DNA

DNA, or deoxyribonucleic acid, is an extremely complex molecule that serves as the "master molecule" of organic life by encoding for proteins that form cellular structures. DNA molecules are made up of two intertwined chains of simpler molecules called bases, assembled on rails of sugar and phosphate molecules like rungs on a ladder.

Four different kinds of molecules form the rungs of the DNA ladder: adenine, thymine, guanine, and cytosine, which are typically referred to by their first letters, A, T, G, and C. In the DNA molecule, A always pairs with T, and G always pairs with C. DNA molecules replicate by "unravelling" their rails. Each rail keeps its associated base. Complementary bases (A for T, G for C) attach to the free bases until two new strands of DNA are created.

In 2369, following clues developed by his mentor, Richard Galen, Captain Jean-Luc Picard discovered that an ancient race of humanoids had seeded many planets with their DNA billions of years earlier. The result of this DNA being spread across the galaxy is the humanoid form itself. This DNA creates a tendency toward a specific external form. The similarity created allows even for inter-species reproduction between species that evolved quadrants apart, however the internal workings even down to the base element of the blood can still be quite diverse.