A-Level生物 DNA复制转录翻译基因表达

1. DNA结构与中心法则 DNA Structure and the Central Dogma

DNA (deoxyribonucleic acid) is a double-stranded polynucleotide that carries the genetic instructions for all living organisms. Each strand consists of a sugar-phosphate backbone with nitrogenous bases (adenine, thymine, cytosine, guanine) projecting inward. The two strands run antiparallel (5′ to 3′ and 3′ to 5′) and are held together by hydrogen bonds between complementary base pairs: adenine pairs with thymine (A-T) via two hydrogen bonds, while cytosine pairs with guanine (C-G) via three hydrogen bonds. The sequence of bases along a DNA strand encodes genetic information, and the double-helix structure ensures faithful copying during cell division.

DNA（脱氧核糖核酸）是携带所有生物体遗传指令的双链多核苷酸。每条链由糖-磷酸骨架和含氮碱基（A、T、C、G）构成。两条链反向平行（5’到3’和3’到5’），通过互补碱基对（A-T和C-G）之间的氢键连接。DNA碱基序列编码遗传信息，双螺旋结构确保细胞分裂时精确复制。

The central dogma of molecular biology describes genetic information flow: DNA is replicated, transcribed into mRNA, and translated into protein. Proposed by Francis Crick in 1958, it explains how genotype determines phenotype. Understanding replication, transcription, and translation is essential for A-Level Biology.

分子生物学的中心法则描述遗传信息流动：DNA复制，转录为mRNA，翻译成蛋白质。由Francis Crick在1958年提出，它解释了基因型如何决定表型。理解复制、转录和翻译对A-Level生物至关重要。

2. DNA复制：半保留机制 DNA Replication: Semiconservative Mechanism

DNA replication is semiconservative: each new DNA molecule has one original strand and one newly synthesised strand. The Meselson-Stahl experiment (1958) proved this using nitrogen isotopes (N-15, N-14) to track DNA across generations of E. coli, ruling out conservative and dispersive models. This experiment is a classic and frequently appears in A-Level exams.

DNA复制是半保留的：每个新DNA分子含一条亲代链和一条子代链。Meselson和Stahl的实验（1958年）用氮同位素（N-15、N-14）追踪大肠杆菌DNA，排除了保守和分散模型。这是经典实验，常出现在A-Level考题中。

Replication begins at specific sequences called origins of replication, where the enzyme DNA helicase unwinds the double helix by breaking hydrogen bonds between base pairs. This creates a replication fork ： a Y-shaped structure where both strands serve as templates. Single-strand binding proteins (SSBs) stabilise the separated strands and prevent them from reannealing. The enzyme DNA gyrase (a type of topoisomerase) relieves the torsional stress that builds up ahead of the replication fork as the DNA unwinds ： without it, the DNA would become supercoiled and replication would stall.

复制从特定的序列（复制起点）开始，DNA解旋酶在此处通过断裂碱基对之间的氢键来解开双螺旋。这产生了一个复制叉：一个Y形结构，两条链都作为模板。单链结合蛋白（SSB）稳定分离的链并防止它们重新配对。DNA旋转酶（一种拓扑异构酶）缓解复制叉前方因DNA解旋而产生的扭转应力：没有它，DNA会变得过度螺旋化，复制就会停滞。

DNA polymerase III cannot initiate synthesis de novo; it requires a short RNA primer synthesised by primase. It then extends the new strand in the 5′ to 3′ direction by adding complementary nucleotides, using the parental strand as a template. The leading strand is synthesised continuously toward the replication fork. The lagging strand is synthesised discontinuously in short fragments called Okazaki fragments, each requiring its own RNA primer. DNA polymerase I removes the RNA primers and replaces them with DNA, and DNA ligase seals the gaps between Okazaki fragments by forming phosphodiester bonds. This process is highly accurate, with an error rate of about one per billion base pairs, due to proofreading by DNA polymerase III.

