Gene Expression and Regulation — 基因表达与调控：从转录到表观遗传

📚 Gene Expression and Regulation | 基因表达与调控

Gene expression is the process by which the information encoded in a gene is used to direct the synthesis of a functional gene product, typically a protein. In A-Level Biology, understanding how genes are switched on and off — and how this regulation can go wrong — is fundamental to topics ranging from cell differentiation to cancer. This article provides a comprehensive bilingual guide to the mechanisms of gene expression and its regulation in both prokaryotes and eukaryotes.

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。在A-Level生物中，理解基因如何被开启和关闭——以及这种调控如何出错——对于从细胞分化到癌症等主题都是基础性的。本文提供了关于原核生物和真核生物中基因表达及其调控机制的全面双语指南。

1. The Central Dogma of Molecular Biology | 分子生物学的中心法则

The central dogma describes the flow of genetic information within a biological system: DNA → RNA → Protein. This framework, first articulated by Francis Crick in 1958, outlines the two key processes of gene expression: transcription (DNA to mRNA) and translation (mRNA to protein). While the central dogma captures the general direction of information flow, modern biology has revealed important exceptions, including reverse transcription (RNA → DNA) in retroviruses and the regulatory roles of non-coding RNAs.

中心法则描述了生物系统中遗传信息的流动方向：DNA → RNA → 蛋白质。这一框架由弗朗西斯·克里克于1958年首次提出，概述了基因表达的两个关键过程：转录（DNA到mRNA）和翻译（mRNA到蛋白质）。虽然中心法则捕捉了信息流动的一般方向，但现代生物学揭示了重要的例外情况，包括逆转录病毒中的逆转录（RNA → DNA）以及非编码RNA的调控作用。

In eukaryotic cells, these processes are spatially separated: transcription occurs in the nucleus, while translation takes place in the cytoplasm on ribosomes. This compartmentalisation allows for additional layers of regulation, including RNA processing and nuclear export. In prokaryotes, both processes occur in the cytoplasm, enabling coupled transcription-translation where ribosomes begin translating mRNA before transcription is even complete.

在真核细胞中，这些过程在空间上是分离的：转录发生在细胞核中，而翻译发生在细胞质的核糖体上。这种区室化允许了额外的调控层次，包括RNA加工和核输出。在原核生物中，两个过程都发生在细胞质中，使得转录-翻译偶联成为可能，即核糖体在转录完成之前就开始翻译mRNA。

2. Transcription: From DNA to mRNA | 转录：从DNA到mRNA

Transcription is the first step of gene expression, in which a particular segment of DNA is copied into messenger RNA (mRNA) by the enzyme RNA polymerase. The process can be divided into three stages: initiation, elongation, and termination.

转录是基因表达的第一步，其中特定的DNA片段被RNA聚合酶复制成信使RNA（mRNA）。该过程可以分为三个阶段：起始、延伸和终止。

Initiation: RNA polymerase binds to a specific DNA sequence called the promoter, located upstream of the gene. In eukaryotes, transcription factors must first assemble at the promoter region (including the TATA box) to recruit RNA polymerase II. In prokaryotes, the sigma factor subunit of RNA polymerase directly recognises the promoter sequences at the -10 (Pribnow box) and -35 regions.

起始：RNA聚合酶与称为启动子的特定DNA序列结合，该序列位于基因上游。在真核生物中，转录因子必须首先在启动子区域（包括TATA盒）组装以招募RNA聚合酶II。在原核生物中，RNA聚合酶的σ因子亚基直接识别-10区（Pribnow盒）和-35区的启动子序列。

Elongation: RNA polymerase unwinds the DNA double helix (creating a transcription bubble of approximately 17 base pairs) and synthesises a complementary RNA strand in the 5′ to 3′ direction. The template strand (also called the antisense strand) is read in the 3′ to 5′ direction. RNA nucleotides are added according to complementary base-pairing rules, with uracil (U) replacing thymine (T) in RNA.

