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

يحتوي الـDNA على الجينات وهي تسلسلات من النيكليوتيدات،بعضها بمثابة تعليمات تشكل شيفرة لسلسلة الأحماض الأمينية في بروتين معين. تدفق المعلومات الجينية من الDNA إلى الRNA ثم إلى البروتين،عملية تُعرف ب لعلم الأحياء الجزيئية. الخطوة الأولى في هذه العملية هي النسخ،حيث يكوّن أحد إنزيمات بوليميراز الـRNA،نسخة عن RNA،أو نسخة عن الجين.يُستخدم الـDNA كقالب بحيث أن كل قاعدة RNA جديدة تُضاف إلى النسخة تكون مكمّلة لسلسلة الـ DNA الأصلية. بعض النسخ،وتُسمى مراسيل أو mRNA،تعطي شيفرة البروتين،بينما تشارك النسخ الأخرى غير المشفرة،في عمليات أخرى داخل الخلية. على سبيل المثال،الـRNA الريبوسومي،والـRNA الناقل،يشاركان في تصنيع البروتين.الخطوة التالية هي الترجمة،حيث تُفك شيفرة mRNA لتكوين سلسلة من الأحماض الأمينية. تُستخدم مجموعة من التعليمات،تُعرف بالشيفرة الجينية،لقراءة mRNA أو مراسيل RNA. تستخدم معظم الكائنات الحية هذه الشيفرة العالمية ذاتها،التي تتألف من مجموعات نيوكليوتيدات تُسمى الكودونات،التي تترجم إلى أحماض أمينية معينة.توجد 64 ثلاثية نيكليوتيدية مختلفة،لكن لا يوجد سوى 20 حمض أميني قياسي في البروتينات،تتسبب في فك الشيفرة،حيث أن مجموعات كودونات متعددة،يمكن أن تُعطي نفس التعليمات. واحد وستون مجموعة تشكل شيفرة الأحماض الأمينية،وثلاثة تعطي إشارة لوقف الترجمة. تحدث الترجمة في الريبوسوم،وهو معقّد كبير يتألف من RNA ريبوسومي وبروتينات،مع مساعدة RNA الناقل يتضمن tRNA من ثلاث بنيات حلَقية على شكل دبوس الشعر.تحتوي إحدى الحلقات على تسلسل يُسمى الكودون المُضاد،والذي يحتوي على قاعدتين مكملتين للكودون. يرتبط حمض أميني مطابق لهذا التسلسل بطرف الـtRNA،الذي ينقله إلى داخل الريبوسوم. تعمل البروتينات،التي تُسمى عوامل ابتداء،على تجميع وحدات الريبوسوم الصغيرة،وtRNA الابتدائي،وmRNA.بعد تجميع المعقّد،ينزلق الريبوسوم على طول الـmRNA باحثًا عن موقع بدء الترجمة. وهنا يرتبط الكودون المضاد و tRNA الابتدائي مع الكودون المكمّل؛ترتبط وحدة الريبوسوم الكبيرة مع المجموعة،وتبدأ الترجمة. عندما يدخل tRNA التالي،ينفصل الحمض الأميني للبادئ،وينتقل إلى الحمض الأميني المجاور،ما ينتج عنه سلسلة متعدد-الببتيد متزايدة.تستمر إضافة الأحماض الأمينية إلى يتم رصد كودون توقف في الـRNA المرسال بعد ذلك يُطلق الريبوسوم السلسلة،كي تستطيع أن تنطوي لتصبح بروتينًا وظيفيًا.

