Is a single polynucleotide necessarily and always a nucleic acid?
There are two types of nucleic acids: 1) DNA --> which is double-stranded, so it's made of only two polynucleotides 2) RNA --> which is single-stranded so it's made of only one polynucleotide. So if we get back to my original question "Is a single polynucleotide necessarily and always a nucleic acid?" --> the answer is no, for DNA is a nucleic acid and it is made of two polynucleotides.