📚 Mutations | 基因突变
Mutations are permanent changes in the nucleotide sequence of DNA or in the structure and number of chromosomes. They are the ultimate source of genetic variation in all living organisms and are essential for evolution, although most individual mutations are neutral or harmful.
突变是指DNA核苷酸序列或染色体结构与数目发生的永久性改变。突变是所有生物体遗传变异的最终来源，对进化至关重要，但大多数单个突变是中性的或有害的。
1. Defining Mutations | 突变的定义
In Cambridge A-Level Biology, a mutation is defined as a change in the base sequence of DNA or a change in chromosome structure or number. Mutations can occur in somatic cells, where they affect only the individual, or in germ cells, where they can be inherited by offspring.
在剑桥A-Level生物中，突变被定义为DNA碱基序列的改变或染色体结构与数目的改变。突变可以发生在体细胞中，只影响个体本身；也可以发生在生殖细胞中，并能遗传给后代。
Mutations are random with respect to the needs of the organism. They generate new alleles and therefore increase genetic diversity. However, most mutations are either neutral or damaging, and only a small fraction are beneficial in a particular environment.
突变对生物体而言是随机的，并不针对生物体的需要。它们产生新的等位基因，从而增加遗传多样性。然而，大多数突变是中性或有害的，只有一小部分在特定环境中是有益的。
2. Gene Mutations: Base Substitution | 基因突变：碱基替换
A gene mutation is a change in the base sequence of a single gene. The simplest type is a base substitution, also called a point mutation, in which one nucleotide is replaced by another. For example, on the coding DNA strand, a cytosine may be replaced by thymine.
基因突变是单个基因中碱基序列的改变。最简单的类型是碱基替换，也称点突变，即一个核苷酸被另一个核苷酸替代。例如，在编码DNA链上，胞嘧啶可能被胸腺嘧啶替换。
Because the genetic code is degenerate, a substitution does not always change the amino acid specified. If it changes the codon to one that codes for a different amino acid, it is a missense mutation. If it changes the codon to a stop codon, it is a nonsense mutation.
由于遗传密码具有简并性，碱基替换并不总是改变所编码的氨基酸。如果密码子变成编码不同氨基酸的密码子，则为错义突变；如果密码子变成终止密码子，则为无义突变。
3. Insertion and Deletion Mutations | 插入突变与缺失突变
Insertion mutations add one or more nucleotides into a DNA sequence, while deletion mutations remove one or more nucleotides. These changes are called indel mutations. If the number of inserted or deleted bases is not a multiple of three, the reading frame of the gene is shifted.
插入突变向DNA序列中增加一个或多个核苷酸，而缺失突变则移除一个或多个核苷酸。这些改变统称为插入缺失突变。如果插入或缺失的碱基数不是3的倍数，基因的阅读框就会发生移位。
A frameshift mutation changes every codon downstream of the mutation site. The resulting polypeptide is usually completely different from the normal polypeptide and is often non-functional. In contrast, insertion or deletion of exactly three bases removes or adds one amino acid without shifting the reading frame.
移码突变会改变突变位点下游的每一个密码子。由此产生的多肽通常与正常多肽完全不同，并且往往没有功能。相反，恰好插入或缺失三个碱基只会删除或添加一个氨基酸，而不会使阅读框移位。
4. Effects of Gene Mutations on Protein Structure | 基因突变对蛋白质结构的影响
The effect of a gene mutation depends on how it changes the amino acid sequence. A silent mutation has no effect on the polypeptide because the new codon still specifies the same amino acid. A missense mutation changes one amino acid, which may alter protein folding and function.
基因突变的影响取决于它如何改变氨基酸序列。沉默突变对多肽没有影响，因为新密码子仍然编码相同的氨基酸。错义突变会改变一个氨基酸，可能改变蛋白质的折叠和功能。
A nonsense mutation creates a premature stop codon, causing translation to stop early and producing a truncated protein that is usually unable to perform its normal role. Frameshift mutations often produce both missense and nonsense effects over a long stretch of the polypeptide.
