📚 Protein Synthesis: Transcription and Translation | 蛋白质合成：转录与翻译
Protein synthesis is the fundamental biological process by which cells generate new proteins based on the genetic information stored in DNA. This two-step mechanism—transcription and translation—is essential for growth, repair, and the maintenance of all living organisms, and it lies at the heart of the Edexcel IGCSE Science (Biology) syllabus. Understanding how the genetic code is read and converted into functional proteins will not only help you tackle exam questions but also give you insight into how life works at a molecular level.
蛋白质合成是细胞根据DNA中储存的遗传信息生成新蛋白质的基本生物学过程。这个两步机制——转录和翻译——对于所有生物的生长、修复和维持至关重要，也是Edexcel IGCSE科学（生物）课程的核心内容。理解遗传密码如何被读取并转化为功能性蛋白质，不仅能帮助你应对考试题目，还能让你从分子层面洞察生命的运作方式。
1. The Genetic Blueprint: DNA and Genes | 遗传蓝图：DNA与基因
Deoxyribonucleic acid (DNA) serves as the master blueprint for protein synthesis. A gene is a specific sequence of nucleotide bases along a DNA molecule that codes for a particular protein. The DNA sequence is written in a four-letter code: adenine (A), thymine (T), cytosine (C), and guanine (G). Every set of three bases, called a triplet or codon when expressed in mRNA, specifies a single amino acid.
脱氧核糖核酸（DNA）是蛋白质合成的主蓝图。基因是DNA分子上编码特定蛋白质的一段特定碱基序列。DNA序列使用由四种字母组成的编码：腺嘌呤（A）、胸腺嘧啶（T）、胞嘧啶（C）和鸟嘌呤（G）。每三个碱基构成一组，称为三联体，在mRNA中则称为密码子，指定一种氨基酸。
In eukaryotes, most DNA is housed inside the nucleus, which physically separates transcription from the ribosomes in the cytoplasm where translation occurs. The double-helix structure of DNA is held together by complementary base pairing: A pairs with T via two hydrogen bonds, and C pairs with G via three hydrogen bonds. This complementarity is crucial for accurate copying of genetic information.
在真核生物中，大部分DNA位于细胞核内，这使得转录过程与发生在细胞质核糖体上的翻译过程在空间上分隔开来。DNA的双螺旋结构通过互补碱基配对维持：A与T通过两个氢键配对，C与G通过三个氢键配对。这种互补性对于遗传信息的精确复制至关重要。
2. The Messenger and the Translators: Types of RNA | 信使与译员：RNA的类型
Protein synthesis requires three major types of ribonucleic acid (RNA): messenger RNA (mRNA), transfer RNA (tRNA), and ribosomal RNA (rRNA). Unlike DNA, RNA is single-stranded and contains uracil (U) instead of thymine. The sugar in RNA is ribose, whereas DNA contains deoxyribose.
蛋白质合成需要三种主要的核糖核酸（RNA）：信使RNA（mRNA）、转运RNA（tRNA）和核糖体RNA（rRNA）。与DNA不同，RNA是单链结构，并且用尿嘧啶（U）代替胸腺嘧啶。RNA中的糖是核糖，而DNA中则是脱氧核糖。
mRNA carries a transcribed copy of the genetic code from DNA to the ribosomes. Its structure is linear, and the sequence of codons it contains determines the order of amino acids in the polypeptide chain. tRNA molecules act as adaptors; each tRNA has a specific anticodon at one end and an attachment site for a particular amino acid at the other. rRNA, together with proteins, forms the ribosome, the molecular machine that catalyses protein synthesis.
mRNA携带着从DNA转录而来的遗传密码副本，并将其送往核糖体。它的结构是线性的，其中所含的密码子序列决定了多肽链中氨基酸的顺序。tRNA分子充当适配器；每个tRNA的一端有一个特定的反密码子，另一端则有一个特定氨基酸的连接位点。rRNA与蛋白质一起构成核糖体，这个分子机器催化蛋白质的合成。
3. Transcription: Rewriting DNA into mRNA | 转录：将DNA改写为mRNA
Transcription is the first major stage of protein synthesis and takes place in the nucleus. The enzyme RNA polymerase binds to a specific region of the DNA called the promoter, which signals the start of a gene. The DNA double helix unwinds, and one of the strands—the template strand—is used to synthesise a complementary mRNA molecule.
