📚 Gene and Gene Mutation: Core Concepts | 基因与基因突变的核心概念
A gene is the fundamental unit of heredity that carries instructions for making proteins or functional RNA molecules. Gene mutations are permanent changes in the DNA sequence that can alter gene function, leading to phenotypic variation, genetic disorders, or evolutionary adaptation. This article discusses the core concepts of genes and gene mutations, following the CIE A-Level Biology syllabus.
基因是遗传的基本单位，携带着制造蛋白质或功能性RNA分子的指令。基因突变是DNA序列中发生的永久性改变，可能改变基因功能，导致表型变异、遗传疾病或进化适应。本文围绕CIE A-Level生物考纲，阐述基因与基因突变的核心概念。
1. What Is a Gene? | 什么是基因？
A gene is a sequence of DNA nucleotides (or RNA in some viruses) that encodes a polypeptide or a functional RNA, such as tRNA or rRNA. In eukaryotic cells, genes are located on chromosomes within the nucleus, while prokaryotic genes are found in the nucleoid region.
基因是一段DNA核苷酸序列（在某些病毒中为RNA），编码一条多肽链或功能性RNA（如tRNA、rRNA）。在真核细胞中，基因位于细胞核内的染色体上；原核生物的基因则位于拟核区域。
Each gene occupies a specific position on a chromosome called a locus (plural: loci). A gene can exist in different forms called alleles, which arise from mutations and contribute to genetic variation.
每个基因在染色体上占据特定位置，称为基因座位。基因可以有不同的形式，称为等位基因，它们由突变产生并导致遗传变异。
- Genes are made of DNA and contain the coded information for protein synthesis.
- 基因由DNA组成，包含蛋白质合成所需的编码信息。
- They are passed from parents to offspring through gametes during reproduction.
- 基因通过生殖过程中的配子从亲代传递给子代。
2. Gene Locus and Alleles | 基因座位与等位基因
In a diploid organism, each gene is present in two copies, one inherited from each parent. These copies are located at the same locus on homologous chromosomes. Alternative versions of the same gene are known as alleles.
在二倍体生物中，每个基因有两份拷贝，一份来自父本，一份来自母本。这些拷贝位于同源染色体的相同座位上。同一基因的不同形式称为等位基因。
For example, the gene for flower colour in pea plants may have a dominant allele (e.g., purple) and a recessive allele (e.g., white). The combination of alleles an individual carries is its genotype, while the observable characteristics are its phenotype.
例如，豌豆花色基因可能有一个显性等位基因（如紫色）和一个隐性等位基因（如白色）。个体所携带的等位基因组合称为基因型，而可观察的特征称为表型。
- Homozygous: two identical alleles at a locus (e.g., AA or aa).
- 纯合子：基因座位上两个等位基因相同（如AA或aa）。
- Heterozygous: two different alleles at a locus (e.g., Aa).
- 杂合子：基因座位上两个等位基因不同（如Aa）。
3. The Genetic Code and Triplets | 遗传密码与三联体
The genetic code is stored in the sequence of bases along the DNA molecule. Three consecutive bases in a gene form a triplet codon, which corresponds to a specific amino acid during translation. The coding strand of DNA is read in groups of three, and the sequence of codons determines the primary structure of a protein.
遗传密码以DNA分子上的碱基序列形式储存。基因中相邻的三个碱基构成一个三联体密码子，对应翻译过程中的一个特定氨基酸。编码链上的DNA按三个一组被读取，密码子序列决定蛋白质的一级结构。
Key features of the genetic code are universality, degeneracy, and non-overlapping nature. Universality means the same codons code for the same amino acids in almost all organisms. Degeneracy means that some amino acids are coded by more than one codon. Non-overlapping means each base is read only once as part of one codon.
遗传密码的主要特点包括通用性、简并性和不重叠性。通用性指几乎在所有生物中，相同密码子编码相同氨基酸；简并性指某些氨基酸可以由多个密码子编码；不重叠性指每个碱基只被读取一次，作为单一密码子的一部分。
DNA bases: A (adenine) ↔ T (thymine), G (guanine) ↔ C (cytosine)
DNA碱基：A（腺嘌呤）↔ T（胸腺嘧啶），G（鸟嘌呤）↔ C（胞嘧啶）
4. 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. This change may involve a single base pair or larger segments of DNA. Mutations can occur spontaneously during DNA replication or be induced by environmental factors such as radiation and chemicals.
