📚 Translation: Protein Synthesis | GCSE OCR 生物：翻译 考点精讲
Translation is the second stage of protein synthesis, following transcription. It uses the mRNA transcript to assemble a polypeptide chain from amino acids. For GCSE OCR Biology, it is essential to understand the roles of ribosomes, tRNA molecules, and the genetic code, as well as the three main steps: initiation, elongation, and termination.
翻译是蛋白质合成的第二阶段，发生在转录之后。它利用信使 RNA（mRNA）转录本将氨基酸组装成多肽链。对于 GCSE OCR 生物学，必须掌握核糖体、转运 RNA（tRNA）和遗传密码的作用，以及翻译的三个主要步骤：起始、延伸和终止。
1. Overview of Translation | 翻译概述
Translation is the decoding of an mRNA sequence into a chain of amino acids. This process takes place in the cytoplasm on ribosomes and requires energy in the form of GTP.
翻译是指将 mRNA 序列解码为氨基酸链的过程。这一过程在细胞质中的核糖体上进行，需要 GTP 形式的能量。
Each group of three mRNA bases – called a codon – specifies one amino acid. The genetic code is universal, non‑overlapping, and degenerate.
mRNA 上每三个碱基为一组，称为一个密码子，决定一种氨基酸。遗传密码具有普遍性、不重叠和简并性。
2. Location and Requirements | 场所与所需物质
In eukaryotic cells, translation occurs on ribosomes either free in the cytoplasm or bound to rough endoplasmic reticulum. In prokaryotes, it occurs in the cytoplasm.
在真核细胞中，翻译发生在游离于细胞质或结合在粗面内质网上的核糖体上。原核生物中翻译在细胞质中进行。
The key players are: mRNA (carries the code), ribosomes (read the code and catalyse peptide bond formation), tRNA molecules (bring amino acids), amino acids, and enzymes such as aminoacyl‑tRNA synthetases.
关键参与者包括：mRNA（携带遗传密码）、核糖体（读取密码并催化肽键形成）、tRNA 分子（携带氨基酸）、氨基酸以及氨酰 tRNA 合成酶等酶类。
3. mRNA and the Genetic Code | mRNA 与遗传密码
mRNA is a single‑stranded copy of a gene. It contains a sequence of codons, each made of three bases (A, U, C, G). The start codon AUG initiates translation and codes for methionine.
mRNA 是基因的单链拷贝。它含有一系列密码子，每个密码子由三个碱基（A、U、C、G）组成。起始密码子 AUG 启动翻译，编码甲硫氨酸。
Stop codons – UAA, UAG, and UGA – do not code for any amino acid; they signal the termination of polypeptide synthesis.
终止密码子（UAA、UAG 和 UGA）不编码任何氨基酸，它们标志着多肽链合成的终止。
4. tRNA and Anticodons | tRNA 与反密码子
Transfer RNA molecules are small, cloverleaf‑shaped structures. Each tRNA has an anticodon of three unpaired bases at one end and an amino acid attachment site at the 3′ end.
转运 RNA 是小分子，呈三叶草形结构。每个 tRNA 一端有三个不成对碱基组成的反密码子，另一端（3’端）为氨基酸结合位点。
The anticodon base‑pairs with the complementary codon on the mRNA, ensuring the correct amino acid is added. For example, the tRNA with anticodon UAC carries methionine and binds to the start codon AUG.
反密码子与 mRNA 上的互补密码子碱基配对，从而保证添加正确的氨基酸。例如，具有反密码子 UAC 的 tRNA 携带甲硫氨酸，并与起始密码子 AUG 结合。
5. Ribosomes: The Translation Machinery | 核糖体：翻译机器
Ribosomes are composed of ribosomal RNA (rRNA) and proteins, and consist of a large subunit and a small subunit. They provide the platform for mRNA and tRNA binding.
核糖体由核糖体 RNA（rRNA）和蛋白质组成，包含一个大亚基和一个小亚基。它们为 mRNA 和 tRNA 的结合提供平台。
The ribosome has three functional sites for tRNA binding – A site (aminoacyl), P site (peptidyl), and E site (exit) – but for GCSE you only need to know that tRNA molecules occupy the ribosome and bring amino acids into the right order.
核糖体有三个 tRNA 结合位点——A 位（氨酰位）、P 位（肽酰位）和 E 位（出口位），但 GCSE 阶段只需知道 tRNA 占据核糖体并按正确顺序将氨基酸带入即可。
6. Initiation | 翻译起始
Initiation begins when the small ribosomal subunit binds to the 5′ end of the mRNA. It scans along the mRNA until it reaches the start codon AUG.
翻译起始于核糖体小亚基与 mRNA 的 5′ 端结合。小亚基沿 mRNA 扫描，直至到达起始密码子 AUG。
An initiator tRNA carrying methionine (tRNA^Met, anticodon UAC) base‑pairs with the start codon. The large ribosomal subunit then joins, forming a functional ribosome with the initiator tRNA in the P site.
携带甲硫氨酸的起始 tRNA（tRNA^Met，反密码子 UAC）与起始密码子配对。随后，大亚基结合，形成功能性核糖体，起始 tRNA 位于 P 位。
7. Elongation: Polypeptide Assembly | 延伸：多肽链的组装
During elongation, a new aminoacyl‑tRNA carrying the next amino acid enters the A site of the ribosome. Its anticodon must be complementary to the mRNA codon in the A site.
在延伸阶段，携带下一个氨基酸的氨酰 tRNA 进入核糖体的 A 位。其反密码子必须与 A 位上的 mRNA 密码子互补。
The ribosome catalyses the formation of a peptide bond between the amino acid in the P site and the incoming amino acid in the A site. The growing polypeptide chain is transferred to the tRNA in the A site.
