Prokaryotic cells are usually microscopic, but some recently discovered prokaryotes are visible to the naked eye. A taxonomic kingdom comprising microorganisms that lack a true nucleus or any other membrane-bound organelle. Instead of a nucleus, prokaryotic cells contain a nucleoid, which is a DNA-dense region of the cell where DNA replication and transcription occur. The chromosome of these cells is circular and, in contrast to eukaryotic cells, continuously codes for cellular proteins and other molecules. Some cells also contain a plasmid, which is a smaller separate piece of circular DNA. The ribosomes of prokaryotes are found floating loose in the cytoplasm, whereas in eukaryotic cells the ribosomes are found in or on the rough endoplasmic reticulum.