📚 2.3 Nucleotides & Nucleic Acids: Exam Practice | 2.3 核苷酸与核酸 真题精练
This targeted revision session covers the most commonly examined topics on nucleotides and nucleic acids for A-Level Biology. Work through typical questions, refine your understanding of DNA replication, RNA types, and key experiments, and pick up tips to avoid common pitfalls.
本场专项精练涵盖 A-Level 生物中核苷酸与核酸的最高频考点。你将重温典型考题，加深对 DNA 复制、RNA 种类、关键实验的理解，并掌握避开常见陷阱的技巧。
1. Nucleotide Structure | 核苷酸结构
A classic exam question asks you to draw and label a nucleotide or identify its components. A nucleotide is the monomer of nucleic acids and consists of a pentose sugar, a phosphate group, and a nitrogenous base. In DNA the pentose sugar is deoxyribose, while in RNA it is ribose. The difference is the absence of an oxygen atom on the 2′ carbon in deoxyribose.
常见真题要求绘制并标记一个核苷酸或识别其组分。核苷酸是核酸的单体，由戊糖、磷酸基团和含氮碱基组成。DNA 中的戊糖是脱氧核糖，RNA 中为核糖。区别在于脱氧核糖的 2′ 碳上缺少一个氧原子。
The nitrogenous bases are classified as purines (double-ring: adenine, guanine) and pyrimidines (single-ring: cytosine, thymine, uracil). Mnemonic: ‘Pure As Gold’ helps recall that adenine and guanine are purines. Thymine is found in DNA; uracil replaces thymine in RNA.
含氮碱基分为嘌呤（双环：腺嘌呤、鸟嘌呤）和嘧啶（单环：胞嘧啶、胸腺嘧啶、尿嘧啶）。助记口诀：嘌呤是 A 和 G。胸腺嘧啶存在于 DNA，RNA 中用尿嘧啶代替。
When drawing a nucleotide, label the 3′ and 5′ carbons correctly. The phosphate is attached to the 5′ carbon, and the base is attached to the 1′ carbon. The 3′ carbon carries a hydroxyl (-OH) group that is essential for forming phosphodiester bonds in the sugar-phosphate backbone.
绘制核苷酸时，要正确标记 3′ 和 5′ 碳。磷酸连接在 5′ 碳上，碱基连接在 1′ 碳上。3′ 碳带有羟基 (-OH)，对于形成糖-磷酸骨架中的磷酸二酯键至关重要。
2. Formation of Polynucleotides | 多核苷酸链的形成
Nucleotides are joined by condensation reactions between the phosphate group of one nucleotide and the 3′ hydroxyl group of another, forming a phosphodiester bond. The process always extends the chain in a 5′ → 3′ direction. A water molecule is released each time a bond forms.
核苷酸通过一个核苷酸的磷酸基团与另一个核苷酸的 3′ 羟基之间发生缩合反应连接在一起，生成磷酸二酯键。此过程总是沿 5′ → 3′ 方向延伸链。每形成一个键会释放一分子水。
Exam hint: many students lose marks by writing ‘peptide bond’ when they mean phosphodiester bond. Be precise. Also, be able to identify the number of phosphodiester bonds in a short stretch of DNA – for a sequence of n nucleotides in a single strand, there are n-1 bonds.
考试提示：许多学生把磷酸二酯键错写成肽键而丢分。务求术语准确。另外，要能算出短链 DNA 中磷酸二酯键的数量——若单链含有 n 个核苷酸，则有 n-1 个磷酸二酯键。
3. DNA Double Helix | DNA 双螺旋结构
The two DNA strands run antiparallel. The sugar-phosphate backbones are on the outside, and the nitrogenous bases pair on the inside via hydrogen bonds. Adenine pairs with thymine (A=T) with two hydrogen bonds; guanine pairs with cytosine (C≡G) with three hydrogen bonds, making C-G pairs slightly more stable.
