IGCSE WJEC Biology: Gene Mutations Exam Focus | IGCSE WJEC 生物：基因突变考点精讲

📚 IGCSE WJEC Biology: Gene Mutations Exam Focus | IGCSE WJEC 生物：基因突变考点精讲

A gene mutation is a permanent change in the DNA base sequence of a gene. In WJEC IGCSE Biology, understanding mutations is essential: they can alter protein structure and function, sometimes causing disease, but also providing the raw material for evolution. This article breaks down the key concepts, types, consequences, and exam-relevant case studies you need to know.

基因突变是指基因的 DNA 碱基序列发生了永久性改变。在 WJEC IGCSE 生物考试中，理解突变是至关重要的：它们可能改变蛋白质的结构与功能，有时引起疾病，但也为进化提供了原始材料。本文梳理了你需要掌握的核心概念、类型、后果以及考试相关的案例研究。

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

A gene mutation is a random and permanent change in the nucleotide sequence of a gene. This can involve a single nucleotide or a larger segment of DNA. Mutations occur spontaneously during DNA replication, but the rate can be increased by mutagens. Mutations can be inherited if they occur in gametes (sex cells).

基因突变是基因的核苷酸序列发生的随机且永久的改变。这可能涉及单个核苷酸，也可能涉及一段较长的 DNA 片段。突变在 DNA 复制过程中会自发产生，但诱变剂会提高突变频率。如果突变发生在配子（生殖细胞）中，则可以遗传给后代。

In IGCSE, you need to know that the order of bases in DNA codes for the order of amino acids in a protein. Therefore, a change in the DNA base sequence may lead to a different amino acid being inserted into a polypeptide chain, altering the shape and function of the final protein.

在 IGCSE 中，你需要明白 DNA 中的碱基顺序决定了蛋白质中氨基酸的顺序。因此，DNA 碱基序列的改变可能导致多肽链中插入一个不同的氨基酸，从而改变最终蛋白质的形状与功能。

Key definition for WJEC: ‘A gene mutation is a change in the sequence of bases in DNA.’ This simple definition is often tested in the early parts of a question.

WJEC 考试中经常考查的关键定义是：“基因突变是指 DNA 中碱基序列的改变。”这个简单的定义常出现在题目的前几小问中。

2. Types of Gene Mutations | 基因突变的类型

Gene mutations can be classified based on how the DNA sequence is altered. The main types tested in WJEC are substitution, insertion, and deletion mutations. These are small-scale mutations affecting one or a few nucleotides.

基因突变可根据 DNA 序列改变的方式进行分类。WJEC 考查的主要类型是替换、插入和缺失突变。这些是影响一个或少数几个核苷酸的小规模突变。

Substitution – one base is replaced by another.
Insertion – an extra base is added into the sequence.
Deletion – a base is removed from the sequence.

替换 – 一个碱基被另一个碱基取代。
插入 – 在序列中添加了一个额外的碱基。
缺失 – 序列中丢失了一个碱基。

Insertion and deletion mutations are more likely to have a dramatic effect because they can cause a frameshift (see Section 5). Substitution may only affect one amino acid, or sometimes none at all, due to the degenerate nature of the genetic code.

插入和缺失突变更可能产生剧烈的影响，因为它们可能引起移码（见第 5 节）。而替换突变可能只影响一个氨基酸，或者由于遗传密码的简并性，有时甚至毫无影响。

3. Substitution Mutations | 替换突变

A substitution mutation occurs when one DNA base is swapped for another. For example, a ‘C’ might be replaced with a ‘T’. This change in the DNA template leads to a different codon in the mRNA during transcription, potentially coding for a different amino acid during translation.

替换突变是指一个 DNA 碱基被换成了另一个。例如，“C”可能被替换为“T”。DNA 模板的这种改变会导致转录过程中 mRNA 上对应的密码子发生变化，从而可能在翻译时编码一个不同的氨基酸。

Because of the redundancy of the genetic code, some substitutions do not change the amino acid – these are called silent mutations. However, if the new codon codes for a different amino acid, it is a missense mutation; if it codes for a stop codon, it is a nonsense mutation.

