Gene Expression in IGCSE Biology | IGCSE 生物：基因表达考点精讲

📚 Gene Expression in IGCSE Biology | IGCSE 生物：基因表达考点精讲

Gene expression is the fundamental process by which the information stored in our DNA is decoded to produce functional molecules, primarily proteins. Understanding this process is essential for IGCSE Biology, as it explains how traits are inherited, how cells specialise, and how organisms develop and function.

基因表达是将储存在 DNA 中的信息解码以产生功能性分子（主要是蛋白质）的基本过程。理解这一过程对于 IGCSE 生物至关重要，因为它解释了性状如何遗传、细胞如何特化以及生物体如何发育和运作。

1. Introduction to Gene Expression | 基因表达简介

A gene is a specific sequence of DNA nucleotides that carries the instructions for making a particular polypeptide or protein. Not all genes are active at all times; cells regulate which genes are expressed, allowing differentiation and response to the environment.

基因是 DNA 核苷酸的一段特定序列，携带着制造特定多肽或蛋白质的指令。并非所有基因都始终活跃；细胞会调控哪些基因被表达，从而实现分化和对环境作出反应。

Gene expression involves two main stages: transcription, where the DNA code is rewritten into messenger RNA (mRNA), and translation, where the mRNA code is read by ribosomes to assemble amino acids into a polypeptide chain. The final protein then folds into its functional shape.

基因表达包含两个主要阶段：转录（将 DNA 代码重写为信使 RNA，即 mRNA）和翻译（核糖体读取 mRNA 代码，将氨基酸组装成多肽链）。最终的蛋白质随后折叠成其功能性形态。

2. DNA Structure and the Gene | DNA 结构与基因

DNA is a double-stranded helix composed of nucleotides. Each nucleotide contains a deoxyribose sugar, a phosphate group, and one of four nitrogenous bases: adenine (A), thymine (T), cytosine (C), and guanine (G). The two strands are held together by hydrogen bonds between complementary base pairs, with A pairing with T and C pairing with G.

DNA 是由核苷酸组成的双链螺旋。每个核苷酸包含一个脱氧核糖、一个磷酸基团以及四种含氮碱基之一：腺嘌呤 (A)、胸腺嘧啶 (T)、胞嘧啶 (C) 和鸟嘌呤 (G)。两条链通过互补碱基对之间的氢键连接在一起，A 与 T 配对，C 与 G 配对。

A gene occupies a specific locus on a chromosome and consists of a unique sequence of these bases. The order of bases along the gene is the genetic code that determines the amino acid sequence of a protein. Sections of DNA that do not code for proteins may have regulatory functions or may be transcribed into other types of RNA.

基因在染色体上占据一个特定的基因座，并由这些碱基的独特序列构成。基因上碱基的排列顺序就是决定蛋白质氨基酸序列的遗传密码。那些不编码蛋白质的 DNA 片段可能具有调控功能，或者被转录为其他类型的 RNA。

3. The Genetic Code | 遗传密码

The genetic code is read in groups of three adjacent bases called codons. Each codon corresponds to one specific amino acid or a stop signal. For example, the codon AUG codes for the amino acid methionine and also serves as a start signal for translation.

遗传密码以三个相邻碱基为一组进行读取，这样的组合称为密码子。每个密码子对应一种特定的氨基酸或一个终止信号。例如，密码子 AUG 编码甲硫氨酸，同时也作为翻译的起始信号。

The code is degenerate because most amino acids are specified by more than one codon. For instance, the codons GUU, GUC, GUA, and GUG all code for valine. The genetic code is nearly universal across all living organisms, indicating a common evolutionary origin.

密码具有简并性，因为大多数氨基酸都有多个密码子来指定。例如，GUU、GUC、GUA 和 GUG 这四个密码子都编码缬氨酸。遗传密码在几乎所有生物体中都是通用的，这表明它们有着共同的进化起源。

Codon (mRNA)	Amino Acid
AUG	Methionine (Start)
UUU, UUC	Phenylalanine
UAA, UAG, UGA	Stop

Table: Examples of mRNA codons and their corresponding amino acids.

表：mRNA 密码子及其对应氨基酸的示例。

4. Transcription: Copying the Code | 转录：复制密码

Transcription takes place in the nucleus. The enzyme RNA polymerase binds to the DNA at the start of a gene, unwinding the double helix. It then uses one of the DNA strands as a template to synthesise a complementary strand of messenger RNA (mRNA).

