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には遺伝子、ヌクレオチドの配列が含まれており、その一部はタンパク質の一連のアミノ酸をコードする命令です。DNAからRNA、タンパク質への遺伝情報の流れは、セントラルドグマとして知られるプロセスです。
このプロセスの最初のステップは転写であり、RNAポリメラーゼ酵素がRNAベースのコピーまたは遺伝子の転写産物を合成します。DNAは、転写産物に追加された新しいRNA塩基がDNAの元の鎖に相補的であるテンプレートとして使用されます。
メッセンジャーまたはmRNAと呼ばれる一部の転写産物はタンパク質をコードしますが、非コード転写産物は他の細胞プロセスに関与します。例えば、リボソームrRNAやトランスファーtRNAはタンパク質合成に関与しています。
次のステップは翻訳で、mRNAを解読してアミノ酸の鎖を合成します。遺伝暗号と呼ばれる一連の命令は、mRNAを読み取るために使用されます。
ほとんどの生物は、特定のアミノ酸に翻訳されるコドンと呼ばれる3つのヌクレオチド基で構成されるこの同じユニバーサルコードを使用しています。
64の異なるヌクレオチドトリプレットがありますが、コードを縮退させるタンパク質には20の標準アミノ酸しかなく、つまり、複数のコドンセットが同じ指示を与えることができます。61 はアミノ酸のコードを設定し、3 は翻訳の停止を示します。
翻訳は、tRNAの助けを借りて、rRNAとタンパク質の大きな複合体であるリボソームで起こります。 
tRNAは3ヘアピンループ構造をしています。1つのループには、コドンに相補的な塩基を持つアンチコドンと呼ばれる配列が含まれています。
この配列に対応するアミノ酸がtRNAの末端に結合し、tRNAがそれをリボソームに輸送します。
開始因子と呼ばれるタンパク質は、小さなリボソームユニット、開始者tRNA、およびmRNAをまとめます。複合体の集合後、リボソームは翻訳開始部位を求めてmRNAに沿って滑る。 
ここで、開始者tRNAアンチコドンが相補的コドンに結合し、大きなリボソーム単位がアセンブリに結合し、翻訳が開始される。次にtRNAが入ってくると、開始剤からのアミノ酸が分離して隣接するアミノ酸に移動し、ポリペプチド鎖が成長します。 
アミノ酸の付加は、mRNAで終止コドンが検出されるまで続きます。その後、リボソームは鎖を放出して、機能的なタンパク質に折り畳むことができます。