两条 DNA 链反向平行。糖-磷酸骨架位于外侧，含氮碱基通过氢键在内侧配对。腺嘌呤与胸腺嘧啶配对 (A=T) 形成两个氢键；鸟嘌呤与胞嘧啶配对 (C≡G) 形成三个氢键，因此 C-G 配对稍更稳定。
If an exam question asks ‘explain how the structure of DNA is related to its function’, mention the double helix providing stability, base sequence carrying genetic code, weak hydrogen bonds allowing strand separation during replication, and the long length coding for many proteins.
若考题要求“解释 DNA 的结构如何与其功能相关”，应提及双螺旋提供稳定性、碱基序列携带遗传密码、弱氢键便于复制时解链，以及长链可编码大量蛋白质。
4. RNA Types and Roles | RNA 的种类与作用
Three main types of RNA are involved in protein synthesis. Messenger RNA (mRNA) is a linear copy of a gene that carries the genetic code from DNA in the nucleus to the ribosome. Transfer RNA (tRNA) is a clover-leaf shaped molecule that delivers specific amino acids and recognises codons via its anticodon. Ribosomal RNA (rRNA) together with proteins makes up the ribosome structure and catalyses peptide bond formation.
参与蛋白质合成的 RNA 主要有三种。信使 RNA (mRNA) 是基因的线性拷贝，将遗传密码从细胞核的 DNA 带到核糖体。转运 RNA (tRNA) 呈三叶草形，运送特定氨基酸并通过反密码子识别密码子。核糖体 RNA (rRNA) 与蛋白质共同构成核糖体并催化肽键形成。
Be ready to distinguish RNA molecules by their relative lengths and shapes. Exam questions may present diagrams of mRNA, tRNA and rRNA and ask you to identify them.
要能根据相对长度与形状区分 RNA 分子。考题可能给出 mRNA、tRNA 和 rRNA 的示意图并让你识别。
5. DNA vs RNA – Key Differences | DNA 与 RNA 的关键区别
Comparison questions appear regularly. Use the table below to structure a full-mark answer.
比较题频繁出现。请用下表构建满分答案。
| Feature | DNA | RNA |
|---|---|---|
| Sugar | Deoxyribose | Ribose |
| Bases | A, C, G, T | A, C, G, U |
| Strands | Double-stranded (usually) | Single-stranded (usually) |
| Helix | Double helix | Various shapes (mRNA linear, tRNA cloverleaf) |
| Length | Very long (millions of base pairs) | Relatively short |
| Location | Mainly in nucleus | Nucleus and cytoplasm |
| Stability | More stable (lacks 2′-OH) | Less stable, easily degraded |
In mark schemes, always note the structural difference in the sugar and the base uracil/thymine substitution. Mention hydrogen bonding and the double vs single strand.
在评分方案中，务必指出糖的结构差异以及尿嘧啶/胸腺嘧啶的替换。还要提到氢键和单双链区别。
6. Evidence for Semi-Conservative Replication | 半保留复制的证据
Meselson and Stahl grew E. coli in a medium containing heavy nitrogen (¹⁵N) for several generations, then transferred the bacteria to a medium with light nitrogen (¹⁴N). They extracted DNA at intervals and centrifuged it in a caesium chloride density gradient. After one round of replication in ¹⁴N, all DNA was of intermediate density, consistent with semi-conservative replication. After two rounds, half of the DNA was light and half was intermediate, ruling out conservative replication.
梅瑟森和斯塔尔将大肠杆菌培养在含有重氮（¹⁵N）的培养基中数代，然后转移至轻氮（¹⁴N）培养基。他们在不同时间提取 DNA，并在氯化铯密度梯度中离心。在 ¹⁴N 中复制一轮后，所有 DNA 均为中间密度，符合半保留复制；两轮后一半为轻链、一半为中间密度，从而排除了全保留复制。
Make sure you can explain why conservative replication would have produced only light and heavy bands after one generation, and that dispersive replication is refuted by the two-generation band pattern showing distinct light and intermediate bands.
务必能解释为何全保留复制会在第一代后就产生轻、重两条带，以及弥散型复制为何被两代后出现的清晰轻带和
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