📚 2.3 Nucleotides and Nucleic Acids | 2.3 核苷酸与核酸
This article covers the structure, types and functions of nucleotides and nucleic acids, including DNA and RNA. It explains how nucleotides join to form polynucleotides, how base pairing stabilises the DNA double helix, and why ATP acts as the universal energy currency. These ideas are essential for understanding replication, transcription and translation in A-level biology.
本文涵盖核苷酸和核酸（包括 DNA 和 RNA）的结构、类型与功能。文章解释核苷酸如何连接成多聚核苷酸、碱基配对如何稳定 DNA 双螺旋，以及为什么 ATP 是通用的能量货币。这些概念是理解 A-level 生物中复制、转录和翻译的基础。
1. Nucleotide structure and components | 核苷酸的结构与组成
A nucleotide is the monomer from which nucleic acids are built. Each nucleotide consists of three components: a pentose sugar, a nitrogenous base and one or more phosphate groups.
核苷酸是构成核酸的单体。每个核苷酸由三个部分组成：一个五碳糖、一个含氮碱基和一个或多个磷酸基团。
The three components are chemically different in character. The pentose sugar and the phosphate group form the structural and electrical backbone of the molecule, while the nitrogenous base carries the information. All living organisms use the same small set of nucleotides, which is why the genetic code is described as universal.
这三个组成部分在化学性质上各不相同。五碳糖和磷酸基团构成分子的骨架并带有电荷，而含氮碱基则携带信息。所有生物都使用同一套小分子核苷酸，这正是遗传密码被称为通用密码的原因。
Nucleotide = pentose sugar + nitrogenous base + phosphate group(s)
The components are joined by condensation reactions. The base is attached to carbon 1 of the sugar, and the first phosphate group is attached to carbon 5 of the sugar. The carbon atoms in the sugar are numbered 1' to 5' with a prime symbol so that they are not confused with the carbon atoms of the base.
各组成部分通过缩合反应连接。碱基连接在糖的 1 号碳上，第一个磷酸基团连接在糖的 5 号碳上。糖上的碳原子用 1' 至 5' 加撇号编号，以免与碱基上的碳原子混淆。
pentose sugar + base → nucleoside + H₂O | nucleoside + phosphate → nucleotide + H₂O
2. The pentose sugars: ribose and deoxyribose | 五碳糖：核糖与脱氧核糖
The pentose sugar in DNA is deoxyribose, which lacks an oxygen atom on carbon 2; in RNA the sugar is ribose. The difference at the 2' position is important because it makes DNA more chemically stable and RNA more reactive.
DNA 中的五碳糖是脱氧核糖，其 2 号碳上缺少一个氧原子；RNA 中的糖是核糖。2' 位的差异很重要，因为这让 DNA 化学上更稳定，而 RNA 更活泼。
In deoxyribose, carbon 2' carries a hydrogen atom instead of a hydroxyl (-OH) group. The hydroxyl group on ribose makes the RNA sugar vulnerable to attack by water, so RNA is hydrolysed more easily and has a shorter lifetime inside the cell. This reactivity suits the short-term roles of mRNA, which must be broken down after a protein has been made.
在脱氧核糖中，2' 位碳上带的是氢原子而不是羟基（-OH）。核糖上的羟基使 RNA 的糖容易被水进攻，因此 RNA 更容易被水解，在细胞内存活时间更短。这种活泼性正好适合 mRNA 的短期功能——蛋白质合成完成后它必须被降解。
Ribose: C₅H₁₀O₅ | Deoxyribose: C₅H₁₀O₄ | 2'-OH (RNA) vs 2'-H (DNA)
You should be able to draw both sugars in the ring form and label carbon 1', 2', 3', 4' and 5'. In an exam, marking the missing oxygen on DNA sugar is usually worth the accuracy mark in a structure question.
