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Gene mutations

Forward mutation

Genetic Mutation from wild-type to mutant or evolution of new mutation from wild-type is called as a forward mutation. Forward mutation leads to the evolution of new traits in the population. It is a natural phenomenon in nature.

Backward mutation

Mutation is a unidirectional process, but sometimes some mutation gives original (wild-type) traits back to population, such mutation is called as the backward or back mutation. Back mutation is a very rare and unusual process in nature.

It gives original phenotype back into the population by true back mutation or by the occurrence of a secondary mutation.

In true back mutation, a mutation occurred at the same location as it occurred during forward mutation. In simple words, it gives the wild-type codon back to population.

In secondary mutation, an occurrence of a new mutation in any place in the sequence of the gene which gives the original function back or sometimes the new mutation suppressed the effect of the previous forward mutation.

Copying error

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

Point mutation: mutation in which a single base is altered, termed as a point mutation (addition or deletion of a single nucleotide). 

We had already covered an article on the nucleotide, read the article here: The Function of dNTPs in PCR reaction

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 codonoriginated from a nucleotide changethat 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

Some of the mutations are extra large; in fragile X syndrome, an alteration in triple repeat number results in an abnormal protein. In the 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.

Read the entire article on fragile X and trinucleotide expansion mechanism: Trinucleotide repeat expansion disorders

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.

Pleiotropy

Mutation in one gene influences more than one trait or phenotype is referred to as pleiotropy. Again recall the situation in sickle cell anaemia. Mutant HBS allele results in anaemic condition as well as protect the heterozygous individual from the malaria parasite. We will discuss pleiotropy broadly in other articles.

Different types of Genetic mutations randomly occur in the population. It creates allelic variation in a genomeand the new allele originates in the population. Polymorphism is a natural phenomenon. Genetic 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”.