dnaからタンパク質まで

DNA→mRNA→タンパク質という細胞内の遺伝情報の流れは、「遺伝子がmRNAの配列を決定し、そのmRNAがすべてのタンパク質を構成するアミノ酸の配列を決定する」というセントラルドグマによって説明されます。ある分子から別の分子への解読は、特定のタンパク質とRNAによって行われます。DNAに保存された情報は細胞機能の中心であるため、DNA自体を保護したまま、細胞がタンパク質合成のためにこの情報のmRNAコピーを作成することは直感的に理解できます。DNAからRNAへのコピーは比較的簡単であり、DNAのヌクレオチドを読み取るごとにmRNA鎖に1つのヌクレオチドが追加されます。mRNAの3つのヌクレオチドがポリペプチド配列の1つのアミノ酸に対応しているため、タンパク質への翻訳は少し複雑になります。タンパク質への翻訳は、ヌクレオチド1〜3がアミノ酸1に、ヌクレオチド4〜6がアミノ酸2に...というように、系統的かつ共線的に行われます。
 遺伝暗号は縮退した普遍的なものである
 各アミノ酸は、トリプレットコドンと呼ばれる3つのヌクレオチド配列によって定義されます。mRNAとタンパク質のアルファベットの数が異なることから、科学者たちは、1つのアミノ酸はヌクレオチドの組み合わせで表されなければならないと考えた。2つのヌクレオチドの組み合わせは16通り（42）しかないため、ヌクレオチドの2重構造だけではすべてのアミノ酸を指定することはできません。一方、ヌクレオチドの3重構造は64通り（43）あり、アミノ酸の数よりもはるかに多いです。科学者たちは、アミノ酸はヌクレオチド3重項によってコード化されており、遺伝暗号は縮退しているという説を提唱しました。これは後に実験的に確認されました。フランシス・クリックとシドニー・ブレナーは、突然変異原であるプロフラビンを使って、ウイルスの遺伝子に1個、2個、3個のヌクレオチドを挿入しました。1、2個のヌクレオチドを挿入した場合、正常なタンパク質は生成されなかった。しかし、3つのヌクレオチドを挿入すると、正常なタンパク質が合成されるようになった。これにより、アミノ酸は3本のヌクレオチドで指定されなければならないことがわかった。この3つのヌクレオチドの集合体をコドンと呼びます。1つまたは2つのヌクレオチドが挿入されると、トリプレットのリーディングフレームが完全に変わり、後続のアミノ酸のメッセージが変化します。3つのヌクレオチドを挿入すると、翻訳時に余分なアミノ酸が挿入されるが、残りのタンパク質の完全性は維持されます。
 ポリペプチド鎖に特定のアミノ酸の付加を指示するコドンに加えて、64個のコドンのうち3個はタンパク質合成を終了させ、ポリペプチドを翻訳から解放します。これらの3つのコドンはナンセンスコドンまたはストップコドンと呼ばれます。また、AUGというコドンも特別な機能を有しています。AUGは、アミノ酸のメチオニンを指定するだけでなく、翻訳を開始するための開始コドンとしても機能します。翻訳のためのリーディングフレームは、AUG開始コドンによってmRNAの5'末端付近に設定されます。開始コドンに続いて、停止コドンに出会うまで、mRNAは3つのグループに分けて読まれます。
 1つのアミノ酸を複数の類似したコドンで指定することを「縮退」といい、縮退はランダムな突然変異の悪影響を軽減するための細胞のメカニズムであると考えられています。同じアミノ酸を指定するコドンは、通常、1つのヌクレオチドだけが異なります。また、化学的に類似した側鎖を持つアミノ酸は、類似したコドンでコード化されます。例えば、GA*ブロックを構成するアスパラギン酸(Asp)とグルタミン酸(Glu)は、どちらも負の電荷を帯びています。このように、1塩基置換の突然変異が起こったとしても、同じアミノ酸が指定されるために効果がなかったり、類似のアミノ酸が指定されることによって、タンパク質が完全に機能しなくなることを防いでいます。
 遺伝暗号は普遍的なものです。 いくつかの例外を除いて、事実上すべての種が同じ遺伝暗号を使ってタンパク質を合成しています。コドンが保存されているということは、馬のグロビンタンパク質をコードするmRNAをチューリップの細胞に移植すれば、チューリップは馬のグロビンを合成することができるということです。20種類のアミノ酸と64個のトリプレットコドンの組み合わせが約1084通りあることを考えると、遺伝暗号が1つしかないということは、地球上のすべての生命が共通の起源を持っていることの強力な証拠となります。
 上記の文章は以下から引用しました。 <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>.

DNAからタンパク質まで

DNAからmRNA、タンパク質への細胞内の遺伝情報の流れは、遺伝子が...

Education

JoVE Core

Chemistry

化学

Biochemistry

生化学

KEY TERMS AND CONCEPTS

主要用語と概念

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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from dna to protein

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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key terms and concepts

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

chemistry

JoVE Core 化学は、簡潔で理解しやすいアニメーションビデオレッスンを使用して、一般的な化学の概念を生き生きとしたものにします。さらに、活動する科学者のビデオでは、現在世界中の実験室で行われている古典的そして独創的な研究・実験をお見せします。

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New York State Science Learning Standards

Life Science

Structure and Function

HS-LS1-1.

matter-and-energy-in-organisms-and-ecosystems

Matter and Energy in Organisms and Ecosystems

HS-LS1-6.

Nucleic Acids and Nucleotides

Nucleic acids are the most important macromolecules for the continuity of life. They carry the cell&#39;s genetic blueprint and have instructions for its ...

Nucleic Acid Structure

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DNA Base Pairing

Erwin Chargaff&#x2019;s rules on DNA equivalence paved the way for the discovery of base pairing in DNA. Chargaff&#x2019;s rules state that in a ...

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 ...

The DNA Helix

Deoxyribonucleic acid, or DNA, is the genetic material responsible for passing traits from generation to generation in all organisms and most viruses. DNA ...

