Gene Expression in AQA A-Level Biology | A-Level AQA 生物：基因表达考点精讲

📚 Gene Expression in AQA A-Level Biology | A-Level AQA 生物：基因表达考点精讲

Gene expression is the process by which the genetic code in DNA is used to synthesise functional gene products, mainly proteins or non-coding RNA molecules. It involves two major stages – transcription and translation – and is tightly regulated in both prokaryotes and eukaryotes. In AQA A-Level Biology, understanding the mechanisms of gene expression, mutations that alter it, and how cells control which genes are switched on or off is essential for explaining everything from cell specialisation to the development of cancer.

基因表达是指利用DNA中的遗传密码合成功能性基因产物（主要是蛋白质或非编码RNA分子）的过程。它包含转录和翻译两个主要阶段，并且在原核生物和真核生物中都受到严格的调控。在AQA A-Level生物学中，理解基因表达的机制、改变基因表达的突变以及细胞如何调控基因的开启或关闭，对于解释从细胞特化到癌症发生的各种生命现象至关重要。

1. Overview of Gene Expression | 基因表达概述

The central dogma of molecular biology states that information flows from DNA to RNA to protein. A gene is a sequence of DNA nucleotides that codes for a polypeptide or a functional RNA. Not all genes are expressed in every cell; differential gene expression allows cells to become specialised. The two key steps are transcription (making mRNA from a DNA template) and translation (using mRNA to assemble a polypeptide at the ribosome).

分子生物学的中心法则指出，信息从DNA流向RNA再到蛋白质。基因是一段编码多肽或功能性RNA的DNA核苷酸序列。并非所有基因都在每个细胞中表达；差异性基因表达使细胞得以特化。两个关键步骤是转录（以DNA为模板合成mRNA）和翻译（在核糖体上利用mRNA组装多肽）。

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

Transcription begins when RNA polymerase binds to a specific DNA sequence called the promoter, located just upstream of the gene. The enzyme unwinds the DNA double helix and uses one strand – the template strand – to build a complementary pre‑mRNA molecule. RNA polymerase adds RNA nucleotides according to base‑pairing rules (A with U, C with G) and forms phosphodiester bonds between them. Transcription stops when the polymerase reaches a terminator sequence. In prokaryotes, the mRNA produced is immediately ready for translation; in eukaryotes, the transcript must be processed first.

转录始于RNA聚合酶与被称为启动子的特定DNA序列结合，启动子位于基因的上游。该酶解开DNA双螺旋，并以其中一条链——模板链——为模板构建互补的前体mRNA分子。RNA聚合酶按照碱基配对原则（A与U配对，C与G配对）添加RNA核苷酸，并在其间形成磷酸二酯键。当聚合酶到达终止子序列时转录停止。在原核生物中，产生的mRNA可直接用于翻译；而在真核生物中，转录产物需要先进行加工。

3. Post-Transcriptional Modification in Eukaryotes | 真核生物的转录后修饰

Eukaryotic pre‑mRNA undergoes three main processing steps before it leaves the nucleus: capping, polyadenylation and splicing. A modified guanine nucleotide (7‑methylguanosine) is added to the 5′ end, forming a 5′ cap that protects the mRNA and aids ribosome binding. At the 3′ end, an enzyme adds a poly(A) tail of around 200 adenine nucleotides, which enhances stability and facilitates export. Splicing removes non‑coding introns and joins together the coding exons; this is carried out by a spliceosome, a complex of small nuclear ribonucleoproteins (snRNPs). Alternative splicing allows a single gene to produce multiple different polypeptides.

真核生物的前体mRNA在离开细胞核前需经历三个主要加工步骤：加帽、加尾和剪接。一个经过修饰的鸟嘌呤核苷酸（7‑甲基鸟苷）被添加到5’端，形成5’帽结构，以保护mRNA并协助核糖体结合。在3’端，一种酶添加大约200个腺嘌呤核苷酸组成的poly(A)尾，这增强了mRNA的稳定性并促进其出核。剪接过程切除非编码的内含子，并将编码的外显子连接在一起；这是由剪接体——一种小核核糖核蛋白（snRNP）复合物——完成的。可变剪接使得一个基因能够产生多种不同的多肽。

4. Translation: Building a Polypeptide | 翻译：构建多肽

Translation takes place on ribosomes in the cytoplasm. The mRNA attaches to the small ribosomal subunit, and a transfer RNA (tRNA) molecule carrying the amino acid methionine binds to the start codon (AUG) via its complementary anticodon. The large subunit then joins, forming a functional ribosome. The ribosome moves along the mRNA in the 5′ to 3′ direction; each new codon is matched with a tRNA that carries the corresponding amino acid. Amino acids are joined by peptide bonds, and the growing polypeptide chain is eventually released when a stop codon (UAA, UAG or UGA) is reached. Multiple ribosomes often translate a single mRNA simultaneously, forming a polysome.

