📚 The Structure of DNA and RNA | DNA与RNA的结构
Nucleic acids, deoxyribonucleic acid (DNA) and ribonucleic acid (RNA), are the universal molecules of heredity. They store the genetic instructions that govern the development, functioning, and reproduction of all known organisms and many viruses. Understanding their molecular architecture is foundational to modern biology, from gene expression to biotechnology.
核酸，即脱氧核糖核酸(DNA)和核糖核酸(RNA)，是遗传的通用分子。它们储存着控制所有已知生物及许多病毒发育、功能和繁殖的遗传指令。理解其分子结构是现代生物学——从基因表达到生物技术——的基础。
1. Introduction to Nucleic Acids | 核酸简介
DNA is the long-term repository of genetic information in cells. In eukaryotes, it is housed mainly within the nucleus, organised into chromosomes. RNA acts as an intermediary, transferring the genetic code from DNA to the protein-synthesis machinery, but it also performs catalytic, structural, and regulatory roles.
DNA是细胞内遗传信息的长期储存库。在真核生物中，它主要位于细胞核内，组形成染色体。RNA则充当中间体，将遗传密码从DNA传递到蛋白质合成机器，但它也发挥催化、结构和调控作用。
Both DNA and RNA are linear polymers called polynucleotides, built from monomeric units known as nucleotides. Each nucleotide contains a phosphate group, a pentose sugar, and a nitrogenous base. The precise chemical identity of the sugar and the set of bases used define the fundamental differences between the two nucleic acids.
DNA和RNA都是称为多核苷酸的线性聚合物，由称为核苷酸的单体单元构建而成。每个核苷酸包含一个磷酸基团、一个戊糖和一个含氮碱基。糖和所用碱基类型的具体化学差异定义了两类核酸的根本区别。
2. The Nucleotide: Building Block | 核苷酸：基本构件
A nucleotide is composed of three parts: a phosphate group attached to the 5′ carbon of the pentose, a nitrogenous base linked to the 1′ carbon via a β‑N‑glycosidic bond, and the pentose sugar itself. If the phosphate is absent, the base–sugar unit is called a nucleoside. The combination of a nucleoside with one, two, or three phosphate groups forms a nucleotide (e.g., AMP, ADP, ATP).
一个核苷酸由三部分组成：连接在戊糖5′碳上的磷酸基团、通过β‑N‑糖苷键连接在1′碳上的含氮碱基，以及戊糖本身。如果没有磷酸，碱基‑糖单元称为核苷。核苷与一、二或三个磷酸基团结合则形成核苷酸（例如AMP、ADP、ATP）。
The phosphate group makes the nucleotide acidic and highly polar. In a polynucleotide chain, the phosphate links adjacent sugar residues via phosphodiester bonds, creating a continuous sugar–phosphate backbone that is negatively charged at physiological pH.
磷酸基团使核苷酸呈酸性且极性很强。在多核苷酸链中，磷酸通过磷酸二酯键连接相邻的糖残基，形成一条在生理pH下带负电荷的糖‑磷酸骨架。
3. Pentose Sugar: Deoxyribose vs Ribose | 戊糖：脱氧核糖与核糖的区别
The pentose in DNA is 2′‑deoxyribose, meaning that the hydroxyl (–OH) group on the 2′ carbon of ribose is replaced by a hydrogen atom (–H). In RNA, the sugar is ribose, which retains the 2′‑OH group. This single chemical difference has profound consequences for the stability and folding of the two nucleic acids.
DNA中的戊糖是2′‑脱氧核糖，这意味着核糖2′碳上的羟基(–OH)被氢原子(–H)取代。RNA中的糖是核糖，保留了2′‑OH基团。这一唯一的化学差异对两种核酸的稳定性和折叠产生了深远的影响。
The 2′‑OH group in RNA increases reactivity: it can act as a nucleophile, attacking the adjacent phosphodiester bond. This renders RNA susceptible to alkaline hydrolysis via a 2′,3′‑cyclic phosphate intermediate. DNA, lacking this hydroxyl, is far more resistant to such cleavage, making it ideally suited for long-term genetic storage.
RNA中的2′‑OH基团提高了反应活性：它可以作为亲核试剂攻击邻近的磷酸二酯键。这使RNA在碱性条件下易通过2′,3′‑环状磷酸中间体发生水解。DNA由于缺少这一羟基，对此类裂解具有更强的抵抗力，因此非常适合长期遗传储存。
4. Nitrogenous Bases: Purines and Pyrimidines | 含氮碱基：嘌呤与嘧啶
The nitrogenous bases are planar, aromatic heterocycles. Purines — adenine (A) and guanine (G) — possess a double ring composed of a six-membered and a five-membered ring fused together. Pyrimidines — cytosine (C), thymine (T), and uracil (U) — have only a single six-membered ring.
含氮碱基是平面的芳香族杂环化合物。嘌呤——腺嘌呤(A)和鸟嘌呤(G)——拥有一个由六元环和五元环融合而成的双环结构。嘧啶——胞嘧啶(C)、胸腺嘧啶(T)和尿嘧啶(U)——仅含有一个六元环。
DNA incorporates A, G, C, and T; RNA uses A, G, C, and U. Thymine differs from uracil only by a methyl group at the 5‑position of the ring. This methylation is thought to help DNA repair mechanisms distinguish thymine from deaminated cytosine, thereby maintaining genetic fidelity.
