Gene Mutations in A-Level WJEC Biology: Key Points Explained | A-Level WJEC 生物：基因突变考点精讲

📚 Gene Mutations in A-Level WJEC Biology: Key Points Explained | A-Level WJEC 生物：基因突变考点精讲

Gene mutations are permanent changes in the DNA sequence that can alter protein synthesis and lead to new traits or disorders. This article covers the essential WJEC A‑Level Biology specification points, from the types of point mutations to their role in disease and evolution, with a strong focus on exam‑ready explanations.

基因突变是DNA序列中永久性的变化，可能改变蛋白质合成并导致新的性状或疾病。本文紧扣WJEC A‑Level生物考试大纲，涵盖从点突变类型到突变在疾病和进化中的作用等核心考点，并提供应试导向的清晰解释。

1. What Is a Gene Mutation? | 什么是基因突变？

A gene mutation is any change in the nucleotide sequence of a gene. It may involve a single base pair or larger segments of DNA. Mutations can arise spontaneously during DNA replication or be induced by mutagens such as chemicals or radiation. In WJEC, you need to distinguish gene mutations from chromosome mutations, which involve whole chromosomes or large segments.

基因突变是指基因中核苷酸序列的任何改变，可能涉及单个碱基对或更大DNA片段。突变可在DNA复制过程中自发产生，也可由化学物质或辐射等诱变剂诱导发生。在WJEC考试中，需要区分基因突变与染色体突变，后者涉及整条染色体或大片段。

2. Types of Point Mutations | 点突变的类型

A point mutation affects a single nucleotide. The three basic types are substitution, insertion, and deletion. Substitution replaces one base with another; insertion adds an extra base; deletion removes a base. All three can alter the genetic code, but their consequences vary dramatically.

点突变影响单个核苷酸。三种基本类型是替换、插入和缺失。替换是用一个碱基替换另一个；插入增加额外碱基；缺失移除一个碱基。三者都可能改变遗传密码，但后果差异很大。

3. Substitution and Its Outcomes | 替换及其后果

In a substitution mutation, one nucleotide is swapped for another. Depending on the new codon, the effect can be silent (same amino acid), missense (different amino acid), or nonsense (a stop codon). For example, the sickle‑cell mutation is a missense substitution where GAG becomes GTG, changing glutamate to valine.

在替换突变中，一个核苷酸被另一个替换。根据新密码子，效果可能是沉默（相同氨基酸）、错义（不同氨基酸）或无义（终止密码子）。例如，镰刀型细胞突变就是一种错义替换，GAG变成GTG，将谷氨酸变为缬氨酸。

4. Insertion and Deletion: Frameshift Mutations | 插入与缺失：移码突变

Insertions or deletions that are not multiples of three cause a frameshift. The ribosome reads the mRNA in triplets, and shifting the reading frame changes every codon downstream. This usually produces a completely different amino acid sequence and an early stop codon, rendering the protein non‑functional. Frameshift mutations are therefore far more disruptive than most substitutions.

不是3的倍数的插入或缺失会导致移码。核糖体以三联体方式阅读mRNA，阅读框的移动会改变下游每一个密码子。这通常产生完全不同的氨基酸序列和提前终止密码子，使蛋白质丧失功能。因此，移码突变的破坏性远大于大多数替换。

5. Silent, Missense and Nonsense Mutations | 沉默突变、错义突变和无义突变

Silent mutations do not change the amino acid because the genetic code is degenerate. Missense mutations code for a different amino acid, which may alter protein structure and function. Nonsense mutations introduce a premature STOP codon, leading to a truncated, usually inactive protein. Exam questions often ask you to predict the outcome from a given codon table.

由于遗传密码的简并性，沉默突变不改变氨基酸。错义突变编码不同的氨基酸，可能改变蛋白质结构和功能。无义突变引入提前终止密码子，导致截短的、通常无活性的蛋白质。考试题经常要求你根据给定的密码子表预测结果。

6. Mutagens: Physical, Chemical and Biological Agents | 诱变剂：物理、化学和生物因素

Mutagens increase the rate of mutation above the spontaneous background level. Physical mutagens include ionising radiation (X‑rays, gamma rays) and ultraviolet light, which can cause thymine dimers. Chemical mutagens such as benzopyrene in tobacco smoke or nitrous acid can directly alter base structures. Biological mutagens include certain viruses that insert their DNA into the host genome, disrupting normal genes.

