Types of Mutations: Spontaneous Mutations Dr. Anuj Rani Department of Botany, T.N.B. College (T.M. B.U., Bhagalpur-812007) [E- learning: B.Sc. Part-III, Botany Hons.]
Definition: A sudden change in the genetic information is called a mutation. Thus the term mutation refers to both the change in the genetic material and the process by which the change occurs. An organism that exhibits a novel phenotype resulting from a mutation is called a mutant.
Types of Mutations on following basis: (i) On the basis of Forward and Reverse Mutations: Forward mutation: A mutation from wild type (original type) to a new type is known as forward mutation. Reverse mutation: The mutated gene may mutate back to the wild type. It is known as reverse mutation.
(ii) On the basis of nature of occurrence: Spontaneous mutations: Spontaneous mutations are result of errors that occur naturally in DNA. Induced mutations: Induced mutations are result of errors that occur by exposure to radiation, chemicals, viruses, or other mutagenic agents. (iii) On the basis of cell type: Gametic Mutations: Mutations occurring in germ cells or gametes produce gametic mutations. These mutations are especially significant because they can be transmitted to offspring and every cell in the offspring will have the mutation. Somatic mutations: Somatic mutations occurring in somatic or non-reproductive cells are called somatic mutations. These mutations may have little effect on the organism because they are confined to just one cell and its daughter cells. Somatic mutations cannot be passed on to offspring because are lost with death of the mutant organism.
(iv) On the basis of part/portion of genetic material affected:
1
Chromosomal Aberration: Physical/Structural/Visible changes in chromosome that might be accidental or induced. Point Mutation: A point mutation is a change in a single nucleotide in DNA. This type of mutation is usually less serious than a chromosomal alteration. An example of a point mutation is a mutation that changes the codon UUU to the codon UCU. Point mutations can be divided into three types:
1. Silent mutation (mutated codons codes for the same amino acid, cause no effect). 2. Missense mutation (mutated codons codes for the different amino acid, cause variable effect). 3. Nonsense mutation (mutated codon is a premature stop codon, generally cause serious effect).
Spontaneous Mutations: Spontaneous mutations are the result of errors in natural biological processes, have a very low frequency. While induced mutations are due to agents in the environment that cause changes in DNA structure.
Origin of Spontaneous Mutations: Spontaneous mutations arise from a variety of sources, including errors in DNA replication, spontaneous lesions, and transposable genetic elements. Mutation involves changes in DNA. Several mechanisms are known that bring about alterations in DNA. These modifications may arise from error in DNA replication, tautomeric shifts and damage to DNA from radiation. Errors occur during replication by substitution of frame shift in DNA sequence. (I) Base pair substitution: Substitution of base pairs is the most common mutation. During replication of DNA repair wrong base pairs are incorporated. Base pair substitutions are of two types, transition and transversion. Mutations resulting from tautomeric shifts in the bases of DNA involve the replacement of a purine in one strand of DNA with the other purine and the replacement of a pyrimidine in the complementary strand with the
2
other pyrimidine. Such base-pair substitutions are called Transitions. Base-pair substitutions involving the replacement of a purine with a pyrimidine and vice versa are called Transversions.
Tautomerism: Tautomerism is the relationship between two structural isomers that are in chemical equilibrium and readily change into one another. Generally the bases exist in keto form, but at a time they can take on either an imino or enol form. (Fig.02). A rare form of adenine can pair with cytosine, and the end form of thymine can pair with guanine. These tautomeric shifts alter the hydrogen bonding characteristics of the bases, and permit for purine substitution or pyrimidine for pyrimidine substitution. These lead to stable modification in nucleotide sequences.
Fig 1: Transition (A) and Transversion (B) mutation (a)Transition mutation: Transition is the replacement of one purine (e.g. A or G) by another purine (e.g. G or A) or one pyrimidine (e.g. T or C) by another pyrimidine (e.g. C or T). Four types of changes are possible in transition such as AT → GC, GC → AT, TA →CG, and CG → TA. (Fig. 01,a). During replication, errors in DNA arise with high frequency. (b) Transversion mutation: Transversion mutation involves the substitution of purine by a pyrimidine or a pyrimidine by a purine. This type of mutation is rare due to steric problems of paring of purines with purines, and pyrimidines with pyrimidines. Eight types of changes are possible in transversion such as AT → TA, AT → GC, GC → CG, GC→ TA, AT → TA, TA → GC, CG → GC, and CG → AT (Fig.02 b).
3
Fig02: Tautomeric Shift for all four Bases
4
Fig 03:Mutation due to Tautomeric shift that causes Base pair substitution
Fig 04: mechanism by which tautomeric shift in the bases in DNA cause mutation
5
(II) Frameshift Mutation: A Frameshift mutation is a deletion or insertion of one or few nucleotides in the DNA molecule ,that changes the reading frame of the base sequence resulting in mutation. Therefore such mutation those results from shifting in reading frame backward or forward. Deletions remove nucleotides, and insertions add nucleotides (Fig 05).
Fig 05: Frameshift Mutation
All three types of point mutations Transitions, Transversions, and Frameshift mutations are present among spontaneously occurring mutations.
*********************************
6