📚 IGCSE Edexcel Biology: Gene Mutations Exam Points | IGCSE Edexcel 生物：基因突变 考点精讲
Welcome to this focused revision guide on gene mutations for the IGCSE Edexcel Biology specification. Mutations are permanent changes in the DNA base sequence; they can alter the structure and function of proteins, leading to new alleles and contributing to genetic diversity. In this article, you will learn about the different types of mutations, how they affect protein synthesis, what causes them, and their role in evolution - all aligned with Edexcel exam requirements.
欢迎阅读本文精心编写的 IGCSE Edexcel 生物基因突变考点精讲。突变是 DNA 碱基序列的永久性改变，它们可能改变蛋白质的结构与功能，从而产生新的等位基因，增加遗传多样性。本文将系统讲解突变的类型、对蛋白质合成的影响、成因以及在进化中的作用，全面贴合 Edexcel 考纲要求。
1. What Are Mutations? | 什么是基因突变？
A gene mutation is a random and permanent alteration in the nucleotide sequence of DNA. These changes may involve a single base pair or larger segments of a chromosome, but in IGCSE we focus on gene mutations (point mutations and small insertions/deletions).
基因突变是 DNA 核苷酸序列中随机的、永久性的改变。这些变化可能涉及单个碱基对或染色体较大片段，但 IGCSE 主要关注基因突变（点突变和小片段插入/缺失）。
Mutations are the ultimate source of new alleles - variants of a gene. Without mutations, all individuals would have identical alleles, and no heritable variation would exist.
突变是产生新等位基因（基因变体）的最终来源。没有突变，所有个体的等位基因完全相同，就不会存在可遗传的变异。
If a mutation occurs in a body cell (somatic), it cannot be passed to offspring. However, if it takes place in a gamete, the altered allele can be inherited, potentially affecting the next generation.
如果突变发生在体细胞中，它不会传递给后代；但如果发生在配子中，改变的等位基因可以遗传，可能影响下一代。
2. Types of Gene Mutations | 基因突变的类型
Gene mutations are broadly divided into substitutions, insertions, and deletions. Substitutions are point mutations where one base is swapped for another. Insertions and deletions (indels) add or remove bases, often causing a frameshift in the genetic code.
基因突变大致分为碱基替换、插入和缺失。替换是点突变，即一个碱基被另一个碱基替换。插入和缺失（indels）会添加或移除碱基，往往导致遗传密码发生移码。
The effect of a mutation depends on where it occurs in the gene and whether it changes the amino acid sequence of the protein. A mutation in a non-coding region may have no effect, while a mutation in a critical coding region can completely destroy protein function.
突变的影响取决于它在基因中发生的位置，以及它是否改变蛋白质的氨基酸序列。非编码区的突变可能没有影响，而关键编码区的突变可能完全破坏蛋白质功能。
Exam tip: always link the type of mutation to the consequence for the polypeptide chain, not just define the mutation. Edexcel often awards marks for explaining the effect on protein structure and function.
考试提示：一定要把突变类型与多肽链的后果联系起来，而不是只定义突变。Edexcel 经常为"解释对蛋白质结构和功能的影响"给分。
3. Substitutions: Silent, Missense and Nonsense | 碱基替换：沉默、错义和无义
A substitution replaces one base with another. Because the genetic code is degenerate, some substitutions do not change the amino acid - these are silent mutations. For example, GGU and GGC both code for glycine, so a change from U to C in the third position has no effect on the protein.
碱基替换是用一个碱基替换另一个碱基。由于遗传密码具有简并性，有些替换不改变氨基酸——这些是沉默突变。例如 GGU 和 GGC 都编码甘氨酸，因此第三位由 U 变为 C 对蛋白质没有影响。
A missense mutation changes one amino acid in the polypeptide chain. This can alter the folding and function of the protein. Sickle cell anaemia is a classic example: in the β-globin gene, the codon GAG changes to GTG, so glutamate is replaced by valine at position 6 of the β-globin chain.
错义突变会改变多肽链中的一个氨基酸。这可能改变蛋白质的折叠和功能。镰状细胞贫血是经典例子：在 β-珠蛋白基因中，密码子 GAG 变为 GTG，因此 β-珠蛋白链第 6 位的谷氨酸被缬氨酸替换。
A nonsense mutation creates a premature stop codon (UAA, UAG or UGA in mRNA; TAA, TAG or TGA in DNA). Translation stops early, producing a shortened polypeptide that is usually non-functional.