DNA聚合酶III不能从头启动合成，它需要由引物酶合成的一条短RNA引物。然后它沿着5’到3’方向延伸新链，以亲代链为模板添加互补核苷酸。前导链朝向复制叉连续合成。滞后链以短片段（称为冈崎片段）不连续合成，每个片段都需要自己的RNA引物。DNA聚合酶I移除RNA引物并用DNA替换，DNA连接酶通过形成磷酸二酯键封闭冈崎片段之间的缺口。这套分子机器非常精确，由于DNA聚合酶III的校对活性，错误率约为每十亿碱基对一次。

3. 转录：从DNA到mRNA Transcription: From DNA to mRNA

Transcription is the synthesis of an RNA molecule from a DNA template, catalysed by the enzyme RNA polymerase. Unlike DNA replication, transcription does not require a primer and only one strand of DNA ： the template strand (antisense strand) ： is transcribed. The other strand (sense strand) has the same sequence as the resulting mRNA, except that thymine (T) is replaced by uracil (U) in RNA. Transcription begins when RNA polymerase binds to a promoter region (including the TATA box in eukaryotes) upstream of the gene, causing the DNA to unwind locally (forming a transcription bubble of about 17 base pairs).

转录是以DNA为模板合成RNA分子的过程，由RNA聚合酶催化。与DNA复制不同，转录不需要引物，且只有一条DNA链：模板链（反义链）：被转录。另一条链（有义链）与产生的mRNA序列相同，只是RNA中的胸腺嘧啶（T）被尿嘧啶（U）取代。当RNA聚合酶与基因上游的启动子区域（包括真核生物中的TATA框）结合时，转录开始，导致DNA局部解旋（形成约17个碱基对的转录泡）。

RNA polymerase moves along the template strand 3′ to 5′, assembling ribonucleotides 5′ to 3′. The three stages are initiation (promoter binding), elongation (nucleotide addition), and termination (transcript release). In prokaryotes, a hairpin-loop terminator causes dissociation; in eukaryotes, the AAUAAA polyadenylation signal triggers cleavage.

RNA聚合酶沿模板链3’到5’移动，以5’到3’方向组装核糖核苷酸。三个阶段为起始（启动子结合）、延伸（核苷酸添加）和终止（转录本释放）。原核生物中发夹环终止子导致解离；真核生物中AAUAAA多聚腺苷酸化信号触发切割。

4. mRNA加工：真核生物特有的步骤 mRNA Processing: A Eukaryote-Specific Step

In eukaryotes, the primary transcript (pre-mRNA) undergoes extensive processing in the nucleus before it becomes mature mRNA ready for translation. Three key modifications occur: capping, polyadenylation, and splicing. The 5′ cap is a modified guanine nucleotide (7-methylguanosine) added to the 5′ end of the pre-mRNA. This cap protects the mRNA from degradation by exonucleases and facilitates ribosome binding during translation initiation. The cap is added co-transcriptionally ： that is, while transcription is still ongoing.

在真核生物中，初级转录本（pre-mRNA）在成为可进行翻译的成熟mRNA之前，需要在细胞核中进行广泛的加工。三种关键修饰发生：加帽、多聚腺苷酸化和剪接。5’帽是一个添加在pre-mRNA 5’端的修饰鸟嘌呤核苷酸（7-甲基鸟苷）。这个帽保护mRNA免受核酸外切酶的降解，并在翻译起始过程中促进核糖体结合。帽是与转录同时添加的：即在转录仍在进行时就加上去了。

The 3′ poly-A tail consists of approximately 200 adenine nucleotides added to the 3′ end of the pre-mRNA after cleavage at the polyadenylation signal. The poly-A tail also protects mRNA from degradation and aids in the export of mRNA from the nucleus to the cytoplasm. The most important modification is splicing: removal of introns (non-coding sequences) and joining of exons (coding sequences) by the spliceosome. Alternative splicing allows one gene to code for multiple proteins, expanding the eukaryotic genome’s capacity.

