Understanding Protein Synthesis: Transcription and Translation | 理解蛋白质合成：转录与翻译

📚 Understanding Protein Synthesis: Transcription and Translation | 理解蛋白质合成：转录与翻译

The ability of cells to manufacture specific proteins is fundamental to life. Every protein your body makes — from the collagen in your skin to the enzymes in your saliva — is built according to precise instructions stored in your DNA. This process, called protein synthesis, takes place in two main stages: transcription and translation. Understanding these stages is essential for grasping how genes determine our traits, how mutations can cause disease, and how modern biotechnology works.

细胞制造特定蛋白质的能力是生命的基础。你身体制造的每一种蛋白质——从皮肤中的胶原蛋白到唾液中的酶——都是按照储存在DNA中的精确指令构建的。这个过程称为蛋白质合成，发生在两个主要阶段：转录和翻译。理解这些阶段对于掌握基因如何决定我们的性状、突变如何导致疾病以及现代生物技术如何运作至关重要。

1. Why Do Cells Need to Make Proteins? | 细胞为何需要制造蛋白质？

Proteins are the workhorses of the cell. They act as enzymes to speed up chemical reactions, form structural components like muscle fibres and cell membranes, function as hormones to send signals, and serve as antibodies to defend against pathogens. Almost every aspect of cell function relies on the correct proteins being produced at the right time and in the right amounts.

蛋白质是细胞的主力军。它们充当酶以加速化学反应，形成肌肉纤维和细胞膜等结构成分，作为激素传递信号，并作为抗体抵御病原体。细胞功能的几乎每个方面都依赖于在正确的时间和正确的数量产生正确的蛋白质。

But proteins cannot be made directly from DNA. The DNA is too precious and too large to leave the nucleus. Instead, the cell uses a two‑step system: first, the DNA’s code is copied into a smaller, messenger molecule (RNA) during transcription. Then, in translation, that messenger is read by ribosomes to assemble the protein. This division of labour also allows one gene to be copied many times, amplifying protein output.

但蛋白质不能直接从DNA制造。DNA太珍贵且太大，无法离开细胞核。相反，细胞使用两步系统：首先，在转录过程中，DNA的编码被复制到一个较小的信使分子（RNA）中。然后，在翻译过程中，核糖体读取这个信使来组装蛋白质。这种分工还允许一个基因被多次复制，从而放大蛋白质的输出量。

2. The Central Dogma of Molecular Biology | 分子生物学的中心法则

The flow of genetic information is often summarised by the central dogma: DNA makes RNA makes protein. This concept, first expressed by Francis Crick, describes the unidirectional transfer of information from the stable storage molecule (DNA) to the transient working molecule (mRNA) and finally to the functional product (protein). Note that some viruses can reverse‑transcribe RNA back to DNA, but that is an exception.

遗传信息的流动通常由中心法则概括：DNA制造RNA，RNA制造蛋白质。弗朗西斯·克里克首次提出的这一概念描述了信息从稳定的储存分子（DNA）单向传递到短暂的工作分子（mRNA），最后到达功能性产物（蛋白质）的过程。请注意，有些病毒可以将RNA逆转录回DNA，但那是一个例外。

IGCSE Biology, you only need to remember the main pathway: DNA → mRNA → protein. The gene — a section of a DNA molecule — , each specifying an amino acid. The chain of amino acids folds into a functional protein.

在IGCSE生物学中，你只需要记住主要途径：DNA → mRNA → 蛋白质。基因——DNA分子的一个片段——包含编码多肽链的碱基序列。在转录过程中，基因被用作模板来产生一条互补的信使RNA（mRNA）链。然后，在翻译过程中，mRNA序列以三个碱基（密码子）为一组被读取，每组指定一个氨基酸。氨基酸链折叠成一个功能性的蛋白质。

3. The Key Players: DNA, RNA and Ribosomes | 关键角色：DNA、RNA和核糖体

Before diving into the steps, let’s clarify the molecules involved. DNA (deoxyribonucleic acid) is a double‑stranded helix composed of nucleotides with the bases adenine (A), thymine (T), cytosine (C) and guanine (G). It is found inside the nucleus and remains there, protecting the original genetic code.

在深入步骤之前，让我们澄清一下所涉及的分子。DNA（脱氧核糖核酸）是一个双链螺旋，由含有腺嘌呤（A）、胸腺嘧啶（T）、胞嘧啶（C）和鸟嘌呤（G）四种碱基的核苷酸组成。它存在于细胞核内并留在那里，保护着原始的遗传密码。

RNA (ribonucleic acid) is a single‑stranded molecule with a slightly different sugar (ribose instead of deoxyribose) and the base uracil (U) instead of thymine. Three types of RNA participate in protein synthesis: messenger RNA (mRNA), transfer RNA (tRNA), and ribosomal RNA (rRNA). mRNA carries the genetic message from DNA to the ribosome. tRNA brings amino acids to the ribosome and recognises codons via its anticodon. rRNA forms part of the ribosome structure and helps catalyse the formation of peptide bonds.

