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 包含基因、核苷酸序列，其中一些是编码蛋白质中 一系列氨基酸的指令。遗传信息从 DNA 到 RNA 再到蛋白质的 流动过程称为"中心法则"这一过程的第一步是转录，RNA 聚合酶合成基于 RNA 的拷贝，或基因的转录本。DNA 用作模板，其中添加到 转录本中的每个新 RNA 碱基 都与原始 DNA 链互补。一些转录本称为信使或 mRNA，它们编码蛋白质，同时非编码转录本 参与其他细胞过程。例如，核糖体 rRNA 和转移 tRNA 参与蛋白质合成。下一步是翻译，mRNA 被解码，合成氨基酸链。一组称为遗传密码的指令 用来读取 mRNA。大多数生物使用相同的通用代码，由三种称为密码子的核苷酸组组成，翻译成特定的氨基酸。有 64 种不同的核苷酸三联体，但在蛋白质中 只有 20 种标准氨基酸使编码简并，也就是说，多个密码子集可以给出 相同的指令。61 组氨基酸编码，3 组表示翻译停止。在 tRNA 的帮助下，翻译发生在核糖体上，核糖体是 rRNA 和蛋白质的 大型复合体。tRNA 具有三发夹环结构。一个环包含一个叫做反密码子的序列，它与密码子 有互补的碱基。与这个序列相对应的氨基酸 附着在 tRNA 的末端，tRNA 将其 转运到核糖体中。称为启动因子的蛋白质将 小核糖体单元、启动因子 tRNA 和 mRNA 聚集在一起。复合体组装完成后，核糖体沿 mRNA 滑动，寻找翻译起始位点。这里，启动子 tRNA 反密码子 与互补密码子结合；大核糖体单元与装配体结合，翻译开始。当下一个 tRNA 进入时，来自启动子的氨基酸被分离，并转移到相邻氨基酸上，形成一个不断增长的多肽链。氨基酸的添加一直持续到 在 mRNA 中检测到终止密码子为止。然后，核糖体释放出这条链，这样，它就可以折叠成一种功能蛋白。

从脱氧核糖核酸（dna）到蛋白质

细胞中的基因信息从 DNA 流向 mRNA 到蛋白质的流动由中心犬进行描述，该犬表示基因指定 mRNA 的序列，而 mRNA 又指定组成所有蛋白质的氨基酸的序列。 一个分子到另一个分子的解码由特定蛋白质和 RNA 执行。 由于 DNA 中存储的信息对细胞功能如此重要，因此很直观的理解是，细胞会制作用于蛋白质合成的 mRNA 副本，同时保持 DNA 本身的完整性和受保护。 DNA 与 RNA 的复制相对简单，在 DNA 链中读取的每一个核苷酸都会在 mRNA 链中添加一个核苷酸。 蛋白质转化为更多化合物，因为三个 mRNA 核苷酸对应多肽序列中的一个氨基酸。 但是，蛋白质转化仍是系统的，共线性的，因此核苷酸 1 至 3 对应于氨基酸 1 ，核苷酸 4 至 6 对应于氨基酸 2 等。 
 遗传密码是简并和通用的
 每种氨基酸均由一种称为三普莱特酸的三核苷酸序列定义。 考虑到 mRNA 和蛋白质“字母”中的“字母”数量不同，科学家们理论家们认为单个氨基酸必须由核苷酸组合来表示。 核苷酸双核不足以指定每个氨基酸，因为只有 16 种可能的双核苷酸组合 (42)。 相比之下，有 64 个可能的核苷酸三重编码 (43) ，这远远超过氨基酸的数量。 科学家们认为，氨基酸是由核苷酸三重编码的，遗传代码是“去生成”。 换言之，一种特定的氨基酸可由多种核苷酸三普莱特编码。 后来经实验确认：弗朗西斯 · 克里克（Francis Crick）和悉尼 · 布伦纳（Sydney Brenner）使用化学多变性素将一、两或三种核苷酸插入病毒基因中。 当插入一到两个核苷酸时，不会产生正常蛋白质。 当插入三个核苷酸时，该蛋白质被合成并起作用。 这表明氨基酸必须由三组核苷酸来指定。 这些核苷酸三重编码称为编码组（codons）。 插入一个或两个核苷酸完全改变了三重编码的读数帧，从而改变了每一种随后的氨基酸的信息。 尽管插入三种核苷酸会导致在翻译过程中插入额外的氨基酸，但蛋白质其余部分的完整性得到了保持。 
 除了指示向多肽链添加特定氨基酸的编码外， 64 种编码中的三种可终止蛋白质合成并从翻译机器中释放多肽。 这些三重编码被称为无效的编码或终止密码子。 另一个编码， AUG，也有一个特殊的功能。 除了指定氨基酸甲氨离子酸外，它还可用作启动转化的起始编码。 用于翻译的读数框架由 AUG开始编码在 mRNA 5 英尺端附近设置。 在开始时， mRNA 以三个一组的形式读取，直到遇到停止时代码为止。 
 一种氨基酸通过多种类似的编码的规格被称为“去变性”。 据认为，变性是一种细胞机理，可减少随机突变的负面影响。 指定相同氨基酸的编码通常仅因一种核苷酸而异。 此外，具有化学相似侧链的氨基酸使用类似的编码进行编码。 例如，占据 Ga* 块的 天冬氨酸 (ASP) 和谷氨酸 (Glu) 都受到了负面指控。 基因代码的这种细微差别确保单核苷酸替代突变可能会指定相同的氨基酸，但不会产生影响或指定相似的氨基酸，从而防止蛋白质完全无法发挥作用。 
 遗传密码几乎是通用的。 除了少数微小的例外，几乎所有物种都使用相同的遗传代码进行蛋白质合成。 维护编码意味着对马中的球蛋白编码的纯化 mRNA 可转移到郁金香细胞中，而郁金香将合成马球蛋白。 只有一种基因代码有力地证明了地球上的所有生命都有一个共同的起源，特别是考虑到大约有 1084 种可能的组合，包括 20 种氨基酸和 64 种三重密码子。 
 本文改编自<a href="https://openstax.org/books/biology-2e/pages/15-1-the-genetic-code"> Openstax, 生物学 2e, 第15.1节：</a><a href="https://openstax.org/books/biology-2e/pages/15-1-the-genetic-code">遗传密码</a>。

从脱氧核糖核酸（DNA）到蛋白质

细胞中遗传信息从DNA到mRNA再到蛋白质的流动由中心法则描述，该法则指出基因决定……

Education

教育

JoVE Core

Chemistry

化学

Biochemistry

生物化学

KEY TERMS AND CONCEPTS

关键术语与概念

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

functional groups

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chemistry of carbohydrates

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amino acids

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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

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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

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Nucleic Acid Structure

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DNA Base Pairing

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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.
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Genomic DNA in Prokaryotes

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Genomic DNA in Eukaryotes

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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
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RNA Stability

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pre-mRNA Processing

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Types of RNA

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DNA as a Genetic Template

What is Gene Expression?

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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

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Ribosomes

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tRNA Activation

Initiation of Translation

Termination of Translation

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Cell Diversity

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HS-LS1-2

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HS-LS1-3

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Replicative Cell Senescence

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HS-LS1-4

The Genetic Code and Codons

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