Name: the genetic code and codons
Uploaded: 2020-09-24T15:06:53.000Z
Duration: 3 min 6 s
Channel: JoVE

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From Molecules To Organisms: Structures And Processes

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From DNA to Protein

L'ADN contient des gènes, des séquences de nucléotides, dont certains sont des instructions qui codent pour la série d'acides aminés dans une protéine. Le flux d'informations génétiques de l'ADN à l'ARN vers la protéine est un processus connu sous le nom de The Central Dogma. La première étape de ce processus est la transcription, où une enzyme ARN polymérase synthétise une copie à base d'ARN, ou transcrit du gène.L'ADN est utilisé comme modèle où chaque nouvelle base d'ARN est ajoutée à la transcription est complémentaire au brin d'origine de l'ADN. Certains transcriptions, appelés messagers ou ARNm, code pour les protéines, tandis que les non codantes participent dans d'autres processus cellulaires. Par exemple, ARNr ribosomal et ARNt de transfert participent à la synthèse des protéines.La prochaine étape est la traduction, où l'ARNm est décodé pour synthétiser une chaîne d'acides aminés. Un ensemble d'instructions connu sous le nom de code génétique est utilisé pour lire l'ARNm. La plupart des organismes utilisent ce même code universel composé de trois groupes nucléotidiques appelés codons qui traduisent à des acides aminés spécifiques.Il existe 64 triplets nucléotidiques différents mais seulement 20 acides aminés standards dans les protéines faisant le code dégénérés, c'est-à-dire que plusieurs ensembles de codons peuvent donner la même instruction. Soixante et un ensembles de codes pour les acides aminés, et trois signalent l'arrêt de la traduction. La traduction se produit au niveau du ribosome, un grand complexe d'ARNr et de protéines, avec l'aide d'ARNt.L'ARNt a une structure à trois boucles en épingle à cheveux. Une boucle contient une séquence appelée l'anticodon, qui a des bases complémentaires au codon. Un acide aminé correspondant à cette séquence est attaché à la fin de l'ARNt, qui le transporte dans le ribosome.Des protéines appelées facteurs d'initiation rassemblent la petite unité de ribosome, un ARNt initiateur et l'ARNm. Après le montage du complexe, le ribosome glisse le long de l'ARNm à la recherche du site de démarrage de la traduction. Ici, l'initiateur de l'ARNt anticodon se lie au codon complémentaire;la grande unité de ribosome se lie à l'assemblage, et la traduction est lancée.Lorsque le prochain ARNt arrive, l'acide aminé de l'initiateur est détaché et transféré à l'acide aminé voisin, résultant en une chaîne polypeptidique croissante. L'ajout d'acides aminés se poursuit jusqu'à un codon d'arrêt est détecté dans l'ARNm. Le ribosome libère alors la chaîne afin qu'il puisse se replier en une protéine fonctionnelle.

