IB Biology: DNA Replication Key Points | IB 生物：DNA复制 考点精讲

📚 IB Biology: DNA Replication Key Points | IB 生物：DNA复制 考点精讲

DNA replication is the fundamental process by which a cell duplicates its entire genome before division. It must be incredibly precise, fast, and tightly regulated. In IB Biology, you are expected to describe the semi-conservative mechanism, identify the roles of key enzymes, and explain the difference between leading strand and lagging strand synthesis.

DNA 复制是细胞在分裂前复制整个基因组的基本过程，必须极其精确、快速且严格调控。在 IB 生物学考试中，你需要描述半保留机制，识别关键酶的作用，并解释前导链与滞后链合成的区别。

1. Overview of DNA Replication | DNA复制概览

DNA replication starts at specific sites called origins of replication. The double helix is unwound, creating two replication forks that move in opposite directions. Each parental strand serves as a template for the synthesis of a new complementary strand, ensuring that the two resulting DNA molecules are identical to the original.

DNA 复制起始于称为复制起点的特定位置。双螺旋被解开，形成两个向相反方向移动的复制叉。每条亲代链都作为合成新互补链的模板，从而保证生成的两个 DNA 分子与原来完全相同。

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2. Semi-Conservative Replication & Meselson-Stahl Experiment | 半保留复制与梅塞尔森-斯塔尔实验

The accepted model for DNA replication is semi-conservative: each new DNA molecule consists of one original (parental) strand and one newly synthesised strand. This was famously demonstrated by Meselson and Stahl using E. coli grown in medium containing heavy nitrogen (¹⁵N) and then transferred to light nitrogen (¹⁴N). After one round of replication, DNA showed an intermediate density; after two rounds, both intermediate and light bands appeared, ruling out conservative and dispersive models.

DNA 复制公认的模型是半保留复制：每一个新 DNA 分子都由一条原始（亲代）链和一条新合成的链组成。梅塞尔森和斯塔尔通过让大肠杆菌先在含重氮（¹⁵N）的培养基中生长，再转移到轻氮（¹⁴N）中，经典地证明了这一点。复制一代后，DNA 呈中间密度；复制两代后，出现中间和轻两条带，从而排除了全保留和分散模型。

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3. Key Enzymes and Proteins at the Replication Fork | 复制叉上的关键酶与蛋白质

Helicase unwinds the parental double helix by breaking the hydrogen bonds between base pairs, forming the replication fork. Topoisomerase (DNA gyrase) relieves the supercoiling tension ahead of the fork by cutting and rejoining DNA strands. Single-strand binding proteins (SSBs) coat the exposed single strands to prevent them from re-annealing or being degraded.

解旋酶通过破坏碱基对之间的氢键来解开亲代双螺旋，形成复制叉。拓扑异构酶（DNA 旋转酶）通过切断并重新连接 DNA 链，缓解复制叉前方的超螺旋压力。单链结合蛋白覆盖在暴露的单链上，防止其重新配对或被降解。

Primase synthesises a short RNA primer (about 10 nucleotides) to provide a free 3′-OH group for DNA polymerase, which can only add nucleotides to an existing 3′ end. DNA polymerase III is the main synthesising enzyme, catalysing the formation of phosphodiester bonds in the 5′ → 3′ direction. DNA polymerase I removes the RNA primers and replaces them with DNA. DNA ligase seals the nicks between Okazaki fragments by forming phosphodiester bonds.

引物酶合成一小段 RNA 引物（约 10 个核苷酸），为 DNA 聚合酶提供游离的 3′-OH 基团，因为 DNA 聚合酶只能将核苷酸添加到已有的 3′ 末端。DNA 聚合酶 III 是主要的合成酶，催化 5′ → 3′ 方向上磷酸二酯键的形成。DNA 聚合酶 I 去除 RNA 引物并用 DNA 替换。DNA 连接酶通过形成磷酸二酯键来封堵冈崎片段之间的缺口。

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4. Initiation at the Origin of Replication | 在复制起点启动

In prokaryotes such as E. coli, replication begins at a single origin (oriC) and proceeds bidirectionally. Initiator proteins bind to the origin, causing a local unwinding and the formation of a replication bubble with two replication forks. Eukaryotic chromosomes have multiple origins, allowing the larger genome to be replicated more quickly.

在原核生物如大肠杆菌中，复制在单一复制起点（oriC）处启动，并双向进行。起始蛋白与起点结合，引起局部解旋，形成一个具有两个复制叉的复制泡。真核染色体有多个复制原点，使其庞大的基因组能够更快完成复制。

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5. Unwinding and Stabilising the DNA | 解旋与稳定DNA

Helicase uses energy from ATP hydrolysis to break the hydrogen bonds between complementary bases, separating the two strands. As helicase advances, it introduces positive supercoiling ahead of the fork; topoisomerase nicks the sugar-phosphate backbone to allow rotation and then reseals it. SSBs bind to the single-stranded regions and keep them extended for complementary base pairing.

解旋酶利用 ATP 水解的能量来断开互补碱基间的氢键，将两条链分开。随着解旋酶向前移动，会在复制叉前方引入正超螺旋；拓扑异构酶切断糖磷酸骨架，使其可以旋转，然后再重新连接。SSB 结合在单链区域使其保持舒展状态，以便进行互补碱基配对。

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6. Leading Strand Synthesis | 前导链的合成

The leading strand is synthesised continuously in the same direction as the movement of the replication fork. Primase lays down a single RNA primer at the origin, and DNA polymerase III then adds DNA nucleotides continuously in the 5′ → 3′ direction, reading the parental template in the 3′ → 5′ direction. Because the newly built strand runs antiparallel, its synthesis towards the fork is seamless.