转录是蛋白质合成的第一个主要阶段，发生在细胞核中。RNA聚合酶结合到DNA上称为启动子的特定区域，该区域标志着基因的起点。DNA双螺旋解开，其中一条链——模板链——被用来合成互补的mRNA分子。
RNA polymerase moves along the template strand in the 3′ to 5′ direction, adding free RNA nucleotides according to the base-pairing rules: C pairs with G, G with C, T with A, and A with U. This process continues until the enzyme reaches a terminator sequence, at which point the newly formed pre-mRNA is released. In eukaryotic cells, the pre-mRNA undergoes processing before it becomes mature mRNA: a 5′ cap and a poly-A tail are added, and non-coding introns are removed by splicing.
RNA聚合酶沿着模板链以3′到5′的方向移动，根据碱基配对规则添加游离的RNA核苷酸：C与G配对，G与C，T与A，而A与U配对。这个过程一直持续到酶到达终止子序列，此时新形成的前体mRNA被释放。在真核细胞中，前体mRNA在成为成熟mRNA之前会经历加工：添加5′帽和poly-A尾，并通过剪接去除非编码的内含子。
The mature mRNA then exits the nucleus through a nuclear pore and enters the cytoplasm, where it will be used by ribosomes. The entire transcription process ensures that the genetic message is faithfully copied into a portable format.
然后成熟的mRNA通过核孔离开细胞核，进入细胞质，在那里被核糖体利用。整个转录过程确保遗传信息被忠实地复制成一种可携带的形式。
4. The Genetic Code: Codons and Amino Acids | 遗传密码：密码子与氨基酸
e codons (4×4×4), but only 20 standard amino acids, meaning the genetic code is degenerate— ode for the same amino acid. The code is also universal, unambiguous, and non-overlapping.
mRNA序列以三个核苷酸为一组读取，这些组称为密码子。总共存在64种可能的密码子（4×4×4），但标准氨基酸只有20种，这意味着遗传密码具有简并性——多个密码子可能编码同一种氨基酸。该密码也是通用的、无歧义的且不重叠的。
Three codons—UAA, UAG, and UGA—do not code for any amino acid. These are stop codons and signal the termination of translation. The codon AUG has a dual role: it codes for methionine and acts as the start codon, marking the point where translation should begin.
有三个密码子——UAA、UAG和UGA——不编码任何氨基酸。它们是终止密码子，标志着翻译的结束。密码子AUG具有双重作用：它编码甲硫氨酸，并且充当起始密码子，指示翻译的起始位置。
5. Translation: Initiation – Setting Up the Ribosome | 翻译：起始 – 装配核糖体
Translation begins when the mature mRNA binds to the small subunit of a ribosome. The ribosome scans the mRNA until it locates the start codon AUG. A special initiator tRNA carrying methionine (in eukaryotes) recognises this codon through its complementary anticodon UAC and pairs with it.
当成熟mRNA与核糖体的小亚基结合时，翻译就开始了。核糖体扫描mRNA，直到找到起始密码子AUG。一种携带甲硫氨酸（在真核生物中）的特殊起始tRNA通过其互补反密码子UAC识别该密码子并与之配对。
Once the initiator tRNA is in place, the large ribosomal subunit joins, forming a complete ribosome. This functional ribosome has three binding sites for tRNA molecules: the A site (aminoacyl), the P site (peptidyl), and the E site (exit). The initiator tRNA initially occupies the P site, leaving the A site available for the next tRNA carrying the second amino acid.