你需要能够画出两种糖的环状结构，并标出 1'、2'、3'、4' 和 5' 位碳。在考试中，标出 DNA 糖上缺失的那个氧原子，通常就是结构题中的准确分点。
3. Nitrogenous bases: purines and pyrimidines | 含氮碱基：嘌呤与嘧啶
Nitrogenous bases are organic rings containing nitrogen atoms. They fall into two structural classes: purines, which have a double ring (a fused six-membered and five-membered ring), and pyrimidines, which have a single six-membered ring.
含氮碱基是含氮原子的有机环状化合物，分为两大类：嘌呤具有双环结构（六元环与五元环稠合），嘧啶只有单个六元环。
| Class | 类别 | Bases | 碱基 | Found in | 存在于 | Ring structure | 环结构 |
|---|---|---|---|
| Purine 嘌呤 | Adenine (A) 腺嘌呤, Guanine (G) 鸟嘌呤 | DNA and RNA | Double ring 双环 |
| Pyrimidine 嘧啶 | Cytosine (C) 胞嘧啶 | DNA and RNA | Single ring 单环 |
| Pyrimidine 嘧啶 | Thymine (T) 胸腺嘧啶 | DNA only | Single ring 单环 |
| Pyrimidine 嘧啶 | Uracil (U) 尿嘧啶 | RNA only | Single ring 单环 |
A useful memory aid is that the names of the purines are shorter than the names of the molecules they are made from: pure silver (Ag) is a purine. Adenine and guanine are purines; cytosine, thymine and uracil are pyrimidines.
一个有用的记忆方法是：嘌呤的名称比合成它们的分子更短——"纯银（Ag）"是嘌呤。腺嘌呤和鸟嘌呤是嘌呤；胞嘧啶、胸腺嘧啶和尿嘧啶是嘧啶。
The bases are hydrophobic and relatively insoluble in water, so they are stacked inside the double helix, away from the aqueous surroundings, which helps to stabilise the structure.
碱基具有疏水性、相对不溶于水，因此它们堆叠在双螺旋内部、远离水环境，这有助于稳定整个结构。
4. Nucleoside, nucleotide and the role of phosphate | 核苷、核苷酸与磷酸基团的作用
When a base is joined to a sugar alone, the molecule is called a nucleoside. Adding one or more phosphate groups converts it into a nucleotide. This distinction is frequently tested, so learn the definitions precisely.
当碱基只与糖相连时，该分子称为核苷。再加上一个或多个磷酸基团后就成为核苷酸。这一区分经常被考查，因此要准确记忆定义。
Phosphate groups are attached to the 5' carbon of the sugar. A nucleotide with one phosphate is a monophosphate, with two a diphosphate, and with three a triphosphate, as in ATP. The phosphate groups carry negative charges, which makes nucleic acids acidic and allows them to interact with positively charged proteins such as histones.
磷酸基团连接在糖的 5' 号碳上。含一个磷酸的核苷酸是一磷酸核苷酸，含两个是二磷酸核苷酸，含三个是三磷酸核苷酸，例如 ATP。磷酸基团带负电荷，这使核酸呈酸性，并能与组蛋白等带正电的蛋白质相互作用。
The bonds between adjacent phosphate groups in a triphosphate such as ATP are high-energy bonds. Hydrolysing them releases a useful quantity of energy, which is the basis of ATP's role in cells.
在 ATP 这类三磷酸分子中，相邻磷酸基团之间的键是高能键。水解这些键可以释放出可用的能量，这正是 ATP 在细胞中发挥作用的基础。
Base + sugar = nucleoside | Nucleoside + phosphate(s) = nucleotide
5. Condensation: from nucleotides to polynucleotides | 缩合反应：从核苷酸到多聚核苷酸
Nucleotides polymerise by condensation reactions. The phosphate group of one nucleotide reacts with the hydroxyl group on carbon 3' of the sugar of the next nucleotide, forming a phosphodiester bond and releasing one molecule of water.