延伸：RNA聚合酶解开DNA双螺旋（产生约17个碱基对的转录泡），并以5’到3’方向合成互补的RNA链。模板链（也称反义链）以3’到5’方向被读取。根据互补碱基配对规则添加RNA核苷酸，其中尿嘧啶（U）在RNA中替代胸腺嘧啶（T）。

Termination: In prokaryotes, termination occurs via two mechanisms: Rho-dependent termination (requiring the Rho protein) and Rho-independent termination (involving a GC-rich hairpin loop followed by a poly-U sequence). In eukaryotes, RNA polymerase II continues past the end of the gene, and the pre-mRNA is cleaved at a specific polyadenylation signal (AAUAAA).

终止：在原核生物中，终止通过两种机制发生：依赖于Rho的终止（需要Rho蛋白）和不依赖于Rho的终止（涉及GC富集的发夹环后跟poly-U序列）。在真核生物中，RNA聚合酶II继续转录超过基因末端，前体mRNA在特定的多聚腺苷酸化信号（AAUAAA）处被切割。

3. RNA Processing in Eukaryotes | 真核生物中的RNA加工

In eukaryotic cells, the primary transcript (pre-mRNA) undergoes extensive processing before it becomes a mature mRNA ready for translation. These modifications are essential for mRNA stability, nuclear export, and efficient translation.

在真核细胞中，初级转录本（前体mRNA）在成为准备翻译的成熟mRNA之前经历广泛的加工。这些修饰对于mRNA的稳定性、核输出和高效翻译至关重要。

5′ Capping: A modified guanine nucleotide (7-methylguanosine) is added to the 5′ end of the pre-mRNA. This 5′ cap protects the mRNA from degradation by exonucleases, facilitates nuclear export, and is recognised by the ribosome during translation initiation.

5’端加帽：一个修饰的鸟嘌呤核苷酸（7-甲基鸟苷）被添加到前体mRNA的5’端。这个5’帽保护mRNA免受外切核酸酶的降解，促进核输出，并在翻译起始时被核糖体识别。

3′ Polyadenylation: A tail of approximately 200 adenine nucleotides (poly-A tail) is added to the 3′ end of the mRNA. The poly-A tail enhances mRNA stability, aids in nuclear export, and plays a role in translation initiation through interaction with poly-A binding proteins (PABPs).

3’端多聚腺苷酸化：约200个腺嘌呤核苷酸组成的尾巴（poly-A尾）被添加到mRNA的3’端。poly-A尾增强mRNA的稳定性，帮助核输出，并通过与poly-A结合蛋白（PABP）的相互作用在翻译起始中发挥作用。

Splicing: Eukaryotic genes contain coding regions called exons and non-coding regions called introns. During splicing, introns are removed and exons are joined together by a large ribonucleoprotein complex called the spliceosome. The spliceosome recognises specific sequences at exon-intron boundaries: the 5′ splice site (GU), the branch point (A), and the 3′ splice site (AG). This process occurs via two transesterification reactions.

剪接：真核基因包含编码区（外显子）和非编码区（内含子）。在剪接过程中，内含子被移除，外显子通过一个称为剪接体的大型核糖核蛋白复合物连接在一起。剪接体识别外显子-内含子边界上的特定序列：5’剪接位点（GU）、分支点（A）和3’剪接位点（AG）。该过程通过两个转酯化反应发生。

Alternative splicing allows a single gene to produce multiple different mRNA transcripts — and therefore multiple protein isoforms — by combining different combinations of exons. It is estimated that over 95% of human multi-exon genes undergo alternative splicing, greatly expanding the diversity of the proteome.

可变剪接允许单个基因通过组合不同的外显子组合产生多种不同的mRNA转录本——因此产生多种蛋白质亚型。据估计，超过95%的人类多外显子基因经历可变剪接，极大地扩展了蛋白质组的多样性。

4. Translation: From mRNA to Protein | 翻译：从mRNA到蛋白质

Translation is the process by which ribosomes decode the mRNA sequence to synthesise a polypeptide chain. This process occurs in the cytoplasm and requires three key players: mRNA (the template), ribosomes (the machinery), and transfer RNA (tRNA) molecules (the adaptors that bring amino acids).