由于遗传密码的冗余性，有些替换不会改变氨基酸——这被称为沉默突变。但若新密码子编码了不同的氨基酸，则为错义突变；若变为终止密码子，则是无义突变。

WJEC students must be comfortable reading a codon table and predicting the effect of a base substitution on the amino acid sequence. Practice linking the complementary base-pairing rules (A-T, C-G, A-U in RNA) to trace the change from DNA → mRNA → polypeptide.

WJEC 考生必须能熟练阅读密码子表，并预测碱基替换对氨基酸序列的影响。要练习利用互补碱基配对规则（DNA 中 A-T、C-G，RNA 中 A-U），追踪变化从 DNA → mRNA → 多肽的全过程。

4. Insertion and Deletion Mutations | 插入与缺失突变

An insertion mutation happens when one or more extra nucleotides are inserted into the DNA sequence. A deletion mutation occurs when nucleotides are removed. These can be as small as one base or involve longer segments.

插入突变是指 DNA 序列中插入了一个或多个额外的核苷酸。缺失突变则是核苷酸被删除。这些改变可以小到单个碱基，也可能涉及较长的片段。

Even a single-base insertion or deletion can drastically alter the whole amino acid sequence downstream of the mutation. This is because the genetic code is read in triplets (codons). Inserting or deleting a base shifts the reading frame, changing every codon from that point onward.

即使只是单碱基的插入或缺失，也可能极大地改变突变位点下游的整个氨基酸序列。这是因为遗传密码是按三联体（密码子）阅读的。插入或删除一个碱基会移动阅读框，从而改变此后所有的密码子。

In WJEC exam questions, you may be given a short DNA sequence and asked to simulate an insertion or deletion, then transcribe and translate it to show the effect. Always show the triplet grouping clearly in your answer.

在 WJEC 考题中，可能给你一段较短的 DNA 序列，要求你模拟一次插入或缺失，然后转录和翻译，以显示其影响。作答时总要清晰地标出三联体分组。

5. Frameshift Mutations | 移码突变

A frameshift mutation is the result of insertions or deletions that are not in multiples of three bases. Because the ribosome reads mRNA in groups of three, adding or removing one or two bases shifts the triplet code, producing a completely different set of amino acids from the mutation point onwards. This usually results in a non-functional protein.

移码突变是由非三的倍数的插入或缺失引起的。由于核糖体以三个碱基为一组阅读 mRNA，增加或缺失一两个碱基会使三联体密码发生位移，从突变位点开始产生一组完全不同的氨基酸。这通常导致蛋白质丧失功能。

Frameshifts often introduce a premature stop codon shortly after the mutation, truncating the polypeptide and severely affecting the protein’s structure. This is a favourite topic for the WJEC higher-tier paper, where you may need to compare the effects of substitution vs. insertion/deletion.

移码突变常会在突变后不远处引入提前的终止密码子，截短多肽链，严重破坏蛋白质结构。这是 WJEC 提高卷中喜欢考查的主题，你可能需要对比替换突变与插入/缺失突变的影响。

If the insertion or deletion is exactly three bases (or a multiple of three), the reading frame is not shifted; you simply add or remove one or more amino acids. This may still affect function, but it is not a frameshift.

若插入或缺失恰好是三个碱基（或三的倍数），阅读框就不会发生移动；此时只是增加或删除了一个或多个氨基酸。这仍可能影响功能，但不属于移码突变。

6. Consequences of Mutations: Missense, Nonsense, Silent | 突变的后果：错义、无义、沉默

Based on how the change affects the resulting protein, point mutations (substitutions) can be classified as missense, nonsense, or silent. These terms are directly examined in WJEC IGCSE Biology.

根据改变对最终蛋白质的影响，点突变（替换）可被分为错义、无义或沉默突变。这些术语在 WJEC IGCSE 生物考试中会直接考查。

Mutation type / 突变类型	Effect on codon / 对密码子的影响	Effect on protein / 对蛋白质的影响
Silent / 沉默	New codon codes for same amino acid / 新密码子仍编码同一氨基酸	No change / 无变化
Missense / 错义	Codes for a different amino acid / 编码一个不同的氨基酸	Amino acid altered; protein may malfunction / 氨基酸改变；蛋白质可能功能异常
Nonsense / 无义	Becomes a stop codon / 变为终止密码子	Translation stops early; short, usually non-functional protein / 翻译提前终止；产生截短、通常无功能的蛋白质

It is critical to explain in an exam that a silent mutation is possible because the genetic code is degenerate (multiple codons can code for the same amino acid). WJEC often rewards this link explicitly.

