Nucleolus: ribosomal RNA synthesis and ribosome assembly
The nucleolus, inside the nucleus, makes ribosomal RNA (the structural RNA of ribosomes) and joins it with proteins into ribosomal subunits (the two halves of a ribosome).
Remember
RNA polymerase I makes most ribosomal RNA in the nucleolus, so an enlarged nucleolus most directly reflects heavy ribosomal RNA synthesis, and blocking that enzyme reduces ribosomal RNA synthesis first and protein synthesis later.
The nucleolus is a dense region inside the nucleus, and its job is to make ribosomes, the machines that build proteins. So the nucleolus works as the cell's ribosome factory.
The nucleolus has two linked tasks. First, the genes for ribosomal RNA (rRNA, the structural RNA that forms the framework of a ribosome) are copied into RNA. The enzyme that copies the genes for most rRNA is RNA polymerase I. Second, the new rRNA is combined with ribosomal proteins that were made in the cytoplasm and imported, which gives partly built ribosomal subunits (the small and large halves of a ribosome). The subunits leave through the nuclear pores (the pores in the double membrane around the nucleus) and finish maturing in the cytoplasm. The copying happens inside the nucleus because rRNA is a copy of DNA, and the copy is made where the DNA is kept.
The other RNA polymerases do different jobs. Polymerase II makes messenger RNA (the copy of a gene that carries the instructions for a protein), and polymerase III makes transfer RNA (the small adaptor molecules that bring amino acids to the ribosome) and 5S rRNA (one small rRNA that polymerase I does not make).
A factory's size follows its workload, and so does the nucleolus. A cell that needs a lot of new protein, such as one that is growing or dividing quickly, needs many ribosomes, so it makes more of them and its nucleolus looks enlarged. Enlarged or numerous nucleoli are a recognised feature of many tumour types. If RNA polymerase I is blocked, ribosomal RNA synthesis falls first, then new subunit assembly, and protein synthesis falls later as a knock-on effect.
Key facts
- Site
- a dense region inside the nucleus
- Polymerase I
- copies the ribosomal RNA genes into RNA in the nucleolus (all rRNAs except 5S)
- Other polymerases
- polymerase II makes messenger RNA, and polymerase III makes transfer RNA and 5S rRNA
- Product
- partly built ribosomal subunits, which leave through the nuclear pores and finish maturing in the cytoplasm
- Enlarged nucleolus
- high ribosome output to meet a high demand for protein
- Order of effects
- ribosomal RNA synthesis falls first, then new subunit assembly, then protein synthesis later
Exam traps
- Messenger RNA is made by RNA polymerase II, so blocking RNA polymerase I does not directly block protein-coding genes.
- A polymerase I inhibitor does not act on ribosomes that already exist, so protein synthesis falls later as a knock-on effect, not as the most direct one.
- Replication of DNA does not explain an enlarged nucleolus, because it copies the chromosomes and is not a nucleolar job.
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