翻译是核糖体解码mRNA序列以合成多肽链的过程。该过程发生在细胞质中，需要三个关键参与者：mRNA（模板）、核糖体（机器）和转运RNA（tRNA）分子（带来氨基酸的适配器）。

Initiation: In eukaryotes, the small ribosomal subunit (40S) binds to the 5′ cap of the mRNA and scans along until it finds the start codon (AUG). The initiator tRNA carrying methionine then binds, and the large ribosomal subunit (60S) assembles to form the complete 80S ribosome. In prokaryotes, the small subunit (30S) recognises the Shine-Dalgarno sequence upstream of AUG.

起始：在真核生物中，小核糖体亚基（40S）与mRNA的5’帽结合并沿其扫描直到找到起始密码子（AUG）。携带甲硫氨酸的起始tRNA随后结合，大核糖体亚基（60S）组装形成完整的80S核糖体。在原核生物中，小亚基（30S）识别AUG上游的Shine-Dalgarno序列。

Elongation: The ribosome has three binding sites for tRNA: the A (aminoacyl) site, P (peptidyl) site, and E (exit) site. A charged tRNA carrying the next amino acid enters the A site. A peptide bond forms between the amino acid in the P site and the incoming amino acid in the A site, catalysed by peptidyl transferase activity of the large subunit rRNA (a ribozyme). The ribosome then translocates, moving the tRNA from A to P to E, and the cycle repeats.

延伸：核糖体有三个tRNA结合位点：A位点（氨酰基位点）、P位点（肽基位点）和E位点（出口位点）。携带下一个氨基酸的负载tRNA进入A位点。在P位点的氨基酸与A位点进入的氨基酸之间形成肽键，由大亚基rRNA的肽基转移酶活性（一种核酶）催化。核糖体然后进行易位，将tRNA从A移到P再到E，循环重复。

Termination: When the ribosome encounters a stop codon (UAA, UAG, or UGA) in the A site, no tRNA recognises it. Instead, release factors bind to the A site, triggering the hydrolysis of the polypeptide from the tRNA in the P site. The ribosomal subunits then dissociate.

终止：当核糖体在A位点遇到终止密码子（UAA、UAG或UGA）时，没有tRNA能识别它。相反，释放因子与A位点结合，触发P位点tRNA上多肽的水解。核糖体亚基随后解离。

5. The Genetic Code | 遗传密码

The genetic code is the set of rules by which the nucleotide sequence of mRNA is translated into the amino acid sequence of a protein. The code is read in triplets called codons, with each codon specifying one amino acid (or a stop signal).

遗传密码是将mRNA的核苷酸序列翻译成蛋白质氨基酸序列的一套规则。密码子以称为密码子的三联体读取，每个密码子指定一个氨基酸（或终止信号）。

Key features of the genetic code include:

遗传密码的主要特征包括：

• Degeneracy (Redundancy): Most amino acids are specified by more than one codon. For example, leucine is encoded by six different codons (UUA, UUG, CUU, CUC, CUA, CUG). This redundancy provides protection against point mutations.
  简并性（冗余性）：大多数氨基酸由多个密码子指定。例如，亮氨酸由六个不同的密码子编码（UUA、UUG、CUU、CUC、CUA、CUG）。这种冗余提供了对点突变的保护。
• Universality: The genetic code is nearly universal across all living organisms, providing strong evidence for a common evolutionary origin. Minor exceptions exist in mitochondria and some protists.
  通用性：遗传密码在所有生物中几乎是通用的，为共同进化起源提供了强有力的证据。线粒体和一些原生生物中存在少量例外。
• Non-overlapping: Each nucleotide is part of only one codon. The reading frame is established by the start codon and maintained throughout translation.
  非重叠性：每个核苷酸只属于一个密码子。阅读框由起始密码子建立并在整个翻译过程中保持。
• Wobble Hypothesis: The third base of a codon can undergo non-standard base pairing (wobble) with the tRNA anticodon, allowing a single tRNA to recognise multiple codons. This is why cells need far fewer than 61 different tRNAs for the 61 sense codons.
  摆动假说：密码子的第三个碱基可以与tRNA反密码子发生非标准碱基配对（摆动），允许单个tRNA识别多个密码子。这就是为什么细胞需要的tRNA种类远少于61种（对应61个有义密码子）。

6. Gene Regulation in Prokaryotes: The Lac Operon | 原核生物的基因调控：乳糖操纵子

The lac operon in E. coli is the classic model of prokaryotic gene regulation, discovered by Francois Jacob and Jacques Monod in 1961 (earning them the Nobel Prize in 1965). The operon contains three structural genes — lacZ (β-galactosidase), lacY (permease), and lacA (transacetylase) — that are transcribed together as a single polycistronic mRNA.

