1. Classification and Localization
In animal cells, there are two main types of protein synthesis machinery. The first type consists of mitochondrial ribosomes, which are localized exclusively inside mitochondria. They are noticeably smaller in size and differ in chemical composition from other ribosomes.
The second, dominant type comprises cytoplasmic ribosomes, which are found in significant numbers in the cytosol. In turn, cytoplasmic structures are subdivided into two categories:
- Free ribosomes — distributed in the hyaloplasm and not bound to membrane structures.
- Membrane-bound ribosomes — fixed to the outer surface of the endoplasmic reticulum (ER) membranes.
- An important clarification is that the terms "free" and "membrane-bound" are more correctly applied not to individual particles, but to their groups (polyribosomes).
2. Chemical Structure and Biogenesis
Physicochemically, a ribosome is a tightly coiled ribonucleoprotein strand. This complex consists of ribosomal RNA (rRNA) molecules and associated ribosomal proteins. In the resting state, when synthesis is not occurring, the organelle is in a dissociated form.
In the functionally assembled state, a ribosome consists of two parts:
- Small subunit: includes one rRNA molecule and approximately thirty ribosomal proteins.
- Large subunit: has a more complex architecture, containing three rRNA molecules (one long and two short) and approximately forty-five proteins.
The site of formation of these structures is the nucleolus. This is where rRNA synthesis and the initial assembly of subunits take place, after which they are transported from the nucleus into the cytoplasm.
3. Function and Mechanism of Action
The key function of ribosomes is the execution of translation. The process starts with the initiation stage, when an active complex is formed on a messenger RNA (mRNA) molecule. First, the small subunit binds to the start region of the mRNA so that the initial codon is positioned in the active site. Next, the initiating transfer RNA (tRNA), loaded with the first amino acid, attaches. This binding is strictly specific: the tRNA anticodon is complementary to the mRNA codon. Finally, the large subunit joins the complex, completing the active system.
During the subsequent elongation phase, the ribosome moves along the mRNA chain. New tRNAs with amino acids enter the active site, complementary codon-anticodon interaction occurs, and the new amino acid is incorporated into the growing polypeptide chain.
4. Dynamics and Types of Polyribosomes
As soon as the first particle moves a sufficient distance away from the start of the mRNA, the next one immediately binds to the vacated start site. A polyribosome (polysome) is formed — a complex consisting of a single mRNA molecule, multiple ribosomes, and peptide chains of varying lengths synthesized by them (the further from the start, the longer the protein). Reaching the finish line, the organelle releases the protein and dissociates into subunits, which can re-bind to the start of the chain.
Polyribosomes are subdivided into two functional classes:
- Membrane-bound polysomes: bind to the ER upon the appearance of a signal sequence on the growing chain. They produce export proteins, lysosomal and peroxisomal enzymes, as well as membrane proteins. They are functionally identical to the rough endoplasmic reticulum (RER).
- Free polysomes: remain in the hyaloplasm and synthesize proteins for the cell's own needs (for the cytosol, nucleus, mitochondria, cytoskeleton). Their number serves as a marker of cellular growth — it is especially high in rapidly growing cells.
5. Visualization of Ribosomes in Histology
Under a light microscope, ribosomes are visualized using a cytochemical reaction for RNA — the Brachet method. Because rRNA predominates among all types of cellular RNA, accumulations of ribosomes stain intensely. As a result, the structures acquire a bright magenta (crimson) color.
Staining is clearly localized in two zones of the cell:
- In the cytoplasm: this causes cytoplasmic basophilia due to massive accumulations of ribosomes.
- In the nucleoli: explained by the high content of newly synthesized rRNA chains and actively forming subunits there.