DNA Replication — DNA 复制

Introduction to DNA Replication — DNA 复制简介

DNA replication is the biological process by which a cell produces two identical copies of its DNA before cell division. This fundamental mechanism ensures that genetic information is faithfully transmitted from one generation of cells to the next. For Cambridge A-Level Biology students, understanding the molecular details of DNA replication is essential – it bridges the gap between the molecular structure of DNA that you learn in earlier topics and the mechanisms of inheritance, mutation, and gene expression covered later in the syllabus.

DNA 复制是细胞在分裂前产生两份完全相同 DNA 拷贝的生物学过程。这一基本机制确保了遗传信息能够忠实地从一代细胞传递到下一代。对于剑桥 A-Level 生物的学生来说，理解 DNA 复制的分子细节至关重要 – 它连接了 DNA 分子结构（早期学习内容）与遗传、突变和基因表达（后续大纲内容）之间的知识桥梁。

The Semi-Conservative Model — 半保留复制模型

The Meselson-Stahl experiment (1958) provided definitive evidence that DNA replication follows a semi-conservative mechanism. In this experiment, E. coli bacteria were grown in a medium containing the heavy nitrogen isotope ¹⁵N for many generations, so that all nitrogen atoms in their DNA were of the heavy isotope. The bacteria were then transferred to a medium containing the normal ¹⁴N isotope and allowed to divide. DNA samples were extracted after one and two rounds of replication and separated by density-gradient centrifugation.

Meselson-Stahl 实验（1958年）提供了确凿证据，证明 DNA 复制遵循半保留机制。在该实验中，大肠杆菌在含有重氮同位素 ¹⁵N 的培养基中培养多代，使其 DNA 中的所有氮原子均为重同位素。然后将细菌转移到含有正常 ¹⁴N 同位素的培养基中，允许其分裂。分别在一轮和两轮复制后提取 DNA 样本，通过密度梯度离心进行分离。

After one generation, all DNA molecules formed a single hybrid band at an intermediate density between ¹⁵N-DNA and ¹⁴N-DNA, indicating that each double helix contained one old (heavy) strand and one new (light) strand. After two generations, two bands appeared: one at the hybrid density and one at the light density. This result is consistent only with the semi-conservative model – if replication were conservative, two bands (one heavy, one light) would appear after just one generation; if dispersive, only a single band of progressively lighter density would be observed.

经过一代后，所有 DNA 分子在 ¹⁵N-DNA 和 ¹⁴N-DNA 之间的中间密度处形成单一杂交带，表明每个双螺旋含有一条旧（重）链和一条新（轻）链。两代后，出现两条带：一条在杂交密度处，一条在轻密度处。这一结果仅与半保留模型一致 – 如果是全保留复制，一代后就会出现两条带（一条重、一条轻）；如果是分散复制，则只会观察到密度逐渐变轻的单一带。

Key Enzymes in DNA Replication — DNA 复制的关键酶

DNA replication in both prokaryotes and eukaryotes depends on a suite of enzymes, each with a specialised function. Understanding what each enzyme does – and the order in which they act – is a core requirement of the Cambridge A-Level specification. The four most important enzymes are DNA helicase, DNA polymerase, primase, and DNA ligase.

原核生物和真核生物中的 DNA 复制都依赖于一系列酶，每种酶都有专门的功能。理解每种酶的作用及其作用的顺序，是剑桥 A-Level 大纲的核心要求。四种最重要的酶分别是 DNA 解旋酶、DNA 聚合酶、引物酶和 DNA 连接酶。

DNA Helicase — DNA 解旋酶

DNA helicase unwinds the double helix by breaking the hydrogen bonds between complementary base pairs (A-T and C-G). This action creates a Y-shaped replication fork where the two parental strands separate and become available as templates. Helicase uses energy from ATP hydrolysis to translocate along the DNA and separate the strands ahead of the replication machinery. In prokaryotes such as E. coli, the main helicase is DnaB, which moves along the lagging strand template in the 5′ to 3′ direction.