RNA（核糖核酸）是一种单链分子，其糖略有不同（核糖而非脱氧核糖），碱基用尿嘧啶（U）代替胸腺嘧啶。三种RNA参与蛋白质合成：信使RNA（mRNA）、转运RNA（tRNA）和核糖体RNA（rRNA）。mRNA将遗传信息从DNA携带到核糖体。tRNA将氨基酸带到核糖体，并通过其反密码子识别密码子。rRNA是核糖体结构的一部分，并帮助催化肽键的形成。

Ribosomes are the molecular machines where translation occurs. They consist of a large and a small subunit, both made of rRNA and proteins. Ribosomes can be found free in the cytoplasm or attached to the rough endoplasmic reticulum. They have sites for mRNA binding and for incoming tRNA molecules.

核糖体是翻译发生的分子机器。它们由一个大的和一个小的亚基组成，两者都由rRNA和蛋白质构成。核糖体可以游离在细胞质中，也可以附着在粗面内质网上。它们拥有结合mRNA和进入的tRNA分子的位点。

4. The Genetic Code: Triplets and Codons | 遗传密码：三联体和密码子

ree nucleotides, called triplets in DNA and codons in mRNA. Each codon corresponds to one amino acid or a stop signal. Because there are 4 bases, a three‑ ons, but only about 20 amino acids are used to build proteins. Therefore, the code is degenerate: several codons can specify the same amino acid.

遗传密码基于三个核苷酸的组合，在DNA中称为三联体，在mRNA中称为密码子。每个密码子对应一个氨基酸或一个终止信号。因为有4种碱基，三字母的编码可产生4³ = 64个可能的密码子，但只有大约20种氨基酸用于构建蛋白质。因此，密码是简并的：多个密码子可以指定同一种氨基酸。

The table below shows a standard mRNA codon chart. Notice that AUG serves as the start codon (specifying methionine), and UAA, UAG, and UGA are stop codons that do not code for any amino acid.

下表显示了标准的mRNA密码子表。请注意，AUG作为起始密码子（指定甲硫氨酸），而UAA、UAG和UGA是终止密码子，不编码任何氨基酸。

First base	U	C	A	G	Third base
U	UUU Phe UUC Phe	UCU Ser UCC Ser	UAU Tyr UAC Tyr	UGU Cys UGC Cys	U C
	UUA Leu UUG Leu	UCU Ser UCC Ser	UAA Stop UAG Stop	UGA Stop UGG Trp	A G
	C	CUU Leu CUC Leu	CCU Pro CCC Pro	CAU His CAC His	CGU Arg CGC Arg	U C
		CUA Leu CUG Leu	CCU Pro CCC Pro	CAA Gln CAG Gln	CGU Arg CGC Arg	A G
A	AUU Ile AUC Ile AUA Ile	ACU Thr ACC Thr	AAU Asn AAC Asn	AGU Ser AGC Ser	U C A
	AUG Met (Start)	ACU Thr ACC Thr	AAA Lys AAG Lys	AGA Arg AGG Arg	G
	G	GUU Val GUC Val	GCU Ala GCC Ala	GAU Asp GAC Asp	GGU Gly GGC Gly	U C
		GUA Val GUG Val	GCU Ala GCC Ala	GAA Glu GAG Glu	GGG Gly	A G

5. Transcription: Copying the Gene into mRNA | 转录：将基因复制到mRNA

Transcription occurs in the nucleus and follows these steps:

• Initiation: The enzyme RNA polymerase binds to a region of the DNA just before the gene, called the promoter. This signals the start of the gene. The DNA double helix unwinds and unzips, breaking the hydrogen bonds between complementary bases.
• Elongation: RNA polymerase moves along one of the DNA strands (the template strand), reading the bases and adding complementary RNA nucleotides one by one. Remember, in RNA, uracil (U) pairs with adenine (A) instead of thymine. So, if the DNA template has the sequence TAC GGA TCG, RNA polymerase will synthesise the mRNA sequence AUG CCU AGC.
• Termination: When RNA polymerase reaches a termination sequence, it detaches. The newly formed pre‑mRNA molecule is released, and the DNA rewinds into its double helix.

