Polynucleotides have directionality !
Adjacent nucleotides in a single strand of DNA (polynucleotide) are joined by a phospho - diester bond between their 3' and 5' carbons. (Recall that an ester bond is R - O - R). This means that the respective 5' and 3' carbons are exposed at either end of the polynucleotide, which are therefore called the 5' end and the 3' end. These are also called the phosphoryl and hydroxyl ends, respectively, because of the chemical groups typically attachd to those ends. A polynucleotide therefore has directionality: the two ends of the molecule have very different biochemical properties, and behave very differently in molecular genetic processes. It is therefore critical to recognize which end is which.
A detailed understanding of this figure is key to the first part of the course !!!