DNA 解旋酶通过断裂互补碱基对（A-T 和 C-G）之间的氢键来解开双螺旋。这一作用产生一个 Y 形的复制叉，两条亲链在此分离并作为模板可用。解旋酶利用 ATP 水解产生的能量沿 DNA 移位，在复制机器前方分离两条链。在原核生物（如大肠杆菌）中，主要解旋酶是 DnaB，它沿滞后链模板以 5′ 到 3′ 方向移动。

DNA Polymerase — DNA 聚合酶

DNA polymerase is the enzyme that synthesises new DNA strands by adding nucleotides to the 3′ end of a growing polynucleotide chain. It can only add nucleotides in the 5′ to 3′ direction, meaning it reads the template strand in the 3′ to 5′ direction. A free 3′-OH group is required as a starting point – this is why a primer is necessary. In E. coli, DNA polymerase III is the main replicative enzyme, while DNA polymerase I removes RNA primers and fills the resulting gaps. Eukaryotic cells have multiple DNA polymerases, with polymerases α, δ, and ε playing major roles in nuclear DNA replication.

DNA 聚合酶是通过向生长中的多核苷酸链的 3′ 端添加核苷酸来合成新 DNA 链的酶。它只能沿 5′ 到 3′ 方向添加核苷酸，这意味着它沿 3′ 到 5′ 方向读取模板链。需要游离的 3′-OH 基团作为起始点 – 这就是为什么需要引物。在大肠杆菌中，DNA 聚合酶 III 是主要的复制酶，而 DNA 聚合酶 I 则移除 RNA 引物并填补由此产生的缺口。真核细胞有多种 DNA 聚合酶，其中聚合酶 α、δ 和 ε 在核 DNA 复制中起主要作用。

Primase — 引物酶

Primase is an RNA polymerase that synthesises short RNA primers (typically 10-12 nucleotides long) complementary to the template DNA strand. These primers provide the free 3′-OH group that DNA polymerase requires to begin synthesis. Without primase, DNA polymerase cannot initiate a new strand. In E. coli, the primase is DnaG, which associates transiently with the helicase at the replication fork.

引物酶是一种 RNA 聚合酶，合成与模板 DNA 链互补的短 RNA 引物（通常 10-12 个核苷酸长）。这些引物提供 DNA 聚合酶开始合成所需的游离 3′-OH 基团。没有引物酶，DNA 聚合酶就无法启动新链的合成。在大肠杆菌中，引物酶是 DnaG，它在复制叉处与解旋酶短暂结合。

DNA Ligase — DNA 连接酶

DNA ligase seals the nicks (breaks in the phosphodiester backbone) between adjacent DNA fragments. Specifically, it joins the 3′-OH end of one fragment to the 5′-phosphate end of the next, forming a phosphodiester bond. This enzyme is essential for joining Okazaki fragments on the lagging strand and for sealing the gaps left after RNA primers are removed and replaced with DNA. DNA ligase uses energy from ATP (in eukaryotes and bacteriophages) or NAD⁺ (in bacteria) to catalyse this reaction.

DNA 连接酶封闭相邻 DNA 片段之间的切口（磷酸二酯骨架中的断裂）。具体来说，它将一个片段的 3′-OH 端连接到下一个片段的 5′-磷酸端，形成磷酸二酯键。该酶对于连接滞后链上的冈崎片段以及封闭 RNA 引物移除并被 DNA 替换后留下的缺口至关重要。DNA 连接酶利用 ATP（在真核生物和噬菌体中）或 NAD⁺（在细菌中）的能量来催化该反应。

The Replication Fork — 复制叉

The replication fork is the active region where DNA replication takes place. At each fork, the two parental strands are unwound and separated, and both serve simultaneously as templates for new DNA synthesis. Because DNA polymerase can only synthesise in the 5′ to 3′ direction, the two strands at the fork are replicated by different mechanisms.