无义突变会产生提前终止密码子（mRNA 中为 UAA、UAG 或 UGA；DNA 中为 TAA、TAG 或 TGA）。翻译提前停止，产生通常无功能的缩短多肽。
| Type 类型 | Effect 效果 | Example 例子 |
| Silent 沉默 | No amino acid change 氨基酸不变 | GGU → GGC (glycine 甘氨酸) |
| Missense 错义 | One amino acid changed 一个氨基酸改变 | GAG → GTG (Glu → Val) |
| Nonsense 无义 | Premature stop codon 提前终止 | CAA → UAA (Gln → Stop) |
4. Insertions, Deletions and Frameshift | 插入、缺失与移码突变
An insertion adds one or more bases to the DNA sequence, while a deletion removes one or more bases. If the number of bases added or removed is not a multiple of three, the reading frame shifts. This is called a frameshift mutation.
插入是向 DNA 序列中添加一个或多个碱基，缺失则是移除一个或多个碱基。如果添加或移除的碱基数不是 3 的倍数，阅读框就会移动。这称为移码突变。
After a frameshift, every codon downstream of the mutation is read incorrectly, so almost all amino acids after that point are changed. This usually produces a completely different and non-functional protein. Frameshift mutations are often more damaging than substitutions.
发生移码后，突变位点下游的每个密码子都会被错误读取，因此该位点之后的几乎所有氨基酸都会改变。这通常产生完全不同且无功能的蛋白质。移码突变通常比碱基替换更具破坏性。
If three bases are inserted or deleted, one amino acid is added or removed but the reading frame stays the same. The CFTR ΔF508 mutation is a deletion of three bases, removing one phenylalanine; this still causes cystic fibrosis because the protein misfolds.
如果插入或缺失三个碱基，则增加或减少一个氨基酸，但阅读框保持不变。CFTR 的 ΔF508 突变就是缺失三个碱基，去掉一个苯丙氨酸；它仍会导致囊性纤维化，因为蛋白质错误折叠。
Exam tip: when asked why a frameshift is harmful, write "it changes the reading frame, so all subsequent codons are different, changing the amino acid sequence and protein structure."
考试提示：当被问到为什么移码有害时，写"它改变阅读框，因此后续所有密码子都不同，改变了氨基酸序列和蛋白质结构"。
5. Effects on Protein Structure and Function | 对蛋白质结构与功能的影响
A mutation changes the DNA base sequence, which changes the mRNA codon sequence, which may change the amino acid sequence. The amino acid sequence determines the primary structure of a protein, and the primary structure determines how the protein folds into its secondary and tertiary structures.
突变改变 DNA 碱基序列，从而改变 mRNA 密码子序列，进而可能改变氨基酸序列。氨基酸序列决定蛋白质的一级结构，而一级结构决定蛋白质如何折叠成二级和三级结构。
If the amino acid sequence changes, the bonds between amino acids (including hydrogen bonds, ionic bonds, and disulfide bridges) may be affected. This can alter the active site of an enzyme or the shape of a receptor, so the protein may no longer function.
如果氨基酸序列改变，氨基酸之间的键（包括氢键、离子键和二硫键）可能受到影响。这可能改变酶的活性位点或受体的形状，因此蛋白质可能不再发挥功能。
Some mutations are neutral because they occur in non-coding DNA or produce a silent change. Others are beneficial, harmful, or lethal. The effect depends on the role of the protein and whether the changed amino acid is critical for structure or function.
有些突变是中性的，因为它们发生在非编码 DNA 中或产生沉默改变。其他突变可能有利、有害或致死。影响取决于蛋白质的作用，以及改变的氨基酸对结构或功能是否关键。
6. Causes of Mutations: Spontaneous and Induced | 突变的原因：自发与诱发
Mutations can be spontaneous or induced. Spontaneous mutations happen randomly during DNA replication, for example when DNA polymerase makes a mistake or when a base undergoes chemical change. These occur at a low rate.
突变可以是自发的或诱发的。自发突变在 DNA 复制过程中随机发生，例如 DNA 聚合酶出错或碱基发生化学变化。这些突变发生率很低。
Induced mutations are caused by mutagens. Mutagens include ionising radiation (such as X-rays and gamma rays), ultraviolet (UV) radiation, and certain chemicals (such as those in tobacco smoke, tar, and some industrial chemicals).