大肠杆菌中的乳糖操纵子是原核生物基因调控的经典模型，由弗朗索瓦·雅各布和雅克·莫诺于1961年发现（为他们赢得了1965年诺贝尔奖）。该操纵子包含三个结构基因——lacZ（β-半乳糖苷酶）、lacY（透性酶）和lacA（转乙酰酶）——它们作为单个多顺反子mRNA一起被转录。

The regulation of the lac operon depends on two key elements working together:

乳糖操纵子的调控依赖于两个共同作用的关键元素：

1. The Repressor (LacI): In the absence of lactose, the lac repressor protein (encoded by the lacI gene, which is constitutively expressed) binds to the operator sequence, physically blocking RNA polymerase from transcribing the structural genes. This is negative regulation. When lactose is present, it is converted to allolactose, which acts as an inducer by binding to the repressor and causing a conformational change that prevents it from binding to the operator. RNA polymerase can then transcribe the genes.

1. 阻遏蛋白（LacI）：在没有乳糖的情况下，lac阻遏蛋白（由组成型表达的lacI基因编码）与操纵基因序列结合，物理上阻止RNA聚合酶转录结构基因。这是负调控。当乳糖存在时，它被转化为别乳糖，别乳糖作为诱导物通过与阻遏蛋白结合并引起构象变化，阻止其与操纵基因结合。RNA聚合酶然后可以转录这些基因。

2. The CAP-cAMP System (Catabolite Repression): Even when lactose is present, the lac operon is only fully active when glucose levels are low. When glucose is scarce, cAMP levels rise, and cAMP binds to the catabolite activator protein (CAP). The CAP-cAMP complex binds to a site upstream of the promoter, bending the DNA and enhancing RNA polymerase binding. This is positive regulation. When glucose is abundant, cAMP levels are low, CAP cannot bind, and transcription remains low even if lactose is present — the bacterium prefers glucose as an energy source.

2. CAP-cAMP系统（分解代谢物阻遏）：即使乳糖存在，乳糖操纵子仅在葡萄糖水平低时才完全激活。当葡萄糖稀缺时，cAMP水平上升，cAMP与分解代谢物激活蛋白（CAP）结合。CAP-cAMP复合物与启动子上游的位点结合，弯曲DNA并增强RNA聚合酶的结合。这是正调控。当葡萄糖充足时，cAMP水平低，CAP不能结合，即使乳糖存在转录也保持低水平——细菌优先使用葡萄糖作为能量来源。

The lac operon demonstrates a beautiful logic: the bacterium only expends energy producing lactose-metabolising enzymes when (a) lactose is available to induce the system, and (b) glucose is not available as a preferred alternative.

乳糖操纵子展示了一个精美的逻辑：细菌仅在（a）乳糖可用以诱导该系统，且（b）葡萄糖不可用作首选替代品时，才消耗能量产生乳糖代谢酶。

7. Gene Regulation in Eukaryotes: Transcription Factors | 真核生物的基因调控：转录因子

Eukaryotic gene regulation is significantly more complex than prokaryotic regulation. Unlike the operon model, eukaryotic genes are typically regulated individually through the combinatorial action of multiple transcription factors that bind to regulatory DNA sequences.

真核生物的基因调控比原核生物的调控复杂得多。与操纵子模型不同，真核基因通常通过与调控DNA序列结合的多个转录因子的组合作用进行个体调控。

Promoter-proximal elements: Located close to the transcription start site (within ~200 bp), these include the TATA box, CAAT box, and GC box. General transcription factors (such as TFIID, which contains the TATA-binding protein or TBP) assemble at these sites to form the pre-initiation complex.

