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

Die DNA enthält Gene, Sequenzen von Nukleotiden, von denen einige Anweisungen sind, die für die Reihe der Aminosäuren in einem Protein kodieren. Der Fluss genetischer Informationen von der DNA über die RNA bis zum Protein ist ein Prozess, der als das zentrale Dogma bekannt ist.
Der erste Schritt dieses Prozesses ist die Transkription, bei der ein RNA-Polymerase-Enzym eine RNA-basierte Kopie oder ein Transkript des Gens synthetisiert. Die DNA wird als Vorlage verwendet, wobei jede neue RNA-Base, die dem Transkript hinzugefügt wird, komplementär zum ursprünglichen DNA-Strang ist.
Einige Transkripte, die als Boten- oder mRNA bezeichnet werden, kodieren für Proteine, während nicht-kodierende Transkripte an anderen zellulären Prozessen beteiligt sind. So sind beispielsweise ribosomale rRNA und Transfer-tRNA an der Proteinsynthese beteiligt.
Der nächste Schritt ist die Translation, bei der mRNA entschlüsselt wird, um eine Kette von Aminosäuren zu synthetisieren. Eine Reihe von Anweisungen, die als genetischer Code bekannt sind, wird verwendet, um die mRNA zu lesen.
Die meisten Organismen verwenden denselben universellen Code, der aus drei Nukleotidgruppen besteht, die als Codons bezeichnet werden und sich in spezifische Aminosäuren übersetzen lassen.
Es gibt 64 verschiedene Nukleotid-Tripletts, aber nur 20 Standardaminosäuren in Proteinen, die den Code degenerieren, d.h. mehrere Codon-Sätze können die gleiche Anweisung geben. Einundsechzig Sätze geben den Code für Aminosäuren an, und drei signalisieren den Stopp der Translation.
Die Translation erfolgt am Ribosom, einem großen Komplex aus rRNAs und Proteinen, mit Hilfe von tRNA. 
tRNA hat eine Struktur mit drei Haarnadeln. Eine Schleife enthält eine Sequenz, die als Anticodon bezeichnet wird und komplementäre Basen zum Codon hat.
Eine Aminosäure, die dieser Sequenz entspricht, wird am Ende der tRNA angehängt, die sie in das Ribosom transportiert.
Proteine, die als Initiationsfaktoren bezeichnet werden, bringen die kleine Ribosomeneinheit, eine Initiator-tRNA und die mRNA zusammen. Nach dem Zusammenbau des Komplexes gleitet das Ribosom entlang der mRNA auf der Suche nach der Startstelle der Translation. 
Hier bindet das Initiator-tRNA-Anticodon an das komplementäre Codon, die große Ribosomeneinheit an die Assemblierung und die Translation wird eingeleitet. Wenn die nächste tRNA eintrifft, wird die Aminosäure vom Initiator abgelöst und auf die benachbarte Aminosäure übertragen, was zu einer wachsenden Polypeptidkette führt. 
Die Zugabe von Aminosäuren wird so lange fortgesetzt, bis ein Stoppcodon in der mRNA nachgewiesen wird. Das Ribosom gibt dann die Kette frei, so dass sie sich zu einem funktionsfähigen Protein falten kann.

