In the eukaryotic cell can be found:
- Free in the cytoplasm, isolated or linked together forming polysomes or polyribosomes, linked by a chain of mRNA (messenger) that are simultaneously translating.
- Adhered to the outer part of the rough endoplasmic reticulum membrane or on the cytoplasmic side of the outer nuclear membrane (thanks to riboforins, proteins that allow its anchorage).
- Free in the matrix of mitochondria (mitorribosomes) and in the stroma of chloroplasts (plastoribosomes), very similar to the ribosomes of prokaryotic cells.
Structure of ribosomes
The ribosome is made up of two subunits, each with a different sedimentation coefficient.
The two ribosomal subunits, are formed in the nucleolus where two components are joined, in addition to the water: the rRNA and ribosomal proteins. RNA is synthesized in the nucleus while proteins are synthesized in the cytoplasm and are subsequently transported to the nucleolus. The two ribosomal subunits exit the cytoplasm through the nuclear pores and it is in the cytoplasm that they assemble to form the ribosome.
Ribosomes are spherical organelles, with a sedimentation rate of 80 S (in eukaryotic cells) and which are made up of two subunits:
- The minor subunit, which sediments at values of 40 S.
- The largest subunit, with a sedimentation rate of 60 S.
The two subunits are separated in the cytoplasm, and only come together when they have to synthesize proteins.
Remember that the prokaryotic ribosome is 70 S, with 50 S being the largest subunit and 30 S the smallest (depending on the surface).