The term 'BASIC BIOLOGICAL BUILDING BLOCK' can be broken down into four crossword solutions: ATOM (4 letters). An ATOM is the smallest unit of matter that retains its chemical properties. It is composed of a nucleus, which contains protons and neutrons, and electrons that orbit around the nucleus. Atoms are the building blocks of all matter, including biological matter. They are essential for life as they form the molecules that make up living organisms.