📚 Gene Expression: Key Concepts for IB & CIE Biology | 基因表达：IB与CIE生物学考点精讲
Gene expression is the process by which the information encoded in a gene is used to direct the assembly of a functional gene product, usually a protein. It involves two key stages: transcription (DNA to mRNA) and translation (mRNA to polypeptide). Understanding gene expression is fundamental for IB and CIE biology syllabi, as it links molecular genetics to the phenotype and explains how cells differentiate and respond to their environment.
基因表达是指基因中编码的信息被用来指导合成功能性基因产物（通常是蛋白质）的过程。它包括两个关键阶段：转录（DNA 到 mRNA）和翻译（mRNA 到多肽）。理解基因表达是 IB 和 CIE 生物学课程的基础，因为它将分子遗传学与表型联系起来，并解释了细胞如何分化以及如何对环境作出反应。
1. Central Dogma of Molecular Biology | 分子生物学的中心法则
The central dogma states that genetic information flows from DNA to RNA to protein. In some viruses, RNA can be reverse-transcribed into DNA, but the basic flow in cells is DNA → RNA → protein. This concept underpins all of gene expression.
中心法则指出，遗传信息从 DNA 流向 RNA，再到蛋白质。在某些病毒中，RNA 可以被逆转录为 DNA，但细胞中的基本流向是 DNA → RNA → 蛋白质。这个概念是所有基因表达的基础。
Key processes: replication (DNA → DNA), transcription (DNA → mRNA), and translation (mRNA → polypeptide). The dogma emphasizes that proteins cannot be used as templates to recreate DNA or RNA directly.
关键过程：复制（DNA → DNA）、转录（DNA → mRNA）和翻译（mRNA → 多肽）。这一法则强调，蛋白质不能直接作为模板来重新生成 DNA 或 RNA。
2. Transcription: From DNA to mRNA | 转录：从 DNA 到 mRNA
Transcription is the synthesis of a complementary mRNA strand from a DNA template. It occurs in the nucleus of eukaryotes and in the cytoplasm of prokaryotes. The enzyme RNA polymerase binds to a promoter region and unwinds the DNA, reading the template strand in the 3′ to 5′ direction and synthesizing mRNA in the 5′ to 3′ direction.
转录是以 DNA 为模板合成互补 mRNA 链的过程。它在真核生物的细胞核和原核生物的细胞质中进行。RNA 聚合酶与启动子区域结合，解开 DNA，以 3′ 到 5′ 方向读取模板链，并以 5′ 到 3′ 方向合成 mRNA。
In prokaryotes, a single type of RNA polymerase carries out all transcription. In eukaryotes, three RNA polymerases exist: RNA polymerase I (rRNA), II (mRNA and snRNA), and III (tRNA and other small RNAs). Promoters contain specific sequences like the TATA box in eukaryotes.
在原核生物中，单一类型的 RNA 聚合酶完成所有转录。在真核生物中，存在三种 RNA 聚合酶：RNA 聚合酶 I（rRNA）、II（mRNA 和 snRNA）和 III（tRNA 及其他小 RNA）。启动子包含特定序列，如真核生物中的 TATA 盒。
- Initiation: RNA polymerase binds to promoter with sigma factor (prok.) / transcription factors (euk.)
- Elongation: RNA nucleotides added complementary to template (A-U, G-C)
- Termination: RNA polymerase reaches terminator sequence; in eukaryotes, poly-A signal aids termination
- 起始：RNA 聚合酶与 sigma 因子（原核）/转录因子（真核）一起结合启动子
- 延伸：与模板互补的 RNA 核苷酸被添加（A-U，G-C）
- 终止：RNA 聚合酶到达终止序列；在真核生物中，poly-A 信号帮助终止
3. RNA Processing in Eukaryotes | 真核生物的 RNA 加工
In eukaryotes, the primary transcript (pre-mRNA) undergoes extensive processing before translation. Three main modifications: 5′ capping with a modified guanine nucleotide, 3′ polyadenylation (poly-A tail of about 200 adenines), and splicing to remove introns and join exons.