核苷酸通过缩合反应聚合成链。一个核苷酸的磷酸基团与另一个核苷酸糖上 3' 号碳的羟基反应，形成磷酸二酯键并释放一分子水。
nucleotide + nucleotide → dinucleotide + H₂O (phosphodiester bond formed)
Repeating this process produces a polynucleotide with a sugar-phosphate backbone. The backbone is therefore a repeating chain of sugar and phosphate units, and the bases project sideways from it. The chain has a definite direction: one end has a free phosphate on carbon 5' (the 5' end) and the other has a free hydroxyl on carbon 3' (the 3' end).
重复这一过程就生成具有糖-磷酸骨架的多聚核苷酸。骨架是糖与磷酸交替连接的长链，碱基则向侧面伸出。链具有明确的方向性：一端是 5' 号碳上自由的磷酸（5' 端），另一端是 3' 号碳上自由的羟基（3' 端）。
- Phosphodiester bonds are strong covalent bonds that hold the backbone together. | 磷酸二酯键是连接骨架的强共价键。
- Hydrogen bonds between bases are much weaker and can be broken during replication and transcription. | 碱基之间的氢键弱得多，可在复制和转录时被断开。
- Directionality 5' → 3' is essential for reading the sequence and for enzyme action. | 方向性 5' → 3' 对读取序列和酶的作用至关重要。
Because each nucleotide differs only in its base, the sequence of bases along the chain is the sequence of information. A gene is simply a length of DNA with a particular base sequence.
由于各核苷酸只在碱基上不同，链上的碱基顺序就是信息顺序。一个基因不过是一段具有特定碱基序列的 DNA。
6. DNA structure: the double helix | DNA 结构：双螺旋
DNA consists of two polynucleotide strands wound around each other to form a right-handed double helix. The two strands are antiparallel, meaning they run in opposite directions: one runs 5' to 3' and the other 3' to 5'.
DNA 由两条多聚核苷酸链相互缠绕形成右手双螺旋。两条链反向平行，即方向相反：一条从 5' 到 3'，另一条从 3' 到 5'。
The sugar-phosphate backbones are on the outside of the helix, where they interact with water, while the bases point inwards and pair with bases on the opposite strand. This arrangement shields the information-carrying bases and makes DNA a very stable molecule.
糖-磷酸骨架位于螺旋外侧，与水相互作用；碱基朝向内侧，与对侧链的碱基配对。这种排列保护了携带信息的碱基，使 DNA 成为非常稳定的分子。
| Feature | 特征 | Value | 数值 |
|---|---|
| Diameter of helix | 螺旋直径 | about 2 nm | 约 2 nm |
| Distance per complete turn | 每圈长度 | about 3.4 nm | 约 3.4 nm |
| Base pairs per turn | 每圈碱基对数 | 10 | 10 对 |
| Rise per base pair | 每对碱基间距 | about 0.34 nm | 约 0.34 nm |
The model was proposed by Watson and Crick in 1953, using the X-ray diffraction data of Rosalind Franklin and Maurice Wilkins, together with Chargaff's base composition data. Exam questions often ask you to explain how the evidence supports the model.
该模型由沃森和克里克于 1953 年提出，依据是罗莎琳德·富兰克林和莫里斯·威尔金斯的 X 射线衍射数据，以及查加夫的碱基组成数据。考试常要求你说明这些证据如何支持该模型。
7. Base pairing and hydrogen bonding | 碱基配对与氢键
The two strands of DNA are held together by hydrogen bonds between complementary bases. Adenine always pairs with thymine, and guanine always pairs with cytosine.
DNA 两条链之间由互补碱基之间的氢键相连。腺嘌呤总是与胸腺嘧啶配对，鸟嘌呤总是与胞嘧啶配对。
- A pairs with T using 2 hydrogen bonds. | A 与 T 之间形成 2 个氢键。
- G pairs with C using 3 hydrogen bonds. | G 与 C 之间形成 3 个氢键。
- A purine always pairs with a pyrimidine, so the helix keeps a constant width. | 嘌呤总与嘧啶配对，因此螺旋宽度保持恒定。
A = T | G ≡ C | A + G = T + C
This consistency is the basis of Chargaff's rules: in double-stranded DNA the percentage of adenine equals the percentage of thymine, and the percentage of guanine equals the percentage of cytosine. Consequently the total percentage of purines equals the total percentage of pyrimidines.