de l'adn à la protéine

Le flux d'informations génétiques dans les cellules de l'ADN à l'ARNm aux protéines est décrit par le dogme central, qui indique que les gènes spécifient la séquence des ARNm, qui à son tour spécifie la séquence des acides aminés constituant toutes les protéines. Le décodage d'une molécule à une autre est effectué par des protéines et des ARN spécifiques. Comme l'information stockée dans l'ADN est si essentielle à la fonction cellulaire, il est logique que la cellule fasse des copies d'ARNm de cette information pour la synthèse des protéines tout en gardant l'ADN lui-même intact et protégé. La copie de l'ADN vers l'ARN est relativement simple, avec un nucléotide ajouté au brin d'ARNm pour chaque nucléotide lu dans le brin d'ADN. La traduction en protéine est un peu plus complexe car trois nucléotides d'ARNm correspondent à un acide aminé dans la séquence polypeptidique. Cependant, la traduction en protéine est toujours systématique et colinéaire, de sorte que les nucléotides 1 à 3 correspondent à l'acide aminé 1, les nucléotides 4 à 6 correspondent à l'acide aminé 2, et ainsi de suite. 
 Le code génétique est dégénéré et universel 
 Chaque acide aminé est défini par une séquence de trois nucléotides appelée codon triplet. Compte tenu des différents nombres de “ lettres ” dans les “ alphabets ” de l’ARNm et des protéines, les scientifiques ont théorisé que les acides aminés uniques doivent être représentés par des combinaisons de nucléotides. Des doublets de nucléotides ne seraient pas suffisants pour spécifier chaque acide aminé car il n'y a que 16 combinaisons possibles de deux nucléotides (42). En revanche, il y a 64 triplets de nucléotidiques possibles (43), ce qui est beaucoup plus que le nombre d'acides aminés. Les scientifiques ont théorisé que les acides aminés étaient codés par des triplets nucléotidiques et que le code génétique était “ dégénéré ”. En d'autres termes, un acide aminé donné pourrait être codé par plus d'un triplet nucléotidique. Ceci a été confirmé plus tard expérimentalement : Francis Crick et Sydney Brenner ont utilisé le produit chimique mutagène proflavine pour insérer un, deux, ou trois nucléotides dans le gène d'un virus. Lorsqu'un ou deux nucléotides étaient insérés, les protéines normales n'étaient pas produites. Lorsque trois nucléotides étaient insérés, la protéine était synthétisée et fonctionnelle. Ceci a démontré que les acides aminés doivent être spécifiés par des groupes de trois nucléotides. Ces triplets nucléotidiques sont appelés des codons. L'insertion d'un ou deux nucléotides a complètement modifié le cadre de lecture du triplet, modifiant ainsi le message pour chaque acide aminé suivant. Bien que l'insertion de trois nucléotides entraîne l'insertion d'un acide aminé supplémentaire pendant la traduction, l'intégrité du reste de la protéine est maintenue. 
 En plus des codons qui indiquent l'ajout d'un acide aminé spécifique à une chaîne polypeptidique, trois des 64 codons arrêtent la synthèse des protéines et libèrent le polypeptide de la machinerie de traduction. Ces triplets sont appelés des codons de terminaison ou codons stop. Un autre codon, AUG, a également une fonction spéciale. En plus de spécifier l'acide aminé méthionine, il sert également de codon de départ pour initier la traduction. Le cadre de lecture pour la traduction est défini par le codon d'initiation AUG près de l'extrémité 5' de l'ARNm. Après le codon d'initiation, l'ARNm est lu par groupes de trois jusqu'à ce qu'un codon de terminaison soit rencontré. 
 La spécification d'un seul acide aminé par plusieurs codons similaires est appelée " dégénérescence ". On croit que la dégénérescence est un mécanisme cellulaire pour réduire l'impact négatif des mutations aléatoires. Les codons qui spécifient le même acide aminé diffèrent généralement d'un seul nucléotide. En outre, les acides aminés ayant des chaînes latérales chimiquement similaires sont codés par des codons similaires. Par exemple, l'aspartate (Asp) et le glutamate (Glu), qui occupent le bloc GA*, sont tous deux chargés négativement. Cette nuance du code génétique garantit qu'une mutation par substitution d'un seul nucléotide peut spécifier le même acide aminé mais n'a aucun effet ou peut spécifier un acide aminé similaire, empêchant ainsi que la protéine ne devienne complètement non fonctionnelle. 
 Le code génétique est presque universel. À quelques exceptions près, pratiquement toutes les espèces utilisent le même code génétique pour la synthèse des protéines. La conservation des codons signifie qu'un ARNm purifié codant la protéine de la globine chez les chevaux pourrait être transféré dans une cellule de tulipe, et la tulipe synthétiserait la globine du cheval. Le fait qu'il n'existe qu'un seul code génétique est une preuve essentielle que toute la vie sur Terre partage une origine commune, en particulier en considérant qu'il y a environ 1084 combinaisons possibles de 20 acides aminés et 64 codons triplets. 
 Ce texte a été adapté d'<a href="https://openstax.org/books/biology-2e/pages/15-1-the-genetic-code">Openstax, Biologie 2e, Chapitre 15.1 :</a><a href="https://openstax.org/books/biology-2e/pages/15-1-the-genetic-code"> Le code génétique</a>.

