📚 Edexcel Biology 2.3 Nucleotides & Nucleic Acids | 核苷酸与核酸
Nucleotides and nucleic acids form the molecular foundation of all living organisms. DNA (deoxyribonucleic acid) stores hereditary information, while RNA (ribonucleic acid) carries and executes that information during protein synthesis. Understanding their structures, functions, and interactions is essential for mastering Edexcel Biology Unit 2, Topic 2.3.
核苷酸与核酸构成了所有生命有机体的分子基础。DNA（脱氧核糖核酸）储存遗传信息，而RNA（核糖核酸）在蛋白质合成过程中传递和执行这些信息。理解它们的结构、功能及其相互作用，是掌握Edexcel生物学第二单元2.3主题的关键。
1. What is a Nucleotide? | 什么是核苷酸？
A nucleotide is the basic structural unit of nucleic acids. Each nucleotide consists of three components: a phosphate group (H₃PO₄), a pentose sugar (a 5-carbon sugar), and a nitrogenous base. The phosphate group is acidic and negatively charged, giving DNA its overall negative charge.
核苷酸是核酸的基本结构单位。每个核苷酸由三个组成部分：一个磷酸基团（H₃PO₄）、一个戊糖（五碳糖）和一个含氮碱基。磷酸基团呈酸性并带负电荷，使DNA整体呈负电性。
The pentose sugar is either ribose (in RNA) or deoxyribose (in DNA). Deoxyribose lacks one oxygen atom at the 2′ carbon position; this subtle difference significantly contributes to DNA’s greater chemical stability compared with RNA, making DNA better suited for long-term genetic storage.
戊糖可以是核糖（存在于RNA中）或脱氧核糖（存在于DNA中）。脱氧核糖在2’碳位置缺少一个氧原子；这一细微差别极大地促进了DNA相较于RNA更高的化学稳定性，使DNA更适合长期储存遗传信息。
The phosphate group is attached to the 5′ carbon of the sugar, while the nitrogenous base is attached to the 1′ carbon. This asymmetric arrangement gives each nucleotide a distinct 5′ end and 3′ end, which is crucial for the directionality of nucleic acid synthesis and for the antiparallel organisation of the two DNA strands.
磷酸基团连接在糖的5’碳上，而含氮碱基连接在1’碳上。这种不对称排列赋予每个核苷酸明确的5’端和3’端，这对于核酸合成的方向性以及DNA两条链的反向平行排列至关重要。
When a base is attached to a sugar only (without phosphate), the molecule is called a nucleoside. Phosphorylation of a nucleoside at the 5′ position produces a nucleotide. In cells, free nucleotides also exist as triphosphates, such as ATP, which serve as energy carriers.
当碱基仅与糖连接（不含磷酸）时，该分子称为核苷。核苷在5’位磷酸化后生成核苷酸。在细胞中，游离核苷酸也以三磷酸形式存在，如ATP，作为能量载体发挥作用。
2. The Structure of DNA | DNA的结构
DNA is a double-stranded helical molecule. Two polynucleotide chains run in opposite directions (antiparallel) and wind around one another to form a right-handed double helix, first described by Watson and Crick in 1953. The diameter of the helix is approximately 2 nm, and one complete turn spans 3.4 nm, containing about 10 base pairs.
DNA是双链螺旋分子。两条多核苷酸链以相反方向（反向平行）排列，相互缠绕形成右手双螺旋，这一结构由Watson和Crick于1953年首先描述。螺旋直径约为2 nm，一个完整螺距为3.4 nm，包含约10个碱基对。
The backbone of each strand consists of alternating sugar and phosphate groups connected by phosphodiester bonds (3′-5′ bonds). These covalent bonds form through condensation reactions between the phosphate group of one nucleotide and the hydroxyl group at the 3′ carbon of the adjacent sugar, eliminating one water molecule per bond.
每条链的骨架由糖和磷酸基团交替连接而成，通过磷酸二酯键（3′-5’键）连接。这些共价键通过一个核苷酸的磷酸基团与相邻糖的3’碳羟基之间的缩合反应形成，每个键释放一个水分子。
The two strands are held together by hydrogen bonds between complementary nitrogenous bases. Specifically, adenine (A) pairs with thymine (T) via two hydrogen bonds, whereas guanine (G) pairs with cytosine (C) via three hydrogen bonds. Because of this pairing rule, the two strands of DNA are said to be complementary — if one strand has the sequence 5′-ATGC-3′, the other strand must be 3′-TACG-5′.