من الحمض النووي إلى البروتين

إن تدفق المعلومات الجينية في الخلايا من DNA إلى mRNA إلى البروتين موصوف بالقاعدة المركزية، والتي تنص على أن الجينات تحدد تسلسل mRNAs، والتي بدورها تحدد تسلسل الأحماض الأمينية التي تتكون منها جميع البروتينات. فك التشفير من جزيء إلى آخر يتم إجراؤه بواسطة بروتينات محددة و RNAs. نظراً لأن المعلومات المخزنة في DNA مركزية جداً للوظيفة الخلوية، فمن المنطقي أن تقوم الخلية بعمل نسخ mRNA من هذه المعلومات لتخليق البروتين مع الحفاظ على الحمض النووي نفسه سليماً و يعتبر نسخ DNA إلى RNA أمراً بسيطاً نسبياً، حيث تتم إضافة نيوكليوتيد واحد إلى خيط mRNA لكل نيوكليوتيد يُقرأ في خيط DNA.تعتبر الترجمة إلى البروتين أكثر تعقيدًا بعض الشيء لأن ثلاثة نيوكليوتيدات mRNA تتوافق مع حمض أميني واحد في تسلسل متعدد الببتيد. ومع ذلك، فإن الترجمة إلى البروتين لا تزال منتظمة وخطية، مثل أن النيوكليوتيدات 1 إلى 3 تتوافق مع الأحماض الأمينية 1، والنيوكليوتيدات 4 إلى 6 تتوافق مع الأحماض الأمينية 2، وهكذا. 
 الكود الجيني منحلّ وشامل 
 يتم تعريف كل حمض أميني من خلال تسلسل ثلاثي النيوكليوتيدات يسمى الكودون الثلاثي. بالنظر إلى الأرقام المختلفة لـ “الحروف” في mRNA والبروتين “الأبجديات,” افترض العلماء أن الأحماض الأمينية الفردية يجب أن يتم تمثيلها من خلال مجموعات من النيوكليوتيدات. لن تكون مضاعفات النوكليوتيدات كافية لتحديد كل حمض أميني لأنه لا يوجد سوى 16 توليفة محتملة من اثنين من النيوكليوتيدات (42). في المقابل ، هناك 64 ثلاثي نيوكليوتيد محتمل (43)، وهو عدد أكبر بكثير من عدد الأحماض الأمينية. افترض العلماء أن الأحماض الأمينية تم تشفيرها بواسطة ثلاثة توائم من النيوكليوتيدات وأن الشفرة الوراثية “منحلّة.” بعبارة أخرى، يمكن ترميز حمض أميني معين بأكثر من ثلاثي نيوكليوتيد واحد. تم تأكيد ذلك لاحقاً تجريبياً: استخدم فرانسيس كريك وسيدني برينر المغير الكيميائي بروفلافين لإدخال واحد أو اثنين أو ثلاثة نيوكليوتيدات في جين الفيروس. عندما تم إدخال واحد أو اثنين من النيوكليوتيدات، لا يتم إنتاج البروتينات الطبيعية. عندما تم إدخال ثلاث نيوكليوتيدات، تم تصنيع البروتين وعمله. أظهر هذا أن الأحماض الأمينية يجب أن تحدد بمجموعات من ثلاثة نيوكليوتيدات. تسمى هذه النوكليوتيدات الثلاثية الكودونات. أدى إدخال واحد أو اثنين من النيوكليوتيدات إلى تغيير إطار القراءة الثلاثي تماماً، وبالتالي تغيير الرسالة لكل حمض أميني لاحق. على الرغم من أن إدخال ثلاثة نيوكليوتيدات تسبب في إدخال حمض أميني إضافي أثناء الترجمة، فقد تم الحفاظ على سلامة بقية البروتين. 
 بالإضافة إلى الكودونات التي ترشد إضافة حمض أميني معين إلى سلسلة بولي ببتيد، تنهي ثلاثة من الكودونات الـ 64 تخليق البروتين وتحرر البولي ببتيد من آلية الترجمة. تسمى هذه المجموعات الثلاثة أكواد غير منطقية أو أكواد الإيقاف. كودون آخر، AUG، له أيضاً وظيفة خاصة. بالإضافة إلى تحديد ميثيونين الأحماض الأمينية، فإنه يعمل أيضاً ككودون بدء لبدء الترجمة. يتم تعيين إطار القراءة للترجمة بواسطة كودون البدء AUG بالقرب من نهاية 5' للـ mRNA. بعد رمز البدء، تتم قراءة mRNA في مجموعات من ثلاثة حتى يتم العثور على كودون الإيقاف. 
 يسمى تحديد حمض أميني واحد بواسطة عدة أكواد متشابهة "انحلال." يُعتقد أن الانحلال هو آلية خلوية لتقليل التأثير السلبي للطفرات العشوائية. عادةً ما تختلف الكودونات التي تحدد نفس الحمض الأميني بنوكليوتيد واحد فقط. بالإضافة إلى ذلك، يتم ترميز الأحماض الأمينية ذات السلاسل الجانبية المتشابهة كيميائياً بواسطة أكواد مماثلة. على سبيل المثال، فإن شحنة كل من الأسبارتات (Asp) والغلوتامات (Glu)، التي تشغل كتلة GA*، هي سالبة. يضمن هذا الفارق الدقيق في الشفرة الجينية أن طفرة استبدال النوكليوتيدات المفردة قد تحدد نفس الحمض الأميني ولكن ليس لها تأثير أو تحدد حمض أميني مشابه، مما يمنع البروتين من أن يصبح غير فعال تماماً. 
 الشفرة الجينية عالمية تقريباً. مع بعض الاستثناءات الطفيفة، تستخدم جميع الأنواع تقريباً نفس الشفرة الجينية لتخليق البروتين. يعني حفظ الكودونات أنه يمكن نقل mRNA المنقى الذي يشفر بروتين الغلوبين في الخيول إلى خلية خزامى، وسيقوم الخزامى بتصنيع غلوبين الحصان. إن وجود رمز جيني واحد فقط هو دليل قوي على أن كل أشكال الحياة على الأرض تشترك في أصل مشترك، لا سيما بالنظر إلى أن هناك حوالي 1084 مجموعات محتملة من 20 حمض أميني و 64 كودون ثلاثي. 
 تم اقتباس هذا النص من <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>.

