What Is A Mutation?
A mutation is a modification while copying or replicating DNA molecules resulting in variants of DNA which can be passed onto the subsequent generations. These are caused by mutagens which are agents causing mutations. Mutation can occur in a part of the DNA or the entire DNA alteration can take place.
Types Of Mutations
There are many ways in which mutations can occur in DNA, listed below are types of mutations:
Base substitution/Point mutation
Point mutations are the single base substitutions. They are further subdivided into two types:
Point mutations can either be of these three mutations:
- Missense mutations – A missense mutation causes a single base pair substitution to modify the genetic code to produce an amino acid differing from the normal amino acid at that location. A missense variant modifies the functionality of a protein.
- Nonsense mutations – This mutation of the DNA causes an incomplete and shortened protein product wherein a base untimely stop reading mRNA causing the polypeptide chain to end abruptly and the resultant is a protein product that is truncated and non-functional. This mutation alters a codon to a stop codon.
- Silent mutations – Silent mutations are the mutations wherein a change in the DNA does not alter the amino acid it generates and hence does not affect the functionality of the amino acid. The phenotype of the individual is usually not affected in this type of mutation.
A frameshift mutation occurs due to an insertion or deletion of a base pair that are usually not in the multiples of three. It causes the transformation of the genetic code in an unreadable format or a reading frame from the location of the mutation up till the end of the gene. It differs from the substitution mutation in a way such that replacement of nucleotides does not occur. Frameshift mutations have no effect on the amino acid sequence, however, there may be a change in the protein regulation.
Addition or insertion of an additional base pair causes a frameshift which is dependant on whether added base pairs are in multiples of three or not.
Frameshift may occur as a result of a deletion of one or multiple base pairs from the DNA sequence. This causes modification in the translational frame due to the deletion of two or more base pairs thereby generating a nonfunctional and a distorted message. But, if three or more base pairs are deleted, the reading frame stays unimpaired.
Three Things That A Substitution Mutation Can Cause
One of the types of mutations is the substitution mutation. A substitution mutation switches one base for another, change in only one chemical letter takes place. This switch can be due to many sources which could be related to the storage and the reading of DNA. Due to the process of depurination, nucleotides tend to fall off. Since there are only four nucleotides to choose from, the proteins that manage the DNA tend to mistake in order to replace these nucleotides. This should be substituted by other proteins in the sequence to look up for errors in DNA.
It is probable to replicate this DNA if the substitution mutation is missed. The process of deamination is another driving factor. The only way a protein can discriminate between nucleotides is via the associated amino groups. The protein machinery tends to misidentify the nucleotide and hence supply the wrong nucleotide pair. The new nucleotide establishes in a new cell line during DNA replication. External factors such as sunlight and carcinogens can also cause nucleotide swaps. Carcinogens are chemicals that hinder the protein machinery and cause a lot of mutations. A substitution mutation can cause the following:
- Change in the coding of amino acids codon to a particular stop codon resulting in an incomplete protein, which is usually non-functional.
- Can cause Silent mutations where a codon change can encode the same amino acid resulting in no changes in the protein synthesized.
- Change a codon which encodes a variant amino acid that brings about a small change in the protein synthesized, an example of this is sickle-cell anaemia.
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