📚 IB Biology: Gene Expression Key Exam Points | IB 生物：基因表达 考点精讲
Gene expression is a fundamental topic in IB Biology that explains how genetic information is decoded to produce functional products, primarily proteins, and how this process is regulated in different organisms. Mastering this topic requires understanding transcription, translation, and the control mechanisms that ensure the right genes are expressed at the right time and in the right cell.
基因表达是 IB 生物的核心主题，解释了遗传信息如何解码以产生功能性产物（主要是蛋白质），以及这一过程在不同生物中是如何被调控的。要精通本主题，需要理解转录、翻译以及确保正确基因在正确时间和正确细胞中表达的控制机制。
1. Central Dogma and the Flow of Genetic Information | 中心法则与遗传信息流
The central dogma of molecular biology states that genetic information flows from DNA to RNA to protein. This unidirectional flow is essential for maintaining the integrity of genetic material while allowing the production of diverse proteins.
分子生物学的中心法则指出，遗传信息从 DNA 流向 RNA，再流向蛋白质。这种单向流动对于维持遗传物质的完整性，同时允许产生多样化的蛋白质至关重要。
Exceptions to this dogma include reverse transcription in retroviruses, where RNA is converted back into DNA, and the existence of non-coding RNAs that never translate into protein. The IB syllabus expects you to recognise these exceptions.
该法则的例外包括逆转录病毒中的逆转录过程（RNA 被逆转录为 DNA），以及从不翻译成蛋白质的非编码 RNA 的存在。IB 大纲要求你了解这些例外。
2. Transcription in Prokaryotes and Eukaryotes | 原核与真核生物的转录
Transcription is the synthesis of an RNA molecule from a DNA template. The enzyme RNA polymerase binds to a promoter region and synthesises a complementary RNA strand in the 5′ to 3′ direction. The process differs significantly between prokaryotes and eukaryotes, which is frequently examined.
转录是以 DNA 为模板合成 RNA 分子的过程。RNA 聚合酶与启动子区域结合，沿 5′ 到 3′ 方向合成互补的 RNA 链。原核生物与真核生物的转录过程存在显著差异，这是常考内容。
| Feature | Prokaryotes | Eukaryotes |
|---|---|---|
| RNA polymerase | Single type of RNA polymerase synthesises all RNA | Three types: RNA pol I (rRNA), II (mRNA), III (tRNA, 5S rRNA) |
| Promoter recognition | Sigma factor guides RNA pol to the promoter (-10 TATAAT and -35 TTGACA regions) | General transcription factors (e.g. TFIID binds TATA box) and specific factors are required |
| Location | Cytoplasm (no nucleus) | Nucleus; transcription and translation are spatially and temporally separated |
| Post-transcriptional modification | Rare; mRNA is used immediately, often polycistronic | Extensive: 5′ capping, 3′ poly-A tail, splicing (removal of introns) |
In prokaryotes, transcription and translation can occur simultaneously because there is no nuclear envelope. In eukaryotes, the primary transcript (pre-mRNA) undergoes processing before it exits the nucleus.
在原核生物中，由于没有核膜，转录和翻译可以同时发生。而在真核生物中，初级转录物（前体 mRNA）需要经过加工才能离开细胞核。
3. Steps of Transcription | 转录的步骤
Transcription consists of three main stages: initiation, elongation, and termination. In initiation, RNA polymerase binds to the promoter, unwinds the DNA helix, and begins RNA synthesis. The first nucleotide is always a purine (A or G).
转录包括三个主要阶段：起始、延伸和终止。在起始阶段，RNA 聚合酶与启动子结合，解开 DNA 双螺旋，开始 RNA 合成。第一个核苷酸总是嘌呤（A 或 G）。
During elongation, RNA polymerase moves along the template strand in the 3′ to 5′ direction, adding ribonucleotides complementary to the DNA template (uracil replaces thymine). The growing RNA chain extends from the 5′ end.
在延伸阶段，RNA 聚合酶沿模板链 3′ 到 5′ 方向移动，添加与 DNA 模板互补的核糖核苷酸（尿嘧啶代替胸腺嘧啶）。生长的 RNA 链从 5′ 端延伸。
Termination in prokaryotes often involves a hairpin loop formed by a GC-rich inverted repeat followed by a poly-U sequence, causing RNA polymerase to dissociate. In eukaryotes, termination signals like the polyadenylation signal AAUAAA lead to cleavage and addition of the poly-A tail.
原核生物的终止通常涉及富含 GC 的反向重复序列形成的发夹结构，其后跟着一段 poly-U 序列，导致 RNA 聚合酶解离。真核生物中，终止信号如多聚腺苷酸化信号 AAUAAA 引起裂解并加上 poly-A 尾。
4. Post-Transcriptional Modification in Eukaryotes | 真核生物的转录后修饰
Pre-mRNA in eukaryotes undergoes three major modifications: addition of a 5′ cap (7-methylguanosine), addition of a 3′ poly-A tail (about 200 adenine nucleotides), and splicing to remove introns and join exons. These modifications protect mRNA from degradation and facilitate nuclear export and translation initiation.