两条链通过互补含氮碱基之间的氢键结合在一起。具体而言，腺嘌呤（A）与胸腺嘧啶（T）通过两个氢键配对，而鸟嘌呤（G）与胞嘧啶（C）通过三个氢键配对。由于这种配对规则，DNA的两条链被称为互补链——如果一条链的序列是5′-ATGC-3’，另一条链必然是3′-TACG-5’。
This base-pairing arrangement means that adenine and guanine (purines, with double-ring structures) pair with thymine and cytosine (pyrimidines, with single-ring structures). The consistent size of each base pair maintains a uniform helix width of 2 nm.
这种碱基配对安排意味着腺嘌呤和鸟嘌呤（嘌呤，双环结构）与胸腺嘧啶和胞嘧啶（嘧啶，单环结构）配对。每个碱基对大小一致，维持了2 nm的均匀螺旋宽度。
3. The Structure of RNA | RNA的结构
RNA is typically a single-stranded polynucleotide, although it can fold into complex secondary structures such as hairpin loops and cloverleaf forms through intramolecular base pairing. RNA contains ribose as its pentose sugar and uses uracil (U) in place of thymine (T).
RNA通常是单链多核苷酸，尽管它可以通过分子内碱基配对折叠成复杂的二级结构，如发夹环和三叶草形。RNA含有核糖作为戊糖，并使用尿嘧啶（U）代替胸腺嘧啶（T）。
There are three main types of RNA, each with a distinct role in protein synthesis:
RNA有三种主要类型，每种在蛋白质合成中具有不同的作用：
- mRNA (messenger RNA) carries the genetic code transcribed from DNA to ribosomes; it is relatively short-lived and rapidly degraded after translation.
- 信使RNA（mRNA）将转录自DNA的遗传密码传递到核糖体；它相对短命，翻译后迅速降解。
- tRNA (transfer RNA) transports specific amino acids to the ribosome during translation; each tRNA has an anticodon that recognises a complementary codon on mRNA.
- 转运RNA（tRNA）在翻译过程中将特定氨基酸运送到核糖体；每个tRNA都有一个反密码子，可识别mRNA上的互补密码子。
- rRNA (ribosomal RNA) combines with proteins to form ribosomes, the site of protein synthesis; rRNA also catalyses the formation of peptide bonds.
- 核糖体RNA（rRNA）与蛋白质结合形成核糖体，即蛋白质合成的场所；rRNA还催化肽键的形成。
Because RNA is single-stranded, it is generally less stable than DNA and is more susceptible to hydrolysis by alkali, which is an important property when carrying out molecular biology experiments.
由于RNA是单链的，它通常不如DNA稳定，更容易被碱水解，这是进行分子生物学实验时的一个重要特性。
4. Purines and Pyrimidines | 嘌呤与嘧啶
The nitrogenous bases found in nucleic acids fall into two categories based on their ring structure. Purines are larger, double-ring molecules: adenine (A) and guanine (G). Pyrimidines are smaller, single-ring molecules: cytosine (C), thymine (T), and uracil (U).
核酸中的含氮碱基根据其环结构分为两类。嘌呤是较大的双环分子：腺嘌呤（A）和鸟嘌呤（G）。嘧啶是较小的单环分子：胞嘧啶（C）、胸腺嘧啶（T）和尿嘧啶（U）。
| Category 类别 | Bases 碱基 | Ring Structure 环结构 | Found In 存在于 |
| Purine 嘌呤 | Adenine (A), Guanine (G) | Double ring 双环 | DNA and RNA |
| Pyrimidine 嘧啶 | Cytosine (C), Thymine (T), Uracil (U) | Single ring 单环 | C, T in DNA; C, U in RNA |
The pairing of one purine with one pyrimidine is geometrically essential: a purine-purine pair would bulge the helix, while a pyrimidine-pyrimidine pair would constrict it. Either situation would destabilise the double helix. The one-to-one ratio of purines to pyrimidines is a fundamental invariant of double-stranded DNA.
嘌呤与嘧啶的配对在几何上是必需的：嘌呤-嘌呤配对会使螺旋膨胀凸起，而嘧啶-嘧啶配对会使螺旋收缩变窄。任何一种情况都会使双螺旋不稳定。嘌呤与嘧啶的一比一比例是双链DNA的基本不变特征。
5. Complementary Base Pairing | 互补碱基配对
Complementary base pairing governs both the storage and the transmission of genetic information. In DNA, A pairs with T, and G pairs with C; in RNA, A pairs with U, and G pairs with C. This specificity arises from the positions of hydrogen donors and acceptors on each base.
互补碱基配对既支配遗传信息的储存，也支配遗传信息的传递。在DNA中，A与T配对，G与C配对；在RNA中，A与U配对，G与C配对。这种特异性源于每个碱基上氢供体和氢受体的位置。
The number of hydrogen bonds differs between the base pairs:
不同碱基对之间的氢键数量不同：
- A-T pair: 2
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