📚 Gene Expression | 基因表达
Gene expression is the process by which the information stored in a gene is used to synthesise a functional gene product, typically a protein. This process is fundamental to all living organisms, as proteins carry out most cellular functions. In IGCSE WJEC Biology, understanding gene expression involves exploring transcription, translation, the genetic code, and how mutations can affect protein synthesis. Mastering these concepts will help you explain how genotype determines phenotype and why organisms differ from one another.
基因表达是指储存在基因中的信息被用于合成功能性基因产物（通常是蛋白质）的过程。这一过程对所有生物体都至关重要，因为蛋白质执行着大多数细胞功能。在IGCSE WJEC生物学中，理解基因表达需要探索转录、翻译、遗传密码以及突变如何影响蛋白质合成。掌握这些概念将帮助你解释基因型如何决定表现型，以及生物体为何彼此不同。
1. DNA and Genes: The Blueprint | DNA 与基因：生命蓝图
DNA (deoxyribonucleic acid) is a long molecule made up of repeating units called nucleotides. Each nucleotide contains a phosphate group, a sugar (deoxyribose), and one of four nitrogenous bases: adenine (A), thymine (T), cytosine (C), or guanine (G). The DNA molecule is a double helix, with the two strands held together by complementary base pairing — A always pairs with T, and C always pairs with G. A gene is a specific sequence of nucleotides along a DNA molecule that codes for a particular polypeptide or protein. Genes are located on chromosomes in the nucleus.
DNA（脱氧核糖核酸）是一种由重复单元（核苷酸）组成的长分子。每个核苷酸包含一个磷酸基团、一个脱氧核糖以及四种含氮碱基之一：腺嘌呤（A）、胸腺嘧啶（T）、胞嘧啶（C）或鸟嘌呤（G）。DNA分子是双螺旋结构，两条链通过互补碱基配对连接在一起——A总是与T配对，C总是与G配对。基因是DNA分子上的一段特定核苷酸序列，它编码了某个特定的多肽或蛋白质。基因位于细胞核中的染色体上。
2. The Genetic Code | 遗传密码
The genetic code is the set of rules by which the sequence of bases in DNA is translated into the sequence of amino acids in a protein. The code is read in groups of three bases called triplets or codons (when referring to mRNA). Each codon corresponds to one specific amino acid. For example, the DNA triplet ‘TAC’ transcribes into the mRNA codon ‘AUG’, which codes for the amino acid methionine and also acts as a start signal for translation.
遗传密码是一套规则，DNA中的碱基序列按照这套规则被翻译成蛋白质中的氨基酸序列。密码以三个碱基为一组进行读取，称为三联体或密码子（指mRNA时）。每个密码子对应一个特定的氨基酸。例如，DNA三联体 ‘TAC’ 转录成mRNA密码子 ‘AUG’，这编码了氨基酸甲硫氨酸，同时也作为翻译的起始信号。
The genetic code has several important features: it is universal (the same codons code for the same amino acids in almost all organisms), degenerate (most amino acids are coded for by more than one codon), and non-overlapping (each base is read only once, as part of one triplet). These properties help to minimise the impact of some mutations.
遗传密码具有几个重要特征：它是通用的（几乎在所有生物体中，相同的密码子编码相同的氨基酸）、简并的（大多数氨基酸由不止一个密码子编码）和非重叠的（每个碱基只被读取一次，作为一个三联体的一部分）。这些特性有助于将某些突变的影响降到最低。
3. Transcription: From DNA to mRNA | 转录：从 DNA 到 mRNA
Transcription is the first stage of gene expression and occurs in the nucleus. The enzyme RNA polymerase binds to a specific region of the DNA at the start of a gene and unwinds the double helix. One strand of the DNA acts as a template. Free RNA nucleotides align opposite their complementary bases on the template strand (A with U, T with A, C with G, and G with C; note that RNA uses uracil (U) instead of thymine). RNA polymerase then joins these nucleotides together to form a molecule of messenger RNA (mRNA). Once the entire gene has been transcribed, the mRNA detaches from the DNA template, and the DNA rewinds. The mRNA molecule then moves out of the nucleus through a nuclear pore into the cytoplasm.