在真核生物中，初级转录本（前体 mRNA）在翻译前经历广泛的加工。三种主要修饰：5′ 端加帽（经修饰的鸟嘌呤核苷酸），3′ 端加多聚腺苷酸尾（约 200 个腺嘌呤的 poly-A 尾），以及剪接以去除内含子并连接外显子。
5′ cap protects mRNA from degradation and aids ribosome binding. The poly-A tail also protects mRNA and facilitates export from the nucleus. Splicing is carried out by spliceosomes, which recognize splice sites at exon-intron boundaries. Alternative splicing allows one gene to code for multiple proteins, greatly increasing proteome diversity.
5′ 帽保护 mRNA 免受降解并帮助核糖体结合。poly-A 尾同样保护 mRNA 并促进其从细胞核输出。剪接由剪接体完成，它识别外显子-内含子边界的剪接位点。可变剪接使一个基因可以编码多种蛋白质，极大地增加了蛋白质组的多样性。
4. Translation: mRNA to Polypeptide | 翻译：mRNA 到多肽
Translation is the process by which ribosomes decode mRNA to synthesize a polypeptide chain. It occurs in the cytoplasm (on free ribosomes or rough ER) using tRNA molecules that carry specific amino acids. Ribosomes have two subunits, which assemble around mRNA.
翻译是核糖体解码 mRNA 以合成多肽链的过程。它在细胞质中进行（游离核糖体或粗面内质网上），利用携带特定氨基酸的 tRNA 分子。核糖体有两个亚基，它们围绕 mRNA 组装。
Key sites in a ribosome: A site (aminoacyl-tRNA entry), P site (peptidyl-tRNA), E site (exit). Translation proceeds in three steps: initiation, elongation, and termination. Initiation involves the small ribosomal subunit, mRNA, initiator tRNA (carrying methionine), and large subunit assembly. In prokaryotes, the Shine-Dalgarno sequence aligns mRNA; in eukaryotes, the 5′ cap and Kozak sequence are crucial.
核糖体中的关键位点：A 位点（氨酰 tRNA 进入）、P 位点（肽基 tRNA）、E 位点（退出）。翻译分三步进行：起始、延伸和终止。起始涉及小核糖体亚基、mRNA、起始 tRNA（携带甲硫氨酸）和大亚基的组装。在原核生物中，Shine-Dalgarno 序列对齐 mRNA；在真核生物中，5′ 帽和 Kozak 序列至关重要。
Elongation: tRNAs bring amino acids matching codons; peptide bonds form between amino acids; ribosome translocates. Termination: stop codon is reached, release factors bind, polypeptide released.
延伸：tRNA 带来与密码子匹配的氨基酸；氨基酸之间形成肽键；核糖体移位。终止：到达终止密码子，释放因子结合，多肽释放。
5. The Genetic Code | 遗传密码
The genetic code is a triplet code: three nucleotides (a codon) specify one amino acid. It is degenerate (multiple codons for most amino acids), unambiguous (each codon specifies only one amino acid), and universal (same in almost all organisms). There are 64 codons, including 61 for amino acids and 3 stop codons (UAA, UAG, UGA).
遗传密码是三联体密码：三个核苷酸（一个密码子）指定一种氨基酸。它具有简并性（大多数氨基酸有多个密码子）、无歧义性（每个密码子只指定一种氨基酸）和通用性（几乎所有生物体中都相同）。共有 64 个密码子，其中 61 个编码氨基酸，3 个终止密码子（UAA, UAG, UGA）。
The start codon AUG codes for methionine (and marks the beginning of translation). The genetic code is often displayed in a codon table. Mutations can alter codons and thus amino acid sequences; silent mutations do not change the amino acid due to degeneracy.
