Nucleus

The Nucleus controls all the activities of the cell and acts as the information centre. It is only found in Eukaryotes (prokaryotic DNA is loose in the cytoplasm). It contains chromosomes, which is the genetic material for the cell. DNA and RNA synthesis occurs here, in addition to being the site of transcription . After transcription, mRNA exits the nucleus through the nuclear membrane where it then travels to the cytoplasm, the site which is responsible for associating the mRNA with ribosomes and protein translation.

It is comprised of:

1. A Nuclear envelope (nuclear membrane) = twin membrane that protects contents from cytoplasm and macromolecules that could interfere with internal processes. Nuclear pores perforate the membrane and allow small molecules to diffuse freely in and out of the structure. Macromolecules (like nucleic acids) are moved into the nucleus via active transport.
2. Nucleoplasm (karyoplasm) = this material comprises mostly of Chromatin and is bound by the nuclear envelope.
3. Nucleolus = a densely packed, small body, known more so as the 'core' of Nucleus, built around the nucleolar organiser. It is a relatively unstable structure without a membrane, it contains rRNA (ribosomal RNA) and is the site of Ribosome synthesis. These ribosomal proteins are then packaged into ribonucleoproteins which later mature.
   

A nucleus is also the centre of an atom, comprised of protons and neutrons.

The nuclear envelope is supported by the nuclear lamina, found on the inner nuclear membrane, which is made up of interconnecting intermediate filaments in a sheet-like formation^[1].

References

1. ↑ Alberts, B., Johnson, A., Lewis, J., Raff, M., Roberts, K. and Walter, P. (2008) Molecular Biology of the Cell, 5th edition, New York, Abingdon: Garland Science, Taylor and; Francis Group, LLC