يتم وصف تدفق المعلومات الجينية في الخلايا من الحمض النووي إلى الرنا المرسال إلى البروتين من خلال العقيدة المركزية ، والتي تنص على أن الجينات تحدد تسلسل ...

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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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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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Chromatin Position Affects Gene Expression

Chromatin is the massive complex of DNA and proteins packaged inside the nucleus. The complexity of chromatin folding and how it is packaged inside the ...

Two structural features of the DNA molecule provide a basis for the mechanisms of heredity: the four nucleotide bases and its double-stranded nature. The ...

Organization of Genes

Chromosome Structure

A functional eukaryotic chromosome must contain three elements: a centromere, telomeres, and numerous origins of replication.
The centromere is a DNA ...

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

Biology is a natural science that studies life and living organisms, including their structure, function, development, interactions, evolution, ...

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Levels of Organization

Biological organization is the classification of biological structures, ranging from atoms at the bottom of the hierarchy to the Earth&#x27;s biosphere. ...

The Scientific Method

The scientific method is a detailed, empirical problem-solving process used by biologists and other scientists. This iterative approach involves ...

Taxonomy

Taxonomy is the science of defining and naming groups of biological organisms based on shared characteristics. It uses a hierarchy of increasingly ...

Phylogeny

Phylogeny is concerned with the evolutionary diversification of organisms or groups of organisms. A group of organisms with a name is called a taxon ...

Develop and use a model to illustrate the hierarchical organization of interacting systems that provide specific functions within multicellular organisms. Emphasis is on functions at the organism system level such as nutrient uptake, water delivery, or organism movement in response to neural stimuli. An example of an interacting system could be an artery depending on the proper function of elastic tissue and smooth muscle to regulate and deliver the proper amount of blood within the circulatory system.

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Overview of Metabolism

Living cells constantly carry out various chemical reactions which are necessary for their proper functioning. These reactions are interlinked to one ...

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 ...

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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

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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

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