MUTATION DEFINITION Mutation can be defined as a

MUTATION

DEFINITION: • Mutation can be defined as a sudden heritable change in the genotype either in structure or in its content. • The term ‘mutation’ was first coined by Hugo de Vries in 1901. De Vries defined mutation as ‘sudden change in heriditary material which was the main cause of evolution. ’ • The affected individuals due to mutation are called ‘mutants. ’ • Mutation is a very rare process, as evident from tremendous constancy in the forms of species from generation to generation.

ROLES: Mutation is the ultimate source of genetic variation. It’s role in living organisms are as follows • It plays the key role in the process of evolution, as mutation provides raw material for evolution. • It brings about diversity in biological entity. • Using it’s beneficial roles; live stocks, crop varieties etc. may be improved for human welfare. • Mutant forms in many cases have more adaptive power over the wild forms.

MAJOR CLASSIFICATION OF DIFFERENT MUTATIONS

TYPES Mutation can be variously classified. An account of them is given here. According to direction: 2 types A. Forward mutation : Forward mutations are mutations that cause the genotype to change from wild type to mutant type. The new form, as well as the wild form, may, however, further mutate to result in still additional allelic forms. B. Reverse mutation: Reverse or back mutations are mutations that cause the genotype to change from mutant type to wild type. This is usually fewer as compared to forward muttion. forward A a (wild form) ( mutant form) reverse

According to magnitude and phenotype: 3 types A. Dominant mutation: The mutation that can express over the wild type is called dominant mutation. So, it express in heterozygous condition. EXAMPLE: Defective tooth enamel in man is due to a X- linked dominant mutation. B. Recessive mutation: The mutation that cannot express over the wild type is called recessive mutation, which can express in homozygous state. EXAMPLE: Black body in Drosophila. C. Lethal mutation: Lethal mutation is the mutation that causes the death of any organism either during development or in adult stage, but before attainment of sexual maturity.

According to origin: A. Spontaneous mutation: Mutations that are going on as a natural process bringing random changes in nucleotide sequences of a gene, but without the help of any specific external agent, is called spontaneous mutation. Most spontaneous mutations occur during enzymatic process of DNA replication which leads to error in the genetic code. The error is then reflected in the protein structure. B. Induced mutation: Mutations that result from influence of any external or artificial factors are considered to be induced mutations. EXAMPLE: X- ray, ultraviolet ray, acridine dyes etc.

According to cell type: A. Gametic or germinal mutation: Mutations that occur in germinal tissues or gametes, known as gametic or germinal mutations. It is usually heritable and has a greater significance in evolution. EXAMPLE: Red green colour blindness in man. B. Somatic mutation: Mutations that occur in somatic cells, known as somatic mutations. It is not heritable, because these are not transmitted to the future generations; and, this has no importance in evolution. Somatic mutations will have a greater impact only if they are dominant and occur early in development.

According to size and quality: A. Gene/ Point mutation: Mutations that involves changes usually in a single ( or some-times few) base paires that affects a locus is called gene mutation. This is also termed as ‘point mutation’, because such mutation occurs at a particular point – the nucleotide of DNA. Gene mutation serves as the source of most new alleles and thus are origin of much genetic variability within a population. TYPES: It is of 2 types. A. Base-substitution mutation: It refers to replacement of one base pair by another pair. This is again of 2 types 1. Transition- Replacement of one purine base by another purine in one DNA strand of a pyrimidine by another pyrimidine on the complementary strand is called transition.

There may be 4 types of transition, as adenine (A) may be replaced by guanine (G) and vice versa; and thymine (T) may be replaced by cytosine (C) and vice versa. 2. Transversion: Replacement of a purine by a pyrimidine or a ptrimidine by a purine is called transversion. There may be 8 types of transversions, such as- A T, G C, A C, G T. MUTATIONS PRODUCED BY BASE-PAIR SUBSTITUTION: a. Silent mutation: This type of mutation is produced by substitution of the third base in a codon, but the change does not alter the amino acid. This is because the 3 rd base in a codon, according to Wobble hypothesis, is less specific and less important in coding the amino acid. EXAMPLE: ‘TTT’ codes for amino acid lysine. When the 3 rd base

of this codon ‘T’ is changed to ‘C’, then the new codon is ‘TTC’, still, the TTC also encodes the amino acid ‘lysine. ’ b. Missense mutation: This type of mutation caused by a nucleotide substitution in a codon alters the amino acid that means, a different amino acid is inserted into the polypeptide. EXAMPLES: ‘CCA’ codes for amino acid ‘pnenyl alanine’. When the 1 st base ‘C’ is changed to ‘U’ , the new codon is ‘UCA’. The codon UCA no more codes phenyl alanine, but it encodes a new amino acide ‘serine. ’ c. Non sense mutation: This type of mutation changes a sense codon to a termination codon or stop codon or non-sense codon by a nucleotide substitution. This mutation results in premature termination of polypeptide chain during translation. EXAMPLE: UAU codes for amino acid tyrosine. The 3 rd base U when is changed to G, the new codon is ‘UAG’. The codon UAG no more codes for tyrone, rather it srecifies the stop codon ‘amber’.

c. Neutral mutation: This type of mutation results in a base- pair change in the DNA, which causes change in the resulting m. RNA, but that particular amino acid substitution produces no detectable change in the fuction of the protein translated from the message. A neutral mutation is a subset of missense mutation in which the new codon codes for a different amino acid that is chemically equivalent to the original and therefore does not affect the protein finction. Consequently, the phenotype does not change. EXAMPLE: ‘TTT’ codes for the amino acid ‘lysine’. When the 2 nd base of this codon ‘T’ is changed to ‘C’, the new codon is ‘TCT’, which codes for a different amino acid ‘arginine’ , but lysine and arginine both are chemically similar, that is both are ‘positively charged amino acids. ’

B. Frame shift mutation: Such mtation results from the addition (or insertion) or deletion of one or more base pairs in a gene. Such change will cause all the codons to be changed, that is , the reading frame in the m. RNA will be altered. This is again of 2 types 1. Nucleotide insertion: This involves insertion of additional nucleotides in a gene, leading to alteration of codons and amino acid sequence. 2. Nucleotide deletion: This involves deletion of nucleotides from a gene, leading to alteration of codons and amino acid sequences. REMARK: Base substitution mutation does not involve loss or gain of nucleotides, but their replacement. But, frame shift mutation involves either gain or loss of nucleotides.