1. Classification and localization
In animal cells, there are two main types of protein synthesis machinery. The first type is mitochondrial ribosomes, which are localized exclusively within mitochondria. They differ markedly from the others by a smaller size and a different chemical composition.
The second, dominant type is cytoplasmic ribosomes, which are present in significant quantities 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 on 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 single particles, but to their groups (polysomes).
2. Chemical structure and biogenesis
In its physicochemical nature, a ribosome is a compactly folded 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 a functionally assembled state, the ribosome consists of two parts:
- Small subunit: includes one rRNA molecule and about 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 nucleoli. It is there that rRNA synthesis and initial assembly of subunits take place, after which they are transported from the nucleus into the cytoplasm.
3. Function and working principle
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 initial region of the mRNA so that the first codon is positioned in the active center. 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, the ribosome moves along the mRNA chain. New tRNAs with amino acids arrive at the active center, complementary interaction of the codon and anticodon occurs, and a new amino acid is incorporated into the growing polypeptide chain.
4. Dynamics and types of polysomes
As soon as the first particle moves a sufficient distance from the start of the mRNA, the next one immediately binds to the freed starting region. A polysome is formed — a complex of a single mRNA molecule, several 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 disintegrates into subunits, which can bind to the beginning of the chain again.
Polysomes are divided 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 ER.
- 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 cell growth — it is especially high in rapidly growing cells.
5. Visualization of ribosomes in histology
Under the light microscope, ribosomes are visualized using a cytochemical reaction for RNA — Brachet's method. Since rRNA predominates among all types of cellular RNAs, accumulations of ribosomes give an intense staining. As a result, the structures acquire a bright magenta color.
The 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.
Briefly: answers to topical questions
How specifically do the chemical composition and size of mitochondrial ribosomes differ from cytoplasmic ones?
Mitochondrial ribosomes differ from cytoplasmic ones by a smaller size and a different composition, but specific chemical differences are not detailed in the sources.
| Sign | Mitochondrial ribosomes | Cytoplasmic ribosomes |
|---|---|---|
| Size | Smaller than cytoplasmic; appear as small granules on electron micrographs | Larger than mitochondrial |
| Structural similarity | Similar to prokaryotic ones | Different from mitochondrial ones |
| Chemical composition | Different composition from cytoplasmic ones; specifics not specified in sources | Ribonucleoprotein complex: rRNA and proteins |
What sedimentation coefficients do the subunits and the whole eukaryotic cytoplasmic ribosome have?
The whole eukaryotic cytoplasmic ribosome and its subunits have different sedimentation coefficients (the S value characterizing the sedimentation rate during ultracentrifugation).
Eukaryotic cells are characterized by the following parameters:
- Whole ribosome — 80S.
- Large subunit — 60S (includes 5S, 5.8S, and 28S rRNA molecules and about 50 types of proteins).
- Small subunit — 40S (includes one 18S rRNA molecule and about 33 types of proteins).
To remember the structure: the LARGE subunit lives up to its name — it needs THREE rRNA molecules at once, whereas the SMALL one needs only one.
Class and exam questions
What is the state of ribosomes when translation is not active?
In the resting state, ribosomes are dissociated, meaning their large and small subunits are separated and exist independently.
What is a polysome?
A polysome is a dynamic working complex consisting of a single mRNA molecule, several ribosomes moving along it, and peptide chains of varying lengths synthesized by them.
Why do free polysomes attach to the ER?
Binding to the ER occurs upon the appearance of a signal sequence on the growing peptide chain; this is necessary for the synthesis of export proteins, membrane proteins, or lysosomal enzymes.
Why do nucleoli stain crimson using Brachet's method?
The staining is due to the reaction for RNA: active synthesis of rRNA and assembly of new ribosomal subunits occur in the nucleoli.
What else to study on this topic
- Structure and role of the signal sequence (SS) in binding polysomes to the ER
- Specific features of the composition of mitochondrial ribosomes
- Mechanisms of messenger RNA degradation by enzymes (RNases)
- Detailed mechanism of ribosomal subunit rotation in the translation cycle
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