Question

Define mutation. List the three ways in which mutations can arise.

Answer 1

Hint:- Any alteration in the DNA sequence which may or may not be expressed in the phenotype of an organism is called mutation. Mutation is one of the key players for evolution. There can be many ways by which mutation can arise. Let us know more about mutation to understand it clearly.

Complete Answer:-
Step 1: Let us first understand mutation.
As said above, any change in the nucleotide sequence of the genome of any organism or even in extrachromosomal DNA is called mutation. There can be many agents or causes for mutation. Any error during the replication process, exposure to ionizing radiations or specific agents called mutagens, or viral infections can result in the mutation. Mutagens are any agents, physical or chemical, which can give rise to mutation. Ex- radioactive substances, UV radiation and some chemical agents. Mutation may or not have an altered phenotype.
Step 2: Three ways in which mutations can arise-
Insertion- addition of one or more extra nucleotides to the DNA is called insertion. They are mainly mediated by transposable elements or due to repeating units replication error. They may induce a change in gene expression when inserted into the coding region of mRNA or by changing the reading frame of a gene. Reversion of insertion can be obtained by excising the transposable element.
Deletion- when one or more nucleotides are removed from the DNA. They can also alter the gene expression. They are generally irreversible.
Substitution- when a nucleotide is exchanged for another nucleotide, it is called substitution. The main causes are chemicals or errors in DNA replication.
Transition- when substitution is done with the same type of base, i.e; purine by purine and pyrimidine by pyrimidine.
Transversion- when purine is substituted by pyrimidine and vice-versa.

Note:-
I. The change in single nucleotide by any of the above said phenomena is called point mutation.
II. The above ways are based on the structural mutation on a small scale. Large scale structural mutations are manifested in the form of chromosomal rearrangements or abnormality.
III. Mutation in the non- coding region of a gene such as promoters of enhancers alters the rate of transcription and is one of the mechanisms for the regulation of gene expression.
IV. Mutations can be beneficial or harmful. Harmful mutations are manifested in the form of some genetic disorders or diseases such as cancer. In some cases though, mutations may render some organisms to survive better by providing resistance to environmental stress or by improving their reproduction capacity and is thus beneficial. Theory of natural selection for evolution is based on this particular phenomenon.