诱发突变由诱变剂引起。诱变剂包括电离辐射（如 X 射线和 γ 射线）、紫外线（UV）辐射以及某些化学物质（如烟草烟雾、焦油和某些工业化学品中的物质）。
UV radiation can cause thymine dimers in DNA, where two adjacent thymine bases bond together. This distorts the DNA double helix and can lead to mutations if not repaired. Ionising radiation can break DNA strands.
紫外线辐射可在 DNA 中引起胸腺嘧啶二聚体，即两个相邻的胸腺嘧啶碱基连接在一起。这会扭曲 DNA 双螺旋，如果未修复就可能导致突变。电离辐射可以断裂 DNA 链。
Exam tip: do not say "mutations are caused by mutagens" without examples. Edexcel expects named mutagens and a clear link to DNA damage.
考试提示：不要只说"突变由诱变剂引起"而不给例子。Edexcel 要求写出具体诱变剂名称，并清楚联系到 DNA 损伤。
7. Somatic vs Gametic Mutations | 体细胞突变与配子突变
Somatic mutations occur in body cells. They are not passed to offspring because body cells do not form gametes. However, somatic mutations can cause problems in the individual, such as cancer if the mutation affects genes controlling cell division.
体细胞突变发生在体细胞中。它们不会传递给后代，因为体细胞不形成配子。然而，体细胞突变可以对个体造成问题，例如如果突变影响控制细胞分裂的基因，就可能导致癌症。
Gametic mutations occur in gametes (sperm or egg cells). These mutations can be inherited by offspring and therefore affect the next generation. This is why gametic mutations are important for evolution and genetic variation.
配子突变发生在配子（精子或卵细胞）中。这些突变可以被后代遗传，因此影响下一代。这就是为什么配子突变对进化和遗传变异很重要。
Example: a mutation in a skin cell caused by UV radiation affects only that person. A mutation in a gamete of that person could be passed to their child and may cause a genetic disorder.
例子：紫外线引起的皮肤细胞突变只影响该个体。该个体配子中的突变可以传给子女，并可能导致遗传疾病。
8. Mutation Rate and DNA Repair | 突变率与 DNA 修复
The mutation rate is the frequency at which new mutations occur. It is usually very low because cells have DNA repair enzymes that detect and correct many errors. If repair fails, the mutation becomes permanent.
突变率是新突变发生的频率。它通常很低，因为细胞有 DNA 修复酶，可以检测并纠正许多错误。如果修复失败，突变就会成为永久性改变。
Some genes control DNA repair and cell division. If a mutation occurs in one of these genes, the rate of mutation in other genes may increase. This can lead to uncontrolled cell division and cancer.
有些基因控制 DNA 修复和细胞分裂。如果这些基因之一发生突变，其他基因的突变率可能增加。这可能导致细胞分裂失控和癌症。
Exam tip: remember that mutations are random, not "directed" by the environment. The environment may increase the chance of mutation, but it does not decide which mutation is useful.
考试提示：记住突变是随机的，不是由环境"定向"产生的。环境可能增加突变几率，但不会决定哪种突变有用。
9. Mutations, New Alleles and Genetic Diversity | 突变、新等位基因与遗传多样性
A mutation can produce a new allele of a gene. An allele is a different version of a gene. New alleles increase genetic diversity within a population, which is the raw material for natural selection.
突变可以产生基因的新等位基因。等位基因是基因的不同版本。新等位基因增加种群内的遗传多样性，而遗传多样性是自然选择的原材料。
Genetic diversity is important because it helps populations survive environmental change. If all individuals have the same alleles, a disease or climate change could wipe out the whole population.
遗传多样性很重要，因为它帮助种群在环境变化中生存。如果所有个体都有相同的等位基因，一种疾病或气候变化可能消灭整个种群。
Exam tip: when writing about genetic diversity, link mutation to new alleles, and new alleles to variation in phenotype, then to natural selection.
考试提示：写遗传多样性时，把突变联系到新等位基因，再把新等位基因联系到表型变异，然后联系到自然选择。
10. Mutation and Evolution: Natural Selection | 突变与进化：自然选择
Mutations provide the variation on which natural selection acts. If a mutation gives an organism a beneficial phenotype, the organism is more likely to survive and reproduce, passing the allele to its offspring. Over many generations, the beneficial allele becomes more common.
突变为自然选择提供了变异。如果突变给生物体带来有利的表型，该生物体更可能生存和繁殖，把等位基因传给后代。经过许多代，有利等位基因变得更常见。
A classic example is antibiotic resistance in bacteria. A random mutation may give a bacterium resistance to an antibiotic. When the antibiotic is used, resistant bacteria survive and reproduce, while non-resistant bacteria die. The population evolves.
