Final answer:
Insertion of an additional nucleotide into DNA can cause a frameshift mutation, altering the amino acid sequence in the protein and potentially leading to a nonfunctional or shorter protein if a premature stop codon is introduced.
Step-by-step explanation:
If an additional nucleotide were inserted into a DNA sequence in place of an adenine nucleotide being substituted, it would cause a frameshift mutation. This type of mutation shifts the reading frame of the genetic code, which can have dramatic effects on the protein that is produced. Every codon after the point of insertion would be changed, likely resulting in a completely different sequence of amino acids in the protein. This can lead to a nonfunctional protein or even a shorter protein if a premature stop codon is introduced as a result of the mutation.
When an insertion mutation occurs, it can disrupt the order in which codons are read during translation, and since codons are read in groups of three nucleotides, an insertion that is not in multiples of three will alter every subsequent codon. For example, the insertion of three nucleotides together after the fifth nucleotide in a coding gene would generally result in the addition of an extra amino acid without changing the rest of the amino acid sequence, although it might still affect the protein's length and function if a stop codon is introduced early.