Here's why:

* Polynucleotides are long chains of nucleotides.

* Nucleotides are the building blocks of nucleic acids. They consist of a sugar, a phosphate group, and a nitrogenous base.

* DNA is a nucleic acid that carries the genetic instructions for the development, functioning, growth, and reproduction of all known living organisms and many viruses. It is composed of two long polynucleotide chains, each made up of a backbone of alternating sugar and phosphate groups. These chains are linked together by hydrogen bonds between the nitrogenous bases.

Therefore, DNA is a specific example of a polynucleotide.