基因突变是指基因中核苷酸序列的改变。这种改变可能涉及单个碱基对或DNA的较大片段。突变可在DNA复制过程中自发产生，也可能由辐射、化学物质等环境因素诱导产生。
Mutations are the ultimate source of new alleles and genetic variation. They may be neutral, harmful, or occasionally beneficial, depending on how they affect the resulting protein and the organism’s environment.
突变是新等位基因和遗传变异的最终来源。根据其对蛋白质功能和生物环境的影响，突变可能是中性的、有害的，偶尔也可能是有益的。
5. Types of Gene Mutations | 基因突变的类型
Gene mutations can be classified into two main categories: base substitutions and frameshift mutations. Each has distinct effects on the protein product.
基因突变可分为两大类：碱基替换和移码突变。每一类对蛋白质产物的影响各不相同。
5.1 Base Substitution | 碱基替换
In a base substitution, one base pair is replaced by another. For example, an A-T pair may change to G-C. This can lead to a different codon at that position. The outcome depends on the nature of the new codon.
碱基替换是指一个碱基对被另一个碱基对替代。例如，A-T对可能变为G-C对。这会导致该位置的密码子发生变化，其结果取决于新密码子的性质。
- Silent mutation: the new codon still codes for the same amino acid (due to degeneracy).
- 同义突变：新密码子仍编码同一种氨基酸（由于密码子简并性）。
- Missense mutation: the new codon codes for a different amino acid.
- 错义突变：新密码子编码另一种氨基酸。
- Nonsense mutation: the new codon is a stop codon, prematurely terminating translation.
- 无义突变：新密码子为终止密码子，导致翻译提前终止。
5.2 Insertion and Deletion | 插入和缺失
Insertion occurs when one or more extra nucleotides are added to the gene, while deletion occurs when one or more nucleotides are removed. If the number of added or removed nucleotides is not a multiple of three, the reading frame shifts, causing a frameshift mutation.
插入是指基因中加入一个或多个额外核苷酸；缺失是指删除一个或多个核苷酸。如果增加或删除的核苷酸数量不是3的倍数，阅读框就会发生移位，导致移码突变。
Original: AUG – UCU – GCA – UUU → Met – Ser – Ala – Phe
原序列：AUG – UCU – GCA – UUU → 甲硫氨酸 – 丝氨酸 – 丙氨酸 – 苯丙氨酸
After deletion of U: AUG – UGC – AUU – U… → Met – Cys – Ile – …
缺失U后：AUG – UGC – AUU – U… → 甲硫氨酸 – 半胱氨酸 – 异亮氨酸 – …
Frameshift mutations usually produce a non-functional protein because the entire amino acid sequence after the mutation is altered, often creating a premature stop codon.
移码突变通常产生无功能的蛋白质，因为突变后的整个氨基酸序列都发生改变，并且常常提前出现终止密码子。
6. Causes of Gene Mutations | 基因突变的原因
Gene mutations can arise spontaneously during DNA replication or be induced by external mutagens. Understanding these causes helps explain how mutations accumulate in populations.
基因突变可在DNA复制过程中自发产生，也可由外部诱变剂诱导产生。了解这些原因有助于解释突变如何在群体中累积。
6.1 Spontaneous Mutations | 自发突变
These occur without any external influence, mainly due to errors in DNA replication. Although DNA polymerase has proofreading ability, mistakes still happen at a low frequency. Depurination and deamination of bases are also spontaneous causes.
自发突变在无外部影响的情况下发生，主要源于DNA复制错误。尽管DNA聚合酶具有校对功能，错误仍以低频率发生。碱基的脱嘌呤和脱氨基也是自发突变的原因。
6.2 Induced Mutations | 诱导突变
Mutagens increase the mutation rate. Physical mutagens include ionising radiation (X-rays, gamma rays, UV radiation). Chemical mutagens include base analogues, alkylating agents, and intercalating agents such as ethidium bromide.
诱变剂会增加突变率。物理诱变剂包括电离辐射（X射线、γ射线、紫外线）。化学诱变剂包括碱基类似物、烷化剂以及溴化乙锭等嵌入剂。
| Mutagen Type | 诱变剂类型 | Examples | 实例 | Mode of Action | 作用方式 |
|---|---|---|
| Ionising radiation 电离辐射 | X-rays, gamma rays | Causes DNA strand breaks and base changes |
| UV radiation 紫外线 | Sunlight | Forms thymine dimers, distorting DNA structure |
| Chemical mutagens 化学诱变剂 | Nitrous acid, base analogues | Cause base mispairing or insertions |
7. Effects of Gene Mutations on Protein Structure | 基因突变对蛋白质结构的影响
The consequences of a mutation depend on how it alters the amino acid sequence. Changes in the primary structure can affect the folding, stability, and function of the protein.
