Question

A DNA sequence that is 15 nucleotides long would give rise into a polypeptide sequence that was how many amino acids long?

Answer 1

A DNA sequence that is 15 nucleotides long would normally give rise to a polypeptide sequence that would be 5 amino acids long. This is assuming that all the nucleotides in the DNA sequences are strictly coded to form only sense codons and not a single nonsense codon, also called termination codon

A sense codon is a set of three nucleotides also called a triplet, that codes for a particular amino acid. A DNA sequence of 15 nucleotides has 5 codons.

A nonsense or termination codon is one that does not code for any amino acid. There are three nonsense codons found on mRNA, and these are UAA, UAG and UGA.  So if the DNA sequence has one of these, then the amino acids  in the polypeptide chain will be 4 in number