Mutation class 1: What is mutation?

“Under the influence of environment and other adverse conditions, any alteration in nucleotide sequences of DNA is termed as mutations”.

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The term mutation was coined by Hugo De Vries. It is an important phenomenon in nature for the creation of variation.

In routine life, we are faces so many unfavourable conditions- like adverse medication, contrast food, UV light and other adverse environmental conditions. Many different nucleotides are deleted and inserted during adverse conditions but these are non-pathogenic. However, some conditions are extreme, as in SRY, if some sequences or entire SRY gene is deleted, individual remain infertile, during entire life.

Different types of mutation are explained here. When a single nucleotide is deleted or inserted in a genome, it creates SNPs (single nucleotide polymorphism). Millions of different SNPs are arising every day but DNA has its own repair mechanism, which repairs almost any variation of the genome.

Polymorphisms is a natural phenomenon. Mutation is occurred to make us adaptive in an adverse environment but it may be harmful sometime. Scientifically, “what we are today, is a result of millions of mutation in the past”.

Copying error

Copying error occurs during cell division while replication. If it remains un-repaired, such a mutation will change the genetic code.

Chromosomal mutation: deletion or insertion of any part in a chromosome is called as a chromosomal mutation. Generally, it may include more than one gene because a part of the chromosome contains many genes. In detail, chromosomal abnormalities are covered in “ chromosomal alteration and disease”.

Point mutation: mutation in which a single base is altered, termed as a point mutation.

the image represents a point mutation in the DNA sequence. image credit: genome research limited

Silent mutation: it is non-expressive. In silent mutation, the new codon is created from the mutation but it codes for the same amino acid as wild-type.

Mis-sense mutation: A codon originated from a nucleotide change that will code for different amino acids. It can lead to alteration or loss of function in protein.

Non-sense mutation: A stop codon is added to the premature protein. It stops protein synthesis because a stop codon ends synthesis of protein and results in a premature protein.

Frameshift mutation: Alteration in a base pair which results in an abnormal reading frame, and leads to abnormal protein. A specific reading frame has a start codon and a stop codon, in between both codons, a definite coding sequence is present. In frameshift mutation, alteration in DNA  leads to shifting of this reading frame hence the position of start or stop codon altered which results in an abnormal protein.

The image represents frameshift in the DNA sequence. image credit: genome research limited

Other mutations are extra large, in fragile X syndrome, an alteration in triple repeat number result in an abnormal protein. In case of DMD, an entire exon is deleted from DMD gene. Some mutations can alter the expression of other genes. For example, a mutation in the SOX9 somatic gene results in the altered expression of SRY gene.

The image represents base substitution mutation in DNA. image credit: genome research limited.

Some mutations are stable, it can be inherited from generation to generation. Some mutations are non-stable,  may occur for some time in a single generation.

Stable mutations are generally congenital, and non-stable mutations arise after birth. Stable mutations incorporated into genome it can never be repaired, while unstable mutation can be repaired and passes the wild-type allele back to the population.

Mutation is a random process it creates allelic variation in a genome and the new allele is arise in the population. This allele is termed as a mutant allele, the normal allele is wild type. Mutation from wild-type to mutant is called as forward mutation, and mutation from mutant to wild-type is called as reverse mutation or back mutation.

Article covered and reviewed by- Tushar Chauhan

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What is DNA replication?

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