无义突变会产生提前的终止密码子，使翻译提前终止，产生截短的蛋白质，通常无法发挥正常功能。移码突变通常在多肽的一大段范围内同时造成错义和无义效应。
| Outcome | 结果 | Codon change | 密码子变化 | Effect on protein | 对蛋白质的影响 |
|---|---|---|
| Silent | 沉默 | Same amino acid encoded | 编码相同氨基酸 | No effect on protein | 对蛋白质无影响 |
| Missense | 错义 | Different amino acid | 不同氨基酸 | May alter folding or function | 可能改变折叠或功能 |
| Nonsense | 无义 | Stop codon formed | 形成终止密码子 | Truncated, usually non-functional protein | 截短，通常无功能蛋白 |
| Frameshift | 移码 | Reading frame shifted | 阅读框移位 | Extensive sequence change, usually non-functional | 序列大量改变，通常无功能 |
5. Chromosome Mutations: Structural Changes | 染色体突变：结构改变
Chromosome mutations involve changes in the structure or number of whole chromosomes. Structural changes include deletion, duplication, inversion and translocation. Deletion removes a segment of a chromosome; duplication copies a segment; inversion reverses a segment; translocation moves a segment to a non-homologous chromosome.
染色体突变涉及整条染色体结构或数目的改变。结构改变包括缺失、重复、倒位和易位。缺失会移除一段染色体；重复会复制一段；倒位会反转一段；易位则把一段染色体移动到非同源染色体上。
These structural changes may disrupt genes at breakpoints or alter gene dosage. Some rearrangements may have little visible effect if they do not involve critical genes, while others can cause serious developmental abnormalities or disease.
这些结构改变可能破坏断裂点处的基因或改变基因剂量。如果重排不涉及关键基因，有些可能没有明显效应；而另一些则可能导致严重的发育异常或疾病。
6. Chromosome Mutations: Aneuploidy and Polyploidy | 染色体突变：非整倍体与多倍体
Aneuploidy is the gain or loss of individual chromosomes. It usually results from non-disjunction during meiosis, when homologous chromosomes or sister chromatids fail to separate. An example is trisomy 21, in which three copies of chromosome 21 cause Down syndrome.
非整倍体是指获得或丢失个别染色体。它通常由减数分裂中的不分离引起，即同源染色体或姐妹染色单体未能分开。例如，21三体即有三条21号染色体，导致唐氏综合征。
Polyploidy is the presence of more than two complete sets of chromosomes. A normal diploid cell is 2n; a triploid cell is 3n and a tetraploid cell is 4n. Polyploidy is common in plants and can be induced by chemicals such as colchicine. Polyploid plants often have larger cells and may be more vigorous, and polyploidy can lead to rapid reproductive isolation.
多倍体是指存在两套以上的完整染色体组。正常二倍体细胞为2n；三倍体细胞为3n，四倍体细胞为4n。多倍体在植物中很常见，可被秋水仙素等化学物质诱导。多倍体植物通常细胞更大、可能更强壮，并且多倍体可导致快速的生殖隔离。
7. Causes of Mutations | 突变的原因
Mutations arise spontaneously during DNA replication because DNA polymerase occasionally inserts an incorrect base. Cells have proofreading and mismatch repair systems that correct most errors, but a low frequency of mutations still escapes repair.
突变可在DNA复制过程中自发产生，因为DNA聚合酶偶尔会插入错误的碱基。细胞具有校对和错配修复系统，可以纠正大多数错误，但仍有低频率的突变未被修复。
Mutagens increase the rate of mutation. Physical mutagens include ionising radiation such as X-rays and gamma rays, and ultraviolet light, which can cause thymine dimers. Chemical mutagens include base analogs, deaminating agents and substances in tobacco smoke such as benzopyrene. Some viruses and transposable elements also cause mutations by inserting DNA into genes.
诱变剂会提高突变率。物理诱变剂包括X射线和γ射线等电离辐射，以及能引起胸腺嘧啶二聚体的紫外线。化学诱变剂包括碱基类似物、脱氨剂和烟草烟雾中的苯并芘等物质。一些病毒和转座因子也通过将DNA插入基因而引起突变。
8. Sickle Cell Anaemia: A Case Study | 镰状细胞贫血：案例研究
Sickle cell anaemia is caused by a single base substitution in the gene for the beta-globin chain of haemoglobin. In the coding DNA strand, the codon GAG is changed to GTG. The mRNA codon changes from GAG to GUG, so glutamic acid is replaced by valine at position 6 of the beta chain.
镰状细胞贫血由血红蛋白β-珠蛋白链基因中的单个碱基替换引起。在编码DNA链中，密码子GAG变为GTG。mRNA密码子从GAG变为GUG，因此β链第6位的谷氨酸被缬氨酸取代。Published by TutorHao | A-Level Biology Revision Series | aleveler.com
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