📚 Gene Expression: Transcription, Translation, and Regulation | 基因表达：转录、翻译与调控 考点精讲
The flow of genetic information from DNA to functional products is the foundation of life. In both IB and OCR A-Level Biology, gene expression encompasses transcription, RNA processing, translation, and the sophisticated regulatory networks that control when and where genes are switched on. Mastering these mechanisms is essential for understanding cell differentiation, development, and how mutations lead to disease. This revision guide distils the key concepts, offering paired bilingual explanations to reinforce your learning.
遗传信息从 DNA 流向功能性产物是生命的基础。在 IB 和 OCR A-Level 生物中，基因表达涵盖转录、RNA 加工、翻译，以及控制基因何时何地开启的精妙调控网络。掌握这些机制对于理解细胞分化、发育及突变如何导致疾病至关重要。本考点精讲浓缩核心概念，以中英双语配对解析，助你巩固所学。
1. The Central Dogma of Molecular Biology | 分子生物学中心法则
The central dogma states that genetic information flows from DNA to RNA to protein. DNA acts as a permanent repository, messenger RNA (mRNA) carries the code, and proteins execute cellular functions.
中心法则指出，遗传信息从 DNA 流向 RNA 再流向蛋白质。DNA 是永久的存储库，信使 RNA (mRNA) 携带编码，蛋白质执行细胞功能。
In retroviruses, an exception occurs: reverse transcriptase synthesises DNA from an RNA template, reversing the flow.
在逆转录病毒中存在例外：逆转录酶以 RNA 为模板合成 DNA，逆转了流向。
Some RNA viruses can replicate their RNA directly, bypassing DNA entirely.
某些 RNA 病毒可直接复制 RNA，完全绕开 DNA。
2. Transcription: DNA-Directed RNA Synthesis | 转录：以 DNA 为模板合成 RNA
Transcription is the synthesis of an RNA molecule complementary to the template strand of DNA. RNA polymerase binds to a promoter region, unwinds the DNA double helix, and elongates nascent RNA in the 5′ → 3′ direction.
转录是合成一条与 DNA 模板链互补的 RNA 分子的过程。RNA 聚合酶与启动子区域结合，解开 DNA 双螺旋，并以 5′ → 3′ 方向延伸新生 RNA。
The template strand is read 3′ → 5′, while the non-template (coding) strand has the same sequence as the mRNA (with T replaced by U).
模板链沿 3′ → 5′ 方向被读取，而非模板链（编码链）与 mRNA 序列相同（仅 T 被 U 取代）。
In prokaryotes, a single type of RNA polymerase produces all RNAs. Eukaryotes use RNA polymerase II for mRNA and distinct polymerases for rRNA and tRNA.
原核生物中，单一 RNA 聚合酶合成所有 RNA。真核生物使用 RNA 聚合酶 II 合成 mRNA，并使用不同的聚合酶合成 rRNA 和 tRNA。
Transcription terminates when a terminator sequence is reached. In prokaryotes, this often involves a hairpin loop; in eukaryotes, a polyadenylation signal triggers cleavage.
当到达终止子序列时转录结束。在原核生物中，常涉及发夹环结构；在真核生物中，多聚腺苷酸化信号触发切割。
3. RNA Processing in Eukaryotes | 真核生物的 RNA 加工
Primary transcript (pre-mRNA) undergoes three major modifications before leaving the nucleus: capping, polyadenylation, and splicing.
初级转录本（前体 mRNA）在离开细胞核前经历三种主要修饰：加帽、加尾和剪接。
A 5′ cap (7-methylguanosine) is added, protecting the RNA from degradation and aiding ribosome binding.
5′ 端添加帽结构（7-甲基鸟苷），保护 RNA 免受降解并协助核糖体结合。
At the 3′ end, a poly-A tail of about 200 adenines is added, enhancing stability and nuclear export.
