Ribosomes are structures present in all cell types , and are therefore present in both prokaryotic and eukaryotic organisms .
Some authors classify ribosomes as non-membranous organelles , others, however, prefer not to assign the name of organelle to these structures due to their absence of membrane.
Responsible for protein synthesis , ribosomes are made up of ribosomal RNA and associated proteins. They can occur in isolation in the cytosol or associated with membranes.
What is a ribosome?
The ribosome is a structure, with 20 nm to 30 nm, present in all cell types. It is composed of two subunits, one larger and one smaller, formed by ribosomal RNA (rRNA) and proteins.
In eukaryotic organisms, ribosomal RNA is produced in the nucleolus region and proteins in the cytoplasm region . Once formed, the proteins migrate to the nucleus, entering this region through the pores, to associate with the ribosomal RNA and give rise to the subunits that will form the ribosomes.
The two subunits are formed in the nucleus , but do not unite there, migrating towards the cytosol independently. It is in the cytosol that they , linked to a molecule of messenger RNA (mRNA), will form a functional ribosome , that is, capable of synthesizing proteins.
It is worth noting that in both prokaryotic and eukaryotic organisms, the subunits will assemble only after the smaller subunit binds to a messenger RNA molecule .
When a ribosome is formed with its two subunits, it is possible to observe the presence of four binding sites: the site for the binding of the messenger RNA molecule, which is present in the smaller unit, and the P, A and E sites, which are present in the larger subunit and are binding sites for transfer RNA.
In the P site, we have a molecule of transfer RNA (tRNA) linked to the polypeptide chain in formation. In site A, there is the presence of a molecule of transfer RNA carrying the next amino acid that will be added to the chain being formed. Finally, we have the E site, which is where the transfer RNA, now without carrying an amino acid, leaves the ribosome.
Difference between prokaryotic and eukaryote ribosomes
The ribosomes of prokaryotes and eukaryotes are responsible for protein synthesis and are very similar. However, some differences can be observed between them.
The first difference concerns size. The ribosomes of eukaryotes are larger than those seen in prokaryotic organisms. The chemical composition of these cellular structures is also different, presenting, for example, different proteins.
This difference is of medical importance, since certain drugs are able to affect the ribosomes of prokaryotes and not affect the ribosomes of eukaryotic cells, and are therefore important for the treatment of bacterial diseases .
Free and bound ribosomes
Ribosomes can be found in different locations in the cell and, depending on their location, can be called free or bound.
The free ribosomes are those that are loose in the cytosol of the cell, the bound ones , in turn, are those that are associated with the membranes of the endoplasmic reticulum and the nuclear envelope.
Free ribosomes produce proteins that, for the most part, will function in the cytoplasm itself. The ligands, however, produce proteins that will act inside organelles or that will be excreted out of the cell. In prokaryotes, the presence of only free ribosomes is observed, since these organisms have cells without a defined nucleus and do not have membranous organelles.
It is also worth noting that, in eukaryotic cells, there are ribosomes found inside chloroplasts and mitochondria . They are smaller than ribosomes that are free in the cytosol or bound in membranes.
Function of ribosomes
Ribosomes are involved in protein synthesis and are therefore found in greater amounts in cells with high rates of protein synthesis, such as pancreas cells . In cells with a high rate of synthesis, the occurrence of polyribosomes, which are groups of ribosomes joined by the same messenger RNA molecule, is common. It is the messenger RNA that contains the code for the sequence of amino acids that will form the polypeptide chain that will be synthesized on the ribosome.
How does protein synthesis occur?
Protein synthesis occurs on ribosomes and involves three steps: initiation, elongation, and termination. Below is a summary of each of them:
- Initiation: In the initiation stage, there is the approximation between the ribosome subunits, messenger RNA and transfer RNA. The latter will be responsible for carrying the first amino acid that will be placed in the polypeptide chain.
- Elongation: in this step we have the addition of the amino acids that will compose the polypeptide chain. The transfer RNA arrives at the A site, and its anticodon pairs, by complementarity, with the codon of the messenger RNA. The amino acid it brings is linked, through a peptide bond, with the forming polypeptide chain that is in the P site. The ribosome then moves the transfer RNA that is in the A site to the P site, and the one that was in the P site. site P, to site E, where it will be released. In this process, the messenger RNA also moves, leaving the codon that will be translated exposed in the A site.
Termination: when the ribosome reaches the termination codon (UAG, UAA and UGA) in the messenger RNA, the releasing factor, a protein that promotes the breaking of the link between the transfer RNA that is in the P site and the last amino acid, comes into action. of the polypeptide chain. At that moment, this chain is released, and the complex formed by the ribosomal subunits, the messenger RNA and the transporter, is broken.