启动子近端元件：位于转录起始位点附近（约200 bp以内），包括TATA盒、CAAT盒和GC盒。通用转录因子（如含有TATA结合蛋白TBP的TFIID）在这些位点组装形成前起始复合物。

Enhancers and silencers: These are regulatory DNA sequences that can be located thousands of base pairs away from the gene they regulate, either upstream or downstream, and even within introns. Enhancers increase transcription rates when bound by activator proteins, while silencers decrease transcription when bound by repressor proteins. The looping of DNA allows these distant elements to interact with the promoter region.

增强子和沉默子：这些是调控DNA序列，可以位于其所调控基因数千碱基对之外，无论是上游还是下游，甚至在内含子内部。增强子在被激活蛋白结合时增加转录速率，而沉默子在被阻遏蛋白结合时降低转录。DNA的环化允许这些远端元件与启动子区域相互作用。

Combinatorial control: A typical eukaryotic gene is regulated by multiple transcription factors acting together. The specific combination of activators and repressors bound to the regulatory regions of a gene determines whether and at what level the gene is expressed. This allows a relatively small number of transcription factors (approximately 1,600 in humans) to generate immense regulatory complexity.

组合调控：一个典型的真核基因由多个转录因子共同作用来调控。结合在基因调控区域上的激活蛋白和阻遏蛋白的特定组合决定了基因是否表达以及表达水平。这允许相对较少数量的转录因子（人类中约1,600个）产生巨大的调控复杂性。

8. Epigenetics: Regulation Beyond the DNA Sequence | 表观遗传学：超越DNA序列的调控

Epigenetics refers to heritable changes in gene expression that do not involve alterations to the underlying DNA sequence. These changes can be stable through cell divisions and, in some cases, can even be passed from one generation to the next.

表观遗传学指的是不涉及DNA序列本身改变的、可遗传的基因表达变化。这些变化可以在细胞分裂中保持稳定，在某些情况下甚至可以从一代传递到下一代。

DNA Methylation: The addition of methyl groups (CH₃) to cytosine bases in CpG dinucleotides, catalysed by DNA methyltransferases (DNMTs). Methylation of CpG islands in promoter regions is generally associated with gene silencing — the methyl groups physically interfere with transcription factor binding and recruit methyl-CpG-binding proteins that promote chromatin compaction. Aberrant DNA methylation patterns are a hallmark of many cancers.

DNA甲基化：在CpG二核苷酸的胞嘧啶碱基上添加甲基（CH₃），由DNA甲基转移酶（DNMT）催化。启动子区域CpG岛的甲基化通常与基因沉默相关——甲基在物理上干扰转录因子的结合，并招募促进染色质压缩的甲基-CpG结合蛋白。异常的DNA甲基化模式是许多癌症的标志。

Histone Modification: DNA in eukaryotic cells is wrapped around histone proteins to form nucleosomes, the basic unit of chromatin. The N-terminal tails of histones can be chemically modified in various ways: acetylation (added by histone acetyltransferases, HATs, and removed by histone deacetylases, HDACs), methylation, phosphorylation, and ubiquitination. Histone acetylation generally relaxes chromatin structure (euchromatin) and promotes transcription, while deacetylation compacts chromatin (heterochromatin) and represses transcription.

组蛋白修饰：真核细胞中的DNA缠绕在组蛋白上形成核小体，这是染色质的基本单位。组蛋白的N端尾部可以以多种方式被化学修饰：乙酰化（由组蛋白乙酰转移酶HAT添加，由组蛋白去乙酰化酶HDAC去除）、甲基化、磷酸化和泛素化。组蛋白乙酰化通常放松染色质结构（常染色质）并促进转录，而去乙酰化压缩染色质（异染色质）并抑制转录。

X-Inactivation: In female mammals, one of the two X chromosomes is randomly inactivated early in embryonic development to achieve dosage compensation. The inactivated X chromosome (Barr body) is coated with Xist RNA and becomes heavily methylated and transcriptionally silent. This is a striking example of epigenetic regulation at the chromosomal level.

