Which one of the following terms is not a part of the nucleus?

1. Ribosomes
2. Nucleolus
3. Chromosome
4. Gene

 The nucleus is a spherical organelle found in eukaryotic cells, enclosed by a nuclear envelope.

It contains chromosomes and is rich in DNA.

Controlling gene expression, and DNA replication are the major functions of the nucleus.
The nucleolus is a sub-organelle found in the nucleus, without a limiting membrane surrounding it.

It does not contain chromosomes, but it is rich in RNA.
The main functions of the nucleolus are ribosome synthesis, and assembling signal recognition particles.