📚 IGCSE Biology: Translation | 翻译 考点精讲
Protein synthesis is a fundamental biological process that occurs in all living cells. After transcription creates a messenger RNA (mRNA) copy of a gene, the next step is translation. Understanding translation is essential for IGCSE Biology, because it explains how the genetic code stored in DNA is converted into functional proteins that build, regulate and defend the cell. This article is a focused revision guide covering the key concepts, the roles of mRNA, tRNA and ribosomes, the three stages of translation, common pitfalls and exam tips.
蛋白质合成是发生在所有活细胞中的基本生物学过程。在转录产生基因的信使RNA (mRNA) 副本后，下一步就是翻译。理解翻译对于IGCSE生物至关重要，因为它解释了储存在DNA中的遗传密码如何转化为构建、调节和防御细胞的功能性蛋白质。本文是一份重点复习指南，涵盖关键概念、mRNA、tRNA与核糖体的作用、翻译的三个阶段、常见错误以及考试技巧。
1. Overview of Translation | 翻译概述
Translation is the process by which the sequence of codons on mRNA is decoded into a sequence of amino acids, forming a polypeptide chain.
翻译是指mRNA上的密码子序列被解码为氨基酸序列，从而形成多肽链的过程。
This process takes place on ribosomes in the cytoplasm. Ribosomes can be free-floating or attached to the rough endoplasmic reticulum (RER), where the resulting proteins may be further processed and folded.
该过程在细胞质中的核糖体上进行。核糖体可以游离漂浮，也可以附着在粗面内质网 (RER) 上，产生的蛋白质可在此进行进一步加工和折叠。
Translation requires mRNA, transfer RNA (tRNA), ribosomes, amino acids and energy in the form of ATP, plus specific enzymes. The accuracy of this process is vital, as errors can lead to non-functional proteins.
翻译需要mRNA、转运RNA (tRNA)、核糖体、氨基酸以及ATP形式的能量，还需要特定的酶。该过程的准确性至关重要，因为错误可能导致非功能性蛋白质。
As a single-sentence definition for exam answers: translation is the synthesis of a polypeptide at a ribosome, using the base sequence of mRNA to determine the order of amino acids.
可用于考试作答的一句话定义是：翻译是在核糖体上合成多肽的过程，利用mRNA的碱基序列决定氨基酸的排列顺序。
2. The Genetic Code and Codons | 遗传密码与密码子
The genetic code is a triplet code: every three bases on mRNA form one codon, and one codon codes for one amino acid.
遗传密码是三联体密码：mRNA上每三个碱基构成一个密码子，一个密码子编码一个氨基酸。
There are 64 possible codons but only about 20 amino acids, so the code is degenerate (redundant). This means more than one codon can code for the same amino acid, which reduces the harmful effect of some mutations.
可能的密码子共有64种，而氨基酸只有约20种，因此遗传密码是简并的（冗余的）。这意味着多个密码子可以编码同一种氨基酸，从而降低某些突变带来的有害影响。
The code is read in a fixed reading frame, without overlapping and without punctuation, in the 5' to 3' direction along the mRNA. This is why an insertion or deletion of a single base is so serious.
密码子沿mRNA按固定的阅读框、从5'到3'方向读取，既不重叠，也没有标点符号分隔。这正是单个碱基插入或缺失后果严重的原因。
Three codons are stop codons: UAA, UAG and UGA. They do not code for any amino acid; they signal the end of the polypeptide chain.
有三个密码子是终止密码子：UAA、UAG和UGA。它们不编码任何氨基酸，而是发出多肽链终止的信号。
The start codon is AUG, which also codes for the amino acid methionine. The genetic code is also described as universal, because the same codons code for the same amino acids in almost all organisms.
起始密码子是AUG，它同时编码氨基酸甲硫氨酸。遗传密码还被描述为通用密码，因为在几乎所有生物中，相同的密码子编码相同的氨基酸。
3. The Role of mRNA | mRNA 的作用
Messenger RNA (mRNA) carries the genetic information from DNA to the ribosome. Each group of three bases on mRNA is called a codon.
信使RNA (mRNA) 将遗传信息从DNA携带到核糖体。mRNA上每三个碱基一组称为一个密码子。
A codon codes for a specific amino acid. For example, the codon AUG codes for methionine, GGG codes for glycine, and UUU codes for phenylalanine.
一个密码子编码一种特定的氨基酸。例如，密码子AUG编码甲硫氨酸，GGG编码甘氨酸，UUU编码苯丙氨酸。
In eukaryotes, mRNA is made in the nucleus during transcription, then leaves the nucleus through a nuclear pore and travels to a ribosome. mRNA is a single-stranded molecule and is much shorter than DNA.