3′ 端添加约 200 个腺苷酸的 poly-A 尾，增强稳定性并促进核输出。
Splicing removes introns (non-coding regions) and joins exons (coding regions). The spliceosome, a complex of small nuclear ribonucleoproteins (snRNPs), catalyses this process.
剪接去除内含子（非编码区）并连接外显子（编码区）。剪接体——由小核核糖核蛋白 (snRNP) 组成的复合物——催化该过程。
Alternative splicing allows one gene to produce multiple protein isoforms, greatly increasing proteome diversity.
可变剪接使一个基因能产生多种蛋白质同工型，极大增加了蛋白质组的多样性。
4. The Genetic Code: Triplets and Codons | 遗传密码：三联体与密码子
The genetic code is a set of rules by which a sequence of three nucleotides (codon) in mRNA specifies an amino acid or a stop signal. There are 64 codons: 61 encode amino acids, and 3 are stop codons (UAA, UAG, UGA).
遗传密码是一套规则，mRNA 中三个核苷酸的序列（密码子）对应一种氨基酸或终止信号。共有 64 个密码子：61 个编码氨基酸，3 个是终止密码子 (UAA、UAG、UGA)。
The code is degenerate: most amino acids are specified by more than one codon, which reduces the impact of mutations.
密码子具有简并性：多数氨基酸由多个密码子编码，这降低了突变造成的影响。
It is nearly universal, with few exceptions in mitochondria and some protozoa. The start codon AUG codes for methionine.
密码子具有近乎通用性，仅在线粒体和某些原生生物中存在少数例外。起始密码子 AUG 编码甲硫氨酸。
The reading frame is set by the start codon. Insertions or deletions shift this frame, altering downstream amino acids.
阅读框由起始密码子确定。插入或缺失会移动阅读框，改变下游氨基酸序列。
5. Translation: From mRNA to Protein | 翻译：从 mRNA 到蛋白质
Translation occurs on ribosomes, using transfer RNAs (tRNAs) as adaptors. Each tRNA carries a specific amino acid and has an anticodon complementary to an mRNA codon.
翻译发生在核糖体上，以转运 RNA (tRNA) 为适配器。每个 tRNA 携带特定氨基酸，并带有与 mRNA 密码子互补的反密码子。
Aminoacyl-tRNA synthetases charge tRNAs with their cognate amino acids, ensuring fidelity using the energy of ATP.
氨酰 tRNA 合成酶利用 ATP 的能量将 tRNA 与其同源氨基酸连接，确保忠实性。
Initiation: the small ribosomal subunit binds mRNA, the initiator tRNAMet pairs with AUG, and the large subunit joins.
起始：小核糖体亚基结合 mRNA，起始 tRNAMet 与 AUG 配对，大亚基加入。
Elongation: the ribosome moves along mRNA codon by codon. Peptide bonds form between amino acids in the P site and incoming aminoacyl-tRNA in the A site; the tRNA in the E site exits.
延伸：核糖体沿 mRNA 逐个密码子移动。P 位点上的氨基酸与进入 A 位点的氨酰 tRNA 之间形成肽键；E 位点上的 tRNA 离开。
Termination occurs when a stop codon reaches the A site; release factors promote hydrolysis of the polypeptide from the tRNA, and the ribosomal subunits dissociate.
当终止密码子到达 A 位点时翻译终止；释放因子促使多肽从 tRNA 水解释放，核糖体亚基解离。
6. Regulation in Prokaryotes: The lac Operon | 原核生物调控：乳糖操纵子
The lac operon in E. coli is a classic model of gene regulation. It consists of a regulatory gene (lacI), a promoter, an operator, and three structural genes (lacZ, lacY, lacA) needed for lactose metabolism.
大肠杆菌的乳糖操纵子是基因调控的经典模型。它包括一个调节基因 (lacI)、一个启动子、一个操纵基因以及乳糖代谢所需的三个结构基因 (lacZ、lacY、lacA)。
In the absence of lactose, the lac repressor (product of lacI) binds the operator, blocking RNA polymerase and preventing transcription.