转录发生在细胞核内。RNA 聚合酶在基因起始处与 DNA 结合，解开双螺旋。然后它以其中一条 DNA 链为模板，合成互补的信使 RNA (mRNA) 链。

During synthesis, complementary base pairing rules apply with a key difference: in RNA, uracil (U) replaces thymine (T). Therefore, where the DNA template strand has an A, RNA polymerase adds a U; where DNA has T, it adds A; C pairs with G, and G pairs with C. This produces an mRNA molecule that is a copy of the coding strand of DNA, except with U instead of T.

合成过程中遵循碱基互补配对原则，但有一个关键区别：在 RNA 中，尿嘧啶 (U) 替代了胸腺嘧啶 (T)。因此，当 DNA 模板链上有 A 时，RNA 聚合酶会添加 U；当 DNA 上有 T 时，则添加 A；C 与 G 配对，G 与 C 配对。这样产生的 mRNA 分子就是 DNA 编码链的副本，只是用 U 取代了 T。

DNA template: 3′ TAC GGA TCA 5′

mRNA transcript: 5′ AUG CCU AGU 3′

5. Translation: Building the Protein | 翻译：构建蛋白质

Translation occurs at ribosomes in the cytoplasm. The mRNA strand binds to a ribosome, and the ribosome reads the codons one by one. Transfer RNA (tRNA) molecules, each carrying a specific amino acid, have an anticodon that is complementary to an mRNA codon.

翻译发生在细胞质中的核糖体上。mRNA 链与核糖体结合，核糖体逐次读取密码子。转运 RNA (tRNA) 分子各携带一个特定的氨基酸，其反密码子与 mRNA 的一个密码子互补。

As each codon is exposed, a matching tRNA molecule binds to it via base pairing between codon and anticodon. The ribosome catalyses the formation of a peptide bond between the amino acid brought by the incoming tRNA and the growing polypeptide chain. The ribosome then moves to the next codon, and the process repeats until a stop codon is reached, at which point the polypeptide is released.

每当一个密码子暴露出来，与之匹配的 tRNA 分子就会通过密码子和反密码子之间的碱基配对与之结合。核糖体催化新进入的 tRNA 所携带的氨基酸与正在延长的多肽链之间形成肽键。随后核糖体移动到下一个密码子，重复此过程，直至遇到终止密码子，此时多肽链被释放。

Multiple ribosomes can translate a single mRNA simultaneously, forming a structure called a polyribosome. This allows rapid production of many copies of the protein.

多个核糖体可以同时翻译一条 mRNA，形成称为多聚核糖体的结构。这使得细胞能够快速合成出大量蛋白质副本。

6. 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 is transcribed into RNA, and RNA is translated into protein. This directional flow from DNA → RNA → protein is the basis of gene expression.

中心法则描述了遗传信息在生物系统内的流动方向：DNA 被转录为 RNA，RNA 再被翻译为蛋白质。这种从 DNA → RNA → 蛋白质的定向流动是基因表达的基础。

While the central dogma is largely accurate, there are exceptions, such as reverse transcription in retroviruses, where RNA is used as a template to make DNA. However, for IGCSE, focus on the main pathway: genes in DNA provide the code to produce specific proteins.

虽然中心法则大体正确，但也存在例外，例如逆转录病毒中的逆转录过程，即用 RNA 作为模板来合成 DNA。但在 IGCSE 阶段，重点在于主要途径：DNA 中的基因提供编码来制造特定的蛋白质。

7. Mutations: Changes in the DNA | 突变：DNA 的改变

A mutation is a random change in the sequence of bases in DNA. Mutations can occur spontaneously during DNA replication or be induced by mutagens such as chemicals or radiation. They are the ultimate source of genetic variation.

突变是指 DNA 碱基序列发生的随机改变。突变可以在 DNA 复制过程中自发产生，也可以由化学物质或辐射等诱变剂诱发。它们是遗传变异的最终来源。

Substitution mutations: a single base is replaced by another. This may change only one codon, resulting in a different amino acid (missense), a stop codon (nonsense), or no change due to the degeneracy of the genetic code (silent mutation).

替换突变：一个碱基被另一个替换。这可能只改变一个密码子，导致不同氨基酸（错义）、产生终止密码子（无义），或因密码简并性而无任何变化（沉默突变）。
Insertion or deletion mutations: one or more bases are added or removed. This causes a frameshift, altering the reading frame of all subsequent codons, which usually results in a completely different and non-functional protein.

