Ribosomes producing proteins, but need proteins to be produced?

Question

So according to my textbook:

RNA is used to create ribosomal RNA (known as rRNA) which is then combined with proteins to form the ribomsomes necessary for protein synthesis.

I'm a bit confused with this statement, as ribosomes are needed to synthesise proteins, but in order to create ribosomes proteins are needed in the first place. How does this work, does the proteins needed for the creation of rRNA come from a different source? Or by synthesising proteins via ribosomes produce more proteins than required to create the ribosomes in the first place?

Also, where are the proteins synthesised? On cytoplasmic ribosomes, or on ribosomes bound to the RER?

Thanks!

Answer 1

Ribosomes are the only means we know by which cells produce proteins. Consequently, all proteins are made by a ribosome, including the proteins that then become part of a new ribosome. It's never a question of "more proteins than required" because there are different types of proteins and to make a ribosome, you need those specific types of proteins known as ribosomal proteins. The process of making ribosomes is known as ribosome biogenesis.

This raises the obvious question of where a cell gets its ribosomes from when it is "born". The answer is simple: from its parent. When a cell divides, each daughter cell is made of the components of the parent. The cytoplasm also gets divided, and each daughter cell receives mitochondria, ER, etc. from the parent cell. By the time those ribosomes break down, they will have produced new ribosomal proteins for new ribosomes already.

According to this review from 2017, it appears the ribosomal proteins made in the cytoplasm (so, cytoplasmic ribosomes) get shuttled into the nucleolus, where they get assembled with rRNA to make new ribosomes, which then get exported out of the nucleus into the cytoplasm.