A point mutation is characterized by a change in a single nucleotide within the DNA sequence. This alteration can occur through various mechanisms, such as the replacement of one base with another (substitution), the addition of a base (insertion), or the deletion of a base (deletion). The significance of point mutations lies in their potential effects on protein synthesis and function, which vary depending on the exact nature of the mutation and its location within the gene.
The other options describe characteristics that do not align with the definition of a point mutation. For example, altering the entire protein structure would imply a more significant change than a single nucleotide substitution, leading to misconceptions about the scope of a point mutation. Saying that it does not affect the polypeptide chain contradicts the very nature of point mutations, as they can lead to changes that affect protein function. Lastly, involving multiple nucleotides suggests a different kind of mutation altogether, such as insertions or deletions shaped by larger segments of the DNA sequence. Therefore, defining point mutations by their primary characteristic of changing a single nucleotide is essential for understanding how these variations influence genetics and molecular biology.