起始密码子 AUG 编码甲硫氨酸（并标记翻译的起点）。遗传密码通常用密码子表表示。突变可以改变密码子从而改变氨基酸序列；由于简并性，沉默突变不会改变氨基酸。
6. Regulation of Gene Expression in Prokaryotes: The lac Operon | 原核生物的基因表达调控：lac 操纵子
The lac operon in E. coli is a classic model of gene regulation. It consists of structural genes (lacZ, lacY, lacA), a promoter, an operator, and a regulatory gene (lacI). The operon allows the bacterium to metabolize lactose only when glucose is absent and lactose is present.
大肠杆菌中的 lac 操纵子是基因调控的经典模型。它由结构基因（lacZ、lacY、lacA）、启动子、操纵基因和调节基因（lacI）组成。该操纵子使细菌只有在没有葡萄糖、有乳糖存在时才代谢乳糖。
In the absence of lactose, the lac repressor protein (product of lacI) binds to the operator, blocking RNA polymerase and preventing transcription. This is negative regulation. When lactose is present, it is converted to allolactose, which binds to the repressor, inactivating it. The repressor dissociates, allowing transcription.
在缺乏乳糖时，lac 阻遏蛋白（lacI 的产物）与操纵基因结合，阻断 RNA 聚合酶，阻止转录。这是负调控。当乳糖存在时，它转化为异乳糖，与阻遏蛋白结合，使其失活。阻遏蛋白脱离，允许转录。
Catabolite activator protein (CAP) and cAMP provide positive control: when glucose levels are low, cAMP increases; cAMP binds CAP, and the complex binds near the promoter, enhancing RNA polymerase binding. Thus, maximum transcription occurs when lactose is present and glucose is absent.
分解代谢物激活蛋白（CAP）和 cAMP 提供正调控：当葡萄糖水平低时，cAMP 升高；cAMP 与 CAP 结合，复合物结合在启动子附近，增强 RNA 聚合酶的结合。因此，当乳糖存在且葡萄糖缺乏时，转录量最大。
7. Regulation in Eukaryotes: Transcription Factors and Enhancers | 真核生物的调控：转录因子与增强子
Eukaryotic gene regulation is more complex, involving transcription factors, enhancers, silencers, and chromatin modifications. Transcription factors are proteins that bind to specific DNA sequences (response elements) to initiate or regulate transcription. General transcription factors (e.g., TFIID) are required for RNA polymerase II binding at the core promoter.
真核生物基因调控更复杂，涉及转录因子、增强子、沉默子和染色质修饰。转录因子是结合特定 DNA 序列（应答元件）以启动或调节转录的蛋白质。通用转录因子（如 TFIID）是 RNA 聚合酶 II 在核心启动子上结合所必需的。
Specific transcription factors bind to enhancers (far from promoter) or silencers and interact with the mediator complex to influence the rate of transcription. This allows precise control of gene expression in response to signals, development, and cell type. For example, steroid hormone receptors are ligand-activated transcription factors.
特异性转录因子与增强子（远距离启动子）或沉默子结合，并通过中介复合物影响转录速率。这使得基因表达可以响应信号、发育和细胞类型而精确调控。例如，类固醇激素受体是配体激活的转录因子。
8. Epigenetics: Heritable Changes Without DNA Sequence Alteration | 表观遗传：不改变 DNA 序列的可遗传变化
Epigenetics involves modifications that affect gene expression without changing the DNA sequence. These changes can be inherited through cell division. Two major mechanisms: DNA methylation (addition of methyl groups to cytosine, often silencing genes) and histone modification (acetylation, methylation, phosphorylation of histone tails).
表观遗传学涉及影响基因表达但不改变 DNA 序列的修饰。这些变化可以通过细胞分裂遗传。两个主要机制：DNA 甲基化（在胞嘧啶上加甲基基团，通常沉默基因）和组蛋白修饰（组蛋白尾巴的乙酰化、甲基化、磷酸化）。
Histone acetylation neutralizes positive charges on histones, reducing their affinity for DNA and creating a more open chromatin structure (euchromatin), which promotes transcription. Deacetylation tightens chromatin, repressing transcription. The epigenome responds to environmental factors such as diet, stress, and toxins.
