Each eukaryotic cell, in the DNA in the nucleus, houses the instructions necessary to summarize the proteinthe “handyman” of our body. These extraordinarily versatile molecules are assembled thanks to the protein synthesisthe biochemical process through which the genetic information contained in mRNA It is converted into proteins. The sequence of nucleotids (ATCG) of DNA contains genetic information that is temporarily copied to the EN (AUCG) – Transcription – and, subsequently, thanks to genetic code He is then translated into a sequence of amino acids -Translation -, the basic units that make up the proteins. They perform a wide range of functions, from speeding up metabolic reactions to the transport of substances, from cellular signal to structural support.
The transcription process: from DNA to RNA MESSAGERE
The DNADesossiribonucleic acid, the molecule of life is defined, inside it contains all the information that makes us unique. We can imagine it as an instruction book written with an alphabet of four letters: adenine (TO), fear (T), cytosine (C) And cheek (G), the nitrogen bases. These letters appear specifically (A with T, C with G) forming a structure a double propeller reminiscent of a spiral staircase. The 4 letters form the “steps” of the scale while the “handrails” consist of a chain of centered sugars (deossiribosio) e phosphate groups. The nitrogen base set, sugar and phosphate group forms the nucleotidethe monomeric unit characteristic of nucleic acids. Each of the two filament of DNA has one precise directionlike a double sense road, indicated by the ends 5 ‘ And 3 ‘which reflect the sugar chemistry present at the end of the filament.
In this book called DNA, there are some “phrases” (ATCG sequences), i genewhich contain the instructions to build proteins. These DNA parts are made up of coding sections, the exonswhich transport the real information for the protein, and non -coding sections, the intronswhich are removed before the message is translated. The information, to get to the proteins manufacturers, the ribosomes, must pass through an intermediary: theRNA Messenger (mRNA).
This occurs through the transcriptionthe process in which an enzyme, a protein that accelerates a chemical reaction, called RNA polymerase IIreads DNA and creates a temporary copythe mRNA, formed by a single filament. This enzyme moves along the DNA in the direction 5 ′ → 3 ‘building the mRNA with an alphabet similar to that of DNA, but with a difference: instead of the T, use the U (urecile). So when he reads one TO On DNA, he puts one U on the MRNA, and vice versa and when he reads one C put a G and vice versa. DNA and RNA differ, as well as for the number of filaments and the letter u, also for or penty sugar that constitutes them, in fact, in the MRNA there is the rebuke.
The messenger thus created is defined pre-mo which, to become a ready for use, must undergo changes:
- Hood at the end 5 ‘: a chemical “hood” is added at the beginning of the MRNA to protect it and help him bind himself to ribosomes in the next phase.
- Poly-a tail at the end 3 ‘: a long tail of Adenine (Poli-A) is added at the end of the MRNA, to stabilize it and facilitate its exit from the nucleus.
- Splicing: At this stage the non -coding RNA traits (introns) and united with each other the coding traits (exons) are removed, those that actually bring the instructions to build proteins.
Now theripe mRNA He is ready to leave the nucleus and bring his message to ribosomes in the cytoplasm, where proteins will be built.
Protein synthesis: what it is and how it works
Protein synthesis or translation It is the following step to the transcription, in which the information contained before the DNA and then in the MRNA comes “translated“In the language of proteins. Just like translating one text from one language to another, the cell decodes the sequence of nucleotids (A, U, C, G Nella AR) in a sequence of amino acids. The proteins, in fact, are polymerslong chains of amino acids in turn composed of carbon, hydrogen, oxygen, nitrogen and sometimes sulfur. These bricks, divided into non -essential (are synthesized by our body) and essential (introduced via the diet), bind together through them peptide tiesa type of covalent bond that is formed between the carboxylic group (-Cooh) of an amino acid and the amino group (-Nh2) of another.
To complete the translation, the amino acids do not assemble independently but the cell uses i ribosomesorgans consisting of two subunits (one greater and a minor) of Ribosomal (rRNA) and proteins. Ribosomes “read” the sequence of MRNA nucleotids in groups of three, called codoniand every codon associate the complementary sequence, theanti-codonepresent on another category of RNA, i tRNA or transport rna. The anti-codone is specific for a amino acidloaded on the trn from the enzyme aminoacil-trna synthetase. To better understand, let’s see step by step how this process takes place.
The rebi, inside it has three “compartments”, i sites a – p – especific for each passage of protein synthesis.
- Beginning of the translation: The site a It welcomes the tRNA with the associated amino acid (the aminoacil-trna) and allows the recognition between the codon (of the MRNA) and the Anticodone (of the tRNA).
- Lengthening of the polypetide: on the P site, if the bond is correct, the peptide link between the amino acid associated with the tRNA and the protein in formation takes place.
- Termination of the translation: In the Site e (from Exitexit) the tRN, now without the amino acid, can leave the rebi.
Specific codons such as Uaa, UAG And Uga They report to the ribosome that the translation must end and this point the protein can be released.
Let’s take a practical example: the rebi flows the mRNA and initially meets the Start codon Augwhich codes for amino acid memoine (Met). A specific trn, with the complementary Anticoon, is linked to the site to the rebi, bringing the methionine with it. The next codon on the MRNA, for example GUCspecifies amino acid valine (Val). Another tRNA, with the Anticodon Cag, is positioned on site A, going Valina.
At this point, the methionine, previously linked to the tRN on the P site, forms a peptide link with Valina on the site A. The “unloading” trn on the P website moves to the site e and is released, while the tRNA with the peptide chain (Met-Val) It moves to the P. site the A is now free to accommodate the tRNA corresponding to the next cod. This process is repeated, with the sequential addition of amino acids to the growing chain, until the ribosome meets a Stop codon (Uga, Uaa or UAG) on the MRNA, reporting the end of the translation and the release of the complete protein.
Each codon, therefore, encodes for a particular amino acid (or a start signal or fine translation), thus determining the precise order in which the amino acids will be assembled to form the protein. Ribosome must be very precise, only one error can cause the loss of the structure of the protein and also of its function.
The genetic code: a universal and degenerated language
The genetic code It is the engine that guides the synthesis of proteins, guiding the process starting from how information is organized in the DNA, passing from the temporary copy of MRNA. Two fundamental aspects of the genetic code are his universality and his degeneration. It is defined universal because it is present and is used in All organisms living with small exceptions. Degeneration, on the other hand, derives from the fact that i 64 possible codons (generated by the four nitrogen bases in triplets) codify for only 20 amino acids; Three of these codons act as signals of termination (UAG, UAA, UGA), while the remaining 61 specify amino acids. This redundancy implies that more codons they can codify for it Same amino acida crucial aspect of the genetic code.
