Parts of ribosomes shift from cytosol to nucleus and from nucleus to cytoplasm: Is this true? If yes, why is it so?
Ribosomal proteins are made in cytosol which then enter the nucleus through nuclear pores in case of eukaryotes. Inside the nucleus, ribosomal proteins are integrated with rRNA to form ribosomal subunits.
Each ribosome is made up of two subunits, and each subunit is made of rRNA and proteins. Ribosomal proteins are translated/synthesised in cytoplasm by other existing ribosomes from specific mRNAs. These ribosomal proteins must be integrated with another component of ribosome i.e. rRNA to form ribosomal subunits.
Assembly of ribosomal proteins and rRNA can happen within eukaryotic nucleus, at a location known as nucleolus. The genetic DNA transcripts rRNA within nucleus and integration of ribosomal proteins with rRNA takes place nearby. The factory of integration where ribosomal subunits are generated appear as nucleolus within nucleus.
Nuclear membrane in eukaryotes is associated with pores through which ribosomal proteins can enter nucleus from cytosol and pores also allow ribosomal subunits to come out in cytoplasm.
Proteins which are delivered in the lumen of endoplasmic reticulum are meant for other membrane bound organelles or are secretory products of the cell.