Protein synthesis is a vital biological process that is observed in the cells of any organism. Proteins, as the main class of organic substances in cells, perform a wide range of functions, ranging from enzymatic to protective. What is a polysome? How is this structure used in protein synthesis?
What is a polysome
In eukaryotic and prokaryotic cells, ribosomes are mandatory structures. These are small bodies, consisting of two subunits and performing the function of protein synthesis during translation. Ribosomes are non-membrane structures.
A polysome is an entity made up of several ribosomes. They communicate with each other using messenger or messenger RNA. Several ribosomes, like beads, are planted on this molecule, resulting in a temporary structure - the polysome.
Cell polysomes are concentrated where rapid synthesis of a large number of proteins is necessary. This is the main biological role of these formations.
The structure of the ribosome
Since they form a polysome of several interconnected ribosomes, it is necessary to consider the structure of the latter.
Any ribosome of a cell consists of two subunits: large and small. A small gap forms between them, exactly matching the diameter of the mRNA molecule that should be there.
Ribosomes are complex nucleoprotein complexes, because the composition of both small and large subunits includes ribosomal RNA molecules and proteins. Proteins perform the function of a framework, and RNAs that have ribozyme (enzymatic) activity are used in the translation process.
Differences between ribosomes of prokaryotes and eukaryotes
Since prokaryotes and eukaryotes are two different groups of organisms, they have differences in the composition of the ribosomes, and hence the polysome too.
In prokaryotes, there are a large number of 70S ribosomes in the cytoplasm (S is the sedimentation constant, which shows the particle deposition rate in a centrifuge). They are formed by the large 50S subunit and the small 30S subunit. The composition of the large subunit in prokaryotes includes 5S and 23S RNA. In turn, the small subunit is formed by 16S RNA.
In bacterial cells , many polysomes are formed. This is due to the fact that their protein synthesis process is very fast: translation begins even before the transcription process ends. For the same reason, the concentration of ribosomes in the cell is the highest near the nucleoid.
In eukaryotes, differences are observed in the values of the sedimentation constant of ribosome elements. For her, this value is 80, for the large and small subunits - 60 and 40, respectively. 5S, 28S, and 5.8S RNA are isolated in the large subunit, and 18S RNA in the small subunit.
In both prokaryotes and eukaryotes, polysomes accelerate the process of protein synthesis. This is very important when rapid formation of copies of protein molecules is necessary. A high synthesis rate is especially characteristic of bacteria, because their matrix RNA exists for a short time, and during this period it is necessary to synthesize as many protein molecules as possible. This is what a polysome is in biology.
The translation process is the main function of the polysome
In the process of translation, the primary structure of the protein molecule is formed. Three stages of the synthesis of polypeptides are distinguished: initiation, elongation and termination.
1. Initiation of the broadcast. The mRNA molecule must attach to 16S RNA according to the principle of complementarity. To do this, there is a special sequence, which in prokaryotes is called a ribosome-binding site, and in eukaryotes it is called a cap. Consequently, messenger RNA binds to the small subunit of the ribosome.
2. Elongation. This stage is characterized by the extension of the polypeptide chain. Transport RNAs bring amino acids to the ribosome that attach to the formed chain via a peptide bond. So, protein synthesis takes place until the moment when the ribosome does not stumble on the stop codon.
3. Termination. Once it reaches the stop codon, protein synthesis stops. A newly formed polypeptide molecule drops out of the ribosome through a special passage, and then a secondary, tertiary, or quaternary structure is formed.
Ribosomal profiling - a method for studying ribosomes
To study what a polysome is, it is necessary to isolate this structure from the cells of the body. The easiest way to conduct research on prokaryotic cells. Ribosomal profiling includes the following steps:
1. Destruction of a prokaryotic cell. It is important that the destruction must be carried out mechanically, because chemicals can damage the ribosomes themselves.
2. Cleavage of RNAs that are not ribosomal.
3. The same principle is used to remove unnecessary polypeptides that are not part of the structure.
4. Reverse transcription of isolated ribosomal RNA.
5. Sequencing of the obtained DNA.
Conclusion
A polysome is a structure that is made up of several ribosomes. Its task is to accelerate the synthesis of proteins. These structures are found in cells of prokaryotes and eukaryotes, therefore they are characteristic of cells of any organism. That's what polysomes are.