if nirenberg and matthaei performed their cell-free translation experiment using an mrna composed of 60% c and 40% a, what radiolabeled amino acids would be incorporated into the precipitated polypeptides? (check all that apply.) check all that apply phenylalanine phenylalanine leucine leucine isoleucine isoleucine proline proline methionine methionine valine valine serine serine threonine threonine alanine alanine tyrosine tyrosine histidine histidine glutamine glutamine