前导链的合成是与复制叉移动方向相同的连续过程。引物酶在起点处放置一个 RNA 引物，然后 DNA 聚合酶 III 沿着 5′ → 3′ 方向不断添加 DNA 核苷酸，同时以 3′ → 5′ 方向阅读亲代模板链。由于新链与模板链反向平行，其朝向复制叉的合成是顺畅连续的。

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7. Lagging Strand Synthesis and Okazaki Fragments | 滞后链合成与冈崎片段

The lagging strand is synthesised discontinuously in the opposite direction of the fork movement. Because DNA polymerase III can only synthesise in the 5′ → 3′ direction, the lagging strand must be built in short segments called Okazaki fragments. Primase repeatedly lays down RNA primers as the fork opens, and DNA polymerase III extends each fragment from these primers.

滞后链的合成是不连续的，方向与复制叉移动方向相反。由于 DNA 聚合酶 III 只能以 5′ → 3′ 方向合成，滞后链必须被分成若干短片段来构建，这些片段被称为冈崎片段。随着复制叉不断打开，引物酶反复放置 RNA 引物，DNA 聚合酶 III 再以这些引物为起点延伸出每一个片段。

In prokaryotes, Okazaki fragments are typically 1000-2000 nucleotides long; in eukaryotes they are shorter, about 100-200 nucleotides. Each fragment begins with an RNA primer and stops just before the next primer, leaving small gaps.

在原核生物中，冈崎片段通常长约 1000-2000 个核苷酸；在真核生物中更短，约 100-200 个核苷酸。每个片段都以 RNA 引物起始，并在下一个引物前停止，留下小段空隙。

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8. Primer Removal, Gap Filling, and Ligation | 引物去除、空隙填补与连接

After DNA polymerase III finishes an Okazaki fragment, DNA polymerase I removes the RNA primer using its 5′ → 3′ exonuclease activity and simultaneously fills the gap with DNA nucleotides. This process leaves a nick, a break in the sugar-phosphate backbone between two adjacent DNA segments. DNA ligase then seals this nick by catalysing the formation of a phosphodiester bond, using energy from ATP or NAD⁺.

当 DNA 聚合酶 III 完成一个冈崎片段后，DNA 聚合酶 I 利用其 5′ → 3′ 外切酶活性去除 RNA 引物，同时用 DNA 核苷酸填补空隙。这一过程留下一个切口，即相邻两个 DNA 片段之间糖磷酸骨架上的断裂。随后 DNA 连接酶利用 ATP 或 NAD⁺ 提供的能量，催化磷酸二酯键的形成，将切口密封。

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9. Proofreading and Error Correction | 校对与纠错

DNA polymerase III possesses 3′ → 5′ exonuclease activity, which acts as a proofreading mechanism. If an incorrect nucleotide is inserted, the polymerase pauses, removes the mismatched nucleotide, and replaces it with the correct one. This reduces the error rate to approximately one mistake per 10⁹ bases replicated, ensuring the fidelity of the genetic code.

DNA 聚合酶 III 具有 3′ → 5′ 外切酶活性，可作为一种校对机制。如果插入了错误的核苷酸，聚合酶会暂停，移除错配的核苷酸，再用正确的替换。这将错误率降至约每复制 10⁹ 个碱基出现一次错误，从而确保遗传密码的忠实性。

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10. Comparing Prokaryotic and Eukaryotic Replication | 原核与真核复制比较

Prokaryotic DNA is circular and has a single origin of replication; eukaryotes have linear chromosomes with multiple origins. Prokaryotes possess DNA polymerase I and III, while eukaryotes use a larger set of polymerases (α, δ, ε) for priming and elongation. Eukaryotic replication also involves the unwrapping of DNA from histone proteins, and the problem of end shortening is addressed by telomerase, though this is beyond the core IB requirement.

原核生物的 DNA 呈环状，只有一个复制起点；真核生物的染色体呈线状，且有多个复制起点。原核生物使用 DNA 聚合酶 I 和 III，而真核生物使用更多种类的聚合酶（α、δ、ε）来负责引物合成与延伸。真核复制还涉及从组蛋白上解开 DNA 的过程，并且通过端粒酶解决末端缩短问题，不过这超出了 IB 核心要求。

Feature | 特征	Prokaryotes | 原核生物	Eukaryotes | 真核生物
Number of origins | 起点数	Single | 单个	Multiple | 多个
DNA shape | DNA形状	Circular | 环状	Linear | 线状
Key polymerases | 关键聚合酶	DNA pol I & III | 聚合酶 I 和 III	Pol α, δ, ε | 聚合酶 α、δ、ε
Speed | 速度	~1000 nucleotides/s | 约 1000 核苷酸/秒	~50 nucleotides/s | 约 50 核苷酸/秒

The table above summarises the main contrasts that occasionally appear in IB exam questions. Remember that both domains share the fundamental semi-conservative mechanism and use the same basic fork machinery.

上表总结了 IB 考题中偶尔出现的主要对比。请记住，两个生物领域都遵循半保留这一基本机制，并使用相同的基本复制叉装置。

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