Question

In eukaryotes, the small and large ribosomal subunits are:

1) Composed entirely of RNA
2) Composed entirely of protein
3) Exported from the nucleus as a single holoenzyme
4) Joined together only during translation
5) Assembled in the endoplasmic reticulum rather than the nucleus

Answer 1

In eukaryotic cells, ribosomes are made up of rRNA and proteins, assembled in the nucleus and exported separately to the cytoplasm where they join during translation. The two subunits---40S and 60S---are only joined when the mRNA template is ready to be translated into protein.

Step-by-step explanation:

Ribosomes in eukaryotic cells are composed of both RNA and proteins and are not made entirely of either component. Specifically, ribosomes have two subunits: a small 40S subunit containing the 18S rRNA, and a large 60S subunit containing the 5S, 5.8S, and 28S rRNA molecules. Ribosomes are assembled in the nucleus, where the rRNA is produced and combined with proteins.

They are then exported to the cytoplasm where they can bind to the endoplasmic reticulum or float freely within the cellular matrix. The two subunits of the ribosomes are not exported as a single holoenzyme but separately. They come together only during translation when protein synthesis occurs. This process involves the small subunit first binding to the mRNA template and then the large subunit joining to form a complete ribosome, capable of translating the mRNA into a polypeptide chain.