Which Organisms Lack a Nucleus? Prokaryotes Explained

Q: Which organisms lack a nucleus?

Organisms that lack a membrane-bound nucleus are called prokaryotes. They include bacteria and archaea. Their DNA floats in a region of the cell called the nucleoid instead of being enclosed in a nucleus. All other life, including animals, plants and fungi, is made of eukaryotic cells, which do have a nucleus.

Prokaryotes vs eukaryotes

	Prokaryotes	Eukaryotes
Nucleus	No	Yes
Examples	Bacteria, archaea	Animals, plants, fungi, protists
Typical size	0.5–5 micrometers	10–100 micrometers
Organelles (like mitochondria)	None	Many
DNA	Usually one circular chromosome	Several linear chromosomes
Body	Almost always single-celled	Single-celled or multicellular

The name comes from Greek: pro (“before”) + karyon (“kernel” or nucleus), while eu means “true.”

Bacteria

Bacteria live almost everywhere: soil, water, inside animals and even deep underground. Most are harmless or helpful. Your gut holds trillions of them, which help digest food. A few cause disease. Some soil bacteria make the smell after rain; see what causes the smell after rain.

Archaea

Archaea look like bacteria under a microscope, but their chemistry is very different. Microbiologist Carl Woese recognized them as a separate group in 1977. Many live in extreme places:

Boiling hot springs, like those at Yellowstone
Very salty water, such as the Great Salt Lake
Acidic pools and the deep seafloor

They also live in ordinary places, including the human gut.

Life’s three domains

Biologists sort all living things into three domains: Bacteria, Archaea and Eukarya. Prokaryotes were the first life on Earth, appearing at least 3.5 billion years ago. Eukaryotic cells evolved much later, partly by absorbing bacteria that became mitochondria.

Do viruses have a nucleus?

No, but viruses aren’t prokaryotes either. They aren’t made of cells at all and can only reproduce inside a host cell. For how cells copy their DNA, see what DNA is.