考试中必须解释沉默突变之所以可能，是因为遗传密码具有简并性（多个密码子可编码同一种氨基酸）。WJEC 通常会对这种明确关联给予加分。

7. Mutagens and Causes of Mutations | 诱变剂和突变原因

Mutagens are agents that increase the rate of mutation above the natural background level. WJEC candidates need to recognise common physical and chemical mutagens and give examples of their effects on DNA.

诱变剂是指能将突变频率提高至自然本底水平之上的因子。WJEC 考生需要识别常见的物理和化学诱变剂，并举例说明它们对 DNA 的影响。

High-energy radiation: UV light, X-rays, and gamma rays can break DNA strands or cause incorrect base pairing. UV radiation can cause adjacent thymine bases to form thymine dimers, distorting the DNA helix.
Chemical mutagens: Chemicals like those in tobacco smoke (e.g., benzopyrene) or certain food preservatives can chemically modify bases, causing mispairing during replication.

高能辐射：紫外线、X 射线和伽马射线能断裂 DNA 链或导致错误的碱基配对。紫外线可使相邻的胸腺嘧啶碱基形成胸腺嘧啶二聚体，扭曲 DNA 双螺旋。
化学诱变剂：烟草烟雾中的化学物质（如苯并芘）或某些食品防腐剂可以化学修饰碱基，在复制时引起错配。

Mutations can also occur spontaneously due to errors in DNA replication, when DNA polymerase occasionally incorporates a wrong nucleotide and the proofreading mechanism fails to correct it.

突变也可因 DNA 复制过程中的错误而自发产生，即 DNA 聚合酶偶尔掺入错误核苷酸，且校对机制未能将其纠正。

WJEC frequently links mutagens to the increased risk of cancer. Explain that mutations in genes that control cell division (proto-oncogenes and tumour suppressor genes) can lead to uncontrolled cell growth.

WJEC 常将诱变剂与癌症风险增加联系起来。要解释若控制细胞分裂的基因（原癌基因和抑癌基因）发生突变，可能导致细胞生长失控。

8. Sickle Cell Anaemia: A Case Study | 镰刀型细胞贫血：案例研究

Sickle cell anaemia is a classic WJEC example of a disease caused by a single-base substitution mutation in the gene for the beta-globin chain of haemoglobin. This makes it an ideal exam case for demonstrating cause and effect at the molecular level.

镰刀型细胞贫血是 WJEC 教学中一个经典的例子，它由编码血红蛋白 β 珠蛋白链的基因发生单碱基替换突变引起。这使其成为在分子水平展示因果关系的理想考试案例。

The mutation changes the DNA triplet from CTT to CAT in the template strand (or the equivalent change in the coding strand from GAG to GTG). Consequently, the mRNA codon changes from GAG to GUG. Instead of the amino acid glutamic acid, valine is inserted at position 6 of the beta-globin chain.

该突变使得模板链上的 DNA 三联体由 CTT 变为 CAT（或在编码链上对应的 GAG 变为 GTG）。因此，mRNA 密码子从 GAG 变为 GUG。在 β 珠蛋白链的第 6 位，原本的谷氨酸被缬氨酸取代。

Hbᴬ (normal): DNA → GAG → mRNA → GAG → Glutamic acid
Hbˢ (sickle): DNA → GTG → mRNA → GUG → Valine

Hbᴬ（正常）: DNA → GAG → mRNA → GAG → 谷氨酸
Hbˢ（镰刀型）: DNA → GTG → mRNA → GUG → 缬氨酸

This single amino acid change makes haemoglobin molecules clump together under low oxygen conditions, forming long fibres that distort red blood cells into a sickle shape. These sickled cells can block capillaries, causing pain and tissue damage, and are fragile, leading to anaemia.

这种单个氨基酸的改变使得血红蛋白分子在低氧条件下聚集成束，形成长纤维，使红细胞扭曲成镰刀形。这些镰刀形细胞会堵塞毛细血管，引起疼痛和组织损伤，同时脆性增大，导致贫血。

WJEC examiners expect you to state that this is a substitution mutation, specifically a missense mutation. You should also be aware of the heterozygote advantage: individuals with one sickle allele (Hbᴬ Hbˢ) have some protection against malaria, which explains why the allele persists at high frequency in some populations.