X染色体失活：在雌性哺乳动物中，两条X染色体中的一条在胚胎发育早期被随机失活以实现剂量补偿。失活的X染色体（巴氏小体）被Xist RNA覆盖，并被高度甲基化和转录沉默。这是染色体水平上表观遗传调控的显著例子。

Genomic Imprinting: For a small subset of genes (~100 in humans), expression depends on whether the allele was inherited from the mother or the father. Imprinted genes are marked by differential DNA methylation during gametogenesis, and the imprint is maintained after fertilisation. Disorders such as Prader-Willi syndrome and Angelman syndrome result from errors in imprinting.

基因组印记：对于一小部分基因（人类中约100个），表达取决于等位基因是从母亲还是父亲遗传的。印记基因在配子发生过程中通过差异DNA甲基化标记，印记在受精后得以维持。普拉德-威利综合征和安格曼综合征等疾病源于印记错误。

9. Post-Translational Modification | 翻译后修饰

After a polypeptide chain is synthesised, it often requires further chemical modifications to become a fully functional protein. These post-translational modifications (PTMs) dramatically expand the functional diversity of the proteome.

多肽链合成后，通常需要进一步的化学修饰才能成为完全功能性的蛋白质。这些翻译后修饰（PTM）极大地扩展了蛋白质组的功能多样性。

Key types of PTMs include: Phosphorylation (addition of phosphate groups by kinases, removal by phosphatases — critical for signal transduction and cell cycle regulation); Glycosylation (addition of carbohydrate groups — important for protein folding, stability, and cell-cell recognition); Ubiquitination (attachment of ubiquitin, often targeting proteins for proteasomal degradation); Proteolytic cleavage (removal of signal peptides or activation of zymogens, e.g., insulin is cleaved from proinsulin); and Disulfide bond formation (covalent cross-links between cysteine residues that stabilise protein tertiary structure).

PTM的关键类型包括：磷酸化（由激酶添加磷酸基团，由磷酸酶去除——对信号转导和细胞周期调控至关重要）；糖基化（添加碳水化合物基团——对蛋白质折叠、稳定性和细胞间识别重要）；泛素化（连接泛素，通常将蛋白质靶向蛋白酶体降解）；蛋白水解切割（去除信号肽或激活酶原，例如胰岛素从前胰岛素原切割而来）；以及二硫键形成（半胱氨酸残基之间的共价交联，稳定蛋白质三级结构）。

10. Mutations and Their Effects on Gene Expression | 突变及其对基因表达的影响

Mutations are changes in the DNA sequence that can profoundly affect gene expression and protein function. Understanding different types of mutations is essential for A-Level Biology examinations.

突变是DNA序列的变化，可以深刻影响基因表达和蛋白质功能。理解不同类型的突变对A-Level生物考试至关重要。

Point mutations affect a single nucleotide: Silent mutations do not change the amino acid sequence due to the degeneracy of the genetic code; Missense mutations change one amino acid to another (e.g., the GAG → GUG mutation in the β-globin gene causes sickle cell anaemia by changing glutamic acid to valine); Nonsense mutations create a premature stop codon, resulting in a truncated, usually non-functional protein.

点突变影响单个核苷酸：沉默突变由于遗传密码的简并性不改变氨基酸序列；错义突变将一个氨基酸变为另一个（例如，β-珠蛋白基因中的GAG→GUG突变通过将谷氨酸变为缬氨酸导致镰刀型细胞贫血症）；无义突变产生提前终止密码子，导致截短的、通常无功能的蛋白质。

Frameshift mutations (insertions or deletions of nucleotides not in multiples of three) shift the reading frame, completely altering the amino acid sequence downstream of the mutation and usually introducing a premature stop codon.

移码突变（非三的倍数的核苷酸插入或缺失）改变阅读框，完全改变突变下游的氨基酸序列，通常引入提前终止密码子。

Mutations in regulatory regions: Mutations in promoters, enhancers, or splice sites can alter when, where, or how much of a protein is produced, without changing the protein sequence itself. For example, mutations in the promoter region of the FMR1 gene lead to Fragile X syndrome, the most common inherited form of intellectual disability.

