Non-sense mutations often occur when:

Nonsense mutations are a specific type of genetic mutation that result in the substitution of a single nucleotide in the DNA sequence, which ultimately leads to the formation of a premature STOP codon. This premature STOP codon terminates the translation process of the mRNA, leading to an incomplete protein that is often nonfunctional.

The creation of a STOP codon can have severe effects on the resulting protein, as the reading frame is interrupted, causing downstream amino acid sequences to be omitted. In many cases, the resulting protein may not perform its intended biological function, which can lead to various phenotypic consequences.

The other choices relate to different types of mutations or genetic phenomena. For example, alterations in amino acid coding occur with missense mutations, where a single nucleotide change results in a different amino acid being incorporated into the protein. If the phenotype remains unchanged, it typically suggests that the mutation is silent or does not affect protein function significantly. Lastly, repeated sequences of nucleotides refer to another form of mutation called expansions, which can lead to disorders but do not specifically relate to nonsense mutations. Thus, the focus on the creation of a premature STOP codon accurately describes the core outcome of a nonsense mutation.