在真核生物中，mRNA在细胞核内通过转录形成，然后通过核孔离开细胞核，到达核糖体。mRNA是单链分子，比DNA短得多。
The base sequence of mRNA is complementary to the template strand of DNA, with uracil (U) replacing thymine (T). This means mRNA faithfully carries the same message as the coding strand of the gene.
mRNA的碱基序列与DNA的模板链互补，其中尿嘧啶 (U) 取代胸腺嘧啶 (T)。这意味着mRNA忠实地携带了与该基因编码链相同的遗传信息。
4. The Role of tRNA | tRNA 的作用
Transfer RNA (tRNA) is a clover-shaped molecule that carries a specific amino acid to the ribosome. Each tRNA has an amino acid binding site at one end and an anticodon at the other.
转运RNA (tRNA) 是一种三叶草形分子，它将特定的氨基酸运送到核糖体。每个tRNA一端有氨基酸结合位点，另一端有反密码子。
The anticodon is a sequence of three bases on tRNA that is complementary to the codon on mRNA. Complementary base pairing follows the rules A with U, and G with C.
反密码子是tRNA上的三个碱基序列，与mRNA上的密码子互补。互补碱基配对遵循A与U配对、G与C配对的规则。
Because each tRNA is specific, only the correct amino acid is delivered to the correct position on the mRNA. This complementary pairing between codon and anticodon is the key to the accuracy of translation.
由于每种tRNA具有特异性，只有正确的氨基酸会被送到mRNA上正确的位置。密码子与反密码子之间的互补配对，是翻译准确性的关键。
Since there are about 20 amino acids, there are also about 20 different types of amino acid-carrying tRNA, and some amino acids have more than one tRNA because the code is degenerate.
由于大约有20种氨基酸，也就有约20种携带氨基酸的tRNA类型；而由于密码子的简并性，某些氨基酸对应不止一种tRNA。
5. Ribosomes: The Site of Translation | 核糖体：翻译的场所
Ribosomes are the organelles on which translation occurs. They are made of ribosomal RNA (rRNA) and protein, and each ribosome has a large subunit and a small subunit.
核糖体是翻译发生的细胞器。它们由核糖体RNA (rRNA) 和蛋白质组成，每个核糖体有一个大亚基和一个小亚基。
The ribosome has two binding sites for tRNA: the A site (aminoacyl site), where a new tRNA carrying an amino acid arrives, and the P site (peptidyl site), where the growing polypeptide chain is held.
核糖体有两个tRNA结合位点：A位点（氨酰位点），携带氨基酸的新tRNA在此进入；P位点（肽基位点），正在延伸的多肽链在此被固定。
The ribosome also acts as an enzyme: it catalyses the formation of peptide bonds between adjacent amino acids. It holds the mRNA and the tRNAs in the correct position so that base pairing can occur.
核糖体还起酶的作用：它催化相邻氨基酸之间肽键的形成。它将mRNA和tRNA固定在正确位置，使碱基配得以发生。
Free ribosomes make proteins that stay inside the cytoplasm, while ribosomes attached to the rough endoplasmic reticulum make proteins that are exported from the cell, such as enzymes and hormones.
游离核糖体合成留在细胞质内使用的蛋白质，而附着在粗面内质网上的核糖体则合成被分泌到细胞外的蛋白质，例如酶和激素。
6. The Three Stages: Initiation, Elongation and Termination | 翻译的三个阶段：起始、延伸与终止
Initiation: the mRNA binds to the small ribosomal subunit, and the ribosome moves along the mRNA until it reaches the start codon AUG. A tRNA carrying methionine pairs its anticodon with AUG and occupies the P site, forming the initiation complex.
起始：mRNA与小核糖体亚基结合，核糖体沿mRNA移动直到到达起始密码子AUG。携带甲硫氨酸的tRNA以其反密码子与AUG配对，占据P位点，形成起始复合物。
Elongation: a second tRNA carrying the next amino acid enters the A site and its anticodon pairs with the next codon. The ribosome catalyses a peptide bond between the two amino acids, then moves one codon along the mRNA (translocation), releasing the first tRNA.
延伸：携带下一个氨基酸的第二个tRNA进入A位点，其反密码子与下一个密码子配对。核糖体催化两个氨基酸之间形成肽键，然后沿mRNA移动一个密码子的距离（易位），并释放第一个tRNA。
This cycle repeats many times: the ribosome moves along the mRNA codon by codon, and the polypeptide chain grows as each new amino acid is added. The mRNA is read in the 5' to 3' direction, so the polypeptide is built from the N-terminus to the C-terminus.