转录发生在细胞核内，遵循以下步骤：

• 起始： RNA聚合酶结合到基因前方的一段DNA区域，称为启动子，这标志着基因的开始。DNA双螺旋解开并解旋，断开互补碱基之间的氢键。
• 延伸： RNA聚合酶沿着其中一条DNA链（模板链）移动，读取碱基并逐一添加互补的RNA核苷酸。请记住，在RNA中，尿嘧啶（U）与腺嘌呤（A）配对，而不是胸腺嘧啶。因此，如果DNA模板链序列为TAC GGA TCG，RNA聚合酶将合成mRNA序列AUG CCU AGC。
• 终止： 当RNA聚合酶到达一个终止序列时，它会脱离。新形成的前体mRNA分子被释放，DNA重新缠绕成双螺旋。

In eukaryotic cells, the pre‑mRNA often undergoes processing before it can be used: a cap is added to the 5′ end, a poly‑A tail to the 3′ end, and non‑coding regions (introns) are cut out. The final mature mRNA then exits the nucleus through a nuclear pore and enters the cytoplasm, carrying the genetic message.

在真核细胞中，前体mRNA通常需要经过加工才能使用：5′端加上帽子，3′端加上poly‑A尾巴，并且非编码区域（内含子）被切除。最终成熟的mRNA然后通过核孔离开细胞核进入细胞质，携带着遗传信息。

6. Translation: Decoding the mRNA into a Polypeptide | 翻译：将mRNA解码成多肽

Translation takes place on ribosomes in the cytoplasm. It can be divided into three phases: initiation, elongation and termination. During this process, the sequence of codons on the mRNA is used to determine the sequence of amino acids in the growing polypeptide chain.

翻译在细胞质的核糖体上进行。它可以分为三个阶段：起始、延伸和终止。在这个过程中，mRNA上的密码子序列被用来确定生长中多肽链的氨基酸序列。

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Here is a simple overview of the translation stages:

以下是翻译阶段的简单概述：

7. Initiation of Translation | 翻译的起始

The small ribosomal subunit binds to the mRNA near the 5′ end. The ribosome scans the mRNA until it finds the start codon AUG. This codon codes for methionine. A special initiator tRNA carrying methionine binds to the start codon via its anticodon UAC. Then, the large ribosomal subunit joins to form the complete ribosome. The initiator tRNA sits in the P site (peptidyl site) of the ribosome, leaving the A site (aminoacyl site) ready for the next tRNA.

核糖体小亚基结合到mRNA的靠近5′端处。核糖体扫描mRNA，直到找到起始密码子AUG。该密码子编码甲硫氨酸。一个携带甲硫氨酸的特殊起始子tRNA通过其反密码子UAC与起始密码子结合。然后，核糖体大亚基加入，形成完整的核糖体。起始子tRNA位于核糖体的P位（肽基位），使A位（氨酰基位）为空，准备接纳下一个tRNA。

8. Elongation: Building the Polypeptide Chain | 延伸：构建多肽链

During elongation, amino acids are added one by one to the growing chain. The process involves three key steps:

1. Codon recognition: The next mRNA codon is exposed in the A site. A tRNA molecule with a complementary anticodon and carrying the correct amino acid enters the A site and base‑pairs with the codon.
2. Peptide bond formation: The ribosome catalyses the formation of a peptide bond between the amino acid in the P site and the amino acid in the A site. The growing polypeptide chain is transferred to the tRNA in the A site.
3. Translocation: The ribosome shifts one codon along the mRNA (from 5′ to 3′). The tRNA that was in the P site is now empty and exits from the E site (exit site). The tRNA carrying the polypeptide chain moves into the P site, and the A site is now open for the next tRNA.

在延伸过程中，氨基酸被逐个添加到不断增长的链上。此过程涉及三个关键步骤：

1. 密码子识别： 下一个mRNA密码子暴露在A位中。一个具有互补反密码子并携带正确氨基酸的tRNA分子进入A位，与密码子碱基配对。
2. 肽键形成： 核糖体催化P位上的氨基酸与A位上的氨基酸之间形成肽键。正在生长的多肽链被转移到A位的tRNA上。
3. 转位： 核糖体沿着mRNA移动一个密码子（从5′到3′方向）。原本在P位的tRNA现在已空，从E位（出口位）离开。携带多肽链的tRNA移入P位，A位现在空出，准备迎接下一个tRNA。

This cycle repeats, with the ribosome reading the mRNA codon by codon and tRNA molecules delivering the specified amino acids. The polypeptide chain grows from the N‑terminus to the C‑terminus.

这个循环不断重复，核糖体逐密码子地读取mRNA，tRNA分子递送指定的氨基酸。多肽链从N端向C端生长。

9. Termination of Translation | 翻译的终止

Elongation continues until the ribosome encounters a stop codon (UAA, UAG, or UGA) in the A site. These codons do not code for any tRNA. Instead, a release factor protein binds to the stop codon, causing the ribosome to break the bond between the polypeptide and the tRNA in the P site. The completed polypeptide is released, and the ribosomal subunits dissociate from the mRNA, ready to be used again.

