Cells showing actin filament cytoskeleton (red), mitochondria (green), and DNA (blue). In cell biology, an organelle is a specialized subunit within a cell that has a specific function, and it is usually separately enclosed within its own lipid bilayer. Organelles are identified by microscopy, and can also be purified by cell fractionation. There are many types of organelles, particularly in eukaryotic cells. Microfilaments or actin filaments are the thinnest filaments of the cytoskeleton, a structure found in the cytoplasm of eukaryotic cells. The mitochondrion (plural mitochondria) is a membrane-bound organelle found in most eukaryotic cells. Deoxyribonucleic acid (DNA) is a molecule that encodes the genetic instructions used in the development and functioning of all known living organisms and many viruses. DNA is a nucleic acid; alongside proteins and carbohydrates, nucleic acids compose the three major macromolecules essential for all known forms of life.