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

Il DNA contiene geni, ossia sequenze di nucleotidi, alcuni dei quali contengono istruzioni che codificano per la serie di amminoacidi in una proteina. Il flusso di informazioni genetiche dal DNA all'RNA alle proteine è un processo definito Dogma Centrale. Il primo passo di questo processo è la trascrizione, in cui un enzima RNA polimerasi sintetizza una copia basata sul RNA, o trascritto del gene.

Il DNA viene utilizzato come modello, in cui ogni nuova base di RNA aggiunta alla trascrizione è complementare al filamento originale di DNA. Alcune trascritti, detti messaggeri o mRNA, codificano per proteine, mentre quelli non codificanti partecipano ad altri processi cellulari. Per esempio, l'rRNA ribosomiale e il tRNA di trasferimento partecipano alla sintesi proteica.

Il passaggio successivo è la traduzione, dove l'mRNA viene decodificato per sintetizzare una catena di amminoacidi. Un insieme di istruzioni conosciuto come codice genetico è usato per leggere l'mRNA. La maggior parte degli organismi usa questo stesso codice universale composto da tre gruppi nucleotidici, detti codoni, che si traducono in amminoacidi specifici.

Ci sono 64 diverse triplette di nucleotidi ma soltanto 20 amminoacidi standard nelle proteine, facendo degenerare il codice, cioè più set di codoni possono fornire la medesima istruzione. Sessantuno di questi impostano il codice per gli amminoacidi, e tre segnalano l'arresto della traduzione. La traduzione avviene nel ribosoma, un grande complesso di rRNA e proteine, con l'aiuto del tRNA.

al quinto carbonio sullo zucchero. tRNA ha una struttura a forcina a tre anelli. Questo nucleotide libero trifosfato Un anello c, 760 trifosfati, con tre fosfati attaccati reagisce con i gruppi idrossilici 3'Questo rappresenta l'OH che è attaccato al terzo carbonio dello zucchero alla fine del filamento in crescita.

La reazione provoca il rilascio di pirofosfato e la formazione di un legame fosfodiesterico fra i due nucleotidi. Dopo la sintesi dei nuovi filamenti, RNasi H, o varianti aggiuntive della DNA polimerasi, rimuovete i primer e sintetizza DNA al loro posto. Gli spazi fra i frammenti vengono dunque sigillati dalla DNA ligasi per generare un filamento continuo.

L'aggiunta di nucleotidi continua fino a quando due forcelle di replicazione si incontrano, determinando la completa replicazione.

dal dna alle proteine

Il flusso dell'informazione genetica nelle cellule dal DNA all'mRNA alle proteine è descritto dal dogma centrale, il quale afferma che i geni specificano la sequenza degli mRNA, che a loro volta specificano la sequenza degli amminoacidi che compongono tutte le proteine. La decodifica di una molecola in un'altra viene eseguita da proteine e RNA specifici. Poiché le informazioni immagazzinate nel DNA sono così centrali per la funzione cellulare, è intuitivo che la cellula produca copie di mRNA di queste informazioni per la sintesi proteica mantenendo il DNA stesso intatto e protetto. La copiatura del DNA nell'RNA è relativamente semplice, poiché un nucleotide viene aggiunto al filamento dell'mRNA per ogni nucleotide letto nel filamento del DNA. La traduzione in proteina è un po' più complessa perché tre nucleotidi dell'mRNA corrispondono a un amminoacido nella sequenza polipeptidica. Tuttavia, la traduzione in proteina è ancora sistematica e collineare, in modo tale che i nucleotidi da 1 a 3 corrispondono all'amminoacido 1, i nucleotidi da 4 a 6 corrispondono all'amminoacido 2 e così via.