插入或缺失突变：添加或删除一个或多个碱基。这会引起移码，改变其后的所有密码子的读框，通常导致产生完全不同的、无功能的蛋白质。

Mutations can be harmful, neutral, or beneficial. Beneficial mutations may give an organism a survival advantage and are subject to natural selection.

突变可能是有害的、中性的或有益的。有益的突变可能会给生物体带来生存优势，并受到自然选择的作用。

8. From Genotype to Phenotype | 从基因型到表现型

The genotype is the genetic makeup of an organism, represented by the combination of alleles it possesses for a particular gene. The phenotype is the observable characteristic resulting from the expression of the genotype and its interaction with the environment.

基因型是指一个生物体的遗传组成，由其所拥有的特定基因的等位基因组合来表示。表现型则是基因型表达及其与环境相互作用所导致的、可观察到的特征。

Different alleles arise through mutations and can be dominant or recessive. A dominant allele is expressed in the phenotype even if only one copy is present, while a recessive allele is only expressed when two copies are present (homozygous recessive). For example, in pea plants, the allele for tall stems (T) is dominant over the allele for short stems (t).

不同的等位基因通过突变产生，可以是显性或隐性。显性等位基因即便只有一个拷贝也会在表现型中表达，而隐性等位基因只有在两个拷贝都存在（纯合隐性）时才表达。例如，在豌豆植株中，高茎等位基因 (T) 对矮茎等位基因 (t) 为显性。

Gene expression determines the protein products that shape the phenotype, but the environment can also influence how genes are expressed, leading to variation in the phenotype.

基因表达决定了塑造表现型的蛋白质产物，但环境也能影响基因的表达方式，从而导致表现型的变异。

9. Gene Expression and the Environment | 基因表达与环境

Environmental factors can affect gene expression without changing the DNA sequence itself. Temperature, light, nutrition, and the presence of certain chemicals can switch genes on or off, influencing the phenotype.

环境因素可以在不改变 DNA 序列本身的情况下影响基因表达。温度、光照、营养以及某些化学物质的存在都可以开启或关闭基因，进而影响表现型。

A classic example is the fur colour of the Himalayan rabbit. At lower temperatures, the extremities produce dark fur because the enzyme responsible for pigment production is active only in cooler regions. This illustrates how the same genotype can produce different phenotypes depending on environmental conditions.

一个经典的例子是喜马拉雅兔的毛色。在较低温度下，其四肢末端会产生深色毛发，因为负责色素生成的酶只在较冷区域有活性。这说明了相同的基因型如何根据环境条件产生不同的表现型。

In humans, identical twins share the same genotype but may develop slight differences in phenotype due to variations in diet, lifestyle, and exposure to environmental agents, demonstrating the interplay between genes and the environment.

在人类中，同卵双胞胎拥有相同的基因型，但由于饮食、生活方式和接触环境物质的差异，可能会在表现型上出现细微差别，这体现了基因与环境之间的相互作用。

10. Exam Tips and Common Mistakes | 考试技巧与常见错误

When describing transcription, always state that it produces mRNA and occurs in the nucleus. Use ‘uracil’ instead of ‘thymine’ for RNA.

描述转录时，务必说明它产生的是 mRNA 且发生在细胞核中。使用 RNA 的‘尿嘧啶’而非‘胸腺嘧啶’。
In translation, highlight the role of ribosomes, tRNA anticodons, and the formation of peptide bonds. Do not confuse codons with anticodons.

在翻译中，要强调核糖体、tRNA 反密码子以及肽键形成的作用。不要混淆密码子和反密码子。
When discussing mutations, distinguish between substitution and frameshift mutations and their likely effects on protein structure.

讨论突变时，要区分替换突变和移码突变及其对蛋白质结构可能造成的影响。
Always link gene expression to phenotype, and be prepared to explain how environmental factors can modify the expression of a gene.

始终将基因表达与表现型联系起来，并准备好解释环境因素如何改变基因的表达。
Use clear terminology: ‘allele’ for variant forms of a gene, ‘homozygous’ and ‘heterozygous’ for genotype description, and ‘dominant’ or ‘recessive’ for expression patterns.

使用清晰的术语：基因的不同形式用‘等位基因’，基因型描述用‘纯合’和‘杂合’，表达模式用‘显性’和‘隐性’。

Almost all marks for gene expression questions come from sequencing the process correctly, so practise drawing flow diagrams and labelling the key molecules involved.

基因表达类题目的得分点几乎都来自能否正确排定过程顺序，因此要多练习绘制流程图并标注所涉及的关键分子。

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Gene Expression in IGCSE Biology | IGCSE 生物：基因表达 考点精讲