经典例子是细菌的抗生素抗性。随机突变可能使细菌对抗生素产生抗性。当使用抗生素时，抗性细菌存活并繁殖，而非抗性细菌死亡。种群因此进化。
Another example is the peppered moth. A mutation may produce dark-coloured moths, which are better camouflaged on polluted trees. These moths survive better and pass on the allele, so the dark form becomes more common.
另一个例子是桦尺蠖。突变可能产生深色蛾，它们在受污染的树干上伪装更好。这些蛾生存得更好并传递等位基因，因此深色类型变得更常见。
Exam tip: mutations are random; natural selection is not. Selection acts on the phenotype, not directly on the DNA.
考试提示：突变是随机的；自然选择不是随机的。选择作用于表型，而不是直接作用于 DNA。
11. Worked Examples and Exam Calculations | 例题与考试计算
Example 1: A DNA template strand reads TAC GGA TTC. Write the mRNA codons. The mRNA is complementary to the template: AUG CCU AAG. Using the genetic code, this gives methionine, proline, lysine.
例 1：DNA 模板链为 TAC GGA TTC。写出 mRNA 密码子。mRNA 与模板链互补：AUG CCU AAG。根据遗传密码，这给出甲硫氨酸、脯氨酸、赖氨酸。
Example 2: A base substitution changes GAG to GTG in the DNA coding strand. The mRNA codon changes from GAG to GUG. GAG codes for glutamate; GUG codes for valine. This is a missense mutation and is the cause of sickle cell anaemia.
例 2：碱基替换使 DNA 编码链中的 GAG 变为 GTG。mRNA 密码子由 GAG 变为 GUG。GAG 编码谷氨酸；GUG 编码缬氨酸。这是一个错义突变，是镰状细胞贫血的原因。
xample 3: A gene has 300 bases in its coding region. How many amino acids does it code for? cids (not counting the stop codon). If one base is inserted, the reading frame shifts and the amino acid sequence after the mutation changes completely.
例 3：某基因编码区有 300 个碱基。它编码多少个氨基酸？300 ÷ 3 = 100 个密码子，因此最多 100 个氨基酸（不计终止密码子）。如果插入一个碱基，阅读框移动，突变后的氨基酸序列完全改变。
Exam tip: always show your working. For calculations, divide the number of bases by 3 to find the number of codons, then remember that one codon codes for one amino acid.
考试提示：一定要写出计算过程。计算时，用碱基数除以 3 得到密码子数，然后记住一个密码子编码一个氨基酸。
12. Exam Technique and Common Mistakes | 考试技巧与常见错误
Common mistake 1: saying "mutations are always harmful." In fact, mutations can be harmful, beneficial, or neutral. Many mutations have no effect because the genetic code is degenerate or because they occur in non-coding DNA.
常见错误 1：说"突变总是有害的"。事实上，突变可能有害、有利或中性。许多突变没有影响，因为遗传密码具有简并性，或因为它们发生在非编码 DNA 中。
Common mistake 2: confusing mutation with natural selection. Mutation creates new alleles randomly; natural selection changes allele frequencies in a population based on survival and reproduction.
常见错误 2：混淆突变与自然选择。突变随机产生新等位基因；自然选择根据生存和繁殖改变种群中的等位基因频率。
Common mistake 3: saying "organisms mutate in order to survive." Organisms do not choose to mutate. Mutations happen randomly, and selection acts on the results.
常见错误 3：说"生物体为了生存而发生突变"。生物体不会选择突变。突变随机发生，选择作用于结果。
Common mistake 4: forgetting that somatic mutations are not inherited. Only mutations in gametes can be passed to offspring.
常见错误 4：忘记体细胞突变不可遗传。只有配子中的突变可以传给后代。
Final revision checklist: define mutation; describe substitution, insertion, deletion; explain silent, missense, nonsense, and frameshift; name mutagens; distinguish somatic and gametic; link mutation to new alleles, genetic diversity, and natural selection; and practise explaining effects on protein structure and function.
最后复习清单：定义突变；描述替换、插入、缺失；解释沉默、错义、无义和移码；说出诱变剂；区分体细胞和配子突变；把突变联系到新等位基因、遗传多样性和自然选择；并练习解释对蛋白质结构和功能的影响。
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