复制叉是 DNA 复制发生的活跃区域。在每个复制叉处，两条亲链被解开并分离，同时作为新 DNA 合成的模板。由于 DNA 聚合酶只能沿 5′ 到 3′ 方向合成，复制叉处的两条链通过不同的机制进行复制。

The leading strand is the strand whose 3′ end points toward the replication fork. DNA polymerase can synthesise this strand continuously in the same direction as the fork moves, adding nucleotides to the 3′ end without interruption. Only one primer is needed at the origin of replication to initiate leading-strand synthesis.

前导链是其 3′ 端指向复制叉的链。DNA 聚合酶可以沿复制叉移动的同一方向连续合成该链，不间断地向 3′ 端添加核苷酸。只需在复制起点处使用一个引物即可启动前导链的合成。

The lagging strand has its 5′ end pointing toward the replication fork, so its template runs 5′ to 3′ in the direction of fork movement. DNA polymerase cannot synthesise continuously on this strand. Instead, synthesis occurs in short, discontinuous segments called Okazaki fragments (approximately 1000-2000 nucleotides long in prokaryotes, 100-200 in eukaryotes). Each fragment requires its own RNA primer, which is later removed and replaced with DNA before the fragments are joined by DNA ligase.

滞后链的 5′ 端指向复制叉，因此其模板沿复制叉移动方向为 5′ 到 3’。DNA 聚合酶无法在该链上进行连续合成。相反，合成以短的、不连续的片段进行，称为冈崎片段（在原核生物中约 1000-2000 个核苷酸长，在真核生物中为 100-200 个核苷酸长）。每个片段都需要自己的 RNA 引物，这些引物随后被移除并用 DNA 替换，然后由 DNA 连接酶将片段连接起来。

Okazaki Fragments in Detail — 冈崎片段详解

Okazaki fragments, named after the Japanese scientists Reiji and Tsuneko Okazaki who discovered them in 1968, are the key to understanding lagging-strand synthesis. Each fragment begins with an RNA primer synthesised by primase. DNA polymerase III then extends the primer, synthesising DNA in the 5′ to 3′ direction until it reaches the next primer. DNA polymerase I subsequently removes the RNA primer and replaces it with DNA nucleotides. Finally, DNA ligase seals the remaining nick between adjacent fragments, creating a continuous DNA strand.

冈崎片段以日本科学家冈崎令治和冈崎恒子（1968年发现）命名，是理解滞后链合成的关键。每个片段始于引物酶合成的 RNA 引物。然后 DNA 聚合酶 III 延伸引物，沿 5′ 到 3′ 方向合成 DNA，直到到达下一个引物。随后 DNA 聚合酶 I 移除 RNA 引物并用 DNA 核苷酸替换。最后，DNA 连接酶封闭相邻片段之间的剩余切口，形成连续的 DNA 链。

A common exam question asks students to explain why the lagging strand is synthesised discontinuously. The answer hinges on two facts: first, DNA polymerase can only add nucleotides to the 3′ end of a growing chain (5′ to 3′ direction); second, the two strands in the double helix are antiparallel. As a result, the template for the lagging strand runs in the 5′ to 3′ direction relative to the movement of the replication fork, meaning the polymerase must work in short segments away from the fork.

一个常见的考题是要求学生解释为什么滞后链是不连续合成的。答案取决于两个事实：第一，DNA 聚合酶只能向生长链的 3′ 端添加核苷酸（5′ 到 3′ 方向）；第二，双螺旋中的两条链是反向平行的。因此，滞后链的模板相对于复制叉的移动方向为 5′ 到 3’，这意味着聚合酶必须以远离复制叉的短片段形式工作。

Stages of DNA Replication — DNA 复制的阶段

Initiation — 起始阶段

DNA replication begins at specific sequences called origins of replication. Prokaryotic chromosomes typically have a single origin (oriC in E. coli), while eukaryotic chromosomes contain multiple origins to allow the replication of large genomes in a reasonable time. Initiator proteins recognise and bind to the origin, causing local unwinding of the DNA. This allows DNA helicase to bind and begin separating the strands bidirectionally, forming two replication forks that move in opposite directions.