翻译在细胞质的核糖体上进行。mRNA与核糖体小亚基结合，一个携带甲硫氨酸的转运RNA（tRNA）分子通过其互补的反密码子与起始密码子AUG结合。随后大亚基加入，形成功能性的核糖体。核糖体沿mRNA的5’到3’方向移动；每一个新的密码子都与一个携带相应氨基酸的tRNA配对。氨基酸通过肽键连接，当到达终止密码子（UAA、UAG或UGA）时，不断延长的多肽链最终被释放。多个核糖体常同时翻译一条mRNA，形成多聚核糖体。

5. Understanding the Genetic Code | 理解遗传密码

The genetic code is the set of rules by which the sequence of bases in mRNA is translated into a sequence of amino acids. The code is read in triplets, or codons, each specifying one amino acid. It is degenerate (most amino acids are coded for by more than one codon), non‑overlapping and universal across almost all organisms. The start codon AUG also codes for methionine, while three stop codons signal the end of translation. Because of degeneracy, some point mutations may not change the amino acid sequence – these are called silent mutations.

遗传密码是一套规则，将mRNA中的碱基序列翻译为氨基酸序列。密码以三联体（即密码子）的形式阅读，每个密码子对应一种氨基酸。遗传密码具有简并性（大多数氨基酸由不止一个密码子编码）、不重叠性，并且在几乎所有生物中都是通用的。起始密码子AUG同时也编码甲硫氨酸，而三个终止密码子标志着翻译的结束。由于简并性，某些点突变可能不会改变氨基酸序列，这些突变称为沉默突变。

6. Types of Gene Mutations | 基因突变类型

A gene mutation is a change in the nucleotide sequence of DNA. Substitution mutations replace one base with another; they may be silent, missense (resulting in a different amino acid) or nonsense (introducing a premature stop codon). Insertions or deletions of nucleotides cause a frameshift if the number added or removed is not a multiple of three, altering every codon downstream and usually producing a non‑functional protein. Mutagens such as ionising radiation, chemicals and viruses increase the rate of mutation. Mutations are the ultimate source of genetic variation but can also lead to genetic disease or cancer.

基因突变是指DNA核苷酸序列的改变。替换突变将一个碱基替换为另一个；它们可能是沉默的、错义的（导致不同的氨基酸）或无义的（提前引入终止密码子）。核苷酸的插入或缺失如果添加或移除的数量不是3的倍数，就会引起移码突变，改变下游的每一个密码子，通常产生无功能的蛋白质。电离辐射、化学物质和病毒等诱变剂会增加突变频率。突变是遗传变异的最终来源，但也可能导致遗传病或癌症。

7. Prokaryotic Gene Regulation: The Lac Operon | 原核生物基因调控：乳糖操纵子

In prokaryotes, genes with related functions are often organised into operons. The lac operon in E. coli contains three structural genes (lacZ, lacY and lacA) needed to metabolise lactose. A promoter region allows RNA polymerase to bind, while an operator sequence controls access. When lactose is absent, a repressor protein binds to the operator, blocking transcription. When lactose is present, it is converted to allolactose, which binds to the repressor, changing its shape so it can no longer bind the operator. This allows RNA polymerase to transcribe the structural genes. The operon is also regulated by cAMP‑CRP in response to glucose levels.

在原核生物中，功能相关的基因通常组织成操纵子。大肠杆菌的乳糖操纵子包含三个代谢乳糖所需的结构基因（lacZ、lacY和lacA）。启动子区域允许RNA聚合酶结合，而操纵基因序列控制着可及性。当乳糖不存在时，阻遏蛋白与操纵基因结合，阻断转录。当乳糖存在时，它被转化为异乳糖，异乳糖与阻遏蛋白结合，改变其形状，使其无法再结合操纵基因。这使RNA聚合酶能够转录结构基因。该操纵子还受cAMP‑CRP根据葡萄糖水平进行调控。

8. Eukaryotic Gene Regulation: Transcription Factors | 真核生物基因调控：转录因子

Eukaryotic gene expression is controlled largely at the level of transcription by transcription factors – proteins that bind to specific DNA sequences near the promoter or at distant enhancer regions. Activator proteins help RNA polymerase form a transcription initiation complex, while repressor proteins block the process. Some hormones, such as steroid hormones, enter the cell, bind to intracellular receptors and the hormone‑receptor complex then acts as a transcription factor, directly regulating target genes. The combinatorial control of multiple transcription factors allows fine‑tuned, tissue‑specific gene expression.

