📚 Mutations and Mutagens | 突变与诱变因素
Mutations are permanent changes in the DNA sequence of an organism. They can occur spontaneously during DNA replication or be induced by environmental agents known as mutagens. Understanding the causes and consequences of mutations is essential in IGCSE Biology, as they underpin topics in genetics, cancer and evolution.
突变是生物体 DNA 序列的永久性改变。它们可以在 DNA 复制过程中自发产生，也可以由称为诱变剂的环境因素诱导产生。理解突变的起因和后果对 IGCSE 生物学至关重要，因为它们是遗传学、癌症和进化等主题的基础。
1. What Are Mutations? | 什么是突变？
A mutation is a change in the nucleotide sequence of DNA. This can involve a single base pair, such as a substitution, or larger segments of a chromosome. Mutations can be harmful, neutral or occasionally beneficial, and they are the ultimate source of genetic variation.
突变是指 DNA 核苷酸序列的改变。它可能只涉及一个碱基对（如替换），也可能涉及染色体较大的片段。突变可能是有害的、中性的，或偶尔有益的，是遗传变异的最终来源。
2. Types of Gene Mutations | 基因突变的类型
The main types of gene mutation include substitution, insertion and deletion. A substitution replaces one base with another, potentially altering a single amino acid. Insertions and deletions cause a frameshift, which changes the reading frame and usually leads to a completely different and non-functional protein.
基因突变的主要类型包括替换、插入和缺失。替换是将一个碱基换成另一个，可能改变单个氨基酸。插入和缺失会引起移码，改变阅读框，通常导致完全不同的、无功能的蛋白质。
For example, in sickle cell anaemia, a single substitution in the haemoglobin gene changes the amino acid glutamic acid to valine, distorting red blood cell shape. This illustrates how even a small mutation can have a significant effect.
例如，在镰状细胞贫血中，血红蛋白基因中的一个单碱基替换将谷氨酸变为缬氨酸，导致红细胞变形。这表明即使微小的突变也可能产生重大影响。
3. Causes of Mutations: Spontaneous and Induced | 突变的原因：自发与诱发
Mutations can arise spontaneously due to errors in DNA replication or as a result of the natural chemical instability of DNA. The rate of spontaneous mutation is typically very low. However, environmental factors called mutagens can greatly increase the frequency of mutations; these are described as induced mutations.
突变可因 DNA 复制错误或 DNA 本身的化学不稳定性而自发产生。自发突变的频率通常极低。然而，称为诱变剂的环境因素可大大增加突变的频率，这类突变被称为诱发突变。
4. Ionising Radiation as a Mutagen | 电离辐射作为诱变剂
Ionising radiation, such as gamma rays, X-rays and some high-energy ultraviolet (UV) radiation, can damage DNA directly. These rays carry enough energy to break chemical bonds, causing single- or double-strand breaks in the DNA backbone or chemically altering bases. This often leads to errors when the cell attempts to repair the damage.
电离辐射，如 γ 射线、X 射线和一些高能紫外线，能够直接破坏 DNA。这些射线携带足够的能量打断化学键，引起 DNA 骨架的单链或双链断裂，或使碱基发生化学变化。当细胞试图修复这些损伤时，常会导致错误。
| Mutagen Type | 诱变剂类型 | Example | 示例 | Effect on DNA | 对 DNA 的影响 |
|---|---|---|
| Ionising radiation | 电离辐射 | Gamma rays, X-rays | DNA strand breaks, base damage |
| Ultraviolet radiation | 紫外线辐射 | UV-B, UV-C from the sun | Thymine dimer formation |
| Chemical mutagens | 化学诱变剂 | Benzopyrene (tobacco smoke), nitrites | Base modification, cross-linking |
5. Ultraviolet (UV) Radiation | 紫外线辐射
Ultraviolet light, particularly UV-B and UV-C, is a powerful mutagen. It causes adjacent thymine bases on the same DNA strand to bond together, forming thymine dimers. These dimers distort the DNA helix and interfere with normal base pairing during replication, leading to mutations if not repaired.
紫外线，特别是 UV-B 和 UV-C，是一种强诱变剂。它使同一条 DNA 链上相邻的胸腺嘧啶碱基键合在一起，形成胸腺嘧啶二聚体。这些二聚体扭曲 DNA 螺旋，干扰复制过程中的正常碱基配对，若未被修复则会引发突变。
6. Chemical Mutagens | 化学诱变剂
Chemical mutagens include substances that alter the structure of nucleotide bases or insert themselves between bases. Examples are nitrous acid, which deaminates bases, and coal tar derivatives. Some chemicals cause base pair substitutions, while others form bulky adducts on DNA, blocking replication and repair.
