Gene Mutations | 基因突变 考点精讲

📚 Gene Mutations | 基因突变 考点精讲

A sudden, permanent change in the sequence of bases in DNA is called a gene mutation. Mutations occur all the time, and sometimes they lead to new forms of a gene – alleles. While many mutations have no effect or are harmful, a few can be beneficial, driving evolution through natural selection.

基因突变是指 DNA 中碱基序列发生突然的、永久性的改变。突变时刻都在发生，有时会产生新形式的基因——等位基因。虽然许多突变没有影响或有害，但少数可能是有益的，通过自然选择推动进化。

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

A gene mutation is a change in the nucleotide sequence of a gene. Genes are made of DNA, and the order of bases (A, T, C, G) codes for the sequence of amino acids in a protein. When that order is altered, the protein may change, leading to a different phenotype.

基因突变是基因核苷酸序列的改变。基因由 DNA 组成，碱基（A、T、C、G）的排列顺序编码蛋白质中的氨基酸序列。当这一顺序改变时，蛋白质可能发生变化，进而导致不同的表现型。

Mutations can involve a single nucleotide (point mutation) or larger segments of a chromosome. In IGCSE CCEA Biology, we focus mainly on gene mutations and their consequences.

突变可能只涉及单个核苷酸（点突变），也可能涉及染色体上较大片段。在 IGCSE CCEA 生物学中，我们主要关注基因突变及其影响。

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2. Types of Gene Mutations | 基因突变的类型

Gene mutations are broadly classified into three main types based on the kind of base change:

根据碱基变化方式，基因突变主要分为三种类型：

Mutation Type | 类型	Description | 描述
Substitution	One base is replaced by another. | 一个碱基被另一个碱基替换。
Insertion	One or more extra bases are added into the sequence. | 序列中插入一个或多个额外碱基。
Deletion	One or more bases are removed from the sequence. | 序列中删除一个或多个碱基。

Insertion and deletion often lead to a frameshift, which usually has a much larger effect on the protein than a substitution. Substitution mutations can be silent, missense, or nonsense.

插入和缺失经常导致移码突变，这通常比替换突变对蛋白质的影响大得多。替换突变可以是沉默突变、错义突变或无义突变。

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3. Substitution Mutations | 替换突变

In a substitution mutation, a single nucleotide is swapped for another. For example, in the DNA sequence 3′-TAC-5′, the T might change to G, giving 3′-GAC-5′. This changes the mRNA codon from AUG to CUG.

在替换突变中，单个核苷酸被交换成另一个。例如，在 DNA 序列 3′-TAC-5′ 中，T 可能变成 G，产生 3′-GAC-5’。这将使 mRNA 密码子从 AUG 变为 CUG。

The effect on the polypeptide depends on which base is changed and where it occurs. Because the genetic code is degenerate, some substitutions do not alter the amino acid (silent), while others change one amino acid (missense) or introduce a premature stop codon (nonsense).

对多肽的影响取决于哪个碱基被改变以及改变的位置。由于遗传密码具有简并性，有些替换不会改变氨基酸（沉默突变），有些会改变一个氨基酸（错义突变），有些会引入提前终止密码子（无义突变）。

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4. Insertion and Deletion – Frameshift | 插入和缺失——移码突变

When a nucleotide is added or removed, the reading frame of the codons shifts. Every codon after the mutation is altered, leading to a completely different amino acid sequence from that point onward.

当增加或移除一个核苷酸时，密码子的阅读框发生移位。突变之后的所有密码子都会改变，导致从那一点开始氨基酸序列完全不同。

A frameshift mutation nearly always produces a non-functional protein because the entire sequence downstream is ‘read’ incorrectly. A premature stop codon is often introduced quickly.

移码突变几乎总是会产生无功能的蛋白质，因为下游的整个序列被错误“读取”。通常很快就会出现提前的终止密码子。

For example, original DNA: 3′-TAC GGA TAA-5′ (mRNA: AUG CCU AUU). Deleting the first G (underlined) gives 3′-TAC GAT AA…-5′ leading to different codons.

