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

K12 Standards

Louisiana Student Standards for Science

High School

Life Sciences

from-molecules-to-organisms-structures-and-processes

From Molecules To Organisms: Structures And Processes

HS-LS1-1

From DNA to Protein

O ADN contém genes, sequências de nucleotídeos, alguns dos quais são instruções de código para a série de aminoácidos numa proteína. O fluxo de informação genética do ADN ao ARN às proteínas é um processo conhecido como O Dogma Central. O primeiro passo deste processo é a transcrição, onde uma enzima de RNA polimerase sintetiza uma cópia baseada em RNA, ou transcrição do gene.O ADN é utilizado como modelo onde cada nova base de RNA acrescentada para a transcrição é complementar para a vertente original do ADN. Algumas transcrições, chamadas mensageiro ou mRNA, código para proteínas, enquanto os não codificadores participam em outros processos celulares. Por exemplo, o rRNA ribossômico e transferência tRNA participam em sínteses proteicas.O passo seguinte é tradução, onde O mRNA é descodificado para sintetizar uma cadeia de aminoácidos. Um conjunto de instruções conhecido como o código genético é utilizado para ler o mRNA. A maioria dos organismos usa isto mesmo código universal composto por três grupos nucleotídeos chamados códons que traduzem aminoácidos específicos.Existem 64 diferentes trigémeos nucleotídeos, mas apenas 20 aminoácidos padrão em proteínas que fazem o código degenerar, ou seja, múltiplos conjuntos de códon podem dar a mesma instrução. Sessenta e um conjuntos de código para aminoácidos, e três sinalizam o paragem da tradução. A tradução ocorre no ribossomo, um grande complexo de rRNAs e proteínas, com a ajuda do tRNA.tRNA tem uma estrutura em laço de três ganchos capilares Um laço contém uma sequência chamada de anticódon, que tem bases complementares para o códon. Um aminoácido correspondente a esta sequência é anexado no final do tRNA, que o transporta para o ribossomo. As proteínas chamadas de fatores de iniciação reúnem a pequena unidade ribossômica, um iniciador tRNA, e o mRNA.Após a montagem do complexo, o ribossomo desliza ao longo do mRNA em busca do local de início da tradução. Aqui, o iniciador tRNA anticódon liga-se ao códon complementar;a grande unidade ribossômica liga-se à montagem, e a tradução é iniciada. Quando o próximo tRNA entrar, o aminoácido do iniciador é destacado e transferido para o aminoácido vizinho, resultando numa cadeia de crescimento de polipeptídios.A adição de aminoácidos continua até um códon de paragem ser detectado no mRNA. O ribossomo então liberta a cadeia de modo a poder dobrar-se numa proteína funcional.