该循环反复进行多次：核糖体沿mRNA一个密码子一个密码子地移动，每加入一个新的氨基酸，多肽链就延长一段。mRNA按5'到3'方向被读取，因此多肽链从N端向C端延伸。
Termination: when a stop codon (UAA, UAG or UGA) reaches the A site, no tRNA has a complementary anticodon, so no amino acid is added. The polypeptide chain is released, and the ribosome and mRNA separate to be used again.
终止：当终止密码子（UAA、UAG或UGA）到达A位点时，没有任何tRNA具有互补的反密码子，因此没有氨基酸被加上。多肽链被释放，核糖体与mRNA分离，以便再次使用。
7. Peptide Bond Formation and Energy Requirements | 肽键形成与能量需求
Each amino acid has an amino group (-NH₂) at one end and a carboxyl group (-COOH) at the other. A peptide bond forms between the carboxyl group of one amino acid and the amino group of the next, releasing a molecule of water.
每个氨基酸一端有氨基 (-NH₂)，另一端有羧基 (-COOH)。肽键在一个氨基酸的羧基与下一个氨基酸的氨基之间形成，同时释放一分子水。
This is a condensation reaction, and the bond formed is -CO-NH-. Two amino acids form a dipeptide, many amino acids form a polypeptide, and one or more folded polypeptides form a protein.
这是一个缩合反应，形成的键为-CO-NH-。两个氨基酸形成二肽，许多氨基酸形成多肽，一条或多条折叠的多肽形成蛋白质。
Translation is an energy-consuming process. ATP is needed to attach the amino acid to its tRNA (activation), and further energy is used for the movement of the ribosome along the mRNA.
翻译是一个耗能过程。将氨基酸连接到其tRNA上（活化）需要ATP，核糖体沿mRNA移动也需要进一步消耗能量。
The order of amino acids is determined by the order of codons on the mRNA, and this order determines how the polypeptide folds, which in turn determines the shape and function of the final protein.
氨基酸的顺序由mRNA上密码子的顺序决定，而这个顺序决定多肽如何折叠，进而决定最终蛋白质的形状和功能。
8. Polysomes and the Efficiency of Translation | 多核糖体与翻译效率
A single mRNA molecule can be translated by many ribosomes at the same time. This structure is called a polysome or polyribosome.
一条mRNA分子可以同时被多个核糖体翻译。这种结构称为多核糖体（多聚核糖体）。
In a polysome, several ribosomes move along the same mRNA one after another, each producing a complete polypeptide chain from the same message.
在多核糖体中，多个核糖体沿同一条mRNA依次移动，每个核糖体都从同一份信息中产生一条完整的多肽链。
This greatly increases the rate of protein synthesis, so a cell can make many identical protein molecules quickly from one mRNA transcript.
这极大地提高了蛋白质合成的速率，使细胞能够从一份mRNA转录本快速制造出许多相同的蛋白质分子。
It also explains why mRNA is short-lived: once it is degraded, protein production from that message stops, giving the cell fine control over which proteins are made and when.
这也解释了为什么mRNA寿命较短：一旦它被降解，由该信息指导的蛋白质生产就会停止，从而使细胞能精细控制制造哪些蛋白质以及何时制造。
9. Translation Compared with Transcription | 翻译与转录的比较
Transcription and translation are easily confused in exams. Transcription copies the base sequence of a gene into mRNA and happens in the nucleus in eukaryotes. Translation converts the mRNA base sequence into an amino acid sequence and happens at the ribosome.
转录与翻译在考试中很容易混淆。转录将基因的碱基序列复制成mRNA，在真核生物中发生于细胞核内。翻译则将mRNA的碱基序列转换为氨基酸序列，发生在核糖体上。
| Feature 特征 | Transcription 转录 | Translation 翻译 |
| Location 场所 | Nucleus (eukaryotes) 细胞核（真核生物） | Ribosome in cytoplasm 细胞质中的核糖体 |
| Template 模板 | DNA template strand DNA模板链 | mRNA mRNA |
| Building blocks 原料 | Free nucleotides 游离核苷酸 | Amino acids 氨基酸 |
| Product 产物 | mRNA | Polypeptide 多肽 |
| Key pairing 关键配对 | DNA base with RNA base DNA碱基与RNA碱基 | Codon with anticodon 密码子与反密码子 |
A useful memory phrase: transcription happens in the nucleus and makes a message; translation happens at the ribosome and makes a machine.
一个实用的记忆口诀：转录在细胞核中制造"信息"，翻译在核糖体上制造"机器"。
10. Errors, Mutations and Their Effects | 错误、突变及其影响
Translation must be accurate. If the wrong amino acid is inserted, the polypeptide may fold incorrectly and become a non-functional protein, which can seriously affect the cell.