真核生物的基因表达在很大程度上是通过转录因子在转录水平上进行调控的，转录因子是能结合到启动子附近或远处增强子区域特定DNA序列的蛋白质。激活蛋白帮助RNA聚合酶形成转录起始复合物，而抑制蛋白则阻断这一过程。某些激素（如类固醇激素）进入细胞后，与胞内受体结合，激素‑受体复合物随后充当转录因子，直接调控靶基因。多种转录因子的组合调控使得基因表达能够精细地、组织特异性地进行。

9. Epigenetics: Beyond the DNA Sequence | 表观遗传学：超越DNA序列

Epigenetic modifications are heritable changes in gene expression that do not involve alterations to the DNA base sequence. Increased methylation of cytosine bases in CpG islands of promoter regions generally represses transcription by preventing transcription factor binding. Histone modification, such as acetylation, reduces the positive charge on histone tails, loosening their grip on DNA and making the chromatin more accessible (euchromatin). This promotes transcription. Deacetylation has the opposite effect, condensing chromatin (heterochromatin) and silencing genes. Epigenetic marks can be influenced by environmental factors and are linked to development and diseases like cancer.

表观遗传修饰是指不涉及DNA碱基序列改变的可遗传的基因表达变化。启动子区域CpG岛中胞嘧啶甲基化程度的增加通常通过阻止转录因子结合来抑制转录。组蛋白修饰，例如乙酰化，会降低组蛋白尾部的正电荷，减弱其对DNA的抓持，使染色质更易接近（常染色质），这促进转录。去乙酰化则产生相反效果，使染色质凝缩（异染色质）并沉默基因。表观遗传标记可受环境因素影响，并与发育以及癌症等疾病有关。

10. RNA Interference (RNAi) | RNA干扰

RNA interference is a post‑transcriptional mechanism that regulates gene expression using small RNA molecules. Small interfering RNA (siRNA) is formed when double‑stranded RNA is cut by the enzyme Dicer into fragments of 21–23 nucleotides. One strand of the siRNA is loaded onto a RISC (RNA‑induced silencing complex) and guides it to a complementary mRNA target. RISC then cleaves the mRNA, preventing translation. MicroRNAs (miRNAs) are similarly processed and typically inhibit translation by binding with imperfect complementarity to target mRNAs. This pathway is important in defending against viruses and controlling development.

RNA干扰是一种利用小RNA分子在转录后水平调控基因表达的机制。当双链RNA被Dicer酶切割成21–23个核苷酸的片段时，就会形成小干扰RNA（siRNA）。siRNA的一条链被装载到RISC复合物（RNA诱导的沉默复合体）上，并引导该复合物与互补的mRNA靶标结合。随后RISC切割mRNA，从而阻止翻译。微小RNA（miRNA）经过类似的加工过程，通常通过与靶mRNA不完全互补结合来抑制翻译。该通路在防御病毒和调控发育中发挥重要作用。

11. Cancer and Gene Expression | 癌症与基因表达

Cancer arises when the normal controls on cell division and apoptosis break down. Mutations in proto‑oncogenes can convert them into oncogenes, which are permanently activated and promote excessive cell division even in the absence of growth signals. For example, a mutation in the ras gene can leave the Ras protein permanently switched on. Tumour suppressor genes, such as p53, normally inhibit cell cycle progression or trigger apoptosis; loss‑of‑function mutations in both alleles disable these brakes. Chronic exposure to carcinogens and epigenetic silencing of tumour suppressor genes can both lead to uncontrolled proliferation and tumour formation.

当细胞分裂和凋亡的正常控制被打破时，癌症就会发生。原癌基因的突变可将其转化为癌基因，癌基因被永久激活，即使在缺乏生长信号的情况下也能促进过度的细胞分裂。例如，ras基因的突变可使Ras蛋白持续处于激活状态。抑癌基因（如p53）通常抑制细胞周期进程或触发凋亡；两个等位基因都发生功能丧失性突变会使这些制动机制失效。长期接触致癌物和抑癌基因的表观遗传沉默均可导致失控的增殖和肿瘤形成。

12. Stem Cells and Differential Gene Expression | 干细胞与差异性基因表达

Stem cells are undifferentiated cells capable of both self‑renewal and differentiation into specialised cell types. In embryonic stem cells, expression of certain transcription factors (such as Oct4, Sox2 and Nanog) maintains pluripotency. As development proceeds, signalling molecules trigger changes in gene expression that commit a cell to a specific lineage. Adult stem cells are multipotent and play a key role in tissue repair. Scientists can reprogramme differentiated cells into induced pluripotent stem cells (iPSCs) by introducing genes for specific transcription factors, demonstrating that differential gene expression, rather than irreversible DNA changes, underlies cellular specialisation.

干细胞是未分化的细胞，既能自我更新，又能分化成特化的细胞类型。在胚胎干细胞中，某些转录因子（如Oct4、Sox2和Nanog）的表达可维持其多能性。随着发育的进行，信号分子触发基因表达的变化，使细胞定向于特定的谱系。成体干细胞是多能的，在组织修复中发挥关键作用。科学家通过引入特定转录因子的基因，可将已分化的细胞重编程为诱导多能干细胞（iPSC），这表明细胞特化的基础是差异性基因表达，而非不可逆的DNA改变。

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Gene Expression in AQA A-Level Biology | A-Level AQA 生物：基因表达 考点精讲