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

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From Molecules To Organisms: Structures And Processes

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From DNA to Protein

ДНК содержит гены, последовательности нуклеотидов, некоторые из которых являются инструкциями, кодирующими последовательность аминокислот в белке. Поток генетической информации от ДНК к РНК и белку это процесс, известный как Центральная догма молекулярной биологии. Первым шагом этого процесса является транскрипция, когда фермент РНК-полимераза синтезирует копию на основе РНК или транскрипт гена.ДНК используется в качестве матрицы, где каждое новое основание РНК, добавляемое к транскрипту, комплементарно исходной цепи ДНК. Некоторые транскрипты, называемые мессенджерами или мРНК, кодируют белки, а некодирующие участвуют в других клеточных процессах. Например, рибосомная рРНК и транспортная тРНК участвуют в синтезе белка.Следующим шагом является трансляция, когда мРНК расшифровывается для синтеза цепочки аминокислот. Набор инструкций, известный как генетический код, используется для чтения мРНК. Большинство организмов используют тот же универсальный код, состоящий из трех групп нуклеотидов, называемых кодонами, которые переводятся в определенные аминокислоты.Существует 64 различных триплета нуклеотидов, но только 20 стандартных аминокислот в белках делают код вырожденным, то есть несколько наборов кодонов могут давать одну и ту же инструкцию. Шестьдесят один устанавливает код для аминокислот, а три сигнализируют об остановке трансляции. Трансляция происходит в рибосоме, большом комплексе рРНК и белков, с помощью тРНК.тРНК имеет трехшпилечную петлевую структуру. Одна петля содержит последовательность, называемую антикодоном, которая имеет основания, дополняющие кодон. Аминокислота, соответствующая этой последовательности, прикрепляется к концу тРНК, которая транспортирует ее в рибосому.Белки, называемые факторами инициации, объединяют небольшую единицу рибосомы, тРНК инициатора и мРНК. После сборки комплекса рибосома скользит по мРНК в поисках стартовой площадки трансляции. Здесь антикодон тРНК инициатора связывается с комплементарным кодоном;большая единица рибосомы связывается со сборкой, и начинается трансляция.Когда входит следующая тРНК, аминокислота от инициатора отделяется и переносится на соседнюю аминокислоту, что приводит к росту полипептидной цепи. Добавление аминокислот продолжается до тех пор, пока в мРНК не будет обнаружен стоп-кодон. Затем рибосома освобождает цепь, чтобы она могла складываться в функциональный белок.

От ДНК к белку

Поток генетической информации в клетках от ДНК к мРНК к белку описывается центральной догмой, которая гласит, что гены определяют последовательность мРНК, которая в свою очередь определяет последовательность аминокислот, образующей все белки. Декодирование одной молекулы в другую осуществляется специфическими белками и РНК. Поскольку информация, хранящаяся в ДНК, является настолько центральной для клеточной функции, интуитивно понятно, что клетка будет делать mRNA копии этой информации для синтеза белков, сохраняя при этом саму ДНК нетронутой и защищенной. Копирование ДНК на РНК относительно прямолинейно, при этом один нуклеотидный прилив добавляется в цепочку мРНК для каждого нуклеотидного прочтения в ДНК. Перевод на белок является несколько более сложным, поскольку три нуклеотидов мРНК соответствуют одной аминокислоте в полипептидной последовательности. Однако, перевод на белок все еще систематический и коллинеарный, так что нуклеотиды 1—3 соответствуют аминокислоте 1, нуклеотиды 4—6 соответствуют аминокислоте 2, и так далее. 
 Генетический кодекс деградирует и универсал 
 Каждая аминокислота определяется трёхнуклеотидной последовательностью, называемой кодоном триплета. Учитывая различное количество «букв» в «алфавитах» мРНК и белка, ученые теоретизировали, что отдельные аминокислоты должны быть представлены комбинациями нуклеотидов. Нуклеотидных дублетов было бы недостаточно, чтобы указать каждую аминокислоту, так как существует только 16 возможных двухнуклеотидных комбинаций (42). В отличие от этого, существует 64 возможных нуклеотидных триплетов (43), что намного больше, чем количество аминокислот. Ученые теоретизировали, что аминокислоты были закодированы нуклеотидными триплетами и что генетический код “вырожден”. Другими словами, данная аминокислота может быть закодирована более чем одним нуклеотидным триплетом. Позже это было подтверждено экспериментально: Фрэнсис Крик и Сидней Бреннер использовали химический мутаген-профлавик для вставки одного, двух или трех нуклеотидов в ген вируса. При вставке одного или двух нуклеотидов нормальные белки не производились. Когда были вставлены три нуклеотида, белок был синтезирован и функциональен. Это показало, что аминокислоты должны быть определены группами из трех нуклеотидов. Эти нуклеотидные триплеты называются кодонами. Введение одного или двух нуклеотидов полностью изменило рамку считывания триплета, тем самым изменив сообщение для каждой последующей аминокислоты. Несмотря на то, что введение трех нуклеотидов привело к введению дополнительной аминокислоты во время перевода, целостность остальной части белка сохранялась. 
 В дополнение к кодонам, которые поручают добавление специфической аминокислоты в полипептидную цепь, три из 64 кодонов прекращают синтез белка и высвобощают полипептид из переводческой машины. Эти триплеты называются чепушными кодонами или прекращают кодонами. Другой кодон, АВГ, также имеет особую функцию. Помимо указания аминокислотного метионина, он также служит в качестве стартового кодона для инициации перевода. Кадр для чтения для перевода задается кудоном AUG start около конца 5' мРНК. После начала кодона мРНК читается группами по три человека до тех пор, пока не будет обнаружен стопный кодон. 
 Характеристика одной аминокислоты несколькими похожими кодонами называется «дегенерация». Считается, что дегенерация является клеточным механизмом для уменьшения негативного воздействия случайных мутаций. Кодоны, определяющие одну и ту же аминокислоту, обычно отличаются лишь на один нуклеотидный. Кроме того, аминокислоты с химически похожими боковыми цепями кодируются подобными кодонами. Например, аспартат (ASP) и глютамат (Glu), которые занимают блок GA*, оба отрицательно заряжаются. Этот нюанс генетического кода гарантирует, что однонуклеотидная замещающая мутация может указать ту же аминокислоту, но не оказывает влияния или не указывает на аналогичную аминокислоту, предотвращая превращение белка в полностью нефункциональную. 
 Генетический код почти универсален. За некоторыми незначительными исключениями, практически все виды используют один и тот же генетический код для синтеза белков. Сохранение кодонов означает, что очищенная мРНК, кодирующая белок глобина в лошадях, может быть перенесена в тюльпанную клетку, а тюльпан синтезирует конный глобин. То, что существует только один генетический код, является мощным доказательством того, что вся жизнь на Земле имеет общее происхождение, особенно учитывая, что существует около 1084 возможных комбинаций 20 аминокислот и 64 триплета кодонов. 
 Этот текст был адаптирован из<a href="https://openstax.org/books/biology-2e/pages/15-1-the-genetic-code"> Openstax, Biology 2e, Глава 15.1:</a><a href="https://openstax.org/books/biology-2e/pages/15-1-the-genetic-code"> Генетический кодекс</a>.