The Central Dogma

The central dogma explains the flow of genetic information from DNA nucleotides to the amino acid sequence of proteins.
RNA is the Missing Link Between ...

Genomic DNA in Prokaryotes

The genome of most prokaryotic organisms consists of double-stranded DNA organized into one circular chromosome in a region of cytoplasm called the ...

Genomic DNA in Eukaryotes

Eukaryotes have large genomes compared to prokaryotes. To fit their genomes into a cell, eukaryotic DNA is packaged extraordinarily tightly inside the ...

Overview

Deoxyribonucleic acid, or DNA, is the genetic material responsible for passing traits from generation to generation in all organisms and most ...

DNA Packaging

Overview

Eukaryotes have large genomes compared to prokaryotes. In order to fit their genomes into a cell, eukaryotes must pack their DNA tightly inside ...

RNA Structure

Overview
The basic structure of RNA consists of a five-carbon sugar and one of four nitrogenous bases. Although most RNA is single-stranded, it can form ...

RNA Stability

Intact DNA strands can be found in fossils, while scientists sometimes struggle to keep RNA intact under laboratory conditions. The structural variations ...

pre-mRNA Processing

In eukaryotic cells, transcripts made by RNA polymerase are modified and processed before exiting the nucleus. Unprocessed RNA is called precursor mRNA or ...

Types of RNA

Overview
Three main types of RNA are involved in protein synthesis: messenger RNA (mRNA), transfer RNA (tRNA), and ribosomal RNA (rRNA). These RNAs ...

DNA as a Genetic Template

What is Gene Expression?

A gene is a stretch of DNA that serves as the blueprint for functional RNAs and proteins. Since DNA is comprised&#160; of nucleotides and proteins are ...

Transcription

Transcription is the synthesis of RNA from a DNA sequence by RNA polymerase. It is the first step in producing a protein from a gene sequence. ...

Translation

Translation is the process of synthesizing proteins from the genetic information carried by messenger RNA (mRNA). Following transcription, it constitutes ...

Ribosomes

Ribosomes translate genetic information encoded by messenger RNA (mRNA) into proteins. Both prokaryotic and eukaryotic cells have ribosomes. Cells that ...

tRNA Activation

Initiation of Translation

Termination of Translation

What are Cells?

Cell Diversity

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Cells of the Epidermis

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Bone Cells and Tissue

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Gross Anatomy of Skeletal Muscles

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Structure of Cardiac Muscles

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structure-and-organization-of-smooth-muscles

Structure and Organization of Smooth Muscles

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Chromosome Replication

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Neurons: The Cell Body and the Dendrites

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Lampbrush Chromosomes

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Nervous Tissue: Neuron Types

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Euchromatin

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Nervous Tissue: Glial Cells

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Heterochromatin

Lymphoid Cells and Tissues

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Chromatin Position Affects Gene Expression

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Organization of Genes

Chromosome Structure

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Construct an explanation based on evidence for how the structure of DNA determines the structure of proteins which carry out the essential functions of life through systems of specialized cells. Emphasis is on the conceptual understanding that DNA sequences determine the amino acid sequence and thus protein structure. Students can produce scientific writing, or presentations, and/or physical models that communicate constructed explanations.

Characteristics of Life

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Levels of Organization

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The Scientific Method

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Taxonomy

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Phylogeny

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HS-LS1-2

Overview of Metabolism

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Regulation of Metabolism

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Positive and Negative Feedback Loops

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Functions of Life

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Requirements for Human Life

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What is Homeostasis?

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Negative and Positive Feedback

Homeostatic Imbalance

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pH Homeostasis

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Homeostatic Imbalances in Body Temperature

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Regulation of Food Intake

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HS-LS1-3

What is the Cell Cycle?

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Interphase

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The Cell Cycle Control System

Positive Regulator Molecules

Inhibition of CDK Activity

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M-Cdk Drives Transition Into Mitosis

Mitogens and the Cell Cycle

Replicative Cell Senescence

Abnormal Proliferation

cells-coordinate-growth-and-proliferation

Cells Coordinate Growth and Proliferation

Use a model to illustrate the role of the cell cycle and differentiation in producing and maintaining complex organisms. Emphasis is on conceptual understanding that mitosis passes on genetically identical materials via replication, not on the details of each phase in mitosis.

HS-LS1-4

The Genetic Code and Codons

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