Which of the following statements are true? (choose all that apply) Some amino acids are specifed by just one codon, whereas others are specifed by multiple codons. Because there are 4 RNA nucleotides and each codon is 3 nucleotides long, there are 64 possible codons. each of which directs the ribosome to incorporate a different amino acid into a growing polypeptide chain. The limitations of the genetic code are such that a single amino acid may be specified by no more than four codons Because there are three different possible teading frames in a messenger RNA (mRNA) molecule, most mRNAs can be translated in a cell into three different proteins..