von der dna zum protein

Der Fluss der genetischen Information in Zellen von der DNA über die mRNA bis zum Protein wird durch das zentrale Dogma beschrieben, das besagt, dass Gene die Sequenz von mRNAs festlegen, die wiederum die Sequenz von Aminosäuren festlegen, aus denen alle Proteine bestehen. Die Dekodierung von einem Molekül zu einem anderen wird durch spezifische Proteine und RNAs durchgeführt. Da die in der DNA gespeicherten Informationen für die Zellfunktion von zentraler Bedeutung sind, liegt es intuitiv nahe, dass die Zelle mRNA-Kopien dieser Informationen für die Proteinsynthese erstellt und gleichzeitig die DNA selbst intakt und geschützt hält. Das Kopieren von DNA in RNA ist relativ einfach, da für jedes im DNA-Strang gelesene Nukleotid ein Nukleotid zum mRNA-Strang hinzugefügt wird. Die Übersetzung in Protein ist etwas komplexer, da drei mRNA-Nukleotide einer Aminosäure in der Polypeptidsequenz entsprechen. Die Übersetzung in Protein erfolgt jedoch immer noch systematisch und kollinear, sodass die Nukleotide 1 bis 3 der Aminosäure 1 entsprechen, die Nukleotide 4 bis 6 der Aminosäure 2 und so weiter. Der genetische Code ist degeneriert und universell Jede Aminosäure wird durch eine Sequenz aus drei Nukleotiden definiert, die als Triplett-Codon bezeichnet wird. Angesichts der unterschiedlichen Anzahl von „Buchstaben“ im mRNA- und Protein-„Alphabet“ stellten Wissenschaftler die Theorie auf, dass einzelne Aminosäuren durch Kombinationen von Nukleotiden dargestellt werden müssen. Nukleotid-Dubletts würden nicht ausreichen, um jede Aminosäure zu spezifizieren, da es nur 16 mögliche Zwei-Nukleotid-Kombinationen gibt (42). Im Gegensatz dazu gibt es 64 mögliche Nukleotidtripletts (43), was weit mehr ist als die Anzahl der Aminosäuren. Wissenschaftler stellten die Theorie auf, dass Aminosäuren durch Nukleotidtripletts kodiert würden und dass der genetische Code „degeneriert“ sei. Mit anderen Worten, eine bestimmte Aminosäure könnte von mehr als einem Nukleotidtriplett kodiert werden. Dies wurde später experimentell bestätigt: Francis Crick und Sydney Brenner verwendeten das chemische Mutagen Proflavin, um ein, zwei oder drei Nukleotide in das Gen eines Virus einzufügen. Wenn ein oder zwei Nukleotide eingefügt wurden, wurden die normalen Proteine nicht produziert. Als drei Nukleotide eingefügt wurden, war das Protein synthetisiert und funktionsfähig. Dies zeigte, dass die Aminosäuren durch Gruppen von drei Nukleotiden spezifiziert werden müssen. Diese Nukleotidtripletts werden Codons genannt. Durch die Einfügung von einem oder zwei Nukleotiden wurde der Triplett-Leserahmen vollständig verändert, wodurch sich die Botschaft für jede nachfolgende Aminosäure änderte. Obwohl das Einfügen von drei Nukleotiden dazu führte, dass während der Translation eine zusätzliche Aminosäure eingefügt wurde, blieb die Integrität des restlichen Proteins erhalten. Zusätzlich zu den Codons, die das Hinzufügen einer bestimmten Aminosäure zu einer Polypeptidkette anweisen, beenden drei der 64 Codons die Proteinsynthese und geben das Polypeptid aus dem Translationsapparat frei. Diese Tripletts werden Nonsense-Codons oder Stop-Codons genannt. Ein weiteres Codon, AUG, hat ebenfalls eine besondere Funktion. Es spezifiziert nicht nur die Aminosäure Methionin, sondern dient auch als Startcodon zur Initiierung der Translation. Der Leserahmen für die Translation wird durch das AUG-Startcodon nahe dem 5'-Ende der mRNA festgelegt. Nach dem Startcodon wird die mRNA in Dreiergruppen gelesen, bis ein Stoppcodon gefunden wird. Die Spezifizierung einer einzelnen Aminosäure durch mehrere ähnliche Codons wird als „Degeneration“ bezeichnet. Es wird angenommen, dass Degeneration ein zellulärer Mechanismus ist, um die negativen Auswirkungen zufälliger Mutationen zu verringern. Codons, die dieselbe Aminosäure spezifizieren, unterscheiden sich typischerweise nur um ein Nukleotid. Darüber hinaus werden Aminosäuren mit chemisch ähnlichen Seitenketten durch ähnliche Codons kodiert. Beispielsweise sind Aspartat (Asp) und Glutamat (Glu), die den GA*-Block besetzen, beide negativ geladen. Diese Nuance des genetischen Codes stellt sicher, dass eine Einzelnukleotid-Substitutionsmutation möglicherweise dieselbe Aminosäure spezifiziert, aber keine Wirkung hat oder eine ähnliche Aminosäure spezifiziert, wodurch verhindert wird, dass das Protein völlig funktionsunfähig wird. Der genetische Code ist nahezu universell. Mit wenigen Ausnahmen verwenden praktisch alle Arten den gleichen genetischen Code für die Proteinsynthese. Die Erhaltung der Codons bedeutet, dass eine gereinigte mRNA, die das Globinprotein von Pferden kodiert, in eine Tulpenzelle übertragen werden könnte, und die Tulpe würde Pferdeglobin synthetisieren. Dass es nur einen genetischen Code gibt, ist ein starker Beweis dafür, dass alles Leben auf der Erde einen gemeinsamen Ursprung hat, insbesondere wenn man bedenkt, dass es etwa 1084 mögliche Kombinationen von 20 Aminosäuren und 64 Triplett-Codons gibt. Dieser Text wurde angepasst von <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">Der genetische Code</a>.

Von der DNA zum Protein

Der Fluss genetischer Information in Zellen von der DNA über die mRNA bis zum Protein wird durch das zentrale Dogma beschrieben, das besagt, dass Gene die Abfolge der ...

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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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DNA Packaging

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The Genetic Code and Codons

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