例如，原始 DNA：3′-TAC GGA TAA-5’（mRNA：AUG CCU AUU）。删除第一个 G（下划线标出）后变成 3′-TAC GAT AA…-5’，产生完全不同的密码子。

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5. Effects on the Protein: Silent, Missense, Nonsense | 对蛋白质的影响：沉默、错义、无义

Silent mutation: The new codon still codes for the same amino acid due to the degeneracy of the genetic code. The protein folds and functions normally.

沉默突变：由于遗传密码的简并性，新密码子仍然编码相同的氨基酸。蛋白质正常折叠和发挥功能。

Missense mutation: The base change causes a different amino acid to be incorporated. The protein’s shape and function may be altered. Sickle cell anaemia results from a missense mutation in the haemoglobin gene.

错义突变：碱基改变导致插入不同的氨基酸。蛋白质的形状和功能可能被改变。镰刀型细胞贫血症就是血红蛋白基因错义突变的结果。

Nonsense mutation: The codon becomes a stop codon (UAA, UAG, UGA) too early. Translation stops prematurely, producing a truncated, usually non-functional protein.

无义突变：密码子过早变为终止密码子（UAA、UAG、UGA）。翻译提前终止，产生一个截短的、通常无功能的蛋白质。

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6. Causes of Mutations | 突变的原因

Mutations arise spontaneously during DNA replication, but their frequency increases when cells are exposed to mutagens. Mutagens are environmental agents that damage DNA or interfere with base pairing.

突变在 DNA 复制过程中会自发产生，但当细胞暴露于诱变剂时发生频率增加。诱变剂是损害 DNA 或干扰碱基配对的环境因素。

Type of Mutagen | 诱变剂类型	Examples | 例子
Physical mutagens	UV light, X-rays, gamma rays
Chemical mutagens	Mustard gas, benzene, nitrous acid
Biological agents	Some viruses (e.g. HPV) can insert into host DNA

Ionising radiation can break DNA strands or cause base oxidation. UV light causes thymine dimers, which distort the DNA helix and lead to errors during replication.

电离辐射可导致 DNA 链断裂或碱基氧化。紫外线会引起胸腺嘧啶二聚体，扭曲 DNA 螺旋，在复制时引发错误。

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7. Sickle Cell Anaemia: A Key Example | 镰刀型细胞贫血症：一个关键例子

Sickle cell anaemia is a genetic disorder caused by a substitution mutation in the gene for the beta chain of haemoglobin. The DNA mutation changes the codon from CTT to CAT (in the template strand).

镰刀型细胞贫血症是一种遗传病，由血红蛋白 β 链基因中的替换突变引起。DNA 突变将模板链上的密码子从 CTT 变为 CAT。

This results in the mRNA codon changing from GAA to GUA, so the amino acid inserted is valine instead of glutamic acid. The change in one amino acid alters the shape of the haemoglobin molecule, causing it to form insoluble fibres under low oxygen conditions. Red blood cells become sickle-shaped, blocking capillaries and causing pain and tissue damage.

这使得 mRNA 密码子从 GAA 变为 GUA，因此插入的氨基酸是缬氨酸，而不是谷氨酸。一个氨基酸的改变影响了血红蛋白分子的形状，使其在低氧条件下形成不溶性纤维。红细胞变成镰刀型，堵塞毛细血管，引发疼痛和组织损伤。

Heterozygous individuals (carriers) have some normal haemoglobin and some sickle haemoglobin. They are usually healthy and have increased resistance to malaria, which explains why the allele remains common in malaria-affected regions.

杂合子个体（携带者）同时拥有正常和镰刀型血红蛋白。他们通常很健康，并且对疟疾有更强的抵抗力，这解释了为什么该等位基因在疟疾流行地区仍然很常见。

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8. Mutations and Genetic Variation | 突变与遗传变异

A mutation creates a new allele. This increases the genetic diversity of a population. Without mutation, all individuals would have identical copies of genes, limiting adaptation.

