nitrogenous base

This term is a technical designation used exclusively within biochemistry and molecular biology. It describes the specific heterocyclic molecules that form the rungs of the DNA and RNA ladders. Because it is a precise scientific term, it lacks a casual or metaphorical equivalent and is always used in a formal, academic, or clinical register.

In a grammatical sense, the term functions as a compound noun. While "nitrogenous" is an adjective modifying "base," the entire phrase acts as a single semantic unit. When referring to multiple types of these molecules, the pluralization occurs on the head noun "base."