The Monera is the largest kingdom of prokaryotic microorganisms. Prokaryotes are single-celled organisms that lack an organized nucleus, i.e., one that is separated from the protoplasm by a membrane-like envelope. (Organisms in all other kingdoms have a cellular nucleus and are termed eukaryotic.) Monerans have their genetic material organized as a single strand of DNA, which occurs throughout their cytoplasm. They have no subcellular organelles, such as chloroplasts, mitochondria, or flagella. Monerans were the first organisms to evolve, about 3.5 million years ago (the first eukaryotes evolved 2 billion years afterwards).

Bacteria and blue-green bacteria (or cyanobacteria) are the major groups in the Monera. They have rigid or semi-rigid cell walls, propagate by binary division of the cell, and do not undergo mitosis or meiosis. The cyanobacteria are photosynthetic, using chlorophyll dispersed within the cytoplasm as their primary light-capturing pigment.

Biologists have named about five thousand species of monerans...