一旦起始tRNA就位，核糖体大亚基便会加入，形成一个完整的核糖体。这个有功能的核糖体具有三个tRNA分子结合位点：A位点（氨酰基位点）、P位点（肽基位点）和E位点（出口位点）。起始tRNA最初占据P位点，使A位点空出来，供携带第二个氨基酸的下一个tRNA结合。
6. Translation: Elongation – Building the Polypeptide Chain | 翻译：延伸 – 构建多肽链
During elongation, tRNAs charged with amino acids enter the ribosome one by one. A tRNA whose anticodon is complementary to the codon exposed in the A site binds there. The ribosome catalyses the formation of a peptide bond between the amino acid in the P site and the newly arrived amino acid in the A site; this reaction requires energy from GTP and is catalysed by peptidyl transferase, an enzymatic activity of the rRNA.
在延伸过程中，携带氨基酸的tRNA逐个进入核糖体。反密码子与暴露在A位点的密码子互补的tRNA结合到该位点。核糖体催化P位点的氨基酸与新到达A位点的氨基酸之间形成肽键；这一反应需要GTP提供能量，并由肽基转移酶（rRNA的一种酶活性）催化。
After the peptide bond forms, the ribosome shifts along the mRNA by one codon—a process called translocation. The tRNA that was in the P site moves to the E site and is ejected, the tRNA in the A site moves to the P site, and the A site becomes vacant. This cycle repeats, adding amino acids one at a time in the order specified by the mRNA codons.
肽键形成后，核糖体沿着mRNA移动一个密码子——这一过程称为易位。原来在P位点的tRNA移动到E位点并被弹出，A位点的tRNA移动到P位点，A位点变空。这个循环不断重复，按照mRNA密码子规定的顺序逐个添加氨基酸。
The polypeptide chain grows from the amino terminus (N-terminus) to the carboxyl terminus (C-terminus). Elongation rates can reach up to 20 amino acids per second in eukaryotes, and multiple ribosomes can translate a single mRNA simultaneously, forming a structure known as a polysome.
多肽链从氨基端（N端）向羧基端（C端）延伸。真核生物中延伸速率可达每秒20个氨基酸，并且多个核糖体可以同时翻译一条mRNA，形成称为多聚核糖体的结构。
7. Translation: Termination – Releasing the New Protein | 翻译：终止 – 释放新蛋白质
Elongation continues until a stop codon (UAA, UAG, or UGA) enters the A site. No tRNA molecules recognise these codons. Instead, a protein called a release factor binds to the stop codon, causing the ribosome to add a water molecule to the end of the polypeptide chain. This hydrolysis reaction releases the completed polypeptide from the tRNA in the P site.
延伸持续进行，直到一个终止密码子（UAA、UAG或UGA）进入A位点。没有tRNA分子能识别这些密码子。相反，一种称为释放因子的蛋白质与终止密码子结合，促使核糖体在多肽链末端添加一个水分子。这一水解反应使完整的多肽链从P位点的tRNA上释放出来。
The ribosomal subunits then dissociate from the mRNA and can be recycled for another round of translation. At this point, the new polypeptide is free to fold into its three-dimensional conformation and undergo post-translational modifications, which are essential for its biological function.
随后核糖体亚基从mRNA上解离，并可回收用于下一轮翻译。此时，新生多肽链可以自由折叠成其三维构象，并经历翻译后修饰，这些修饰对于其生物学功能至关重要。
8. From Polypeptide to Functional Protein | 从多肽到功能性蛋白质
A freshly synthesised polypeptide is not yet a functional protein. It must fold into a specific three-dimensional shape, determined largely by its amino acid sequence. Chaperone proteins often assist in proper folding. In some cases, polypeptides are modified by the addition of sugar groups (glycosylation), phosphate groups (phosphorylation), or by cleavage of certain segments.
