Types of Mutation-Deletions and Insertions-Structural Effects of Mutations on the Protein-Non-synonymous mutation

Types of Mutation

•Mutations can range from single base substitutions, through insertions and deletions of single or multiple bases to loss or gain of entire chromosomes. Base substitutions are most prevalent.

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•A substitution is the replacement of a single nucleotide by another. These are the most common type of mutation.

•If the substitution involves replacement by the same type of nucleotide—a pyrimidine for a pyrimidine (C for T or vice versa) or a purine for a purine (A for G or vice versa); this is termed a transition.

•Substitution of a pyrimidine by a purine or vice versa is termed a transversion.

•Transitions occur more frequently than transversionsDeletions and Insertions

•A Deletion involves the loss of one or more nucleotides.

•An Insertion involves the addition of one or more nucleotides into a gene.

•Frameshift mutation: If deletion or insertion  occurs in coding sequences and involves one, two, or more nucleotides that are not a multiple of three, the reading frame will be disrupted. 

•Larger deletions or insertions may result in partial or whole gene deletions and may arise through unequal crossover between repeat sequences.

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•Structural Effects of Mutations on the Protein

•Mutations can also be subdivided into two main groups according to the effect on the polypeptide sequence of the encoded protein, being either:

• Synonymous or silent mutation: If a mutation does not alter the polypeptide product of the gene. A single base-pair substitution, particularly if it occurs in the third position of a codon because of the degeneracy of the genetic code, will often result in another triplet that codes for the same amino acid with no alteration in the properties of the resulting protein.

•Non-synonymous mutation: If a mutation leads to an alteration in the encoded polypeptide. The alteration of the amino acid sequence of the protein product of a gene is likely to result in abnormal function, which is usually associated with disease, or lethality, which has an obvious selective disadvantage.

•Non-synonymous mutations can occur in one of three main ways:

–Missense

–None-sense

–Framshift