3′ poly-A尾由大约200个腺嘌呤核苷酸组成，在多聚腺苷酸化信号处切割后添加到pre-mRNA的3’端。poly-A尾同样保护mRNA免受降解，并帮助mRNA从细胞核输出到细胞质。最重要的修饰是剪接：去除内含子（非编码序列）并将外显子（编码序列）由剪接体连接在一起。可变剪接使单个基因能编码多种蛋白质，扩展了真核基因组的功能储备。

5. 翻译：从mRNA到蛋白质 Translation: From mRNA to Protein

Translation is the synthesis of a polypeptide chain from an mRNA template, occurring on ribosomes in the cytoplasm. Ribosomes are composed of two subunits (large and small) made from ribosomal RNA (rRNA) and proteins. The small subunit binds to the mRNA and recognises the start codon, while the large subunit catalyses the formation of peptide bonds between adjacent amino acids. The ribosome has three binding sites for transfer RNA (tRNA): the A site (aminoacyl ： incoming tRNA), the P site (peptidyl ： tRNA carrying the growing polypeptide), and the E site (exit ： tRNA ready to leave).

翻译是从mRNA模板合成多肽链的过程，在细胞质中的核糖体上进行。核糖体由两个亚基（大亚基和小亚基）组成，由核糖体RNA（rRNA）和蛋白质构成。小亚基与mRNA结合并识别起始密码子，而大亚基催化相邻氨基酸之间形成肽键。核糖体有三个转运RNA（tRNA）结合位点：A位点（氨酰位：进入的tRNA），P位点（肽基位：携带正在生长的多肽的tRNA），和E位点（出口位：准备离开的tRNA）。

Translation proceeds through three phases: initiation, elongation, and termination. Initiation begins when the small ribosomal subunit binds to the 5′ cap of mRNA and scans along until it finds the start codon (AUG, which codes for methionine). The initiator tRNA, carrying methionine, binds to the start codon in the P site, and the large subunit joins, forming the complete initiation complex. During elongation, a new aminoacyl-tRNA enters the A site, a peptide bond forms between the P-site and A-site amino acids, then the ribosome translocates one codon: A-site tRNA moves to P site, empty tRNA exits via E site. Each cycle adds one amino acid to the growing chain.

翻译通过三个阶段进行：起始、延伸和终止。起始时，核糖体小亚基与mRNA的5’帽结合并沿mRNA扫描，直到找到起始密码子（AUG，编码甲硫氨酸）。携带甲硫氨酸的起始tRNA与P位点的起始密码子结合，然后大亚基加入，形成完整的起始复合物。在延伸过程中，新氨酰tRNA进入A位点，P位点与A位点氨基酸之间形成肽键，核糖体移位一个密码子：A位点tRNA移到P位点，空tRNA经E位点离开。每个循环向生长中的多肽链添加一个氨基酸。

6. 遗传密码 The Genetic Code

The genetic code is the set of rules by which the nucleotide sequence of mRNA is translated into the amino acid sequence of a protein. Three consecutive nucleotides (a codon) specify one amino acid. With four different nucleotides, there are 4³ = 64 possible codons, but only 20 standard amino acids ： the code is therefore degenerate (redundant). Most amino acids are specified by more than one codon: for example, leucine is encoded by six different codons (UUA, UUG, CUU, CUC, CUA, CUG). This degeneracy provides a buffer against the effects of point mutations; a change in the third base of a codon often does not alter the amino acid specified (the “wobble” effect).