De l'ADN à la protéine

Le flux d’informations génétiques dans les cellules, de l’ADN à l’ARNm et aux protéines, est décrit par le dogme central, qui stipule que les gènes spécifient la séquence de ...

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the genetic code and codons

The genetic code and codons explain how cells turn DNA information into proteins. Genes first direct the sequence of mRNA, and mRNA then directs the sequence of amino acids that make up proteins. Specific proteins and RNAs carry out this decoding. Cells copy DNA into mRNA so the original DNA stays protected while the cell uses the message for protein synthesis.

Copying DNA to RNA is relatively simple. One nucleotide is added to the mRNA strand for every nucleotide read on the DNA strand. Translating mRNA into protein is more complex because three mRNA nucleotides correspond to one amino acid. Even so, the process is orderly and stays collinear. Nucleotides 1 to 3 specify amino acid 1, nucleotides 4 to 6 specify amino acid 2, and the pattern continues in the same way.

Each amino acid is defined by a three-nucleotide codon, also called a triplet codon. Scientists proposed this idea because the mRNA and protein alphabets are different sizes. Two-nucleotide combinations would not be enough, since there are only 16 possible doublets. Three-nucleotide combinations provide 64 possible triplets, which is far more than the number of amino acids. This led to the idea that the code is degenerate, meaning one amino acid can be encoded by more than one codon.

Francis Crick and Sydney Brenner tested this idea with the chemical mutagen proflavin. They inserted one, two, or three nucleotides into a virus gene. Inserting one or two nucleotides prevented normal proteins from being made. Inserting three nucleotides still allowed a functional protein to be synthesized. This showed that amino acids are read in groups of three. One- or two-nucleotide insertions shift the reading frame and change every later amino acid, while a three-nucleotide insertion adds one amino acid but preserves the rest of the protein.

Some codons do not add amino acids. Three of the 64 codons are stop codons, also called nonsense codons, and they end protein synthesis. Another codon, AUG, has two roles. It codes for methionine and also acts as the start codon that begins translation. The AUG start codon near the 5' end of the mRNA sets the reading frame. After that, the mRNA is read in groups of three until a stop codon appears.

Degeneracy may help reduce the effects of random mutations. Codons that specify the same amino acid usually differ by only one nucleotide. Amino acids with similar side chains are also often encoded by similar codons. For example, aspartate and glutamate are both negatively charged and fall in the GA* codon block. This means a single-nucleotide substitution may have no effect, or it may lead to a similar amino acid instead of a harmful change.

The genetic code is nearly universal. With a few minor exceptions, most species use the same code to make proteins. This shared code means a purified mRNA for horse globin could be placed in a tulip cell, and the tulip would make horse globin. The near-universal code is also strong evidence that life on Earth shares a common origin, especially since there are about 10^84 possible combinations of 20 amino acids and 64 triplet codons.

The Genetic Code and Codons

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The genetic code and codons explain how cells turn DNA information into proteins. Genes first direct the sequence of mRNA, and mRNA then directs the sequence of amino acids that make up proteins. Specific proteins and RNAs carry out this decoding. Cells copy DNA into mRNA so the original DNA stays protected while the cell uses the message for protein synthesis. Copying DNA to RNA is relatively simple. One nucleotide is added to the mRNA strand for every nucleotide read on the DNA strand.

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Learn how codons, stop signals, and the AUG start codon guide protein synthesis, and why the genetic code is nearly universal across species.

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The Genetic Code and Codons

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