调控区域的突变：启动子、增强子或剪接位点的突变可以改变蛋白质产生的时间、地点或数量，而不改变蛋白质序列本身。例如，FMR1基因启动子区域的突变导致脆性X综合征，这是最常见的遗传性智力障碍形式。

11. Cancer: When Gene Regulation Fails | 癌症：当基因调控失效

Cancer is fundamentally a disease of gene regulation. The transformation of a normal cell into a cancer cell typically requires mutations in multiple genes that control cell division, differentiation, and apoptosis (programmed cell death).

癌症从根本上说是一种基因调控疾病。正常细胞转化为癌细胞通常需要多个控制细胞分裂、分化和凋亡（程序性细胞死亡）的基因发生突变。

Oncogenes are mutated forms of normal genes (proto-oncogenes) that promote cell growth and division. A single gain-of-function mutation in one allele of a proto-oncogene can convert it into an oncogene, acting like a stuck accelerator. Examples include RAS (mutated in ~30% of human cancers) and MYC (a transcription factor that drives proliferation).

癌基因是促进细胞生长和分裂的正常基因（原癌基因）的突变形式。原癌基因单个等位基因中的一个功能获得性突变就可以将其转化为癌基因，就像卡住的油门。例子包括RAS（约30%的人类癌症中发生突变）和MYC（驱动增殖的转录因子）。

Tumour suppressor genes normally restrain cell division or promote apoptosis. Both alleles must typically be inactivated (Knudson’s two-hit hypothesis) for cancer to develop — they act like failed brakes. Key tumour suppressors include TP53 (the ‘guardian of the genome’, mutated in over 50% of cancers), RB1 (retinoblastoma protein, which regulates the G1/S checkpoint), and BRCA1/BRCA2 (involved in DNA repair; mutations dramatically increase breast and ovarian cancer risk).

肿瘤抑制基因通常限制细胞分裂或促进凋亡。两个等位基因通常都必须被失活（Knudson的二次打击假说）才能发展成癌症——它们就像失灵的刹车。关键的肿瘤抑制基因包括TP53（”基因组守护者”，超过50%的癌症中发生突变）、RB1（视网膜母细胞瘤蛋白，调控G1/S检查点）以及BRCA1/BRCA2（参与DNA修复；突变显著增加乳腺癌和卵巢癌风险）。

Epigenetic changes also play a critical role in cancer. Hypermethylation of tumour suppressor gene promoters (silencing them epigenetically) and global hypomethylation (leading to genomic instability) are common features of cancer cells. This understanding has led to the development of epigenetic therapies, such as DNA methyltransferase inhibitors and histone deacetylase inhibitors.

表观遗传变化在癌症中也起着关键作用。肿瘤抑制基因启动子的超甲基化（表观遗传沉默它们）和整体低甲基化（导致基因组不稳定性）是癌细胞的常见特征。这一理解导致了表观遗传疗法的发展，如DNA甲基转移酶抑制剂和组蛋白去乙酰化酶抑制剂。

12. Summary Table: Key Molecules in Gene Expression | 总结表：基因表达的关键分子

Molecule / 分子	Function / 功能	Process / 过程
RNA Polymerase / RNA聚合酶	Synthesises mRNA from DNA template / 以DNA为模板合成mRNA	Transcription
Transcription Factors / 转录因子	Bind DNA to regulate transcription / 结合DNA调控转录	Transcription
Spliceosome / 剪接体	Removes introns from pre-mRNA / 从前体mRNA中移除内含子	RNA Processing
Ribosome / 核糖体	Decodes mRNA and catalyses peptide bond formation / 解码mRNA并催化肽键形成	Translation
tRNA / 转运RNA	Delivers amino acids to the ribosome / 将氨基酸运送到核糖体	Translation
Lac Repressor / Lac阻遏蛋白	Blocks transcription of lac operon in absence of lactose / 在无乳糖时阻断乳糖操纵子转录	Prokaryotic Regulation
CAP-cAMP / CAP-cAMP复合物	Activates lac operon when glucose is low / 在葡萄糖低时激活乳糖操纵子	Prokaryotic Regulation
DNA Methyltransferase / DNA甲基转移酶	Adds methyl groups to DNA, silencing genes / 向DNA添加甲基，沉默基因	Epigenetics
Histone Acetyltransferase / 组蛋白乙酰转移酶	Adds acetyl groups to histones, relaxing chromatin / 向组蛋白添加乙酰基，放松染色质	Epigenetics

