The cells are not made of it, it's a part of the cell. It's like the DNA part of it, most animals and plants and humans have it, but there are times when animals and plant don't have any at all. Also the mature red blood cells don't have a nucleus to make room for oxygen or something.
well cells have a nucleus at the center, doesnt mean its made of it. there are other components like cytoplasm ect.
Nope. Eukaryotic cells have a nucleus, however prokaryotic cells lack a nucleus.
No, there are cells that have no nucleus