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

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Louisiana Student Standards for Science

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Life Sciences

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

From Molecules To Organisms: Structures And Processes

HS-LS1-1

From DNA to Protein

DNA zawiera geny, sekwencje nukleotydów, z których niektóre są instrukcjami kodującymi szereg aminokwasów w białku. Przepływ informacji genetycznej od DNA przez RNA do białka jest procesem znanym jako Główny Dogmat.
Pierwszym etapem tego procesu jest transkrypcja, w której enzym polimerazy RNA syntetyzuje kopię lub transkrypt genu oparty na RNA. DNA jest używane jako matryca, w której każda nowa zasada RNA dodana do transkryptu jest komplementarna do oryginalnej nici DNA.
Niektóre transkrypty, zwane przekaźnikami lub mRNA, kodują białka, podczas gdy niekodujące uczestniczą w innych procesach komórkowych. Na przykład rybosomalne rRNA i transferowe tRNA uczestniczą w syntezie białek.
Następnym krokiem jest translacja, w której mRNA jest dekodowane w celu syntezy łańcucha aminokwasów. Zestaw instrukcji znany jako kod genetyczny służy do odczytu mRNA.
Większość organizmów używa tego samego uniwersalnego kodu składającego się z trzech grup nukleotydowych zwanych kodonami, które tłumaczą się na określone aminokwasy.
Istnieją 64 różne tryplety nukleotydów, ale tylko 20 standardowych aminokwasów w białkach powoduje, że kod ulega degeneracji, to znaczy wiele zestawów kodonów może dać tę samą instrukcję. Sześćdziesiąt jeden ustawia kod dla aminokwasów, a trzy sygnalizują zatrzymanie translacji.
Translacja zachodzi w rybosomie, dużym kompleksie rRNA i białek, za pomocą tRNA. 
tRNA ma strukturę trzech pętli spinki do włosów. Jedna pętla zawiera sekwencję zwaną antykodonem, która ma zasady komplementarne do kodonu.
Aminokwas odpowiadający tej sekwencji jest przyłączony na końcu tRNA, który transportuje go do rybosomu.
Białka zwane czynnikami inicjacji łączą małą jednostkę rybosomową, inicjator tRNA i mRNA. Po złożeniu kompleksu rybosom ślizga się wzdłuż mRNA w poszukiwaniu miejsca rozpoczęcia translacji. 
Tutaj inicjator antykodon tRNA wiąże się z kodonem komplementarnym; duża jednostka rybosomu wiąże się ze złożeniem i inicjowana jest translacja. Kiedy pojawia się następny tRNA, aminokwas z inicjatora jest odłączany i przenoszony do sąsiedniego aminokwasu, co powoduje wzrost łańcucha polipeptydowego. 
Dodawanie aminokwasów trwa do momentu wykrycia kodonu stop w mRNA. Rybosom następnie uwalnia łańcuch, aby mógł się złożyć w funkcjonalne białko.

od dna do białka

Przepływ informacji genetycznej w komórkach od DNA przez mRNA do białka opisuje centralny dogmat, który głosi, że geny określają sekwencję mRNA, które z kolei określają sekwencję aminokwasów tworzących wszystkie białka. Dekodowanie jednej cząsteczki na drugą odbywa się za pomocą określonych białek i RNA. Ponieważ informacja przechowywana w DNA ma kluczowe znaczenie dla funkcjonowania komórki, intuicyjnie można przyjąć, że komórka wykonałaby kopie mRNA tej informacji w celu syntezy białek, zachowując jednocześnie nienaruszone i chronione DNA. Kopiowanie DNA na RNA jest stosunkowo proste, polega na dodaniu jednego nukleotydu do nici mRNA na każdy nukleotyd odczytany z nici DNA. Tłumaczenie na białko jest nieco bardziej złożone, ponieważ trzem nukleotydom mRNA odpowiadają jednemu aminokwasowi w sekwencji polipeptydowej. Jednakże translacja na białko jest nadal systematyczna i kolinearna, tak że nukleotydy 1 do 3 odpowiadają aminokwasowi 1, nukleotydy 4 do 6 odpowiadają aminokwasowi 2 i tak dalej.