突变的影响取决于它如何改变氨基酸序列。一级结构的变化会影响蛋白质的折叠、稳定性和功能。
- Silent mutations: no change in amino acid sequence; protein function is usually unaffected.
- 同义突变：氨基酸序列不变，蛋白质功能通常不受影响。
- Missense mutations: one amino acid is changed; the effect ranges from mild to severe depending on the role of the altered amino acid.
- 错义突变：一个氨基酸发生改变；影响从轻微到严重，取决于该氨基酸的功能角色。
- Nonsense mutations: a stop codon appears early, producing a truncated protein that is usually non-functional.
- 无义突变：提前出现终止密码子，产生截短的蛋白质，通常无功能。
- Frameshift mutations: the entire reading frame after the mutation is altered, usually causing a complete loss of function.
- 移码突变：突变后的整个阅读框改变，通常导致功能完全丧失。
For example, sickle cell anaemia is caused by a missense mutation in the β-globin gene, where adenine is replaced by thymine, changing the sixth amino acid from glutamic acid to valine. This single amino acid substitution causes the haemoglobin to polymerise abnormally, leading to distorted red blood cells.
例如，镰状细胞性贫血由β-珠蛋白基因中的错义突变引起，腺嘌呤被胸腺嘧啶替代，使第六位氨基酸从谷氨酸变为缬氨酸。这一单个氨基酸替换导致血红蛋白异常聚合，红细胞变形。
Original DNA: TTC → mRNA: AAG → Glutamic acid
突变DNA: ATC → mRNA: UAG → Valine
8. Significance of Gene Mutations in Evolution and Disease | 基因突变在进化与疾病中的意义
Gene mutations are important in both evolutionary biology and medicine. They provide the raw material for natural selection. Beneficial mutations increase an organism’s fitness and may become more common in a population over generations. Harmful mutations can cause genetic disorders or predispose individuals to diseases such as cancer.
基因突变在进化生物学和医学中都具有重要意义。它们为自然选择提供了原材料。有利突变能提高生物体的适合度，并可能在世代中变得常见。有害突变则可能导致遗传疾病，或使个体易患癌症等疾病。
8.1 Role in Evolution | 在进化中的作用
Mutations introduce new alleles into a population, increasing genetic diversity. In changing environments, some mutations may confer a survival advantage. For example, mutations in bacterial genes can produce resistance to antibiotics, which is a classic example of natural selection acting on new variants.
突变将新等位基因引入群体，增加了遗传多样性。在不断变化的环境中，某些突变可能赋予生存优势。例如，细菌基因中的突变可能产生抗生素耐药性，这是自然选择作用于新变异的经典实例。
8.2 Role in Disease | 在疾病中的作用
Mutations in oncogenes and tumour suppressor genes are linked to cancer. For instance, a mutation in the p53 tumour suppressor gene reduces the cell’s ability to repair DNA damage, leading to uncontrolled cell division. In inherited disorders, mutations in gametes are passed to offspring, causing diseases like cystic fibrosis and haemophilia.
癌基因和肿瘤抑制基因的突变与癌症密切相关。例如，p53肿瘤抑制基因的突变降低了细胞修复DNA损伤的能力，导致细胞分裂失控。在遗传性疾病中，生殖细胞中的突变可传给子代，导致如囊性纤维化和血友病等疾病。
9. Conclusion | 总结
In summary, genes are the physical units of heredity that encode proteins via the triplet genetic code. Gene mutations are changes in DNA sequences that may arise spontaneously or be induced by mutagens. They can be classified as base substitutions or frameshift mutations, with varying effects on protein function. Understanding gene mutations is essential for explaining genetic variation, evolution, and the molecular basis of many human diseases.
总之，基因是通过三联体遗传密码编码蛋白质的遗传单位。基因突变是DNA序列的改变，可自发产生或由诱变剂诱导。它们可分为碱基替换和移码突变，对蛋白质功能有不同影响。理解基因突变是解释遗传变异、进化以及许多人类疾病分子基础的关键。
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