延伸一直持续到核糖体在A位遇到终止密码子（UAA、UAG或UGA）。这些密码子不对应任何tRNA。取而代之的是，一个释放因子蛋白与终止密码子结合，促使核糖体断开多肽与P位tRNA之间的键。完成的多肽链被释放，核糖体亚基从mRNA上解离，准备再次使用。

10. The Role of tRNA: Matching Anticodons to Codons | tRNA的角色：反密码子与密码子的匹配

Transfer RNA molecules are the interpreters of the genetic code. Each tRNA molecule has a specific anticodon at one end and an attachment site for its corresponding amino acid at the other. The enzyme aminoacyl‑tRNA synthetase ‘charges’ each tRNA by attaching the correct amino acid. This high specificity ensures that when the tRNA anticodon pairs with an mRNA codon, the right amino acid is added to the polypeptide.

转运RNA分子是遗传密码的翻译者。每个tRNA分子的一端有一个特定的反密码子，另一端有一个对应氨基酸的附着位点。氨酰tRNA合成酶通过附着正确的氨基酸给每个tRNA“充电”。这种高度的特异性确保了当tRNA的反密码子与mRNA的密码子配对时，正确的氨基酸被添加到多肽中。

For example, a tRNA with the anticodon UAC will carry methionine, recognising the start codon AUG. Another tRNA with anticodon AAG will carry phenylalanine, recognising the codon UUC. This precise matching is the basis of the genetic code’s reliability.

例如，带有反密码子UAC的tRNA将携带甲硫氨酸，识别起始密码子AUG。另一个带有反密码子AAG的tRNA将携带苯丙氨酸，识别密码子UUC。这种精确的配对是遗传密码可靠性的基础。

11. From Polypeptide to Functional Protein | 从多肽到功能性蛋白质

Once the polypeptide chain is released, it does not remain as a linear strand. It begins to fold spontaneously into its specific three‑dimensional shape, determined by the sequence of amino acids. This folding is driven by hydrogen bonds, ionic bonds, hydrophobic interactions and sometimes disulphide bridges between cysteine residues. Some proteins require assistance from chaperone proteins to fold correctly.

一旦多肽链被释放，它并不会保持为一条直线。它开始自发折叠成由其氨基酸序列决定的特定三维形状。这种折叠是由氢键、离子键、疏水相互作用，有时还有半胱氨酸残基之间的二硫桥推动的。有些蛋白质需要伴侣蛋白的协助才能正确折叠。

Many proteins also undergo post‑translational modifications: they may have sugar groups added (glycosylation), be cut by enzymes to remove a signalling peptide, or be assembled with other polypeptide chains to form a quaternary structure (e.g., haemoglobin consists of four polypeptide subunits). Only after this processing does the protein become fully functional and able to perform its specific job in the cell.

许多蛋白质还会经历翻译后修饰：它们可能被添加糖基（糖基化），被酶切除去除信号肽，或与其他多肽链组装形成四级结构（例如，血红蛋白由四个多肽亚基组成）。只有经过这些加工后，蛋白质才具有完全功能性，并能够在细胞中执行其特定的工作。

12. Putting It Together: A Worked Example | 综合示例

Let’s follow a short DNA template sequence coding for the first three amino acids of a protein. Original DNA template strand: TAC AAG CGA ACT. Table for conversion:

让我们跟着一个简短的DNA模板序列走一遍，它编码一个蛋白质的前三个氨基酸。原始DNA模板链：TAC AAG CGA ACT。转换表：

DNA template	TAC	AAG	CGA	ACT
mRNA codon	AUG	UUC	GCU	UGA
Amino acid	Methionine (Start)	Phenylalanine	Alanine	Stop

Thus, this gene fragment would produce the peptide: Met – Phe – Ala. The ribosome would stop at UGA.

因此，这个基因片段将产生肽链：Met – Phe – Ala。核糖体将在UGA处终止。

This simplified example illustrates the direct relationship between a DNA sequence and the amino acid sequence of a protein. Mutations that change even a single base in the DNA can alter an mRNA codon and potentially insert a different amino acid, which may render the protein non‑functional — a concept central to understanding many genetic diseases.

这个简化的例子说明了DNA序列与蛋白质氨基酸序列之间的直接关系。即使DNA中单个碱基的改变也可能改变mRNA密码子，并可能插入不同的氨基酸，这可能导致蛋白质功能丧失——这是理解许多遗传疾病的核心概念。

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