Directionality of nucleic acids
A strand of a nucleic acid (DNA or RNA) has directionality. The information contained in the sequence of nitrogenous bases can only be read in one direction.
The directionality implies that a base sequence such as GTA is not equivalent to ATG. For example, UUG codes for the amino acid leucine while GUU codes for valine.
Directionality also applies to natural language. The sentences "Tom likes Jane." and "Jane likes Tom." have quite different meanings.
The asymmetry of the phosphodiester bond ensures that the direction of the strand is always evident.