📚 Genes and mutations | 基因与突变
Genes are the fundamental units of heredity, carrying the instructions that build and maintain an organism. Mutations are permanent changes in the DNA sequence of a gene or chromosome, and they provide the raw material for evolution while also being responsible for many genetic disorders and cancers.
基因是遗传的基本单位，携带着构建和维持生物体的指令。突变是基因或染色体 DNA 序列中永久性的改变，它们既为进化提供原材料，也与许多遗传病和癌症的发生密切相关。
1. The gene as a unit of heredity | 基因作为遗传单位
A gene is a specific sequence of DNA nucleotides that codes for a functional product, usually a polypeptide or an RNA molecule. In Cambridge A-Level Biology, a gene is defined as a length of DNA that occupies a particular locus on a chromosome and carries the information needed to produce one polypeptide or a functional RNA such as tRNA or rRNA.
基因是一段特定的 DNA 核苷酸序列，编码一种功能性产物，通常是一条多肽或一种 RNA 分子。在剑桥 A-Level 生物课程中，基因被定义为占据染色体上特定基因座的一段 DNA，它携带产生一条多肽或一种功能性 RNA（如 tRNA 或 rRNA）所需的信息。
Most genes consist of coding regions called exons and non-coding regions called introns. In eukaryotic cells, introns are removed during RNA splicing before translation, so only the exons contribute directly to the amino acid sequence of the protein.
大多数基因由编码区（称为外显子）和非编码区（称为内含子）组成。在真核细胞中，内含子在翻译前经过 RNA 剪接被去除，因此只有外显子直接决定蛋白质的氨基酸序列。
The sequence of nucleotide bases along a gene is read in triplets, with each triplet of bases forming a codon. The order of codons determines the order of amino acids in a polypeptide, giving the gene its information content.
基因中核苷酸碱基的序列以三联体形式读取，每三个碱基组成一个密码子。密码子的顺序决定多肽中氨基酸的顺序，从而赋予基因信息内容。
2. Alleles and genetic variation | 等位基因与遗传变异
Different forms of the same gene are called alleles. Alleles arise through mutation and occupy the same gene locus on homologous chromosomes, but they may differ in one or more base pairs and therefore produce slightly different versions of the same polypeptide.
同一基因的不同形式称为等位基因。等位基因通过突变产生，占据同源染色体上相同的基因座，但它们可能在一个或多个碱基对上存在差异，因此产生略有不同的同一多肽版本。
If the two alleles at a locus are identical, the individual is homozygous for that gene. If the alleles are different, the individual is heterozygous. The phenotype expressed depends on dominance relationships between the alleles and on environmental influences.
如果某一基因座上的两个等位基因相同，个体在该基因上是纯合的。如果等位基因不同，个体则是杂合的。表现型取决于等位基因之间的显隐性关系以及环境的影响。
For example, the gene for haemoglobin beta chain has a normal allele HbA and a mutant allele HbS. Heterozygous individuals carry both alleles and usually show no severe symptoms because the normal allele produces enough functional haemoglobin.
例如，血红蛋白 β 链基因有一个正常等位基因 HbA 和一个突变等位基因 HbS。杂合个体同时携带两个等位基因，通常不表现严重症状，因为正常等位基因能产生足够的功能性血红蛋白。
3. The genetic code and protein synthesis | 遗传密码与蛋白质合成
The genetic code is the set of rules by which information encoded in mRNA is translated into a polypeptide sequence. It is degenerate, meaning that most amino acids are specified by more than one codon, and it is universal across almost all organisms.
遗传密码是将 mRNA 中编码的信息翻译成多肽序列的一套规则。它具有简并性，意味着大多数氨基酸由不止一个密码子编码；同时它几乎是所有生物通用的。
Protein synthesis occurs in two main stages: transcription and translation. During transcription, the DNA sequence of a gene is copied into messenger RNA. During translation, ribosomes read the mRNA codons and attach transfer RNA molecules carrying the matching amino acids to build a polypeptide chain.
蛋白质合成主要包括两个阶段：转录和翻译。在转录过程中，基因的 DNA 序列被复制为信使 RNA。在翻译过程中，核糖体读取 mRNA 密码子，并让携带相应氨基酸的转运 RNA 分子结合，以构建多肽链。
Because the genetic code is read in non-overlapping triplets, a small change in a single DNA base can sometimes alter a codon and change the amino acid inserted at that position. This is the molecular basis of many gene mutations.
由于遗传密码以不重叠的三联体方式读取，单个 DNA 碱基的微小改变有时会改变一个密码子，从而改变该位置插入的氨基酸。这是许多基因突变的分子基础。
4. What is a mutation? | 什么是突变？
A mutation is a permanent change in the nucleotide sequence of DNA. Mutations can occur in somatic cells or in germ cells. Only germ-line mutations can be passed on to offspring, while somatic mutations affect only the individual and may lead to diseases such as cancer.