Поток генетической информации в клетках от ДНК к мРНК и белку описывается центральной догмой, которая гласит, что гены определяют последовательность ...

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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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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

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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

The pentose sugar in DNA is deoxyribose, while in RNA the pentose sugar is ribose. The difference between the sugars is the presence of the hydroxyl group ...

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

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pre-mRNA Processing

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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?

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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

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

Ribosomes

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tRNA Activation

Initiation of Translation

Termination of Translation

What are Cells?

Cell Diversity

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Structure of Cardiac Muscles

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Structure and Organization of Smooth Muscles

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Chromosome Replication

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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

Cellular needs and conditions vary from cell to cell and change within individual cells over time. For example, the required enzymes and energetic demands ...

Positive and Negative Feedback Loops

Animal organs and organ systems constantly adjust to internal and external changes through a process called homeostasis (&#34;steady state&#34;). Examples ...

Functions of Life

Human life is characterized by a variety of functions that are essential for survival and well-being. These functions include metabolism, movement, ...

Requirements for Human Life

The Earth and its atmosphere have provided humans with air, water, and food, but these are not the only requirements for survival. Humans also require a ...

What is Homeostasis?

Maintaining homeostasis requires that the body continuously maintain its internal conditions. Each physiological condition has a particular set point, ...

Negative and Positive Feedback

Homeostatic Imbalance

Homeostasis is the maintenance of a stable internal environment within the body, which is crucial for the proper functioning of cells, tissues, organs, ...

pH Homeostasis

Acid-base homeostasis is essential for maintaining normal physiological activities in humans. The pH of various body fluids is strictly regulated because ...

homeostatic-imbalances-in-body-temperature

Homeostatic Imbalances in Body Temperature

Hyperthermia is an umbrella term for elevated body temperature that occurs when heat production exceeds the body&#39;s ability to dissipate it.
...

Regulation of Food Intake

The hypothalamus releases peptides influencing feeding behavior.
Two hypothalamic neuronal groups participate in this process.
Neuropeptide Y (NPY) and ...

Plan and conduct an investigation to provide evidence that feedback mechanisms maintain homeostasis in living organisms. Examples of investigations could include heart rate responses to exercise, stomate responses to moisture and temperature, root development in response to water levels, or cell response to hypertonic and hypotonic environments.

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What is the Cell Cycle?

The cell cycle refers to the sequence of events occurring throughout a typical cell&#8217;s life. In eukaryotic cells, the somatic cell cycle has two ...

Interphase

The cell cycle occurs over approximately 24 hours (in a typical human cell) and in two distinct stages: interphase, which includes three phases of the ...

The Cell Cycle Control System

Positive Regulator Molecules

Inhibition of CDK Activity

S-Cdk Initiates DNA Replication

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.

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The Genetic Code and Codons

Description

Transcript

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