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

DNA bevat genen, sequenties van nucleotiden, waarvan sommige instructies zijn die coderen voor de reeks aminozuren in een eiwit. De stroom van genetische informatie van DNA naar RNA naar eiwit is een proces dat bekend staat als het centrale dogma. De eerste stap van dit proces is transcriptie, waarbij een RNA-polymerase-enzym een op RNA gebaseerde kopie of transcriptie van het gen synthetiseert.Het DNA wordt gebruikt als een sjabloon waarbij elke nieuwe RNA-base die aan het transcript wordt toegevoegd, complementair is aan de oorspronkelijke DNA-streng. Sommige transcripten, genaamd messenger of mRNA, coderen voor eiwitten, terwijl niet-coderende transcripten deelnemen aan andere cellulaire processen. Ribosomaal rRNA en transfer-tRNA nemen bijvoorbeeld deel aan eiwitsynthese.De volgende stap is de vertaling, waarbij mRNA wordt gedecodeerd om een keten van aminozuren te synthetiseren. Er wordt een reeks instructies gebruikt die bekend staan als de genetische code om het mRNA te lezen. De meeste organismen gebruiken dezelfde universele code die is samengesteld uit drie nucleotidegroepen die codons worden genoemd en die zich vertalen in specifieke aminozuren.Er zijn 64 verschillende nucleotide-tripletten, maar slechts 20 standaardaminozuren in eiwitten die de code degenereren, dat wil zeggen, meerdere codonsets kunnen dezelfde instructie geven. Eenenzestig sets coderen voor aminozuren, en drie signaleren het stoppen van de vertaling. Met behulp van tRNA vindt translatie plaats op het ribosoom, een groot complex van rRNA's en eiwitten.tRNA heeft een lusstructuur met drie haarspeldbochten. Een lus bevat een sequentie genaamd anticodon, die complementaire basen heeft aan het codon. Een aminozuur dat overeenkomt met deze sequentie is aan het einde van het tRNA bevestigd, dat het naar het ribosoom transporteert.Eiwitten die initiatiefactoren worden genoemd, brengen de kleine ribosoomeenheid, een initiator-tRNA en het mRNA samen. Na de montage van het complex glijdt het ribosoom langs het mRNA op zoek naar de startplaats van de vertaling. Hier bindt het initiator-tRNA-anticodon aan het complementaire codon;de grote ribosoomeenheid bindt zich aan het samenstel en de vertaling wordt geïnitieerd.Wanneer het volgende tRNA binnenkomt, wordt het aminozuur van de initiator losgemaakt en overgebracht naar het naburige aminozuur, wat resulteert in een groeiende polypeptideketen. De toevoeging van aminozuren gaat door totdat een stopcodon wordt gedetecteerd in het mRNA. Het ribosoom laat vervolgens de keten los zodat deze zich kan vouwen tot een functioneel eiwit.

