📚 IGCSE AQA Biology: Gene Expression | IGCSE AQA 生物：基因表达 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 protein. This fundamental mechanism determines an organism’s traits and functions. In IGCSE AQA Biology, you need to understand transcription, translation, and how mutations and the environment can affect gene expression.
基因表达是指基因中的编码信息被用于指导蛋白质合成的过程。这一基本机制决定了生物体的性状和功能。在 IGCSE AQA 生物课程中，你需要理解转录、翻译以及突变和环境如何影响基因表达。
1. What is Gene Expression? | 什么是基因表达？
Gene expression involves two main stages: transcription and translation. A gene is a segment of DNA that carries the code for a specific polypeptide. Not all genes are active at all times; cells regulate which genes are turned on or off.
基因表达包括两个主要阶段：转录和翻译。基因是携带特定多肽编码的 DNA 片段。并非所有基因都时刻保持活跃；细胞能够调控基因的开启与关闭。
2. The Genetic Code and Proteins | 遗传密码与蛋白质
The sequence of DNA bases (A, T, C, G) determines the order of amino acids in a protein. Every three bases, called a triplet or codon, specify one amino acid. There are 64 possible codons, but only 20 amino acids, so the code is degenerate. The genetic code is universal across almost all organisms.
DNA 碱基（A、T、C、G）的顺序决定了蛋白质中氨基酸的序列。每三个碱基（称为三联体或密码子）编码一种氨基酸。共有 64 种可能的密码子，但只有 20 种氨基酸，因此密码子具有简并性。遗传密码在几乎所有生物中都是通用的。
3. Transcription – DNA to mRNA | 转录——从 DNA 到 mRNA
Transcription occurs in the nucleus (in eukaryotes). An enzyme called RNA polymerase binds to a region of DNA ahead of the gene (promoter). The DNA helix unwinds and the enzyme moves along the template strand, adding complementary RNA nucleotides. RNA has uracil (U) instead of thymine (T), so A pairs with U, T with A, C with G, and G with C. The resulting pre‑mRNA is processed: introns (non‑coding) are removed and exons are spliced together; a 5′ cap and poly‑A tail are added. The mature mRNA then leaves the nucleus through a nuclear pore.
转录在真核细胞的细胞核中进行。RNA 聚合酶与基因前方的启动子区域结合。DNA 双螺旋解开，酶沿模板链移动，添加互补的 RNA 核苷酸。RNA 含有尿嘧啶（U）而不含胸腺嘧啶（T），因此 A 与 U 配对，T 与 A 配对，C 与 G 配对，G 与 C 配对。生成的 pre‑mRNA 经过加工：内含子（非编码区）被切除，外显子剪接；添加 5′ 端帽和 poly‑A 尾巴。成熟的 mRNA 随即通过核孔离开细胞核。
4. Translation – mRNA to Polypeptide | 翻译——从 mRNA 到多肽
Translation takes place on ribosomes in the cytoplasm. The ribosome binds to the mRNA at the start codon (AUG). Transfer RNA (tRNA) molecules carry specific amino acids and have an anticodon that base‑pairs with the complementary codon on mRNA. The ribosome moves along the mRNA, and tRNA molecules deliver amino acids in the correct sequence. Peptide bonds form between adjacent amino acids, creating a polypeptide chain. When a stop codon (UAA, UAG, UGA) is reached, translation terminates and the polypeptide is released.
翻译发生在细胞质的核糖体上。核糖体与 mRNA 上的起始密码子（AUG）结合。转运 RNA（tRNA）分子携带特定的氨基酸，并具有与 mRNA 密码子互补的反密码子。核糖体沿 mRNA 移动，tRNA 按正确序列运送氨基酸。相邻氨基酸之间形成肽键，生成多肽链。当遇到终止密码子（UAA、UAG、UGA）时，翻译终止，多肽被释放。
5. Protein Folding and Function | 蛋白质折叠与功能
The polypeptide chain folds into a specific three‑dimensional shape, determined by the sequence of amino acids. This shape is essential for the protein’s function, such as enzymes, antibodies, structural proteins, or hormones. Chaperone proteins assist in proper folding.
