Here's a breakdown:

* Polynucleotide: A long chain of nucleotides linked together. Examples include DNA and RNA.

* Nucleotide: The building block of a polynucleotide. It consists of three parts:

* A nitrogenous base: Adenine (A), guanine (G), cytosine (C), thymine (T) in DNA, or uracil (U) in RNA.

* A pentose sugar: Deoxyribose in DNA, or ribose in RNA.

* A phosphate group: A chemical group containing phosphorus.

So, when nucleotides join together, they form a polynucleotide chain.