翻译必须准确。如果插入了错误的氨基酸，多肽可能折叠错误，成为非功能性蛋白质，从而严重影响细胞。
A substitution mutation changes one base, and therefore possibly one codon. Because the code is degenerate, this is often a silent mutation, but it can be a missense mutation (one amino acid changed) or a nonsense mutation (a stop codon created early).
替换突变改变一个碱基，因此可能改变一个密码子。由于密码子具有简并性，这常常是沉默突变，但也可能是错义突变（改变一个氨基酸）或无义突变（提前产生终止密码子）。
An insertion or deletion of bases changes the reading frame of all the following codons. This is a frameshift mutation, and it usually produces a completely different and non-functional polypeptide.
碱基的插入或缺失会改变其后所有密码子的阅读框。这就是移码突变，通常会产生完全不同且无功能的多肽。
Some mutations produce a protein that still works, and occasionally a mutation gives an advantage, which is important for natural selection and evolution. However, most random mutations that change protein structure are harmful.
有些突变产生的蛋白质仍然能正常工作，偶尔突变还会带来优势，这对自然选择和进化很重要。然而，大多数改变蛋白质结构的随机突变都是有害的。
11. Common Exam Mistakes and Answer Technique | 常见错误与答题技巧
Common mistake 1: confusing the positions. Transcription occurs in the nucleus; translation occurs at the ribosome in the cytoplasm.
常见错误1：混淆发生场所。转录发生在细胞核内；翻译发生在细胞质中的核糖体上。
Common mistake 2: mixing up codons and anticodons. The codon is on mRNA; the anticodon is on tRNA. Never write that tRNA carries a codon.
常见错误2：混淆密码子与反密码子。密码子在mRNA上；反密码子在tRNA上。绝不要写tRNA携带密码子。
Common mistake 3: saying that the stop codon codes for an amino acid. It does not - it has no matching tRNA, so the chain stops and is released.
常见错误3：说终止密码子编码某个氨基酸。事实并非如此——它没有匹配的tRNA，因此肽链终止并被释放。
Common mistake 4: forgetting that mRNA is read in triplets, in the 5' to 3' direction, in a non-overlapping way.
常见错误4：忘记mRNA是按三联体、沿5'到3'方向、以不重叠的方式被读取的。
When describing translation in a longer answer, use the correct sequence of key points: mRNA binds to the ribosome; a tRNA carrying an amino acid pairs its anticodon with the start codon; a peptide bond forms between amino acids; the ribosome moves along the mRNA; the chain grows; a stop codon ends the process and the polypeptide is released.
在长答题中描述翻译时，要按正确的要点顺序作答：mRNA与核糖体结合；携带氨基酸的tRNA以其反密码子与起始密码子配对；氨基酸之间形成肽键；核糖体沿mRNA移动；肽链延长；终止密码子结束过程并释放多肽。
Always use the correct command words. "State" needs a short answer; "describe" needs a logical sequence; "explain" needs a reason, usually linking complementary base pairing to the accuracy of protein synthesis.
始终注意题目指令词。"陈述"只需简短作答；"描述"需要符合逻辑的先后顺序；"解释"需要给出原因，通常要把互补碱基配对与蛋白质合成的准确性联系起来。
12. Quick Revision Table and Self-Test | 快速复习表与自测
| Term 术语 | Meaning 含义 |
| Codon 密码子 | Three bases on mRNA coding for one amino acid mRNA上编码一个氨基酸的三个碱基 |
| Anticodon 反密码子 | Three bases on tRNA complementary to a codon tRNA上与密码子互补的三个碱基 |
| Start codon 起始密码子 | AUG, codes for methionine AUG，编码甲硫氨酸 |
| Stop codons 终止密码子 | UAA, UAG, UGA - end the chain UAA、UAG、UGA——终止肽链 |
| Polysome 多核糖体 | Many ribosomes on one mRNA 一条mRNA上的多个核糖体 |
| Peptide bond 肽键 | -CO-NH- bond between amino acids 氨基酸之间的-CO-NH-键 |
Self-test: Where does translation occur? What molecule brings amino acids to the ribosome? How many bases make one codon? What happens at a stop codon? Why does a frameshift mutation usually destroy protein function?
自测：翻译发生在哪里？哪种分子将氨基酸带到核糖体？一个密码子由几个碱基组成？在终止密码子处会发生什么？为什么移码突变通常会破坏蛋白质功能？
If you can answer all five questions in full sentences using the correct terms, and can write the sequence of the three stages without prompting, you are well prepared for IGCSE Biology questions on translation.
如果你能用正确的术语、完整的句子回答以上五个问题，并且能不看提示写出三个阶段的顺序，那么你就为IGCSE生物中关于翻译的题目做好了充分准备。
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