化学诱变剂包括能够改变核苷酸碱基结构或插入碱基之间的物质。例如，亚硝酸会使碱基脱氨基，煤焦油衍生物也可致突变。有些化学物引起碱基对替换，另一些则在 DNA 上形成巨大的加合物，阻碍复制与修复。
7. Tobacco Chemicals and Mutation | 烟草化学物与突变
Chemicals in tobacco smoke, such as benzopyrene, are carcinogenic because they act as mutagens. Benzopyrene can bind to DNA, forming adducts that lead to mutations in genes that control cell division, such as the tumour suppressor gene p53. This dramatically increases the risk of lung and other cancers.
烟草烟雾中的化学物质，如苯并芘，是致癌物，因为它们充当诱变剂。苯并芘能与 DNA 结合，形成加合物，导致控制细胞分裂的基因（如抑癌基因 p53）发生突变，从而大幅增加患肺癌及其他癌症的风险。
8. Effects of Mutations on Proteins | 突变对蛋白质的影响
Since genes code for proteins, a mutation may alter the amino acid sequence. In the case of sickle cell anaemia, the substitution causes valine to replace glutamic acid, making haemoglobin molecules stick together under low oxygen conditions. This produces sickle-shaped red blood cells that block capillaries and cause pain.
由于基因编码蛋白质，突变可能改变氨基酸序列。在镰状细胞贫血中，碱基替换导致缬氨酸代替谷氨酸，使血红蛋白分子在低氧条件下黏在一起，产生镰刀状红细胞，堵塞毛细血管并引起疼痛。
Frameshift mutations often result in a completely non-functional protein because every codon downstream of the insertion or deletion is altered. This can be lethal if it affects an essential enzyme.
移码突变通常导致完全无功能的蛋白质，因为插入或缺失位点下游的每个密码子都发生了改变。如果影响的是必需酶，可能致死。
9. Mutations and Cancer | 突变与癌症
Cancer is essentially a disease of uncontrolled cell division caused by the accumulation of mutations in oncogenes and tumour suppressor genes. Oncogenes, when mutated, become overactive, while inactivated tumour suppressor genes fail to halt the cell cycle. Multiple mutations are usually required for a cell to become fully cancerous.
癌症本质上是一种由于癌基因和抑癌基因突变积累而导致的细胞分裂失控疾病。癌基因突变后过度活跃，而失活的抑癌基因无法阻止细胞周期。一个细胞通常需要累积多个突变才会完全癌变。
10. Mutations in Reproductive Cells vs. Body Cells | 生殖细胞突变与体细胞突变
Mutations that occur in gametes (sperm or egg cells) can be passed to offspring, potentially causing inherited disorders. In contrast, mutations in somatic (body) cells affect only the individual in which they occur, contributing to diseases such as cancer but not being inherited.
发生在配子（精子或卵细胞）中的突变可以传给后代，可能导致遗传性疾病。相反，体细胞中的突变只影响该个体本身，可导致癌症等疾病，但不会遗传。
11. Beneficial Mutations and Evolution | 有益突变与进化
Although many mutations are neutral or harmful, some confer an advantage. For example, a mutation that gives bacteria resistance to an antibiotic allows them to survive and reproduce in the presence of the drug. Such beneficial mutations are raw material for natural selection, driving evolution.
虽然许多突变是中性的或有害的，但有些会带来优势。例如，使细菌对抗生素产生抗性的突变让它们能够在药物存在下存活并繁殖。这类有益突变是自然选择的原材料，推动进化。
The sickle cell trait, in which heterozygous individuals have some altered haemoglobin, offers protection against malaria. This demonstrates how a harmful mutation in one context can be beneficial in another environment.
镰状细胞特征中，杂合子个体拥有一些异常血红蛋白，却对疟疾有保护作用。这表明在一个环境中有害的突变，在另一种环境中可能是有益的。
12. Summarising Mutation Frequency and Risk Factors | 总结突变频率与风险因素
The incidence of mutations can be increased by exposure to ionising radiation (gamma rays, X-rays, UV light) and chemical mutagens (including those in tobacco smoke). Lifestyle choices, such as avoiding smoking, using sun protection and minimising unnecessary medical X-rays, can reduce mutation risk. Understanding these factors helps in cancer prevention and genetic counselling.
接触电离辐射（γ 射线、X 射线、紫外线）和化学诱变剂（包括烟草烟雾中的物质）会增加突变的发生率。选择健康的生活方式，如避免吸烟、使用防晒和尽量减少不必要的医疗 X 射线检查，可以降低突变风险。了解这些因素有助于癌症预防和遗传咨询。
Published by TutorHao | Biology Revision Series | aleveler.com
更多咨询请联系16621398022（同微信）
或截图后用「扫一扫 → 相册」选择本图