诱变剂使突变率高于自发背景水平。物理诱变剂包括电离辐射（X射线、γ射线）和紫外线，紫外线可引起胸腺嘧啶二聚体。化学诱变剂如烟草烟雾中的苯并芘或亚硝酸可直接改变碱基结构。生物诱变剂包括某些病毒，它们将DNA插入宿主基因组，破坏正常基因。

7. Consequences of Mutations: Sickle Cell Anaemia | 突变的后果：镰刀型细胞贫血症

Sickle cell anaemia is a classic WJEC example. A substitution mutation in the HBB gene converts the codon GAG (Glu) to GTG (Val). This single amino acid change causes haemoglobin molecules to polymerise under low oxygen, deforming red blood cells into a sickle shape. The sickle cells block capillaries, causing pain and organ damage. Heterozygous carriers have some protection against malaria, which illustrates heterozygote advantage.

镰刀型细胞贫血症是WJEC的经典例子。HBB基因中的一个替换突变将密码子GAG（谷氨酸）变为GTG（缬氨酸）。单个氨基酸的改变导致血红蛋白分子在低氧条件下聚合，使红细胞变形为镰刀状。镰状细胞堵塞毛细血管，引起疼痛和器官损伤。杂合子携带者对疟疾有一定防护，体现了杂合子优势。

8. Mutations and Protein Structure | 突变与蛋白质结构

A protein’s primary structure determines its secondary, tertiary and quaternary folding. A single amino acid substitution can disrupt hydrogen bonds, ionic bonds or hydrophobic interactions, altering the tertiary structure. In enzymes, a change near the active site can destroy catalytic activity. Some mutations, however, may be neutral if the amino acid substitution occurs in a non‑critical region or the new amino acid has similar properties.

蛋白质的一级结构决定其二、三、四级折叠。单个氨基酸替换可能破坏氢键、离子键或疏水相互作用，改变三级结构。在酶中，活性中心附近的变化可能摧毁催化活性。然而，如果氨基酸替换发生在非关键区域，或者新氨基酸具有相似性质，有些突变可能是中性的。

9. Gene Mutations and Genetic Variation | 基因突变与遗传变异

Mutations are the ultimate source of new alleles in a population. Most mutations are harmful or neutral, but occasionally a mutation confers a selective advantage. This genetic variation is the raw material for natural selection and evolution. WJEC expects you to link mutation to the production of different proteins and therefore new phenotypes.

突变是种群中新等位基因的最终来源。大多数突变有害或中性，但偶尔有突变会带来选择性优势。这种遗传变异是自然选择和进化的原始材料。WJEC要求你将突变与不同蛋白质的产生以及因此出现的新表型联系起来。

10. Repair Mechanisms and the Control of Mutations | 修复机制与突变的控制

Cells have several DNA repair mechanisms, such as base excision repair and mismatch repair, which correct errors before they become permanent. Enzymes like DNA polymerase also proofread during replication. However, when repair systems fail or are overwhelmed by mutagens, mutations persist. The balance between mutation and repair is essential for genomic stability.

细胞有多种DNA修复机制，如碱基切除修复和错配修复，可在错误成为永久之前进行纠正。像DNA聚合酶这样的酶在复制过程中也会校对。但当修复系统失效或被诱变剂压垮时，突变就会留存。突变与修复之间的平衡对基因组稳定性至关重要。

11. Exam Tips: Interpreting Mutation Questions | 应试技巧：解答突变题目

WJEC papers often provide a DNA or mRNA sequence and ask you to determine the type of mutation and its effect. Always use the codon table provided. Check if the mutation is a substitution, insertion or deletion. For frameshifts, write out the new sequence of codons to identify early stop codons. Practise explaining the effect on protein structure using key terms like primary structure, tertiary structure, and active site.

WJEC试卷常给出DNA或mRNA序列，要求你判断突变类型及其影响。务必使用提供的密码子表。检查突变是替换、插入还是缺失。对于移码，写出新的密码子序列以识别提前终止密码子。练习运用一级结构、三级结构和活性中心等关键术语解释对蛋白质结构的影响。

12. Summary and Key Points to Remember | 总结与记忆要点

Gene mutations are permanent changes in DNA base sequence. Point mutations include substitution, insertion and deletion. Substitutions can be silent, missense or nonsense; insertions and deletions often cause frameshifts. Mutagens raise mutation rates. Sickle cell anaemia is a missense mutation example with heterozygote advantage. Mutations create genetic variation and can be harmful, neutral or beneficial. Be ready to interpret sequences and predict outcomes.

基因突变是DNA碱基序列的永久改变。点突变包括替换、插入和缺失。替换可能是沉默、错义或无义；插入和缺失常导致移码。诱变剂提高突变率。镰刀型细胞贫血症是一个具有杂合子优势的错义突变例子。突变创造遗传变异，可能有害、中性或有利。做好解读序列并预测结果的准备。

Published by TutorHao | Biology Revision Series | aleveler.com

更多咨询请联系16621398022（同微信）

Gene Mutations in A-Level WJEC Biology: Key Points Explained | A-Level WJEC 生物：基因突变 考点精讲