Cell organelles, or subcellular structures, are specialised to carry out certain functions. The main function of the nucleus is to contain the genetic material (DNA) of the cell. And where better to begin A-Level biology revision than at the information centre of the cell.
The Nucleus
The nucleus (plural nuclei) is often drawn as a dark ‘blob’ within the cell, but its structure is actually a little more complex. Separating the nucleus from the cytoplasm, there is a double membrane called the nuclear envelope. This membrane has ‘holes’ called nuclear pores which allows some molecules to pass in or out. For example, messenger RNA (mRNA) must leave through the nuclear pores to carry genetic information to other organelles called the ribosomes so that proteins can be synthesised.
There is, in fact, another ‘blob’ within the nucleus itself. This is called the nucleolus, just to be confusing. This gives the nucleus a sort of ‘googly eye’ appearance in a more advanced diagram. The function of the nucleolus is to produce ribosomal RNA (rRNA), which makes up part of the structure of ribosomes. You can head over and read the articles about RNA and ribosomes if you are unsure about those.
The nucleus contains DNA, the instructions to make the proteins that the cell needs to carry out its function and give it its structure. DNA is a clever and complex molecule which is very carefully replicated each time a cell divides. We will take a look at DNA in more detail in our biological molecules series. Amazingly, if you stretched out the DNA contained with one human cell, it would be about 2 metres in length. Given that the human body is estimated to have about 37 trillion cells, you can see what a phenomenal length of DNA is contained within the human body! It simply must be condensed down. So, in eukaryotic cells, DNA is wrapped around proteins called histones to form chromatin. Chromatin makes up what we know as chromosomes. Humans have 23 pairs of chromosomes (46 in total), but the number varies depending on species.
Want to see an image of a real nucleus? Check out the electron micrograph at this link.
Summary
So there you have an overview of the nucleus, the information centre of a eukaryotic cell. Don’t forget that prokaryotic cells do not have a true nucleus because they don’t have membrane-bound organelles.
In summary:
- The nucleus contains DNA stored as chromatin.
- It is surrounded by a double membrane called the nuclear envelope, which contains nuclear pores.
- The nucleolus produces rRNA.
- Eukaryotic cells have nuclei, prokaryotic cells do not.