13. Exam Tips and Common Pitfalls | 考试技巧与常见错误

Tip 1 — Use precise terminology: Examiners award marks for accurate scientific language. Distinguish clearly between transcription (DNA → mRNA, using RNA polymerase) and translation (mRNA → protein, using ribosomes). Do not confuse the two processes or their locations — in eukaryotes, transcription is nuclear, translation is cytoplasmic.

技巧1 — 使用精确术语：考官为准确的科学语言打分。清晰区分转录（DNA→mRNA，使用RNA聚合酶）和翻译（mRNA→蛋白质，使用核糖体）。不要混淆这两个过程或它们的位置——在真核生物中，转录在细胞核，翻译在细胞质。

Tip 2 — Explain the logic, not just the mechanism: When answering questions about the lac operon, do not simply list the components. Explain why the regulation exists: the bacterium conserves energy by only producing lactose-metabolising enzymes when lactose is present AND glucose is absent. This demonstrates deeper understanding.

技巧2 — 解释逻辑，而不仅仅是机制：在回答关于乳糖操纵子的问题时，不要仅仅列出组分。解释为什么存在这种调控：细菌仅在乳糖存在且葡萄糖不存在时才产生乳糖代谢酶，从而节省能量。这展示了更深入的理解。

Tip 3 — Connect topics: Gene expression links to many other A-Level topics. Connect mutations to their effects on protein structure and function. Link epigenetics to cell differentiation (how genetically identical cells can have different phenotypes). Relate transcription factors to development and cancer.

技巧3 — 联系各个主题：基因表达与许多其他A-Level主题相关。将突变与其对蛋白质结构和功能的影响联系起来。将表观遗传学与细胞分化联系起来（基因相同的细胞如何具有不同的表型）。将转录因子与发育和癌症联系起来。

Common pitfalls: (1) Confusing the template strand and coding strand — the mRNA sequence matches the coding strand (with U replacing T), not the template strand. (2) Forgetting that transcription factors bind DNA, not RNA. (3) Stating that all mutations are harmful — silent mutations have no effect, and some mutations can be beneficial. (4) Mixing up acetylation and methylation — acetylation of histones activates transcription; methylation of DNA represses transcription.

常见错误：（1）混淆模板链和编码链——mRNA序列匹配编码链（U替代T），而不是模板链。（2）忘记转录因子结合DNA而不是RNA。（3）声称所有突变都是有害的——沉默突变没有影响，有些突变可能是有益的。（4）混淆乙酰化和甲基化——组蛋白乙酰化激活转录；DNA甲基化抑制转录。

Tip 4 — Practise describing processes sequentially: Many A-Level questions ask you to describe transcription or translation. Practise writing clear, step-by-step descriptions using proper terminology: initiation → elongation → termination for both processes. Include enzyme names (RNA polymerase, peptidyl transferase) and key sequences (promoter, start codon, stop codon).

技巧4 — 练习按顺序描述过程：许多A-Level问题要求你描述转录或翻译。练习使用正确的术语按步骤清晰地写：起始→延伸→终止（适用于两个过程）。包括酶名称（RNA聚合酶、肽基转移酶）和关键序列（启动子、起始密码子、终止密码子）。

---

This article provides a comprehensive overview of gene expression and regulation suitable for A-Level Biology (CIE, AQA, Edexcel, OCR, and WJEC specifications). Regular review of these concepts — particularly the lac operon, transcription factors, and epigenetics — will strengthen your understanding for both structured questions and essay-style answers.

本文提供了适合A-Level生物（CIE、AQA、Edexcel、OCR和WJEC考纲）的基因表达与调控全面概述。定期复习这些概念——特别是乳糖操纵子、转录因子和表观遗传学——将加强你对结构化问题和论文式答案的理解。