How do ribosomes get to the ER from the nucleus?

Answer 1

A ribosome contains RNA and proteins. The following description applies to eukaryotes (plants, animals, and fungi). (The ribosomes of prokaryotes (bacteria) and mitochondria are essentially similar but differ in detail, and no nucleus is involved.)

A ribosome in eukaryotes, including humans, contains four different molecules of rRNA (ribosomal RNA) and about 80 protein molecules. The proteins are synthesized in the cytoplasm, then transported through nuclear pores into the nucleus. The rRNA molecules are synthesized directly from genes in the nucleus. The nucleolus is a region of the nucleus in which three of these rRNA genes occur.

Inside the nucleus the RNA and protein molecules are assembled into two subunits, which leave the nucleus, again through nuclear pore complexes. Once in the cytoplasm, the two subunits are combined to form the functional ribosome.

Answer 2

In eukaryotes, rRNA transcription and ribosome assembly take place in the nucleolus. Nascent ribosomes then exit the nucleolus and move into the cytoplasm. Multiple ribosomal proteins assemble with rRNA in the nucleolus and may facilitate proper processing of the pre-rRNA primary transcript. Additionally, a significant number of nonribosomal proteins bind to these formative ribosomes in the nucleolus and also in the nucleoplasm after the nascent ribosomal subunits leave the nucleolus. The binding of particular proteins in a prescribed order is probably necessary for nucleocytoplasmic transport of the processed ribosomal subunits

Answer 3

Ribosomes are assembled in the nucleus and then exported to the cytoplasm where they can either remain free or get targeted to the endoplasmic reticulum (ER) to assist in protein synthesis. Ribosomes bound for the ER have a specific signal sequence that is recognized by a complex called the signal recognition particle (SRP). The SRP binds to the signal sequence and guides the ribosome to the ER where it can start synthesizing proteins destined for secretion or membrane insertion.