无乳糖时，lac 阻遏蛋白 (lacI 产物) 结合在操纵基因上，阻挡 RNA 聚合酶，阻止转录。
When lactose is present, it is converted to allolactose, which acts as an inducer. Allolactose binds the repressor, inactivating it, so transcription proceeds.
存在乳糖时，它被转化为别乳糖，作为诱导物。别乳糖与阻遏蛋白结合使其失活，从而转录得以进行。
Glucose levels also modulate the operon via catabolite activator protein (CAP). When glucose is scarce, cAMP accumulates, binds CAP, and the CAP-cAMP complex activates the promoter.
葡萄糖水平也通过代谢物激活蛋白 (CAP) 调节该操纵子。当葡萄糖缺乏时，cAMP 积累，与 CAP 结合，CAP-cAMP 复合物激活启动子。
7. Regulation in Eukaryotes: Transcription Factors and Enhancers | 真核生物调控：转录因子与增强子
Eukaryotic genes are regulated by specific transcription factors (activators and repressors) that bind to DNA sequences such as enhancers, silencers, and promoter-proximal elements.
真核基因由特定的转录因子（激活因子和阻遏因子）调控，它们与增强子、沉默子和启动子近端元件等 DNA 序列结合。
Enhancers can be located far upstream or downstream from the promoter. DNA looping brings the enhancer-bound activators into contact with the basal transcription complex via mediator proteins.
增强子可位于启动子上游或下游远处。DNA 形成环状，通过中介蛋白将结合在增强子上的激活因子与基础转录复合物拉近。
Combinatorial control allows a limited set of transcription factors to regulate a vast array of genes, enabling cell-type-specific expression.
组合调控使有限的转录因子得以调控大量基因，从而实现细胞类型特异性表达。
Chromatin remodelling and histone modifications (e.g., acetylation by histone acetyltransferases) open chromatin, allowing access to the transcriptional machinery.
染色质重塑和组蛋白修饰（例如组蛋白乙酰转移酶催化的乙酰化）打开染色质结构，容许转录机器接近 DNA。
8. Epigenetics: Beyond the DNA Sequence | 表观遗传学：超越 DNA 序列
Epigenetics refers to heritable changes in gene expression that do not involve alterations to the nucleotide sequence. These marks can be influenced by environment and development.
表观遗传学指不涉及核苷酸序列改变的、可遗传的基因表达变化。这些标记可受环境和发育影响。
DNA methylation typically adds methyl groups to cytosine in CpG dinucleotides, leading to transcriptional silencing when promoter regions are methylated.
DNA 甲基化通常向 CpG 二核苷酸中的胞嘧啶添加甲基，当启动子区域被甲基化时导致转录沉默。
Histone modifications, such as acetylation, methylation, and phosphorylation, alter chromatin compaction. Acetylation generally correlates with active transcription.
组蛋白修饰——如乙酰化、甲基化和磷酸化——改变染色质紧密程度。乙酰化通常与活跃转录相关。
X-inactivation in female mammals illustrates epigenetic regulation: one X chromosome is randomly inactivated and packaged as heterochromatin, forming a Barr body.
雌性哺乳动物的 X 染色体失活诠释了表观遗传调控：一条 X 染色体被随机失活，包装成异染色质，形成巴氏小体。
9. Post-Translational Modifications and Protein Folding | 翻译后修饰与蛋白质折叠
Nascent polypeptides must fold into specific three-dimensional shapes to become functional. Chaperone proteins assist this process, preventing aggregation.
新生多肽必须折叠成特定的三维结构才具有功能。分子伴侣蛋白协助该过程，防止聚集。
Post-translational modifications (PTMs) extend protein functionality. Examples include phosphorylation (addition of phosphate groups), glycosylation (addition of sugars), and ubiquitination (tagging for degradation).