DNA 复制在称为复制起点的特定序列处开始。原核染色体通常只有一个起点（大肠杆菌中的 oriC），而真核染色体含有多个起点，以便在合理时间内完成大基因组的复制。起始蛋白识别并结合到起点，引起 DNA 的局部解开。这使得 DNA 解旋酶能够结合并开始双向分离链，形成两个向相反方向移动的复制叉。

Elongation — 延伸阶段

During elongation, the replication machinery – called the replisome – moves along the DNA synthesising new strands. The replisome is a large multi-protein complex that includes helicase, primase, DNA polymerases, sliding clamps, and single-strand binding proteins (SSBs). SSBs bind to the separated single-stranded DNA to prevent the strands from re-annealing and to protect them from nucleases. The sliding clamp (beta-clamp in prokaryotes, PCNA in eukaryotes) encircles the DNA and tethers DNA polymerase to the template, greatly increasing the enzyme’s processivity – the number of nucleotides added before dissociation – from tens to thousands.

在延伸阶段，复制机器 – 称为复制体 – 沿 DNA 移动，合成新链。复制体是一个大型多蛋白复合体，包括解旋酶、引物酶、DNA 聚合酶、滑动夹和单链结合蛋白。单链结合蛋白与分开的单链 DNA 结合，防止链重新退火并保护它们免受核酸酶的降解。滑动夹（原核生物中的 beta-夹，真核生物中的 PCNA）环绕 DNA 并将 DNA 聚合酶拴在模板上，大大提高了酶的延伸性 – 即解离前添加的核苷酸数量 – 从数十个增加到数千个。

Termination — 终止阶段

In prokaryotes, termination occurs when the two replication forks meet at termination (Ter) sequences opposite the origin. Tus proteins bind to Ter sequences and act as a barrier to helicase, stopping fork progression. In eukaryotes, replication termination is less well-defined; forks simply converge and the replisome disassembles. The final gaps and nicks are filled and sealed by the combined action of DNA polymerases and DNA ligase.

在原核生物中，当两个复制叉在起点对面的终止序列处相遇时，复制终止。Tus 蛋白与终止序列结合，作为解旋酶的障碍，阻止复制叉前进。在真核生物中，复制终止的定义不太明确；复制叉简单汇聚，复制体解体。最后的缺口和切口由 DNA 聚合酶和 DNA 连接酶共同作用填补和封闭。

DNA Proofreading and Error Correction — DNA 校对与纠错

DNA replication is remarkably accurate, with an error rate of approximately one mistake per 10⁹ to 10¹⁰ nucleotides replicated. This extraordinary fidelity is achieved through a combination of base-pairing specificity, proofreading, and post-replicative mismatch repair. The proofreading function is carried out by DNA polymerase itself: the enzyme possesses 3′ to 5′ exonuclease activity, which allows it to detect and remove incorrectly paired nucleotides immediately after they are added.

DNA 复制非常精确，每复制 10⁹ 到 10¹⁰ 个核苷酸才出现约一个错误。这种非凡的保真度是通过碱基配对特异性、校对和复制后错配修复的组合实现的。校对功能由 DNA 聚合酶自身执行：该酶具有 3′ 到 5′ 外切核酸酶活性，使其能够检测并立即移除刚刚添加的错误配对核苷酸。

When an incorrect nucleotide is incorporated, the resulting mismatch distorts the 3′ end of the growing strand, causing the polymerase to pause. The mispaired nucleotide is then excised by the 3′ to 5′ exonuclease domain, and the polymerase resumes synthesis with the correct nucleotide. This proofreading step reduces the error rate by a factor of approximately 100-1000. Importantly, exonuclease activity works in the opposite direction to polymerase activity – this is why it is called 3′ to 5′ exonuclease.