Kod genetyczny jest zdegenerowany i uniwersalny

Każdy aminokwas jest zdefiniowany przez sekwencję trzech nukleotydów zwaną kodonem tripletowym. Biorąc pod uwagę różną liczbę „liter” w mRNA i „alfabetach” białek, naukowcy wysunęli teorię, że pojedyncze aminokwasy muszą być reprezentowane przez kombinacje nukleotydów. Dublety nukleotydów nie wystarczą do określenia każdego aminokwasu, ponieważ istnieje tylko 16 możliwych kombinacji dwóch nukleotydów (42). Dla kontrastu, istnieją 64 możliwe triplety nukleotydów (43), czyli znacznie więcej niż liczba aminokwasów. Naukowcy wysunęli teorię, że aminokwasy są kodowane przez trójki nukleotydów, a kod genetyczny jest „zdegenerowany”. Innymi słowy, dany aminokwas może być kodowany przez więcej niż jedną trójkę nukleotydów. Zostało to później potwierdzone eksperymentalnie: Francis Crick i Sydney Brenner użyli chemicznego mutagenu proflawiny do wstawienia jednego, dwóch lub trzech nukleotydów do genu wirusa. Po wstawieniu jednego lub dwóch nukleotydów normalne białka nie były wytwarzane. Po wstawieniu trzech nukleotydów białko zostało zsyntetyzowane i funkcjonalne. Wykazało to, że aminokwasy muszą być określone przez grupy trzech nukleotydów. Te triplety nukleotydów nazywane są kodonami. Insercja jednego lub dwóch nukleotydów całkowicie zmieniła ramkę odczytu tripletu, zmieniając w ten sposób komunikat dla każdego kolejnego aminokwasu. Chociaż insercja trzech nukleotydów spowodowała wstawienie dodatkowego aminokwasu podczas translacji, integralność reszty białka została zachowana.

Oprócz kodonów, które instruują dodanie określonego aminokwasu do łańcucha polipeptydowego, trzy z 64 kodonów kończą syntezę białka i uwalniają polipeptyd z mechanizmu translacyjnego. Te trojaczki nazywane są kodonami nonsensownymi lub kodonami stop. Inny kodon, AUG, również pełni specjalną funkcję. Oprócz określenia aminokwasu metioniny służy on również jako kodon start inicjujący translację. Ramka odczytu dla translacji jest ustawiana przez kodon start AUG w pobliżu końca 5' mRNA. Po kodonie start mRNA jest odczytywane w grupach po trzy, aż do napotkania kodonu stop.

Specyfikacja pojedynczego aminokwasu za pomocą wielu podobnych kodonów nazywana jest „degeneracją”. Uważa się, że degeneracja jest mechanizmem komórkowym zmniejszającym negatywny wpływ przypadkowych mutacji. Kodony określające ten sam aminokwas zazwyczaj różnią się tylko jednym nukleotydem. Ponadto aminokwasy o podobnych chemicznie łańcuchach bocznych są kodowane przez podobne kodony. Na przykład asparaginian (Asp) i glutaminian (Glu), które zajmują blok GA*, są naładowane ujemnie. Ten niuans w kodzie genetycznym gwarantuje, że mutacja polegająca na podstawieniu pojedynczego nukleotydu może określać ten sam aminokwas, ale nie mieć żadnego efektu lub określać podobny aminokwas, zapobiegając całkowitej niefunkcjonalności białka.

Kod genetyczny jest niemal uniwersalny. Z kilkoma drobnymi wyjątkami, praktycznie wszystkie gatunki wykorzystują ten sam kod genetyczny do syntezy białek. Zachowanie kodonów oznacza, że oczyszczony mRNA kodujący białko globiny u koni może zostać przeniesiony do komórki tulipana, a tulipan będzie syntetyzował końską globinę. To, że istnieje tylko jeden kod genetyczny, stanowi mocny dowód na to, że całe życie na Ziemi ma wspólne pochodzenie, zwłaszcza biorąc pod uwagę, że istnieje około 1084 możliwych kombinacji 20 aminokwasów i 64 kodonów tripletowych.

Od DNA do białka

Przepływ informacji genetycznej w komórkach od DNA przez mRNA do białka jest opisany przez centralny dogmat, który mówi, że geny określają sekwencję ...

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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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