Here's why:

* The nucleus is the control center of the cell. It contains the cell's DNA, which directs all cellular activities. Without a nucleus, a cell cannot replicate, produce proteins, or carry out the basic functions of life.

* Prokaryotic cells, like bacteria and archaea, do not have a true nucleus. However, they do have a nucleoid region where their DNA is concentrated. This region is not enclosed by a membrane, unlike the nucleus in eukaryotic cells.

Therefore, a cell without a nucleus is not considered a complete and functioning cell.