## Translation of genetic information from RNA molecules into the sequence of amino acids

After transcription and the necessary modification, the mRNA molecules pass from the nucleus to the cytoplasm, where the second stage of the expression of genetic information, which is translation, takes place. The result of this process are polypeptide chains forming proteins. Living cells expend more energy on protein synthesis than on anything else.

Besides mRNA, ribosomes and transfer RNA (tRNA) also play an important role in the translation process. tRNA molecules have a specific hairpin structure, which enables them to function as amino acid carriers. Based on the anticodon, i.e. the sequence found in their structure and which is complementary to the codon on the mRNA strand, they act as adapters between the mRNA molecules and the amino acids that make up the resulting proteins. The entire process of translation takes place in ribosomes, which can be stored separately in the cytoplasm of cells, or as part of the rough endoplasmic reticulum. They are composed of a small and a large subunit, and functional ribosomes are created only by joining both subunits together.

An interesting feature of ribosomes is that they have sufficient capacity to synthesize any protein that is encoded in mRNA, even if it comes from cells of another species. This is because the process of translation follows the rules of the genetic code, which is universal. The sequence of nucleotides in the mRNA molecule are read by the translation machinery as codons, i.e. three nucleotides, which determine the inclusion of specific amino acids in the emerging protein. Each amino acid is represented by one or more codons in this code. Out of all 64 possible codons, up to 61 encode different amino acids (20 different in total). Two of them simultaneously function as initiation signals (so-called start codons – AUG, exceptionally also GUG) and three signal the end of translation, which are called termination signals (stop codons – UAA, UAG and UGA) (Figure 3.9). With the help of anticodons present in the tRNA structure, specific amino acids are arranged in the order encoded in the mRNA molecule during the translation process. The result is a chain of amino acids that is released and forms a functional protein.

Like the previous two mechanisms, the process of translation can also be divided into initiation, elongation and termination. Initiating tRNAs, which have a specific structure and always carry the amino acid methionine (in some cases the methionine is chemically modified), play an important role at the start of the process. At the same time, initating tRNAs participate in the assembly of the ribosome, which is why they follow a slightly different mechanism of interaction with its structurally significant sites (Figure 3.10).

Initiating tRNAs bind to the large subunit of the ribosome directly through its ‘P‘ site. Their subsequent binding to the mRNA molecule leads to the joining of both subunits and the formation of a functional ribosome. The translation then enters the elongation phase. Other tRNAs already enter the ribosome by default through the ‘A‘ site. Based on the codons present in the mRNA sequence, they bring with them the corresponding amino acids. A peptide bond is formed between adjacent amino acids in the ‘P‘ site of the ribosome. At the same time, this creates an "empty" tRNA that leaves the ribosome through the ‘E‘ site. There is a shift, which again makes room for the arrival of another tRNA molecule. In this way, translation takes place until the moment when the ribosome encounters a termination signal. Then the translation is terminated and the resulting polypeptide (protein) chain is released. At the same time, the ribosome breaks down into subunits, which can later be used again in the translation of another polypeptide (Figure 3.10).