Mutation can be defined as any kind or form of change in the genetic code of a cell or organism. Point mutations can be defined as the genetic mutation of DNA or RNA by substituting one base with the other or loss of a nucleotide. This substitution or loss of a nucleotide will change the coding part of gene; this will result in a different form and structure of protein. Even though point mutation is a simplest form of mutation, the different form and structured protein is responsible for many kinds of genetic disorders in humans.
Types of Point Mutation:
Based on the effect of point mutations on the genome, they are classified into four types, such as
Silent mutation (Synonymous Mutation) :
In humans twenty amino acids are coded by sixty one codons. Many of the codons (synonyms) code for the same amino acids. For example amino acid leucine can be coded by six different codons, and in case of amino acid methionine two codons and amino acid tryptophan is coded by only one type of codon. In most of the cases, synonym codons differ only by one base, therefore in some cases point mutation will result in a codon, which codes for the same amino acid.
The DNA codons CAA, CAG, CAT and CAC (A for adenine, T for thymine, C for cytosine and G for guanine bases) all code for a valine amino acid. For example if the DNA strand codes for the codon CAA undergoes point mutations and codes as CAG, there will be no change in the structure of the resulting protein as both the codons code for the valine amino acid. This kind of nucleotide substitution is called as silent or synonymous mutations. This type of mutation is known as Silent mutation because this will not induce any changes in the resulting proteins.
A point mutation which results in a codon whih codes for a different amino acid is known as missense mutation. This kind of mutations is responsible for some of the genetic conditions in humans, for example sickle cell anemia. In sickle cell anemia, one of the codon has been changed from CTC to CAC, in the haemoglobin gene. This mutation substitutes glutamic acid with valine aminoacid, this changes the structure of protein haemoglobin, causing the genetic disorder.
A missense mutation can be of less significant, if the substitution occur between two similar amino acids. For example, DNA sequence changes from CTC (glutamic acid) to CTG (aspartic acid) may not induce a major structural and functional change in the resulting protein as both the amino acids are acidic in nature.
Nonsense Mutation and Stop Codon Mutation:
Mutations that change the coding codons into one of the stop codons are known as nonsense mutations. Nonsense mutations produce incomplete and non functional proteins as a result of stop codon, which is present in between the coding region of mRNA as a result of mutation. For example cystic fibrosis disease is caused as a result of nonsense mutation.
Mutation which changes the stop codon into a codon for an amino acid is known as stop codon mutations. This type of mutation results in production of too large protein. Making protein non functional and in some cases harmful proteins. Stop codon mutation causes a rare disease called as familial British dementia where the mutated amyloid protein clogs the brain.
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