Il codice genetico è degenerato e universale

Ogni amminoacido è definito da una sequenza di tre nucleotidi chiamata codone tripletta. Dato il diverso numero di “lettere” negli “alfabeti” dell’mRNA e delle proteine, gli scienziati hanno teorizzato che i singoli amminoacidi devono essere rappresentati da combinazioni di nucleotidi. I doppietti nucleotidici non sarebbero sufficienti per specificare ogni amminoacido perché ci sono solo 16 possibili combinazioni di due nucleotidi (4^2). Al contrario, ci sono 64 possibili triplette di nucleotidi (4^3), che sono molto più del numero di amminoacidi. Gli scienziati hanno teorizzato che gli amminoacidi fossero codificati da triplette di nucleotidi e che il codice genetico fosse “degenerato”. In altre parole, un dato amminoacido potrebbe essere codificato da più di una tripletta di nucleotidi. Ciò fu successivamente confermato sperimentalmente: Francis Crick e Sydney Brenner usarono il mutageno chimico proflavina per inserire uno, due o tre nucleotidi nel gene di un virus. Quando venivano inseriti uno o due nucleotidi, le proteine normali non venivano prodotte. Quando furono inseriti tre nucleotidi, la proteina fu sintetizzata e funzionale. Ciò ha dimostrato che gli amminoacidi devono essere specificati da gruppi di tre nucleotidi. Queste triplette di nucleotidi sono chiamate codoni. L'inserimento di uno o due nucleotidi ha cambiato completamente il frame di lettura della tripletta, alterando così il messaggio per ogni amminoacido successivo. Sebbene l'inserimento di tre nucleotidi abbia causato l'inserimento di un amminoacido extra durante la traduzione, l'integrità del resto della proteina è stata mantenuta.

Oltre ai codoni che istruiscono l'aggiunta di uno specifico amminoacido alla catena polipeptidica, tre dei 64 codoni terminano la sintesi proteica e rilasciano il polipeptide dal meccanismo di traduzione. Queste triplette sono chiamate codoni senza senso o codoni di stop. Anche un altro codone, AUG, ha una funzione speciale. Oltre a specificare l'amminoacido metionina, serve anche come codone di inizio per avviare la traduzione. Il frame di lettura per la traduzione è fissato dal codone di inizio AUG vicino all'estremità 5' dell'mRNA. Dopo il codone di inizio, l'mRNA viene letto in gruppi di tre finché non si incontra un codone di stop.

La specificazione di un singolo amminoacido da parte di più codoni simili è chiamata "degenerazione". Si ritiene che la degenerazione sia un meccanismo cellulare atto a ridurre l'impatto negativo delle mutazioni casuali. I codoni che specificano lo stesso amminoacido tipicamente differiscono solo per un nucleotide. Inoltre, gli amminoacidi con catene laterali chimicamente simili sono codificati da codoni simili. Ad esempio, l'aspartato (Asp) e il glutammato (Glu), che occupano il blocco GA*, sono entrambi caricati negativamente. Questa sfumatura del codice genetico garantisce che una mutazione di sostituzione di un singolo nucleotide possa specificare lo stesso amminoacido ma non avere alcun effetto o specificare un amminoacido simile, impedendo che la proteina venga resa completamente non funzionale.

Il codice genetico è quasi universale. Con poche eccezioni minori, praticamente tutte le specie utilizzano lo stesso codice genetico per la sintesi proteica. La conservazione dei codoni significa che un mRNA purificato che codifica la proteina globina nei cavalli potrebbe essere trasferito in una cellula di tulipano e il tulipano sintetizzerebbe la globina di cavallo. Il fatto che esista un solo codice genetico è una prova evidente del fatto che tutta la vita sulla Terra condivide un’origine comune, soprattutto considerando che esistono circa 1084 possibili combinazioni di 20 aminoacidi e 64 codoni tripletti.

Questo testo è adattato da <a href="https://openstax.org/books/biology-2e/pages/15-1-the-genetic-code">Openstax, Biology 2e, Chapter 15.1: </a><a href="https://openstax.org/books/biology-2e/pages/15-1-the-genetic-code">The Genetic Code</a>.

Dal DNA alle proteine

Il flusso di informazioni genetiche nelle cellule dal DNA all'mRNA alle proteine è descritto dal dogma centrale, che afferma che i geni specificano la sequenza di ...

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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.

The Genetic Code and Codons | JoVE K12

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Structure and Function

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HS-LS1-2

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The Genetic Code and Codons

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