真核生物的前体 mRNA 经历三种主要修饰：添加 5′ 帽（7-甲基鸟苷）、添加 3′ poly-A 尾（约 200 个腺嘌呤核苷酸）以及剪接以去除内含子并连接外显子。这些修饰保护 mRNA 免受降解，并促进核输出和翻译起始。
Alternative splicing allows a single gene to produce multiple protein isoforms by combining different exons. This is a key source of protein diversity in eukaryotes and is frequently examined in the context of gene expression regulation.
可变剪接允许单个基因通过组合不同的外显子产生多种蛋白质同工型。这是真核生物蛋白质多样性的关键来源，是基因表达调控背景下常考的内容。
5. The Genetic Code and tRNA Structure | 遗传密码与 tRNA 结构
The genetic code is a set of rules by which nucleotide triplets (codons) specify amino acids. It is almost universal, degenerate (more than one codon per amino acid), and non-overlapping. Wobble in the third base of the codon allows some tRNAs to recognise multiple codons.
遗传密码是一套由核苷酸三联体（密码子）指定氨基酸的规则。它几乎是通用的、简并的（一个氨基酸可以有多个密码子），且不重叠。密码子第三位碱基的摆动允许某些 tRNA 识别多个密码子。
A tRNA molecule has a cloverleaf secondary structure, with the anticodon loop complementary to the mRNA codon and the 3′ acceptor stem where the amino acid is attached. Aminoacyl-tRNA synthetases catalyse the charging of tRNA with the correct amino acid, a critical step ensuring fidelity of translation.
tRNA 分子具有三叶草形二级结构，反密码子环与 mRNA 密码子互补，3′ 接纳臂是连接氨基酸的位置。氨酰-tRNA 合成酶催化 tRNA 与正确氨基酸的连接，这是确保翻译保真度的关键步骤。
6. Translation: Initiation, Elongation, and Termination | 翻译：起始、延伸和终止
Translation occurs on ribosomes, which consist of a large and a small subunit. In prokaryotes, initiation involves the small ribosomal subunit binding to the Shine–Dalgarno sequence on mRNA, followed by the initiator tRNA carrying formylmethionine (fMet). In eukaryotes, the small subunit scans from the 5′ cap to find the first AUG codon.
翻译在核糖体上进行，核糖体由大、小亚基组成。原核生物中，起始涉及小亚基与 mRNA 上的 Shine–Dalgarno 序列结合，随后携带甲酰甲硫氨酸（fMet）的起始 tRNA 加入。真核生物中，小亚基从 5′ 帽开始扫描以找到第一个 AUG 密码子。
Elongation proceeds through a cycle of codon recognition, peptide bond formation, and translocation. The ribosome has three sites: A (aminoacyl), P (peptidyl), and E (exit). Peptide bond formation is catalysed by peptidyl transferase, an rRNA ribozyme.
延伸通过密码子识别、肽键形成和易位的循环进行。核糖体有三个位点：A（氨酰位）、P（肽酰位）和 E（出口位）。肽键形成由肽基转移酶催化，该酶是一种 rRNA 核酶。
Termination occurs when a stop codon (UAA, UAG, UGA) enters the A site. Release factors bind and trigger hydrolysis of the polypeptide from the tRNA, and the ribosomal subunits disassemble.
当终止密码子（UAA、UAG、UGA）进入 A 位点时终止发生。释放因子结合并触发多肽从 tRNA 上水解，核糖体亚基解离。
7. Gene Regulation: The Lac Operon in Prokaryotes | 原核生物的基因调控：乳糖操纵子
The lac operon is a classic example of inducible gene regulation in E. coli. It consists of a promoter, operator, and three structural genes (lacZ, lacY, lacA) encoding enzymes for lactose metabolism. The regulatory gene lacI encodes a repressor protein.
乳糖操纵子是大肠杆菌中可诱导基因调控的经典例子。它由启动子、操纵基因以及编码乳糖代谢酶的三个结构基因（lacZ、lacY、lacA）组成。调节基因 lacI 编码阻遏蛋白。
In the absence of lactose, the repressor binds to the operator, blocking RNA polymerase and preventing transcription. When lactose is present, it is converted to allolactose, which binds the repressor and inactivates it, allowing transcription to proceed. This is an example of negative inducible control.
当缺乏乳糖时，阻遏蛋白与操纵基因结合，阻止 RNA 聚合酶，从而阻止转录。当存在乳糖时，它被转化为异乳糖，异乳
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