转录是基因表达的第一阶段，发生在细胞核中。RNA聚合酶与基因起始处DNA的一个特定区域结合，并解开双螺旋。DNA的一条链充当模板。游离的RNA核苷酸根据互补配对原则与模板链上的碱基对齐（A与U配对，T与A，C与G，G与C；注意RNA使用尿嘧啶U而不是胸腺嘧啶）。随后RNA聚合酶将这些核苷酸连接在一起，形成信使RNA（mRNA）分子。一旦整个基因被转录完毕，mRNA从DNA模板上脱离开，DNA重新螺旋化。然后mRNA分子通过核孔从细胞核进入细胞质。
4. Translation: From mRNA to Protein | 翻译：从 mRNA 到蛋白质
Translation takes place in the cytoplasm on ribosomes. The mRNA molecule attaches to a ribosome, and the ribosome moves along the mRNA reading the codons one by one. Transfer RNA (tRNA) molecules carry specific amino acids to the ribosome. Each tRNA has an anticodon — a sequence of three bases complementary to a particular mRNA codon — and a binding site for a specific amino acid. When a tRNA anticodon matches the mRNA codon, the tRNA temporarily binds and deposits its amino acid. The ribosome catalyses the formation of peptide bonds between adjacent amino acids, forming a polypeptide chain. The process continues until a stop codon (UAA, UAG, or UGA) is reached; no tRNA matches a stop codon, so translation terminates and the polypeptide is released.
翻译发生在细胞质的核糖体上。mRNA分子附着在核糖体上，核糖体沿着mRNA移动，逐个读取密码子。转运RNA（tRNA）分子携带特定的氨基酸到达核糖体。每个tRNA都有一个反密码子——一段与特定mRNA密码子互补的三个碱基序列——以及一个特定氨基酸的结合位点。当tRNA的反密码子与mRNA密码子匹配时，tRNA短暂结合并放下其氨基酸。核糖体催化相邻氨基酸之间形成肽键，从而形成多肽链。这一过程持续进行，直到遇到终止密码子（UAA、UAG或UGA）；没有tRNA匹配终止密码子，因此翻译终止，多肽链被释放。
In eukaryotes, multiple ribosomes can translate a single mRNA molecule simultaneously, forming a structure called a polysome. This allows the cell to produce many copies of a protein quickly from one mRNA transcript.
在真核生物中，多个核糖体可以同时翻译同一个mRNA分子，形成一种称为多聚核糖体的结构。这使得细胞能够从一条mRNA转录本快速生产出许多蛋白质拷贝。
5. The Roles of mRNA and tRNA | mRNA 和 tRNA 的作用
mRNA (messenger RNA) serves as a transient copy of a gene that carries the genetic instructions from the nucleus to the ribosome. Its sequence of codons directly determines the order of amino acids in the polypeptide. Because mRNA is single-stranded and relatively short-lived, it allows the gene in the DNA to be protected while the information is used in the cytoplasm.
mRNA（信使RNA）作为基因的暂态拷贝，将遗传指令从细胞核运送到核糖体。其密码子序列直接决定了多肽链中氨基酸的顺序。由于mRNA是单链且寿命相对较短，它使得DNA中的基因在信息被利用于细胞质的同时得到保护。
tRNA (transfer RNA) acts as an adaptor molecule that interprets the genetic code. Each tRNA molecule is folded into a cloverleaf shape and carries a single type of amino acid. Its anticodon loop contains a triplet complementary to an mRNA codon. This ensures that the correct amino acid is added to the growing polypeptide chain according to the codon sequence in the mRNA.
tRNA（转运RNA）充当了解读遗传密码的接头分子。每个tRNA分子折叠成三叶草形状，只携带一种类型的氨基酸。它的反密码子环含有一个与mRNA密码子互补的三联体。这确保了根据mRNA中的密码子序列，正确的氨基酸被添加到不断延长的多肽链上。
6. Protein Structure and Function | 蛋白质的结构与功能
The polypeptide chain produced during translation folds into a specific three-dimensional shape to become a functional protein. The sequence of amino acids (primary structure) determines the folding pattern and the final conformation of the protein. This shape is critical for the protein’s function. For example, enzymes have an active site that precisely fits their substrate; structural proteins like collagen provide support; and haemoglobin carries oxygen. If the amino acid sequence is altered, the protein may not fold correctly and may lose its function.