遗传密码是将mRNA的核苷酸序列翻译为蛋白质的氨基酸序列的一套规则。三个连续核苷酸（一个密码子）指定一个氨基酸。由于有四种不同的核苷酸，共有4³ = 64种可能的密码子，但只有20种标准氨基酸：因此密码子是简并的（冗余的）。大多数氨基酸由多个密码子指定：例如，亮氨酸由六个不同的密码子（UUA、UUG、CUU、CUC、CUA、CUG）编码。这种简并性为点突变的影响提供了缓冲；密码子第三位碱基的变化通常不会改变所指定的氨基酸（”摆动”效应）。

Three codons ： UAA, UAG, and UGA ： do not code for any amino acid; they are stop (termination) codons that signal the end of translation. When a stop codon enters the A site, a release factor protein binds instead of a tRNA, causing the ribosome to hydrolyse the bond between the completed polypeptide and the tRNA in the P site, releasing the polypeptide. The ribosomal subunits then dissociate and are recycled. The start codon, AUG, codes for methionine (in eukaryotes) or N-formylmethionine (in prokaryotes), establishing the reading frame for translation.

三个密码子：UAA、UAG和UGA：不编码任何氨基酸，它们是终止密码子，发出翻译结束的信号。当终止密码子进入A位点时，一个释放因子蛋白而非tRNA与之结合，导致核糖体水解完成的多肽与P位点tRNA之间的键，释放多肽。然后核糖体亚基解离并被回收。起始密码子AUG编码甲硫氨酸（真核生物）或N-甲酰甲硫氨酸（原核生物），确立了翻译的阅读框。

7. 基因表达调控 Regulation of Gene Expression

Not all genes are expressed in every cell ： differential gene expression gives rise to the specialised functions of different cell types. In eukaryotes, gene expression can be regulated at multiple levels: transcriptional control (whether and how often a gene is transcribed), post-transcriptional control (mRNA processing and stability), translational control (whether and how much mRNA is translated), and post-translational control (protein modification and degradation). The most important level of control is transcriptional, mediated by transcription factors ： proteins that bind to specific DNA sequences near a gene’s promoter and either activate or repress transcription.

并非所有基因都在每个细胞中表达：差异基因表达产生了不同细胞类型的特化功能。在真核生物中，基因表达可以在多个水平进行调控：转录控制（基因是否转录以及转录频率）、转录后控制（mRNA加工和稳定性）、翻译控制（mRNA是否被翻译以及翻译量）和翻译后控制（蛋白质修饰和降解）。最重要的控制水平是转录控制，由转录因子介导：这些蛋白质与基因启动子附近的特定DNA序列结合，激活或抑制转录。

In prokaryotes, gene regulation is often organised into operons ： clusters of functionally related genes controlled by a single promoter. The lac operon in E. coli is the classic example: it contains genes for lactose metabolism (lacZ, lacY, lacA) and is regulated by the lac repressor protein. When lactose is absent, the repressor binds the operator and blocks transcription. When present, allolactose binds the repressor, causing it to dissociate, allowing transcription. This inducible system conserves energy by producing enzymes only when needed.

在原核生物中，基因调控通常组织成操纵子：由单个启动子控制的功能相关基因簇。大肠杆菌中的lac操纵子是经典例子：它包含乳糖代谢基因（lacZ、lacY、lacA），并由lac阻遏蛋白调控。当乳糖不存在时，阻遏蛋白与操纵基因结合，阻止转录。当乳糖存在时，异乳糖与阻遏蛋白结合使其解离，转录得以进行。这种诱导性系统确保细胞只在需要时产生代谢酶，节省能量和资源。

8. 突变及其影响 Mutations and Their Effects

A mutation is a permanent change in the DNA sequence of an organism. Mutations can arise spontaneously during DNA replication (due to errors that escape proofreading) or be induced by mutagens ： physical agents (UV radiation, X-rays, gamma rays) or chemical agents (benzopyrene from tobacco smoke, nitrous acid, ethidium bromide). The rate of spontaneous mutation is approximately one in 10⁹ base pairs per cell division, demonstrating the extraordinary fidelity of DNA replication. However, environmental mutagens can dramatically increase this rate.