do dna à proteína

O fluxo de informação genética nas células do DNA para mRNA para proteína é descrito pelo dogma central, que afirma que os genes especificam a sequência de mRNAs, que por sua vez especificam a sequência de aminoácidos que compõem todas as proteínas. A descodificação de uma molécula para outra é realizada por proteínas e RNAs específicos. Uma vez que a informação armazenada no DNA é tão central para a função celular, faz sentido que a célula faça cópias de mRNA desta informação para a síntese de proteínas, mantendo o próprio DNA intacto e protegido. A cópia de DNA para RNA é relativamente simples, sendo adicionado um nucleótido à cadeia de mRNA por cada nucleótido lido na cadeia de DNA. A tradução para a proteína é um pouco mais complexa porque três nucleótidos de mRNA correspondem a um aminoácido na sequência polipeptídica. No entanto, a tradução para proteína é também sistemática e colinear, de modo que os nucleótidos 1 a 3 correspondem ao aminoácido 1, os nucleótidos 4 a 6 correspondem ao aminoácido 2, e assim por diante.
O Código Genético é Redundante e Universal
Cada aminoácido é definido por uma sequência de três nucleótidos chamada de codão tripleto. Dados os diferentes números de “letras” nos “alfabetos” de mRNA e proteína, os cientistas teorizaram que aminoácidos únicos deviam ser representados por combinações de nucleótidos. Dupletos de nucleótidos não seriam suficientes para especificar cada aminoácido porque existem apenas 16 combinações possíveis de dois nucleótidos (42). Em contraste, existem 64 tripletos de nucleótidos possíveis (43), que são muito mais do que o número de aminoácidos. Os cientistas teorizaram que os aminoácidos eram codificados por tripletos de nucleótidos e que o código genético era “redundante”. Por outras palavras, um determinado aminoácido pode ser codificado por mais de um tripleto de nucleótidos. Isto foi confirmado experimentalmente mais tarde: Francis Crick e Sydney Brenner usaram o agente mutagénico químico proflavina para inserir um, dois, ou três nucleótidos no gene de um vírus. Quando um ou dois nucleótidos foram inseridos, não houve produção de proteína normal. Quando três nucleótidos foram inseridos, foi sintetizada proteína e era funcional. Isto demonstrou que os aminoácidos deviam estar especificados por grupos de três nucleótidos. Estes tripletos de nucleótidos são chamados de codões. A inserção de um ou dois nucleótidos mudou completamente o quadro de leitura do tripleto, alterando assim a mensagem para cada aminoácido subsequente. Embora a inserção de três nucleótidos tenha causado a inserção de um aminoácido extra durante a tradução, a integridade do resto da proteína foi mantida.
Além dos codões que instruem a adição de um aminoácido específico em uma cadeia polipeptídica, três dos 64 codões terminam a síntese proteica e libertam o polipeptídeo da maquinaria de tradução. Estes tripletos são chamados de codões sem sentido ou codões de terminação. Outro codão, AUG, também tem uma função especial. Além de especificar o aminoácido metionina, ele também serve como codão de iniciação para iniciar a tradução. O quadro de leitura para a tradução é definido pelo codão de iniciação AUG perto da extremidade 5' do mRNA. Após o codão de iniciação, o mRNA é lido em grupos de três até que um codão de terminação seja encontrado.
A especificação de um único aminoácido por vários codões semelhantes é chamada de "redundância". Acredita-se que a redundância seja um mecanismo celular para reduzir o impacto negativo de mutações aleatórias. Os codões que especificam o mesmo aminoácido diferem normalmente apenas em um nucleótido. Além disso, os aminoácidos com cadeias laterais quimicamente semelhantes são codificados por codões semelhantes. Por exemplo, o aspartato (Asp) e o glutamato (Glu), que ocupam o bloco GA*, são ambos negativamente carregados. Esta nuance do código genético garante que uma mutação de substituição de um único nucleótido possa especificar o mesmo aminoácido e não ter efeito, ou especificar um aminoácido semelhante impedindo que a proteína seja completamente não funcional.
O código genético é quase universal. Com algumas pequenas excepções, praticamente todas as espécies usam o mesmo código genético para a síntese de proteínas. A conservação dos codões significa que um mRNA purificado que codifique a proteína globina em cavalos poderia ser transferido para uma célula de tulipa, e a tulipa sintetizaria a globina de cavalo. Existir apenas um código genético é uma evidência poderosa de que toda a vida na Terra compartilha uma origem comum, especialmente considerando que existem cerca de 1084 combinações possíveis de 20 aminoácidos e 64 codões tripleto.
Este texto foi adaptado de <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>.

Do DNA à Proteína

O fluxo de informação genética nas células do DNA para o mRNA e para a proteína é descrito pelo dogma central, que afirma que os genes especificam a sequência 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.

The Genetic Code and Codons | JoVE K12

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New York State Science Learning Standards

Life Science

Structure and Function

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Matter and Energy in Organisms and Ecosystems

HS-LS1-6.

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HS-LS1-2

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HS-LS1-4

The Genetic Code and Codons

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