Which cells contain ribosomes


Ribosomes are the cell's "protein factories". Ribosomes cannot be seen at all under the light microscope; under the electron microscope they appear as tiny dots, they are so tiny. And yet they are extremely important, because the genetic information stored in the DNA is translated into proteins on the ribosomes (protein biosynthesis, translation). These proteins in turn fulfill important functions in the cell as enzymes and structural proteins. Cells that make many proteins also have a correspondingly high number of ribosomes.


Ribosomes consist of two subunits that can also be found individually in the cytoplasm. Only during translation do a large subunit and a small subunit come together to form a complete ribosome.

In prokaryotes, the small subunit consists of an rRNA molecule (see nucleic acids) and 21 different proteins. The large subunit consists of two rRNA molecules and 31 proteins.

The ribosomes of the eukaryotes are larger than those of the prokaryotes. The small subunit of the eukaryotic ribosomes consists of one rRNA molecule and 33 proteins, while the large subunit consists of three rRNA molecules and 49 proteins.

The mitochondria and chloroplasts of the eukaryotes also contain ribosomes, the smaller type that closely resembles the ribosomes of the prokaryotes. According to the endosymbiotic theory, mitochondria and chloroplasts are former prokaryotes that were ingested by a "ureukaryote".


In prokaryotes, the ribosomes occur freely in the cell plasma, and they cannot be anywhere else because there are no organelles in the prokaryote cell.

In eukaryotes, on the other hand, a distinction is made between free and membrane-bound ribosomes. Some of the ribosomes are - similar to prokaryotes - free in the cell plasma. Another part of the ribosomes is bound to the membrane of the endoplasmic reticulum.


The task of the ribosomes is protein biosynthesis, or more precisely, the translation of the mRNA into a peptide. This process is also known as translation. I have described exactly how the translation works on the corresponding genetics page; this complex process may not be of great interest to EF students.


The ribosomes are made up of proteins and ribosomal RNA. The proteins are produced quite normally via protein synthesis, i.e. on the ribosomes. The rRNA is produced by transcribing the corresponding rRNA genes in the DNA. The assembly of the rRNA and protein molecules takes place with the help of approx. 200 different enzymes.