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Original Author(s): Aarushi Khanna
Last updated: 13th June 2018
Revisions: 12

Original Author(s): Aarushi Khanna
Last updated: 13th June 2018
Revisions: 12

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Every eukaryotic cell contains a nucleus. This article will discuss the structure and contents of the nucleus.


The nucleus has three main functions:

  • Gene expression – the nucleus houses the cell’s genomic DNA wrapped around histone proteins to form a structure called chromatin. This allows it to fit in the nucleus and protects it from damage. The DNA is used to control gene expression through a process called transcription, where a DNA template is used to produce pre-mRNA. This requires enzymes such as helicase which are also present in the nucleus.
  • Processing of pre-mRNA – newly synthesised nascent mRNA undergoes post-transcriptional modification to form mature mRNA before leaving the nucleus. These modifications include splicing, polyadenylation and capping
  • Compartmentalisation – a double membrane encloses the nucleus, protecting its DNA from the cytosolic environment, while perforations called nuclear pores allow passage of ions and small molecules.

In addition, the nucleus contains a nucleolus within it, which is the primary site for ribosome synthesis.

Fig 1 – Nucleus with the cisternae of a continuous endoplasmic reticulum highlighting its main features.

Euchromatin vs Heterochromatin

When the nucleus is observed through an electron microscope we can see areas of light and dark. Both of these areas are chromatin.

The light areas are known as euchromatin. This is a more loosely packed chromatin which is normally under active transcription.

The darker areas are heterochromatin. This is a tightly packed chromatin that is not being transcribed.

Fig 2 – Electron micrograph of a nucleus with its darkly stained heterchromatin and lighter stained euchromatin