新合成的多肽还不是功能性蛋白质。它必须折叠成特定的三维形状，这主要由其氨基酸序列决定。分子伴侣蛋白通常协助正确折叠。在某些情况下，多肽会通过添加糖基团（糖基化）、磷酸基团（磷酸化）或切割某些片段而得到修饰。
Many proteins consist of more than one polypeptide subunit, and the final quaternary structure, such as that of haemoglobin (made of two alpha and two beta chains), is assembled only after individual chains have folded. The correct folding and modification of proteins are essential, as misfolded proteins can cause diseases like Alzheimer's and cystic fibrosis.
许多蛋白质由多个多肽亚基组成，例如血红蛋白（由两条α链和两条β链组成）的四级结构只有在各条链折叠后才会组装起来。蛋白质的正确折叠和修饰至关重要，因为错误折叠的蛋白质可导致阿尔茨海默病和囊性纤维化等疾病。
9. Mutations: When the Code Goes Wrong | 突变：当遗传密码出错
A mutation is a change in the DNA base sequence. Mutations can arise spontaneously during DNA replication or be induced by mutagens such as UV radiation and certain chemicals. In the context of protein synthesis, even a single base change can have dramatic effects.
突变是指DNA碱基序列的改变。突变可在DNA复制过程中自发产生，也可由紫外线辐射和某些化学物质等诱变剂诱导。在蛋白质合成中，哪怕一个碱基发生改变都可能导致戏剧性的后果。
Substitution mutations replace one base with another, potentially altering a single codon and leading to a different amino acid (missense) or a stop codon (nonsense).
替换突变将一个碱基换成另一个碱基，可能改变单个密码子，导致氨基酸改变（错义突变）或提前出现终止密码子（无义突变）。
Insertion or deletion of bases can cause a frameshift, changing the reading frame of the entire sequence downstream. This often produces a completely different and usually non-functional protein.
碱基的插入或缺失可导致移码突变，改变下游整个序列的读码框。这通常会产生完全不同且通常无功能的蛋白质。
For Edexcel IGCSE, you should be able to use the genetic code table to predict how a mutation might alter the amino acid sequence of a protein, and link this to potential loss of function.
在Edexcel IGCSE中，你需要能够使用遗传密码表预测突变如何改变蛋白质的氨基酸序列，并将其与功能丧失联系起来。
10. Key Exam Concepts for Edexcel IGCSE Science | Edexcel IGCSE科学重点考查概念
When answering questions on protein synthesis, use precise scientific terminology. Distinguish clearly between transcription (DNA to mRNA in the nucleus) and translation (mRNA to polypeptide at the ribosome). Remember that mRNA codons are complementary to the DNA template strand, and tRNA anticodons are complementary to mRNA codons.
在回答蛋白质合成相关问题时，要使用准确的科学术语。清晰区分转录（细胞核内DNA → mRNA）和翻译（核糖体上mRNA → 多肽）。记住mRNA密码子与DNA模板链互补，而tRNA反密码子与mRNA密码子互补。
Pay attention to base pairing: in RNA, adenine pairs with uracil, not thymine. You may be asked to determine the DNA template strand sequence from a given mRNA sequence, or to use a codon table to identify the amino acid sequence. Practise writing out the steps in order, and be prepared to explain how enzymes such as RNA polymerase, as well as energy from ATP and GTP, are involved.
注意碱基配对：在RNA中腺嘌呤与尿嘧啶配对，而非胸腺嘧啶。你可能会被要求根据给定的mRNA序列推断DNA模板链序列，或者使用密码子表确定氨基酸序列。练习按顺序写出步骤，并准备解释RNA聚合酶等酶以及来自ATP和GTP的能量是如何参与的。
Key topics also include the structure and roles of mRNA, tRNA, and ribosomes, and how mutations can lead to genetic disorders. Use labelled diagrams where appropriate to illustrate the process, and always refer back to the universality and degeneracy of the genetic code in longer answer questions.
重点主题还包括mRNA、tRNA和核糖体的结构和作用，以及突变如何导致遗传疾病。在适当情况下使用带标注的图表来说明过程，并且在较长的问题回答中始终提及遗传密码的通用性和简并性。
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