced and leads to mutant phenotypes.
4. Frameshift mutation— It shifts the reading frame by insertion or deletion of a base. As the number of bases inserted or deleted is not a multiple of three the reading frame will be altered, creating numerous missense or nonsense codons through the remainder of the cistron.
(B) Gross Mutation Changes involving more than one nucleotide pair, may involve the entire gene, the entire chromosome, or sets of chromosomes (polyploidy).
II. BASED ON CELL TYPE
(1) Somatic mutation: Occurs in non-reproductive cells of the body, often producing a mutant phenotype in only a sector of the organism (mosaic or chimera).
(2) Gametic mutation: Occurs in the sex cell, producing a heritable change.
III. BASED ON QUALITY
(A) Structural Mutation: Changes in the nucleotide content of the gene.
1. Substitution mutation - Substitution of one nucleotide for another.
a. Transition mutation substitute one purine for another or one pyrimidine for another.
b. Transversion mutation substitute a purine for a pyrimidine or vice versa.
2. Deletion mutation: It involves the loss of a portion of the DNA sequence. The amount lost varies greatly. Deletion mutation can be as small as a single base or much larger in some cases corresponding to the entire gene sequence.
3. Insertion mutation: Insertion or addition of extra bases, usually from another part of a chromosome causes insertion mutation. The amount inserted may be one or two bases or may be much larger.
(B) Rearrangement Mutation: Changing the location of a gene within the genome often leads to “position effects”.
1. Within a gene - two mutations within the same functional gene can produce different effects, depending on whether they occur in the cis or trans position.
2. Number of genes per chromosome - different phenotype efforts can be produced if the number of gene replicas are nonequivalent on the homologues chromosomes.
3. Moving the gene locus may create new phenotypes, especially when the gene is relocated near heterochromatin.
a. Translocation - movement to a non-homologous chromosome.
b. Inversion — a portion of the DNA sequence is excised then reinserted at the same position but in the opposite orientation.
IV. BASED ON ORIGIN
(A) Spontaneous mutation: Occurs naturally and is of unknown origin. It is also known as background mutation. It occurs at a frequency of 10-6
(B) Genetic control: The mutability of some genes is known to be influenced by other ‘mutator genes’.
1. Specific mutator — It affects one locus.
2. Nonspecific mutator - It simultaneously affects many loci.
(C) Induced mutation: Mutations produced due to the treatment with either a chemical or a physical agent are called as induced mutation. The agents capable of inducing mutations are known as mutagens. Induced mutations are useful in genetic and biochemical studies and also in crop improvement. The process of inducing mutations through treatment with a mutagen is known as mutagenesis, while the exploitation of induced mutation for crop improvement is called mutation breeding.
V. BASED ON DIRECTION
(A) Forward mutation : Creates a change from wild type to abnormal phenotype.
(B) Reverse or back mutation: Produces a change from abnormal phenotype to wild type.
i). Mutation suppressor — A gene change which occurs at a different site from the primary mutation, yet reverses its effect.
a. Extragenic (intergenic) suppressor — It occurs in a gene different from that of the mutant gene.
b. Intragenic suppressor — It occurs at a different nucleotide within the same gene. It shifts the reading frame back to original position.
VI. BASED ON MAGNITUDE OF PHENOTYPIC EFFECT
(A) Change in mutation rate: Some alleles can be distinguished only by the frequency with which they mutate.
(B) Isoalleles: Produce identical phenotypes in homozygous or heterozygous combinations with each other, but prove to be distinguishable when in combination with other alleles.
(C) Mutations affecting viability:
i) Subvitals - relative viability is greater than 10% but less than 100% compared to wild type.
ii) Semilethals - cause more than 90% but < 100% mortality.
iii) Lethals — kill all individuals before adult stage.
MUTAGENS
The agent that induces mutation is termed as mutagen. Mutagens include physical agents, chemical agents and radiations. The most commonly used mutagens are presented below.
1. ionizing radiation : Changes in chemical valence through the ejection of e-.
a. Particulate radiation o-rays, p—rays, fast neutrons, thermal neutrons.
b.Nonparticulate/electromagnetic radiation—X rays, y-rays etc.
2. Non-ionising radiation: It raises the energy levels of atoms (excitation), rendering them less stable. eg. UV-rays and heat. UV produces thymine dimmers i.e., bonding between thymines on the same strand.
3. Chemical mutagens: These are chemical substances that increase the mutability of genes. They are as follows;
1. Alkylating agents - sulphur mustards, nitrogen mustards, epoxides, ethylene imine (EI), sulphate and sulphonates (eg. Ethyl Methane Sulphonate (EMS),Methyl Methane Sulphonate (MMS), diazoalkanes, nitroso compounds.
2. Acrdine dyes - acriflavine, proflavine, acridine orange, acridine yellow,ethidium bromide(EB).
3. Base analogues — 5- bromouracil, 5-chlorouracil.
4. Others — nitrous acid, hydroxyl amine, sodium azide.
DETECTION OF MUTATION
A large number of testing methodologies are now available to screen for mutagenicity.
1. CIB Method
The CIB method was used by H.J. Muller for detecting X-linked (sex linked) mutations in male Drosophila. In CIB, B designates dominant mutation gene for bar-shaped eyes, which is used as a marker for flies. 1’ stands for recessive lethal gene present in the X chromosome. The | and B are always inherited together in the same chromosome. All the female Drosophila with bar-shaped eyes are heterozygous for this chromosome as homozygous females and males with CIB chromosome do not survive because of the lethal gene 1.
The CIB technique was used for identification of sex-linked recessive lethal mutants in male Drosophila after treating them with X-rays. The male Drosophila were first exposed to X-rays and were called X generation as X-rays were used as the mutagen. They are then crossed with bar-eyed CIB females. The resulting X2 generation consisted of a normal-eyed and bar-eyed CIB females alone and no males were recovered. As all the males with the CIB chromosome dies due to the lethal gene1
The F1 CIB female will have one CIB chromosome and one X chromosome from the irradiated male. When this F1 CIB female is mated to a normal male, the half of the male progenies will receive one CIB chromosome and die due the lethal gene 1. The remaining half of the male progeny will receive the X chromosome from the irradiated grandfather. If a lethal mutation was induced by X-rays in the X chromosome then this male progenies will also die.
The detection of sex linked recessive lethal gene in this method is based solely on the presence or absence of male progenies in crosses between Fl CIB female and normal male. If the mutation induced is not lethal, then the X2 will have 50% males. The frequency of the recessive lethal mutation in X chromosome is the ratio between the number of F1 CIB females producing no males to the total number of females tested.
AMES TEST
Ames in 1974 developed a simple test for evaluating the potential of chemicals to cause mutations. The Ames test is based on the principle that both cancer and mutations result from damage of DNA. The results have confirmed that 90 % of known carcinogens are also mutagens and the mutagenesis in bacteria could serve as an indicator of carcinogens in humans.
The Ames test uses four strains of Salmonella typhimurium that have defective coat protein. This coat normally protects the bacteria from chemicals in the surrounding. Also, their DNA repair system has been inactivated to enhance their susceptibility to mutagens. One of the four strains detects base-pair substitution while the other three detect different types of frameshift mutations. The strains carry a mutation rendering them incapable of producing the aminoacid histidine (his). Only bacteria that have mutated (hist) alone are able to synthesize histidine and grow on the medium.
Then the chemicals to be tested are added to the plates inoculated with the bacteria and the number of mutant colonies developed is compared with the control plates with no chemicals. Any chemical which increases the number of colonies is mutagenic and is also mostly carcinogenic in nature.