Biology – Protein synthesis | e-Consult

A gene mutation is fundamentally a change in the nucleotide sequence of DNA. These changes can manifest as substitutions, deletions, or insertions of individual nucleotides. These alterations can have varying consequences for the resulting polypeptide, ranging from no effect to significant changes in protein structure and function.

Substitution Mutation: A substitution mutation involves one nucleotide being replaced by another. There are three main types of substitution:

• Silent Mutation: This is a substitution where the codon changes, but the resulting amino acid remains the same due to the redundancy of the genetic code. Therefore, the polypeptide sequence is unchanged.
• Missense Mutation: This is a substitution that results in a different amino acid being incorporated into the polypeptide. The effect on the polypeptide depends on the nature of the amino acid change. If the substituted amino acid has similar chemical properties to the original, the effect may be minimal. However, if the change is drastic (e.g., changing a hydrophobic amino acid to a charged one), it can significantly alter the protein's structure and function.
• Nonsense Mutation: This is a substitution that creates a premature stop codon. This results in a truncated polypeptide, which is often non-functional. The severity of the effect depends on where the premature stop codon occurs within the gene. A stop codon early in the gene will result in a severely shortened, non-functional protein.

Deletion Mutation: A deletion mutation involves the loss of one or more nucleotides from the DNA sequence. This can lead to a frameshift mutation if the number of deleted nucleotides is not a multiple of three.

• Frameshift Mutation: If the number of deleted nucleotides is not a multiple of three, the reading frame of the mRNA is shifted. This means that the codons are read incorrectly downstream of the deletion. This typically results in a completely different amino acid sequence from the point of the deletion onwards, and often leads to a non-functional polypeptide.
• In-frame Deletion: If the number of deleted nucleotides *is* a multiple of three, no frameshift occurs. Only one amino acid is missing from the polypeptide sequence. The effect on the polypeptide depends on the importance of the missing amino acid.

Insertion Mutation: An insertion mutation involves the addition of one or more nucleotides to the DNA sequence. Similar to deletion mutations, this can also cause frameshift mutations if the number of inserted nucleotides is not a multiple of three.

• Frameshift Mutation: If the number of inserted nucleotides is not a multiple of three, the reading frame of the mRNA is shifted. This typically results in a completely different amino acid sequence from the point of the insertion onwards, and often leads to a non-functional polypeptide.
• In-frame Insertion: If the number of inserted nucleotides *is* a multiple of three, no frameshift occurs. Only one amino acid is added to the polypeptide sequence. The effect on the polypeptide depends on the importance of the added amino acid.

In summary, the impact of a mutation depends on the type of mutation and the specific change it causes to the DNA sequence. Frameshift mutations are generally more severe than substitution mutations, as they often lead to non-functional polypeptides. The specific effect of a substitution mutation depends on the chemical properties of the substituted amino acid and its location within the protein.