在翻译过程中产生的多肽链折叠成特定的三维形状，从而成为有功能的蛋白质。氨基酸序列（一级结构）决定了折叠模式和蛋白质的最终构象。这种形状对蛋白质的功能至关重要。例如，酶具有一个活性位点，能精确地与其底物结合；结构蛋白如胶原蛋白提供支撑；血红蛋白运输氧气。如果氨基酸序列发生改变，蛋白质可能无法正确折叠，从而失去其功能。
7. Gene Mutations: Types and Causes | 基因突变：类型与原因
A gene mutation is a change in the sequence of nucleotide bases in DNA. Mutations can occur spontaneously during DNA replication or can be induced by mutagens such as radiation and certain chemicals. There are several types of gene mutations. A substitution mutation replaces one base with another, which may alter a single codon and possibly change one amino acid. An insertion or deletion mutation adds or removes one or more bases. Because the genetic code is read in triplets, insertions and deletions often cause a frameshift — all subsequent codons are read differently, leading to a completely different amino acid sequence from the point of mutation onwards. Frameshift mutations usually have a severe impact on the protein.
基因突变是指DNA中核苷酸碱基序列的改变。突变可以在DNA复制过程中自发产生，也可以由诱变因素（如辐射和某些化学物质）诱导。基因突变有几种类型：置换突变将一个碱基替换为另一个碱基，这可能改变单个密码子，并可能改变一个氨基酸。插入或缺失突变会增加或移除一个或多个碱基。由于遗传密码以三联体形式读取，插入和缺失通常导致移码——从突变点开始，所有后续密码子都被错误读取，导致完全不同的氨基酸序列。移码突变通常对蛋白质产生严重影响。
8. Effects of Mutations on Protein Function | 突变对蛋白质功能的影响
The effect of a mutation depends on where it occurs and how it changes the amino acid sequence. A substitution may have no effect if it codes for the same amino acid (due to the degeneracy of the genetic code), or it may cause a minor change if the new amino acid has similar properties. However, a substitution that replaces an amino acid critical for the protein’s structure or active site can render the protein non-functional. Insertion and deletion mutations are more likely to produce a non-functional protein because they disrupt the entire reading frame. In some cases, a mutation can lead to genetic disorders; for example, the inherited disorder cystic fibrosis is often caused by the deletion of three bases in the CFTR gene, resulting in the loss of a single amino acid and a defective protein.
突变的影响取决于其发生的位置以及它对氨基酸序列的改变方式。如果置换突变编码了同一个氨基酸（由于遗传密码的简并性），则可能没有影响；如果新氨基酸具有相似的特性，也可能只引起较小的变化。然而，如果置换突变替换了对蛋白质结构或活性位点至关重要的氨基酸，则可能使蛋白质丧失功能。插入和缺失突变更有可能产生无功能的蛋白质，因为它们扰乱了整个阅读框。在某些情况下，突变会导致遗传性疾病；例如，遗传病囊性纤维化通常是由CFTR基因中三个碱基的缺失引起的，导致一个氨基酸丢失，产生有缺陷的蛋白质。
Not all mutations are harmful; some are neutral, and very rarely a mutation can confer an advantage, which is the raw material for evolution by natural selection.
并非所有的突变都是有害的；有些是中性的，极少数情况下突变可能带来优势，这正是自然选择进化的原材料。
9. Regulation of Gene Expression | 基因表达的调控
In multicellular organisms, every cell contains the same set of genes, yet different cells express different genes. This differential gene expression allows cells to become specialised (differentiated) for particular functions. For example, a red blood cell produces haemoglobin, while a muscle cell produces actin and myosin. Gene expression can be regulated at the transcriptional level — proteins called transcription factors can bind to DNA near a gene and either promote or inhibit the binding of RNA polymerase. This ensures that genes are switched on or off at the correct time and in the correct cell type. WJEC IGCSE candidates should understand that only certain genes are ‘switched on’ in any given cell, producing the specific proteins needed for that cell’s function.