翻译后修饰 (PTM) 扩展蛋白质功能。例子包括磷酸化（添加磷酸基团）、糖基化（添加糖类）和泛素化（标记降解）。
Phosphorylation, catalysed by kinases and reversed by phosphatases, is a rapid on/off switch that regulates enzyme activity and signal transduction.
磷酸化由激酶催化、磷酸酶逆转，是一种快速开关，调节酶活性和信号转导。
Improperly folded proteins can lead to diseases such as Alzheimer’s and Parkinson’s, where protein aggregates accumulate.
蛋白质错误折叠可导致疾病，如阿尔茨海默病和帕金森病，此时蛋白质聚集体积聚。
10. Mutations and Their Effects on Gene Expression | 突变及其对基因表达的影响
Mutations are permanent changes in the DNA sequence. Point mutations include silent (no amino acid change), missense (different amino acid), and nonsense (premature stop codon).
突变是 DNA 序列的永久性改变。点突变包括沉默突变（氨基酸不变）、错义突变（不同氨基酸）和无义突变（提前产生终止密码子）。
Insertions or deletions can cause frameshift mutations, altering the entire downstream amino acid sequence and often producing a non-functional protein.
插入或缺失可导致移码突变，改变整个下游氨基酸序列，通常产生无功能蛋白质。
Mutations in promoter or enhancer regions can change the level of gene expression without altering the protein itself, while splice-site mutations disrupt normal mRNA processing.
启动子或增强子区域的突变可改变基因表达水平而不改变蛋白质本身，而剪接位点突变则破坏正常 mRNA 加工。
Mutations are the raw material for evolution, but many genetic disorders result from loss-of-function or gain-of-function mutations.
突变是进化的原材料，但许多遗传病由功能丧失或获得性突变引起。
11. Gene Expression and Cell Differentiation | 基因表达与细胞分化
All somatic cells of a multicellular organism contain the same genome, yet they differentiate into distinct types by selectively expressing specific sets of genes.
多细胞生物的所有体细胞含相同的基因组，但它们通过选择性地表达特定基因集分化成不同的细胞类型。
Master regulatory genes encode transcription factors that trigger cascades of downstream gene expression, committing cells to particular lineages (e.g., MyoD in muscle cells).
主控调节基因编码的转录因子触发下游基因表达级联，使细胞定型至特定谱系（如肌肉细胞中的 MyoD）。
Stem cells retain the ability to divide and give rise to differentiated progeny; their fate depends on intrinsic factors and signals from the niche.
干细胞保持分裂和产生分化子代的能力；其命运取决于内在因素和微环境信号。
Homeobox (Hox) genes contain a conserved homeodomain and are crucial for body plan development, laying out the anterior-posterior axis.
同源异型框 (Hox) 基因含保守的同源域，对躯体蓝图发育至关重要，确立前后轴格局。
12. Techniques to Study Gene Expression | 研究基因表达的技术
Reverse transcription PCR (RT-PCR) converts mRNA into cDNA, then amplifies it to quantify transcript levels. Real-time qPCR enables precise comparison between samples.
逆转录 PCR (RT-PCR) 将 mRNA 转化为 cDNA，然后扩增以定量转录本水平。实时荧光定量 PCR 能够精确比较样品间差异。
DNA microarrays (gene chips) screen the expression of thousands of genes simultaneously, revealing patterns in disease or development.
DNA 微阵列（基因芯片）同时筛查成千上万个基因的表达，揭示疾病或发育中的表达模式。
Reporter genes, such as GFP, are fused to regulatory sequences to visualise where and when a gene is active in living organisms.
报道基因（如绿色荧光蛋白 GFP）与调控序列融合，用于在活体生物中可视化基因活跃的时空位置。
Northern blotting and in situ hybridisation detect specific mRNA molecules, providing information on transcript size and spatial distribution.
Northern 印迹和原位杂交检测特定 mRNA 分子，提供有关转录本大小和空间分布的信息。
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