Prokaryotic and eukaryotic cells: is there a nucleus?

0:00Cell theory says all living things are made of cells. But does every cell have a nucleus?

0:07There are two types of cell: prokaryotic and eukaryotic.

0:12Prokaryotes are bacteria and archaea, each made of just one cell. Here is a bacterium.

0:20Animals, plants and fungi are made of eukaryotic cells. Here is an animal cell.

0:28Both cells have a cell membrane, a thin layer that controls what goes in and out.

0:34Both are filled with cytoplasm, a jelly-like substance where many reactions happen.

0:40Both also have ribosomes, tiny parts that make proteins.

0:45Only the bacterium has a cell wall, a stiff outer layer that supports the cell.

0:51The key difference: the bacterium's DNA, its genetic code, lies loose in the cytoplasm.

0:59The animal cell keeps its DNA inside a nucleus, which has its own membrane.

1:05Organelles are cell parts with their own jobs. Mitochondria release energy. Like the nucleus, they are membrane-bound.

1:14So not every cell has a nucleus. Prokaryotic cells have no nucleus and no membrane-bound organelles. Eukaryotic cells have both.

1:25But no nucleus does not mean no DNA. The bacterium still has its DNA.

1:32Prokaryotic cells are usually 1 to 5 micrometres. A micrometre is a thousandth of a millimetre.

1:40Eukaryotic cells are usually 10 to 100 micrometres, so this drawing is not to scale.

1:47Both types of cell have DNA, but only eukaryotic cells keep it in a nucleus.