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# The nucleolus

Another example of the way that the organisation of chromosomes within the nucleus relates to function is in the nucleolus (plural, nucleoli), where ribosomes are assembled. When viewed by electron microscopy, nucleoli appear as large rounded patches of an electron-dense material with a granular appearance (Figure 11a). Ribosomes are composed of protein and RNA molecules (Section 4.4) and the genes needed for the production of the ribosomal RNA components are located in clusters on several of the chromosomes. The relevant sections of these chromosomes loop into the nucleolus (Figure 13), where the ribosomal RNA molecules transcribed from this DNA are packaged together with ribosomal proteins imported from the cytosol. The assembled ribosome subunits are responsible for the electron-dense appearance of the nucleoli in TEM.

The size of the nucleolus reflects the activity of the cell.

- Given the function of the nucleolus, what might its size indicate about the activity of a cell?

- The presence of a large nucleolus (or multiple nucleoli as in Figure 11a) suggests that the cell is synthesising a large amount of protein, as the nucleolus is the site of ribosome assembly, and ribosomes are required for protein synthesis in the cytosol. A small nucleolus suggests that a cell is not synthesising much protein.

Figure 13 Schematic illustration of the arrangement of chromosomes with respect to the nucleolus. The regions of the chromosomes that contain clusters of genes encoding ribosomal RNA loop to the nucleolus.
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