这种一致性就是查加夫法则的基础：在双链 DNA 中，腺嘌呤的百分比等于胸腺嘧啶，鸟嘌呤的百分比等于胞嘧啶。因此嘌呤的总百分比等于嘧啶的总百分比。
The more G-C pairs a DNA molecule contains, the more stable it is, because three hydrogen bonds must be broken instead of two. DNA with a high G-C content therefore requires a higher temperature to separate its strands, which is why the melting temperature of DNA depends on its base composition.
DNA 分子含 G-C 对越多就越稳定，因为需要断开三个氢键而不是两个。因此 G-C 含量高的 DNA 需要更高的温度才能使两条链分开，这就是 DNA 解链温度取决于碱基组成的原因。
Complementary base pairing also explains how DNA can be copied accurately and how messenger RNA can be transcribed from a DNA template.
互补碱基配对还解释了 DNA 如何被准确复制，以及信使 RNA 如何从 DNA 模板转录而来。
8. RNA structure and types | RNA 的结构与类型
RNA is a single-stranded polynucleotide containing ribose and the bases adenine, guanine, cytosine and uracil. It is much shorter than DNA and, because it is single-stranded, it can fold into complex three-dimensional shapes.
RNA 是含核糖的单链多聚核苷酸，碱基为腺嘌呤、鸟嘌呤、胞嘧啶和尿嘧啶。它比 DNA 短得多，而且由于是单链，可以折叠成复杂的三维形状。
- Messenger RNA (mRNA) is transcribed from a gene and carries the base sequence to the ribosome. | 信使 RNA（mRNA）由基因转录而来，把碱基序列带到核糖体。
- Transfer RNA (tRNA) is a clover-leaf shaped molecule with an anticodon that pairs with a codon, and an amino acid binding site at the other end. | 转运 RNA（tRNA）呈三叶草形，带有与密码子配对的反密码子，另一端是氨基酸结合位点。
- Ribosomal RNA (rRNA) is a structural component of ribosomes and helps catalyse peptide bond formation. | 核糖体 RNA（rRNA）是核糖体的结构成分，并协助催化肽键形成。
Because RNA is single-stranded, base pairing is not continuous; intramolecular hydrogen bonds form between complementary regions, producing the loops and stems seen in tRNA. Uracil replaces thymine, and uracil pairs with adenine.
由于 RNA 是单链，碱基配对并不连续；分子内互补区域之间形成氢键，产生 tRNA 中看到的环与臂。尿嘧啶取代了胸腺嘧啶，并与腺嘌呤配对。
9. DNA and RNA compared | DNA 与 RNA 的比较
A comparison table is an efficient way to revise the differences, and these are common one-mark recall questions.
用对比表格复习差异非常高效，而这些也常是一分记忆题的考点。
| Feature | 特征 | DNA | RNA |
|---|---|---|
| Sugar | 糖 | Deoxyribose 脱氧核糖 | Ribose 核糖 |
| Strands | 链数 | Two, forming a double helix 双链双螺旋 | Usually one 通常单链 |
| Bases | 碱基 | A, T, G, C | A, U, G, C |
| Length | 长度 | Very long 很长 | Shorter 较短 |
| Stability | 稳定性 | Very stable 非常稳定 | Less stable, easily hydrolysed 较不稳定，易水解 |
| Location | 位置 | Nucleus, mitochondria, chloroplasts 细胞核、线粒体、叶绿体 | Nucleus and cytoplasm 细胞核与细胞质 |
The stability of DNA makes it suitable as a long-term store of genetic information, while the instability and single-stranded flexibility of RNA make it suitable as a short-lived working copy.
DNA 的稳定性使其适合长期储存遗传信息，而 RNA 的不稳定性和单链柔性使其适合作临时的工作拷贝。
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