核糖体催化 P 位上的氨基酸与进入 A 位的氨基酸之间形成肽键。正在延长的多肽链被转移到 A 位的 tRNA 上。
The ribosome then translocates – it moves one codon along the mRNA. The empty tRNA in the P site moves to the E site and is released, while the tRNA carrying the growing chain shifts to the P site, leaving the A site empty for the next tRNA.
随后核糖体移位——沿 mRNA 移动一个密码子。P 位的空载 tRNA 移至 E 位并被释放，携带增长链的 tRNA 转移到 P 位，A 位腾空等待下一个 tRNA。
8. Peptide Bond Formation | 肽键形成
A peptide bond is a covalent bond formed between the carboxyl group of one amino acid and the amino group of another. This reaction is catalysed by peptidyl transferase activity of the ribosome’s large subunit (rRNA acts as a ribozyme).
肽键是一种共价键，由一个氨基酸的羧基与另一个氨基酸的氨基之间形成。该反应由核糖体大亚基的肽基转移酶活性催化（rRNA 起核酶作用）。
Amino acid 1 –CO–NH– Amino acid 2 (peptide bond)
氨基酸 1 –CO–NH– 氨基酸 2（肽键）
Water is not released in this condensation reaction; it is a simple acyl substitution driven by the ribosome.
这一缩合反应不释放水，而是由核糖体驱动的一种简单酰基取代反应。（注意：GCSE 阶段只需知道肽键形成连接氨基酸即可。）
9. Termination | 翻译终止
Elongation continues until a stop codon (UAA, UAG, or UGA) enters the A site. There are no tRNA molecules with anticodons that recognise stop codons.
延伸一直持续到终止密码子（UAA、UAG 或 UGA）进入 A 位。没有反密码子能与终止密码子互补的 tRNA 存在。
Instead, a release factor protein binds to the stop codon. This causes the ribosome to add a water molecule to the end of the polypeptide chain, which releases the chain from the final tRNA.
取而代之的是释放因子蛋白与终止密码子结合。这促使核糖体在多肽链末端加上一分子水，使多肽链从最后一个 tRNA 上释放。
The ribosomal subunits, mRNA, and remaining tRNA dissociate, and the newly formed polypeptide is set free to fold into its functional three‑dimensional shape.
核糖体亚基、mRNA 和剩余 tRNA 解离，新形成的多肽被释放并折叠成具有功能的三维形状。
10. The Result: Polypeptide Release | 结果：多肽链的释放
Translation produces a linear chain of amino acids – the primary structure of a protein. This polypeptide will undergo folding and possibly modification (e.g., in the Golgi) to become a functional protein.
翻译产生一条线性的氨基酸链——蛋白质的一级结构。这条多肽链会进行折叠，可能还会在（例如）高尔基体中进行修饰，最终成为有功能的蛋白质。
A single mRNA can be translated multiple times by several ribosomes simultaneously, forming a polyribosome (polysome). This amplifies the rate of protein production.
一条 mRNA 可被多个核糖体同时翻译，形成多聚核糖体（多体）。这大大提高了蛋白质的合成速率。
11. Comparison with Transcription | 与转录的对比
| Feature | Transcription | Translation |
|---|---|---|
| Location (eukaryotes) | Nucleus | Cytoplasm / rough ER |
| Template | DNA gene | mRNA |
| Product | pre‑mRNA / mRNA | Polypeptide (protein) |
| Key enzyme / machinery | RNA polymerase | Ribosome (rRNA, proteins) |
| Start signal | Promoter region on DNA | Start codon (AUG) on mRNA |
| Stop signal | Terminator sequence | Stop codon (UAA, UAG, UGA) |
| Base pairing | DNA–RNA (A‑U, T‑A, C‑G, G‑C) | Codon–anticodon (mRNA–tRNA) |
Knowing these differences helps in answering compare‑and‑contrast exam questions. Both stages are essential for gene expression.
了解这些差异有助于回答比较对照类考题。两个阶段都是基因表达必不可少的环节。
12. Common Exam Questions and Tips | 常见考题与答题技巧
Typical OCR GCSE questions ask: ‘Describe what happens during translation.’ Always mention the role of mRNA carrying the code, tRNA bringing specific amino acids, the ribosome as the site of protein synthesis, and the formation of peptide bonds.
OCR GCSE 的典型考题有：“描述翻译过程中发生的事件。”务必提及 mRNA 携带密码、tRNA 携带特定氨基酸、核糖体是蛋白质合成场所、以及肽键的形成。
- Keywords: codon, anticodon, complementary base pairing, peptide bond, polypeptide.
- 关键词：密码子、反密码子、互补碱基配对、肽键、多肽。
Many marks are lost by confusing transcription and translation. Practise sequencing diagrams that show the movement of the ribosome along mRNA and the order of tRNA arrival.
许多失分是由于混淆转录与翻译造成的。多练习排序题，展示核糖体沿 mRNA 移动以及 tRNA 进入的顺序。
When explaining the genetic code, point out that it is degenerate (more than one codon can code for the same amino acid) but specific (each codon codes for only one amino acid).
解释遗传密码时，要指出其简并性（多个密码子可编码同一种氨基酸）和专一性（每个密码子只编码一种氨基酸）。
If a question asks why translation stops at a stop codon, state that no tRNA has a matching anticodon; instead, a release factor binds, causing the polypeptide and ribosomal subunits to separate.
如果考题问及为什么在终止密码子处翻译停止，要说明没有 tRNA 拥有与之匹配的反密码子，而是释放因子结合，导致多肽和核糖体亚基分离。
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