多肽链折叠成特定的三维形状，这取决于氨基酸的序列。这种形状对蛋白质的功能至关重要，例如酶、抗体、结构蛋白或激素。伴侣蛋白协助正确的折叠过程。
6. Control of Gene Expression | 基因表达的调控
In multicellular organisms, all cells contain the same DNA, but different genes are expressed in different cell types (cell differentiation). Transcription factors are proteins that bind to specific DNA sequences, either promoting or blocking the binding of RNA polymerase. For example, in the lac operon of bacteria, the presence of lactose triggers gene expression. In eukaryotes, environmental signals can activate transcription factors, turning genes on or off.
在多细胞生物中，所有细胞都包含相同的 DNA，但不同类型的细胞表达不同的基因（细胞分化）。转录因子是能够结合特定 DNA 序列的蛋白质，它们可促进或阻断 RNA 聚合酶的结合。例如，在细菌的乳糖操纵子中，乳糖的存在会触发基因表达。在真核生物中，环境信号可以激活转录因子，从而开启或关闭基因。
7. Mutations and Gene Expression | 突变与基因表达
A mutation is a change in the DNA base sequence. Substitution mutations may change a single amino acid (missense) or create a stop codon (nonsense), truncating the protein. Insertions or deletions cause a frameshift, altering all subsequent codons and usually producing a non‑functional protein. Mutations can be neutral, harmful, or occasionally beneficial. For instance, the sickle cell trait arises from a substitution that changes glutamic acid to valine in haemoglobin.
突变是指 DNA 碱基序列的改变。替换突变可能改变单个氨基酸（错义）或产生终止密码子（无义），导致蛋白质截短。插入或缺失会引起移码，改变后续所有密码子，通常产生无功能的蛋白质。突变可以是中性、有害或偶尔有益的。例如，镰状细胞性状源于一个替换突变，将血红蛋白中的谷氨酸变为缬氨酸。
8. Environmental Influences on Gene Expression | 环境对基因表达的影响
Environmental factors such as temperature, pH, UV radiation, and chemical mutagens can affect gene expression. For example, UV light can cause thymine dimers in DNA, leading to mutations that may trigger skin cancer. In plants, the expression of genes for chlorophyll synthesis is light‑dependent. Hormones can also regulate gene expression by acting as signals that activate transcription factors.
温度、pH、紫外线辐射和化学诱变剂等环境因素可影响基因表达。例如，紫外线可引起 DNA 中的胸腺嘧啶二聚体，导致突变，可能诱发皮肤癌。在植物中，叶绿素合成基因的表达是光依赖的。激素也能通过作为激活转录因子的信号来调节基因表达。
9. Comparing Transcription and Translation | 转录与翻译对比
The table below compares transcription and translation:
| Feature | Transcription | Translation |
|---|---|---|
| Location (eukaryotes) | Nucleus | Cytoplasm (ribosomes) |
| Template | DNA (template strand) | mRNA |
| Molecule produced | mRNA | Polypeptide/protein |
| Key players | RNA polymerase | Ribosomes, tRNA |
| Bond formed | Phosphodiester bonds | Peptide bonds |
下表比较了转录与翻译：转录在细胞核中进行，模板为 DNA 模板链，产物为 mRNA，主要参与者是 RNA 聚合酶，形成磷酸二酯键；翻译在细胞质核糖体上进行，模板为 mRNA，产物是多肽，参与者包括核糖体和 tRNA，形成肽键。
10. Exam Tips and Common Confusions | 考试技巧与常见误区
Students often confuse transcription and translation, or mix up DNA and mRNA bases. Remember: mRNA has U, not T. The template strand is read 3′ to 5′, but mRNA builds 5′ to 3′. Mutations in non‑coding DNA may have no effect. Use correct terminology: ‘transcription’ for DNA→mRNA, ‘translation’ for mRNA→polypeptide. In AQA papers, you may be asked to interpret a DNA sequence and determine the polypeptide sequence.
学生常混淆转录与翻译，或搞混 DNA 和 mRNA 的碱基。请记住：mRNA 含有 U，没有 T。模板链按 3′ 到 5′ 方向阅读，但 mRNA 是 5′ 到 3′ 合成。非编码 DNA 的突变可能无影响。使用正确术语：“转录”表示 DNA→mRNA，“翻译”表示 mRNA→多肽。在 AQA 考试中，你可能需要解读 DNA 序列并确定多肽序列。
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