A nucleolus within an animal cell produces and assembles ribosome compounds, for instance ribosomal RNA (rRNA). It does this by assembling rRNA molecules with many protein molecules, into two 'subunits' of future ribosomes whilst still within the nucleolus. After this these 'subunits' leave the nucleus through pores in the nuclear membrane, and enter the cytoplasm. Once the ribosome subunits have reached the cytoplasm they are bought together to form a functional ribosome during an early stage of change then separate when the ribosome has fully changed/translated into a molecule of messenger RNA (mRNA).
