UAC to UAG-
Ans. is 'a' i.e., Nonsense mutation o Appearance of stop codon (UAG) due to mutation results in premature termination and is called nonsense mutation.Point mutation (Base substitution mutation)o Substitution of a single base by another is known as point mutation. If a purine is replaced by another purine base or if a pyrimidine base is replaced by another pyrimidine base, it is known as transition. Substitution of a purine base by a pyrimidine base or vice versa is known as transversion. A point mutation mil affect only one codon. Point mutation can be of following types >Silent mutationo If the substitution occurs in the third base of a codon, the code word may remain unchanged due to degeneracy of the genetic code. For example, GGC to GGG mutation has no effect as both code for glycine thus, codon containing the changed base codes for the same amino acidQ, therefore there is no change in amino acid sequence of the protein. The gene is expressed in term of same protein, Le. there is no change in expression of proteinQMis-sense mutationso If the base substitution changes the code words, i.e. codon containing the changed base codes for different amino acid(AIIMS-02),it is called mis-sense mutation. The sequence of amino acids in the protein changes. The effect of amino acid substitution is variable :-i) Acceptable missense mutation:- The substitution of amino acid has minimal or no effect because either the substituted amino acid is similar to that of original amino acid or the substitution occurs in non critical area. For example, normally hemoglobin (HbA) has valine at position 67 of b-chain in place of valine. This is non-critical position, i.e. substitution of different amino-acid 'at this position produces normally functional hemoglobin. For example, 'hemoglobin Milwaukee' has glutamic acid at this positionQ (in place of valine), 'hemoglobin bristoF has aspartic acidQ and hemoglobin Sydney' has alnine (in place of valine).Partially acceptable mis-sence mutation:- Substitution of one amino acid with other is partially acceptable, i.e. the resultant protein functions normally under certain conditions but not always. For example, substitution of glutamate by valine at position six of b-chain produces HbS. HbS is able to function normally at high oxygen tension but gets precipitated at low oxygen tension.Unacceptable mis-sense mutation:- Substitution of one amino acid with other produces non-functional protein. For example, in methemoglobin (HbM) histidine is replacement by tyrosine at position 58 of a- chain. HbM is incapable of combining with oxygen.C) Nonsence MutationQo Base substitution changes a sense codon into a nonsense (stop or termination) codon. In such cases, protein synthesis will be terminated prematurely and the resulting protein will be usually non-functional.Frame-shift mutationo Frameshift mutationsQ occur due to insertion or deletion0 of one or two bases which causes change in the reading frame distal to the mutation. If 1 or 2 base pair change, whole reading frame is changed distal to the mutation, resulting into entirely different protein molecule.If 3 base pairs change, single amino acid is incorporated or deleted. The rest of amino acid sequence is same.Trinucleotide repeat mutationQo In this type of mutation a codon (i.e. trinucleotide sequence) undergoes amplification and the same codon is repeated continuously so many times in the chain. Diseases associated with trinucleotide repeat mutation are Huntington's diseaseQ (CAG repeat), Spinocerebellar ataxiaQ (CAG repeat), friedreich ataxia (GAA repeat), fragile-X-syndrome (GGG or GCC repeat), dystrophia myotonica (CTG/CUG repeat), X-linked spinobulbar muscular atrophy (CAG repeat) and dentorubral pallidoiusian atrophy (CAG repeat).
Nonsense mutation
Frameshift mutation
Deletion
Missense mutation
Molecular Genetics