WJEC 考官期望你能明确说明这是一种替换突变，具体来说是错义突变。你还应了解杂合子优势：携带一个镰刀型等位基因的个体（Hbᴬ Hbˢ）对疟疾有一定抵抗力，这解释了为何该等位基因在某些人群中仍保持较高频率。

9. Mutations and Evolution | 突变与进化

Mutations are the ultimate source of genetic variation. They produce new alleles, which is the raw material for natural selection and evolution. In the WJEC specification, this is a key concept: without mutation, populations would have no new traits on which selection could act.

突变是遗传变异的最终来源。它们产生新的等位基因，这是自然选择和进化的原始材料。在 WJEC 考纲中，这是一个关键概念：没有突变，种群就没有可供选择作用的新性状。

A small proportion of mutations may be beneficial, giving an organism a survival advantage in its environment. For example, a mutation in a bacterium that confers antibiotic resistance allows it to survive and reproduce when antibiotics are present. Over generations, the frequency of the resistant allele increases in the population – this is evolution by natural selection.

一小部分突变可能是有益的，赋予生物体在所处环境中的生存优势。例如，细菌中赋予抗生素抗性的突变使其在抗生素存在时也能存活和繁殖。经过若干代后，抗性等位基因在种群中的频率上升——这就是通过自然选择发生的进化。

Most mutations, however, are neutral or harmful. Neutral mutations have no effect on survival and may accumulate through genetic drift. Harmful mutations are often removed from the gene pool because affected individuals have lower reproductive success.

然而，大多数突变是中性的或有害的。中性突变对生存无影响，可能通过遗传漂变积累。有害突变常会从基因库中被淘汰，因为受影响的个体繁殖成功率较低。

When answering WJEC questions, always link mutation to the generation of new alleles, variation, and natural selection. Use precise language: ‘Mutations create new alleles, increasing genetic variation, upon which natural selection can act.’

回答 WJEC 题目时，始终要将突变与新等位基因的产生、变异和自然选择联系起来。使用精确的语言：“突变产生新等位基因，增加遗传变异，从而为自然选择提供作用对象。”

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

To maximise marks on WJEC gene mutation questions, pay close attention to the following points that examiners frequently highlight in reports:

要在 WJEC 基因突变题目中拿到最高分，请密切注意以下考官报告中经常强调的要点：

Precise definitions: Always define mutation as a ‘change in the base sequence of DNA’, not just a ‘change in DNA’ or ‘change in genes’. Mention permanence when appropriate.
Use of codon tables: If asked to translate a mutated sequence, always show the step from DNA → mRNA (using U instead of T) and then use the mRNA codon to look up amino acids. Many candidates lose marks by using the DNA triplet directly with the codon table.

精确定义：始终将突变定义为“DNA 碱基序列的改变”，而不仅仅是“DNA 的改变”或“基因的改变”。在适当的时候要提到永久性。
密码子表的使用：若要求翻译一段突变序列，总要先展示 DNA → mRNA 的步骤（用 U 代替 T），再用 mRNA 密码子查找氨基酸。很多考生直接用 DNA 三联体查密码子表而丢分。

Frameshift vs substitution: Know that a single-base insertion/deletion causes a frameshift and is likely to have a major impact, while a substitution may have no effect at all. Use the terms ‘reading frame’ and ‘triplet code’ in your explanation.
Sickle cell depth: For high-mark questions, go beyond the amino acid change; mention the effect on haemoglobin solubility and red blood cell shape, and link to symptoms and heterozygote advantage.

移码 vs 替换：了解单碱基插入/缺失会引起移码，并很可能产生重大影响；而替换可能完全没有影响。解释时使用“阅读框”和“三联体密码”等术语。
镰刀型细胞贫血的深度：在高分题目中，不仅要提氨基酸的改变，还要提及对血红蛋白溶解度和红细胞形状的影响，并关联症状和杂合子优势。

Finally, remember that mutations only affect the organism if the altered protein has a changed function. If the mutation is in a non-coding region or does not alter the final protein, there may be no phenotype change.

最后，记住只有当真蛋白质的功能发生改变时，突变才会影响生物体。如果突变位于非编码区，或未改变最终蛋白质结构，就可能没有表型变化。

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IGCSE WJEC Biology: Gene Mutations Exam Focus | IGCSE WJEC 生物：基因突变 考点精讲