突变是 DNA 核苷酸序列的永久性改变。突变可以发生在体细胞或生殖细胞中。只有生殖细胞突变才能传递给后代，而体细胞突变只影响个体本身，并可能导致癌症等疾病。
Mutations may involve a single nucleotide or larger sections of a chromosome. Gene mutations affect one gene and are often caused by base substitution, insertion or deletion. Chromosome mutations change the structure or number of whole chromosomes, such as in Down syndrome.
突变可能涉及单个核苷酸或染色体中较大的片段。基因突变影响一个基因，通常由碱基替换、插入或缺失引起。染色体突变改变整条染色体的结构或数目，例如唐氏综合征。
The rate of spontaneous mutation is usually very low because DNA polymerase proofreads newly synthesised DNA and mismatch repair systems correct errors. However, environmental mutagens and replication mistakes can still introduce permanent changes.
自发突变的频率通常很低，因为 DNA 聚合酶会校对刚合成的 DNA，错配修复系统也会纠正错误。然而，环境诱变剂和复制错误仍可能引入永久性改变。
5. Types of gene mutations | 基因突变的类型
Base substitution is a gene mutation in which one nucleotide is replaced by another. This may be a transition or a transversion. A substitution can be silent, missense or nonsense depending on its effect on the polypeptide.
碱基替换是一种基因突变，其中一个核苷酸被另一个核苷酸替换。这可以是转换或颠换。根据对多肽的影响，替换可以是沉默突变、错义突变或无义突变。
A silent mutation changes a codon but still codes for the same amino acid because of the degeneracy of the genetic code. A missense mutation changes the codon so that a different amino acid is inserted. A nonsense mutation changes an amino acid codon into a stop codon, causing premature termination of translation and usually a non-functional protein.
沉默突变改变了密码子，但由于遗传密码的简并性，仍然编码相同的氨基酸。错义突变改变密码子，使插入不同的氨基酸。无义突变将一个氨基酸密码子变为终止密码子，导致翻译提前终止，通常产生无功能的蛋白质。
Insertion and deletion mutations, also called frameshift mutations, add or remove one or more nucleotides. If the number of inserted or deleted bases is not a multiple of three, the reading frame of the whole gene downstream is shifted, which usually produces a completely different amino acid sequence from that point onward.
插入和缺失突变，也称移码突变，会添加或移除一个或多个核苷酸。如果插入或缺失的碱基数不是 3 的倍数，基因下游的整个读码框就会发生移动，通常从该点起产生完全不同的氨基酸序列。
6. Causes of mutations | 突变的原因
Mutations can arise spontaneously through errors in DNA replication, such as base mispairing or slippage of DNA polymerase at repetitive sequences. Spontaneous mutations are rare, typically occurring at a rate of about 10⁻⁹ per base pair per replication in bacteria.
突变可以通过 DNA 复制中的错误自发产生，例如碱基错配或 DNA 聚合酶在重复序列处滑动。自发突变很罕见，在细菌中每次复制每个碱基对的发生率约为 10⁻⁹。
Mutations can also be induced by mutagens. Physical mutagens include ionising radiation such as X-rays and ultraviolet light, which can break DNA strands or cause thymine dimers. Chemical mutagens include base analogues, deaminating agents and alkylating agents that modify nucleotide structure.
突变也可以由诱变剂诱发。物理诱变剂包括 X 射线等电离辐射和紫外线，它们可以断裂 DNA 链或引起胸腺嘧啶二聚体。化学诱变剂包括碱基类似物、脱氨剂和烷化剂，它们会改变核苷酸结构。
Some viruses and transposable elements can insert their genetic material into a host gene and disrupt its function. This type of insertional mutagenesis can cause gene inactivation or abnormal expression.
一些病毒和转座元件可以将自身遗传物质插入宿主基因并破坏其功能。这种插入诱变可导致基因失活或异常表达。
7. Effects of mutations on proteins | 突变对蛋白质的影响
The effect of a gene mutation depends on which codon is changed and how the altered amino acid influences protein folding and function. Silent mutations have no effect on the amino acid sequence, while missense mutations may change one amino acid and alter protein shape or activity.
基因突变的影响取决于哪个密码子发生改变，以及被替换的氨基酸如何影响蛋白质折叠和功能。沉默突变对氨基酸序列没有影响，而错义突变可能改变一个氨基酸，从而改变蛋白质的形状或活性。
A nonsense mutation inserts a premature stop codon, resulting in a truncated polypeptide that is usually rapidly degraded. A frameshift
Published by TutorHao | A-Level Biology Revision Series | aleveler.com
更多咨询请联系16621398022（同微信）
屏轩国际教育cambridge primary/secondary checkpoint, cat4, ukiset,ukcat,igcse,alevel,PAT,STEP,MAT, ibdp,ap,ssat,sat,sat2课程辅导，国外大学本科硕士研究生博士课程论文辅导