在多细胞生物中，每个细胞都含有相同的一套基因，但不同细胞表达不同的基因。这种差异化的基因表达使得细胞能够特化（分化）出特定的功能。例如，红细胞产生血红蛋白，而肌细胞产生肌动蛋白和肌球蛋白。基因表达可以在转录水平上进行调控——称为转录因子的蛋白质能结合到基因附近的DNA上，促进或抑制RNA聚合酶的结合。这确保了基因在正确的时间和正确的细胞类型中被开启或关闭。WJEC IGCSE考生应理解，在任何特定细胞中只有部分基因被“开启”，产生该细胞功能所需的特定蛋白质。
10. Enzymes and Gene Expression | 酶与基因表达
Enzymes are proteins that act as biological catalysts. Since genes code for proteins, they also determine the structure of enzymes. The active site of an enzyme is shaped precisely to bind its substrate. A mutation in the gene coding for an enzyme can alter the active site’s shape, preventing the substrate from binding and stopping the enzyme from functioning. This can lead to metabolic disorders. For example, a mutation in the gene for the enzyme phenylalanine hydroxylase disrupts the breakdown of the amino acid phenylalanine, causing the condition phenylketonuria (PKU). Understanding the link between gene expression and enzyme activity helps explain many inborn errors of metabolism.
酶是起生物催化作用的蛋白质。因为基因编码蛋白质，它们也决定了酶的结构。酶的活性位点形状精确，能够结合其底物。编码酶的基因发生突变可能会改变活性位点的形状，阻止底物结合，使酶无法发挥功能。这可能导致代谢紊乱。例如，编码苯丙氨酸羟化酶的基因发生突变会破坏氨基酸苯丙氨酸的分解，引起苯丙酮尿症（PKU）。理解基因表达与酶活性之间的联系有助于解释许多先天性代谢缺陷。
11. Experimental Evidence and Key Terminology | 实验证据与关键术语
In WJEC IGCSE Biology, you may be expected to recall key experiments that contributed to our understanding of gene expression, such as the work of Beadle and Tatum, who proposed the ‘one gene–one enzyme’ hypothesis. While the hypothesis has been refined to ‘one gene–one polypeptide’, the principle remains that a gene carries the information for a specific protein. Be able to define important terms: genotype (the genetic makeup of an organism), phenotype (the observable characteristics resulting from the genotype and its interaction with the environment), allele (a variant form of a gene), homozygous, heterozygous, dominant, and recessive.
在WJEC IGCSE生物学中，你可能需要回忆起一些关键实验，这些实验有助于我们理解基因表达，例如Beadle和Tatum的工作，他们提出了“一个基因一个酶”的假说。虽然该假说已被完善为“一个基因一条多肽”，但其原理依然是：一个基因携带了一个特定蛋白质的信息。要能够定义重要术语：基因型（一个生物体的遗传组成）、表现型（由基因型及其与环境互作产生的可观察特征）、等位基因（基因的变异形式）、纯合子、杂合子、显性和隐性。
12. Summary of the Gene Expression Process | 基因表达过程总结
The overall flow of genetic information in cells follows the framework of the ‘Central Dogma’: DNA → RNA → Protein. DNA is transcribed into mRNA in the nucleus; mRNA is then translated into a specific sequence of amino acids at the ribosome, with the help of tRNA. The resulting polypeptide folds into a functional protein. This multi-step process is tightly regulated and any disruption can lead to altered proteins and potential disease. For your WJEC IGCSE exam, ensure you can describe each stage in detail, explain the roles of different RNA molecules, and discuss how a mutation in a gene can affect protein structure and function.
细胞中遗传信息的总体流向遵循“中心法则”的框架：DNA → RNA → 蛋白质。DNA在细胞核中被转录为mRNA；随后mRNA在核糖体上借助tRNA被翻译成特定的氨基酸序列。生成的多肽链折叠成为有功能的蛋白质。这一多步骤过程受到严格调控，任何中断都可能导致蛋白质改变和潜在的疾病。为了你的WJEC IGCSE考试，确保你能详细描述每个阶段，解释不同RNA分子的作用，并讨论基因突变如何影响蛋白质的结构和功能。
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