组蛋白乙酰化中和组蛋白上的正电荷，降低其对 DNA 的亲和力，形成更开放的染色质结构（常染色质），从而促进转录。去乙酰化使染色质紧缩，抑制转录。表观基因组会对饮食、压力和毒素等环境因素作出反应。
9. Mutations and Their Effects on Gene Expression | 突变及其对基因表达的影响
A mutation is a change in the DNA sequence. Substitution mutations (point mutations) can be silent, missense (different amino acid), or nonsense (early stop codon). Insertions or deletions cause frameshift mutations if the number of nucleotides added or removed is not a multiple of three, altering the entire downstream amino acid sequence.
突变是 DNA 序列的改变。替换突变（点突变）可以是沉默的、错义的（不同氨基酸）或无义的（提前终止密码子）。插入或缺失如果添加或移除的核苷酸数目不是三的倍数，则导致移码突变，改变下游整个氨基酸序列。
Mutations in promoter regions can affect the binding of transcription factors, altering the level of gene expression. Mutations in splice sites can disrupt normal splicing, producing aberrant proteins. Some mutations cause genetic diseases (e.g., sickle cell anemia due to a single missense mutation in the β-globin gene).
启动子区域的突变会影响转录因子的结合，改变基因表达水平。剪接位点的突变会破坏正常剪接，产生异常蛋白质。一些突变会导致遗传病（例如，β-珠蛋白基因中的单个错义突变导致镰状细胞贫血）。
| Mutation Type | 突变类型 | Effect on Protein | 对蛋白质的影响 |
|---|---|
| Silent | 沉默 | No change | 无变化 |
| Missense | 错义 | One amino acid change | 一个氨基酸改变 |
| Nonsense | 无义 | Truncated protein | 截短的蛋白质 |
| Frameshift | 移码 | Altered sequence from mutation site onward | 从突变位点起序列改变 |
10. Exam Tips for Gene Expression | 基因表达考点应试技巧
For both IB and CIE exams, be prepared to describe transcription and translation step-by-step, often with diagrams. Know the differences between prokaryotic and eukaryotic gene expression: absence/presence of RNA processing, coupling of transcription and translation (simultaneous in prokaryotes), operons vs. transcription factors.
对于 IB 和 CIE 考试，要准备好逐步描述转录和翻译，通常要配图。知道原核和真核基因表达的区别：RNA 加工的有无、转录和翻译的偶联（原核中同时进行）、操纵子 vs. 转录因子。
Common essay or extended response questions: Explain how the lac operon is regulated. Compare transcription in prokaryotes and eukaryotes. Discuss the relationship between genes, proteins, and phenotype, including environmental effects and epigenetics. Remember to link structure to function, e.g., ribosome sites, activator-repressor binding.
常见的 essay 或长篇问答题：解释 lac 操纵子如何被调控。比较原核和真核的转录。讨论基因、蛋白质与表型之间的关系，包括环境影响和表观遗传。记住要将结构与功能联系起来，例如核糖体位点、激活因子-阻遏蛋白结合。
Use precise terminology: promoter, operator, transcription factor, RNA polymerase, splicing, anticodon, degenerate code. When explaining experiments (e.g., Jacob and Monod lac operon), describe the logic and conclusions. In data-based questions, interpret graphs showing enzyme activity or gene expression levels under different conditions.
使用精确的术语：启动子、操纵基因、转录因子、RNA 聚合酶、剪接、反密码子、简并密码。当解释实验（例如 Jacob 和 Monod 的 lac 操纵子实验）时，要描述逻辑和结论。在数据分析题中，解释显示不同条件下酶活性或基因表达水平的图表。
Published by TutorHao | Biology Revision Series | aleveler.com
更多咨询请联系16621398022（同微信）
屏轩国际教育cambridge primary/secondary checkpoint, cat4, ukiset,ukcat,igcse,alevel,PAT,STEP,MAT, ibdp,ap,ssat,sat,sat2课程辅导，国外大学本科硕士研究生博士课程论文辅导