van dna tot eiwit

De stroom van genetische informatie in cellen van DNA naar mRNA naar eiwit wordt beschreven door het centrale dogma, dat stelt dat genen de sequentie van mRNA's specificeren, die op hun beurt de sequentie van aminozuren specificeren waaruit alle eiwitten bestaan. Het decoderen van het ene molecuul naar het andere wordt uitgevoerd door specifieke eiwitten en RNA's. Omdat de informatie die in het DNA is opgeslagen zo centraal staat in de cellulaire functie, is het intuïtief logisch dat de cel mRNA-kopieën van deze informatie maakt voor de eiwitsynthese, terwijl het DNA zelf intact en beschermd blijft. Het kopiëren van DNA naar RNA is relatief eenvoudig, waarbij voor elk nucleotide dat in de DNA-streng wordt gelezen, één nucleotide aan de mRNA-streng wordt toegevoegd. De vertaling naar eiwit is iets complexer omdat drie mRNA-nucleotiden overeenkomen met één aminozuur in de polypeptidesequentie. De vertaling naar eiwit is echter nog steeds systematisch en collineair, zodat nucleotiden 1 tot 3 overeenkomen met aminozuur 1, nucleotiden 4 tot 6 overeenkomen met aminozuur 2, enzovoort. De genetische code is gedegenereerd en universeel Elk aminozuur wordt gedefinieerd door een sequentie van drie nucleotiden, het tripletcodon genoemd. Gezien de verschillende aantallen ‘letters’ in de mRNA- en eiwit-alfabetten, theoretiseerden wetenschappers dat afzonderlijke aminozuren moeten worden weergegeven door combinaties van nucleotiden. Nucleotidenparen zouden niet voldoende zijn om elk aminozuur te specificeren, omdat er slechts 16 mogelijke combinaties van twee nucleotiden zijn (42). Daarentegen zijn er 64 mogelijke nucleotidetripletten (43), wat veel meer is dan het aantal aminozuren. Wetenschappers theoretiseerden dat aminozuren werden gecodeerd door nucleotidetripletten en dat de genetische code ‘gedegenereerd’ was. Met andere woorden: een bepaald aminozuur zou gecodeerd kunnen worden door meer dan één nucleotidetriplet. Dit werd later experimenteel bevestigd: Francis Crick en Sydney Brenner gebruikten het chemische mutagene middel proflavine om één, twee of drie nucleotiden in het gen van een virus in te voegen. Wanneer één of twee nucleotiden werden ingevoegd, werden de normale eiwitten niet geproduceerd. Toen drie nucleotiden werden ingevoegd, was het eiwit gesynthetiseerd en functioneel. Dit toonde aan dat de aminozuren gespecificeerd moeten worden door groepen van drie nucleotiden. Deze nucleotidetripletten worden codons genoemd. De insertie van één of twee nucleotiden veranderde het triplet-leesframe volledig, waardoor de boodschap voor elk volgend aminozuur veranderde. Hoewel de insertie van drie nucleotiden ervoor zorgde dat er tijdens de translatie een extra aminozuur werd ingevoegd, bleef de integriteit van de rest van het eiwit behouden. Naast codons die de toevoeging van een specifiek aminozuur aan een polypeptideketen instrueren, beëindigen drie van de 64 codons de eiwitsynthese en geven ze het polypeptide vrij uit de translatiemechanismes. Deze tripletten worden nonsense-codons of stopcodons genoemd. Een ander codon, AUG, heeft ook een speciale functie. Naast het specificeren van het aminozuur methionine, dient het ook als startcodon om de translatie te initiëren. Het leesraam voor vertaling wordt bepaald door het AUG-startcodon nabij het 5'-uiteinde van het mRNA. Na het startcodon wordt het mRNA in groepen van drie gelezen totdat een stopcodon wordt aangetroffen. De specificatie van een enkel aminozuur door meerdere vergelijkbare codons wordt 'degeneratie' genoemd. Er wordt aangenomen dat degeneratie een cellulair mechanisme is om de negatieve impact van willekeurige mutaties te verminderen. Codons die hetzelfde aminozuur specificeren, verschillen doorgaans slechts één nucleotide. Bovendien worden aminozuren met chemisch vergelijkbare zijketens gecodeerd door vergelijkbare codons. Aspartaat (Asp) en glutamaat (Glu), die het GA*-blok bezetten, zijn bijvoorbeeld beide negatief geladen. Deze nuance van de genetische code zorgt ervoor dat een substitutiemutatie van één nucleotide hetzelfde aminozuur kan specificeren, maar geen effect heeft of een vergelijkbaar aminozuur specificeert, waardoor wordt voorkomen dat het eiwit volledig niet-functioneel wordt gemaakt. De genetische code is vrijwel universeel. Op enkele kleine uitzonderingen na gebruiken vrijwel alle soorten dezelfde genetische code voor de eiwitsynthese. Behoud van codons betekent dat een gezuiverd mRNA dat codeert voor het globine-eiwit bij paarden naar een tulpencel kan worden overgebracht, en de tulp zou paardenglobine kunnen synthetiseren. Dat er maar één genetische code is, is een krachtig bewijs dat al het leven op aarde een gemeenschappelijke oorsprong heeft, vooral gezien het feit dat er ongeveer 1084 mogelijke combinaties zijn van 20 aminozuren en 64 tripletcodons.

Van DNA tot Eiwit

The flow of genetic information in cells from DNA to mRNA to protein is described by the central dogma, which states that genes specify the sequence of ...

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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

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HS-LS1-2

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HS-LS1-4

The Genetic Code and Codons

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