Final answer:
The reading frame of a nucleotide sequence is set by the start codon AUG, which divides the sequence into codons. Frameshift mutations, like nucleotide insertions or deletions that are not in multiples of three, change the reading frame, leading to different proteins. The addition of three nucleotides adds an extra amino acid but retains the reading frame.
Step-by-step explanation:
The reading frame of a nucleotide sequence is set by the start codon AUG, which specifies the amino acid methionine. This codon establishes the division of the sequence into consecutive triplets, or codons, each of which corresponds to an amino acid. An insertion or deletion of nucleotides (referred to as frameshift mutations) can alter this reading frame. For example, an insertion of a single nucleotide after the start codon changes all subsequent codons, potentially leading to a completely different protein due to the mix-up of amino acid sequence. This explains why an insertion or deletion that is not in multiples of three is more detrimental than the addition of three nucleotides; the latter does not disrupt the reading frame and only introduces an extra amino acid without affecting the remaining protein structure.