Introduction to the Cell Quiz

See Nucleolus. Option A is correct because the nucleolus, a dense region inside the nucleus, is where ribosomal RNA (rRNA) is made and combined with ribosomal proteins to form the two ribosomal subunits. Ribosomes are the molecular machines that build proteins by translating messenger RNA, and each one is made of a small and a large subunit. The nucleolus contains the genes that code for rRNA and the machinery that processes it, so it acts as the cell's ribosome factory. The subunits are then exported from the nucleus into the cytoplasm, where they mature and either attach to the rough endoplasmic reticulum or float free in the cytosol. Cells that make a lot of protein need a lot of ribosomes, so they tend to have a prominent nucleolus. Memory aid: the nucleolus is the ribosome factory, and the nucleus is its head office.

• Site: the nucleolus is a dense region inside the nucleus.
• Job: it makes ribosomal RNA and combines it with ribosomal proteins to form the two ribosomal subunits.
• After assembly: the subunits are exported to the cytoplasm, where they mature.