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# 4.3 The nucleus

The nucleus is the largest, and so usually the most easily identified cell organelle. In a typical animal cell, the nucleus is roughly spherical in shape (Figure 11) and is often, but not always, situated near the centre of the cell. The nucleus contains the majority of the genetic information of the cell. Two of the major activities that take place in the nucleus are: DNA replication (the synthesis of new DNA in preparation for cell division) and transcription (the production of RNA copies of parts of the DNA sequence). The production of a messenger RNA (mRNA) is the first step in the synthesis of proteins. Another important process that takes place in the nucleus is the assembly of ribosomes, the complex structures in the cytoplasm where mRNA is translated into protein.

Figure 11 (a) Electron micrograph of the nucleus of a rat liver cell. Heterochromatin and euchromatin, which are described in the text, can be clearly differentiated as electron-dense (dark) and electron-lucent (pale) areas, respectively. Two electron-dense nucleoli (see below) are also clearly visible. (b) Schematic diagram showing the structure of the nucleus, which is linked to the endoplasmic reticulum.

In order for these three major activities to take place, much coordination and many other 'subsidiary' activities are necessary. For example, replication and transcription do not take place at the same time. The enzymes needed to synthesise DNA or transcribe it into RNA must be present at the right places and times, and, since protein synthesis occurs in the cytoplasm, there must be a means of exporting mRNA (and ribosomes) out of the nucleus, and specific proteins into it. How does the organisation of the nucleus and its component molecules allow all of these activities to take place?

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