DNA含有A、G、C、T；RNA使用A、G、C、U。胸腺嘧啶与尿嘧啶的区别仅在于环的5‑位多了一个甲基。这一甲基化被认为有助于DNA修复机制区分胸腺嘧啶与脱氨后的胞嘧啶，从而维持遗传准确性。
5. Phosphodiester Bonds and Polynucleotide Formation | 磷酸二酯键与多核苷酸链的形成
Nucleotides polymerise through a condensation reaction between the 3′‑OH group of one nucleotide and the 5′‑phosphate group of the next, forming a phosphodiester bond and releasing a water molecule. This reaction is catalysed in vivo by DNA or RNA polymerases, which require a template strand to synthesize the complementary chain.
核苷酸通过一个核苷酸的3′‑OH基团与下一个核苷酸的5′‑磷酸基团之间的缩合反应聚合，形成磷酸二酯键并释放一分子水。该反应在体内由依赖模板链的DNA聚合酶或RNA聚合酶催化合成互补链。
The resulting polynucleotide has directionality: one end carries a free 5′‑phosphate (the 5′ end) and the other a free 3′‑hydroxyl (the 3′ end). By convention, nucleic acid sequences are written from 5′ to 3′, matching the direction in which polymerases synthesise the new strand. This directionality is crucial for processes such as replication, transcription, and translation.
生成的多核苷酸具有方向性：一端带有游离的5′‑磷酸（5′端），另一端带有游离的3′‑羟基（3′端）。按照惯例，核酸序列按5′到3′方向书写，与聚合酶合成新链的方向一致。这一方向性对复制、转录和翻译等过程至关重要。
6. DNA Double Helix: Watson–Crick Model | DNA双螺旋结构：沃森‑克里克模型
The three‑dimensional structure of DNA, elucidated by Watson and Crick in 1953, is a right‑handed double helix. Two antiparallel polynucleotide strands coil around a common axis, with the sugar–phosphate backbones on the outside and the nitrogenous bases stacked in the interior. The diameter of the helix is approximately 2 nm.
1953年，沃森和克里克阐明了DNA的三维结构：一个右手双螺旋。两条反平行的多核苷酸链围绕同一轴心缠绕，糖‑磷酸骨架位于外侧，含氮碱基堆叠在内侧。螺旋直径约2纳米。
Base stacking interactions — hydrophobic and van der Waals forces between the planar rings — contribute significantly to the stability of the double helix. Hydrogen bonds between complementary bases on opposite strands provide specificity. The helix completes a full turn every 10 base pairs, which is a distance of 3.4 nm. The asymmetry of the helix creates a wider major groove and a narrower minor groove, which are recognised by DNA‑binding proteins and transcription factors.
碱基堆积作用——平面环之间的疏水力和范德华力——对双螺旋的稳定性贡献显著。互补碱基之间的氢键则提供了特异性。螺旋每10个碱基对完成一周旋转，长度为3.4纳米。螺旋的不对称性形成了较宽的大沟和较窄的小沟，这些沟槽可被DNA结合蛋白和转录因子识别。
7. Base Pairing in DNA: Complementary Strands | DNA中的碱基配对：互补链
Watson– two hydrogen bonds (A=T), and guanine pairs with cytosine through three hydrogen bonds (C≡ f’s ratios (A=T, G=C), ensures that the two strands of the double helix are complementary.
沃森‑克里克碱基配对遵循严格的互补规则：腺嘌呤与胸腺嘧啶通过两个氢键配对（A=T），鸟嘌呤与胞嘧啶通过三个氢键配对（C≡G）。这一模式与查加夫比值(A=T, G=C)一致，确保双螺旋的两条链是互补的。
owing the strands to be separated (melted) during replication and transcription. The three‑hydrogen‑bonded C≡ issociate than the A=T pair, influencing the melting temperature of DNA depending on its G–C content.
氢键虽然单个较弱，但大量累积后可赋予结构显著的稳定性，同时仍允许链在复制和转录过程中解链（熔解）。三氢键的C≡G配对比A=T配对需要更多能量解离，因此DNA的解链温度受其G–C含量的影响。
8. RNA Structure and Functional Types | RNA的结构与功能类型
Unlike the predominantly double‑stranded DNA, RNA molecules are generally single‑stranded. However, they can fold back on themselves to form short double‑ n loops, and complex tertiary structures stabilised by intramolecular base pairing (A=U, C≡G) and interactions involving the 2′‑OH group.
与主要为双链的DNA不同，RNA分子通常为单链。然而，它们可以向后折叠，形成短的双螺旋区域、发夹环以及由分子内碱基配对（A=U, C≡G）和涉及2′‑OH基团的相互作用所稳定的复杂三级结构。
Cells produce several classes of RNA with specialised functions. Messenger RNA (mRNA) carries the protein‑coding sequence from DNA. Ribosomal RNA (rRNA) combines with proteins to form ribosomes, the catalytic centres for protein synthesis. Transfer RNA (tRNA) acts as the adapter that decodes mRNA codons into specific amino acids. Small nuclear RNAs (snRNAs) and microRNAs (miRNAs) are involved in RNA processing and gene silencing.
细胞产生多种具有特殊功能的RNA类别。信使RNA (mRNA) 携带来自DNA的蛋白质编码序列。核糖体RNA (rRNA) 与蛋白质结合形成核糖体——蛋白质合成的催化中心。转运RNA (tRNA) 充当解码mRNA密码子为特定氨基酸的接头分子。小核RNA (snRNA) 和微小RNA (miRNA) 参与RNA加工与基因沉默。
9. tRNA: Cloverleaf and Anticodon | 转运RNA：三叶草结构与反密码子
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