突变是生物体DNA序列的永久性改变。突变可以在DNA复制过程中自发产生（由于逃过校对的错误），也可以由诱变剂诱导：物理因素（紫外线、X射线、伽马射线）或化学因素（烟草烟雾中的苯并芘、亚硝酸、溴化乙锭）。自发突变的速率约为每次细胞分裂中每10⁹个碱基对有一个突变，这证明了DNA复制的非凡精确性。然而，环境诱变剂可以显著提高这一速率。

Point mutations involve a change in a single nucleotide and can be classified as: substitution (replacement of one base by another), insertion (addition of one or more bases), or deletion (removal of one or more bases). Substitutions can be silent (no amino acid change due to degeneracy), missense (different amino acid, e.g., sickle cell anaemia), or nonsense (premature stop codon, truncated protein). Insertions and deletions often cause frameshift mutations, where the reading frame is shifted and every codon downstream of the mutation site is read differently, usually producing a non-functional protein. Frameshift mutations are typically far more severe than substitutions.

点突变涉及单个核苷酸的改变，可分为：替换（一个碱基被另一个替换）、插入（添加一个或多个碱基）或缺失（移除一个或多个碱基）。替换可以是无义突变（氨基酸不变）、错义突变（不同氨基酸，如镰刀型贫血症）或无义突变（提前终止密码子，截断蛋白质）。插入和缺失通常导致移码突变，即阅读框移动，突变位点下游的每个密码子都被不同地读取，通常产生无功能的蛋白质。移码突变通常比替换严重得多。

9. 考试技巧与常见错误 Exam Tips and Common Pitfalls

When answering questions about DNA replication, always name the specific enzymes and describe their functions in the correct order. A common mistake is confusing the roles of DNA polymerase I and DNA polymerase III: polymerase III is the main replicative enzyme that extends the new strand, while polymerase I removes RNA primers and replaces them with DNA. Another pitfall is forgetting to mention the antiparallel nature of DNA strands when explaining why the lagging strand is synthesised discontinuously ： the enzyme can only synthesise in the 5′ to 3′ direction, and the lagging strand template runs 5′ to 3′ toward the fork, necessitating Okazaki fragments.

在回答有关DNA复制的问题时，始终按正确顺序列出特定酶并描述它们的功能。一个常见错误是混淆DNA聚合酶I和DNA聚合酶III的作用：聚合酶III是延伸新链的主要复制酶，而聚合酶I移除RNA引物并用DNA替换。另一个陷阱是在解释为什么滞后链不连续合成时忘记提到DNA链的反向平行性质：酶只能沿5’到3’方向合成，而滞后链的模板朝向复制叉是5’到3’，因此需要冈崎片段。

For transcription and translation questions, use precise terminology and never confuse the two processes. The genetic code is universal, degenerate, and non-overlapping. Distinguish eukaryotic transcription factors from prokaryotic operons. For mutations, classify the type first, and note that frameshifts alter every downstream amino acid.

对于转录和翻译问题，术语要精确，不要混淆两者。遗传密码是通用的、简并的、非重叠的。区分真核转录因子和原核操纵子。对于突变，先分类再讨论影响，移码突变会改变下游每一个氨基酸。

10. 核心双语术语 Key Bilingual Terms

These processes form the molecular basis of inheritance and gene expression. A thorough understanding of DNA replication, transcription, and translation will not only prepare you for exam questions on molecular biology but also lay the foundation for more advanced topics such as genetic engineering, gene therapy, and cancer biology, where disruptions to these fundamental processes play central roles.

这些过程构成了遗传和基因表达的分子基础。透彻理解DNA复制、转录和翻译不仅能为分子生物学的考试题目做好准备，还能为更高级的专题如基因工程、基因疗法和癌症生物学奠定基础，在这些领域中，对这些基本过程的破坏起着核心作用。

A-Level生物 DNA复制 转录 翻译 基因表达