突变会产生新的等位基因。这增加了种群的遗传多样性。没有突变，所有个体都会拥有相同的基因副本，限制了适应能力。

Most mutations that are passed to offspring occur in gametes. Somatic (body cell) mutations affect only the individual and are not inherited.

大多数遗传给后代的突变发生在配子中。体细胞突变只影响个体，不会遗传。

A neutral mutation neither improves nor reduces fitness, but can be acted upon by natural selection if the environment changes.

中性突变既不会提高也不降低适应度，但如果环境发生变化，自然选择可能对其起作用。

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9. Mutations and Evolution | 突变与进化

Mutation is the ultimate source of genetic variation upon which natural selection acts. In a changing environment, a previously neutral or even harmful allele can become beneficial.

突变是遗传变异的根本来源，自然选择在其基础上发挥作用。在变化的环境中，原本中性甚至有害的等位基因可能变得有益。

For example, antibiotic resistance in bacteria arises from spontaneous mutations in genes coding for binding sites or enzymes. When antibiotics are used, those with the resistance allele survive and reproduce, increasing the frequency of the allele – evolution in action.

例如，细菌对抗生素的耐药性源自编码结合位点或酶的基因的自发突变。使用抗生素时，那些拥有耐药等位基因的细菌存活并繁殖，使该等位基因的频率增加——这就是进化在发生。

Thus, mutation and natural selection together drive the formation of new species over time.

因此，突变与自然选择共同推动新物种的形成。

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10. Comparing Mutation Types | 突变类型对比

Feature	Substitution	Insertion / Deletion
Effect on reading frame	Does not shift	Causes a frameshift
Number of affected amino acids	Usually only one	All amino acids downstream are altered
Potential outcomes	Silent, missense, nonsense	Usually leads to non-functional protein
Example	Sickle cell mutation	Cystic fibrosis (often involves deletion of 3 bases; some are frameshift mutations)

While a substitution may have a mild effect, frameshift mutations are more dramatic and almost always destroy the protein’s function.

虽然替换可能影响轻微，但移码突变更加剧烈，几乎总是破坏蛋白质的功能。

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11. Detecting Mutations and Their Impact | 检测突变及其影响

Mutations can be observed by comparing DNA or protein sequences. In the lab, gel electrophoresis can separate DNA fragments of different lengths. A frameshift mutation changes the size of the DNA fragment, whereas a substitution may not be detected by size alone.

通过比较 DNA 或蛋白质序列可以观察突变。在实验室中，凝胶电泳可分离不同长度的 DNA 片段。移码突变会改变 DNA 片段的大小，而仅凭大小可能检测不到替换突变。

At the protein level, a change in function can be observed – for example, enzymes may lose catalytic activity if the active site is altered.

在蛋白质水平，可以观察到功能的改变——例如，如果活性位点被改变，酶可能失去催化活性。

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12. Exam Tips for CCEA IGCSE Biology | CCEA IGCSE 生物学考试技巧

• Define mutation precisely: A random, permanent change in the base sequence of DNA.
• 精确地给出定义： DNA 碱基序列的随机、永久性改变。
• Link to the genetic code: Always explain how a change in DNA sequence leads to a different mRNA codon, which can change the amino acid sequence.
• 与遗传密码相联系： 始终解释 DNA 序列的改变如何导致不同 mRNA 密码子，从而可能改变氨基酸序列。
• Use sickle cell anaemia as a case study: Be able to describe the DNA mutation, the amino acid change (Glu → Val), and the consequences.
• 以镰刀型细胞贫血症为案例： 能够描述 DNA 突变、氨基酸变化（谷氨酸→缬氨酸）及其后果。
• Distinguish between types: Know that substitution leads to point mutation effects, while insertion/deletion cause frameshifts.
• 区分不同类型： 知道替换导致点突变效应，而插入/缺失引起移码。
• Mention mutagens: Name examples and explain how they increase the rate of mutation.
• 提及诱变剂： 举出例子并解释它们如何增加突变率。
• Evolution context: Stress that mutation introduces variation, which is the raw material for natural selection.
• 进化背景： 强调突变引入变异，这是自然选择的原材料。

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