Explain in detail about the types of gene mutations.

Explanation

Types of Mechanism of Gene Mutations. The smallest part of a gene that can undergo mutation is known as muton. It can be as small as a single nucleotide. Most of the gene mutations involve a change in only a single nucleotide. A gene may undergo several point mutations. This produces multiple alleles. Gene mutations occur by three methods-inversion, substitution (of two types-transition and transversion) and frameshift (of two types-insertion and deletion).
1. Inversion. A distortion of DNA by mutagen can change the base sequence of a cistron in the reverse order. The process is called inversion. The new sequence will naturally have different codons,. e.g.,

2. Substitution (Replacement). In substitution a nitrogen base is changed with another. It is of two types, transition and transversion. (i) Transition. A nitrogen base is replaced by another of its type, that is, one purine is replaced by another purine (adenine guanine) while one pyrimidine by another pyrimidine (cytosine = thymine or uracil). e.g.,

(ii) Transversion. In this a purine base is substituted or replaced by pyrinidine base and vice versa. e.g., Cytosine with Guanine (C`hArr` G) Adenine with thymine (A`hArr` T)

3. Frame-Shift. It is gene mutation where the reading of codons is changed. due to insertion or deletion of nucleotides. It is just like changing the sentence-The Fat Dog Ate the cat by insertion of alaphabet T. It will read-The Tfa Tdo cat Eth Eca T which is meaningless. Similarly, deletion of letter F will produce another meaningless sentence-The Atd oga Tet Hec At. Insertion of one or more nucleotides shifts the reading of nucleotide frame in the forward direction while deletion will cause a backward reading of frame. (i) Insertion. One or more nuleotides are added in the DNA segment representing a gene.

(ii) Delection. One or more nucleotides are lost from a segment of DNA representing a cistron or gene.

Nonsense, Same-sense and Mis-sense Mutations. A nonsense mutation is the one which stops polypeptide synthesis due to formation of a terminating or nonsense codon, viz., ATT(UAA), ATC(UAG), ACT(UGA). A mis-sense mutation is the one which involves change in a codon that produces a-different arnino acid at the specific site in polypeptide, often resulting in its nonfunctioning. A same-sense mutation is silent mutation in which the codon is changed but the change does not alter the amino acid specificity (e.g., GCA`to `GCT or GCC or GCG).