当错误的核苷酸被掺入时，产生的错配会扭曲生长链的 3′ 端，使聚合酶暂停。然后错配的核苷酸被 3′ 到 5′ 外切核酸酶结构域切除，聚合酶用正确的核苷酸恢复合成。这一校对步骤将错误率降低约 100-1000 倍。重要的是，外切核酸酶活性沿与聚合酶活性相反的方向工作 – 这就是为什么它被称为 3′ 到 5′ 外切核酸酶。

Comparison of Prokaryotic and Eukaryotic Replication — 原核与真核复制的比较

Feature / 特征	Prokaryotes (e.g. E. coli) / 原核生物	Eukaryotes / 真核生物
Genome size / 基因组大小	Single circular chromosome (~4.6 Mbp) / 单环状染色体	Multiple linear chromosomes (~3.2 Gbp in humans) / 多条线性染色体
Origins of replication / 复制起点	Single origin (oriC) / 单个起点	Multiple origins (thousands) / 多个起点
Replication speed / 复制速度	~1000 nucleotides per second / 约1000个核苷酸/秒	~50 nucleotides per second / 约50个核苷酸/秒
Main DNA polymerase / 主要DNA聚合酶	DNA polymerase III / DNA聚合酶III	Polymerases α, δ, ε / 聚合酶α、δ、ε
Primer removal / 引物移除	DNA polymerase I / DNA聚合酶I	RNase H and FEN1 / RNase H和FEN1
Telomeres / 端粒	Not applicable (circular chromosome) / 不适用（环状染色体）	Present; maintained by telomerase / 存在；由端粒酶维持
Location / 位置	Cytoplasm / 细胞质	Nucleus / 细胞核

While the basic mechanism of semi-conservative replication is conserved across all domains of life, these differences reflect the greater complexity and larger genome size of eukaryotic cells. The presence of multiple origins in eukaryotes is essential to complete replication within the S phase of the cell cycle, which typically lasts 6-8 hours in mammalian cells despite the slower replication rate.

虽然半保留复制的基本机制在所有生命域中都是保守的，但这些差异反映了真核细胞更大的复杂性和更大的基因组大小。真核生物中存在多个起点，对于在细胞周期的 S 期内完成复制至关重要 – 尽管复制速率较慢，哺乳动物细胞的 S 期通常持续 6-8 小时。

Telomeres and the End-Replication Problem — 端粒与末端复制问题

Linear eukaryotic chromosomes face a unique challenge: the ends of lagging strands cannot be fully replicated by conventional mechanisms. The very last RNA primer on the lagging strand is removed, leaving a short gap at the 5′ end that cannot be filled because there is no upstream 3′-OH for DNA polymerase to extend from. As a result, chromosomes would progressively shorten with each round of replication – a phenomenon known as the end-replication problem.

线性真核染色体面临一个独特的挑战：滞后链的末端无法通过常规机制完全复制。滞后链上最后的 RNA 引物被移除后，在 5′ 端留下一个短缺口，由于没有上游 3′-OH 供 DNA 聚合酶延伸，该缺口无法被填补。因此，染色体将随着每一轮复制而逐渐缩短 – 这一现象称为末端复制问题。

Telomeres – repetitive, non-coding sequences (TTAGGG in vertebrates) at chromosome ends – provide a solution. These sequences do not carry essential genetic information, so their gradual loss does not immediately threaten the cell. Telomerase, a specialised reverse transcriptase containing its own RNA template, extends the 3′ overhang of telomeres, allowing the replication machinery to synthesise the complementary strand normally. Telomerase is active in germ cells, stem cells, and most cancer cells, but is largely inactive in normal somatic cells, contributing to cellular ageing.

端粒 – 染色体末端的重复非编码序列（脊椎动物中为 TTAGGG） – 提供了解决方案。这些序列不携带必需的遗传信息，因此它们的逐渐丢失不会立即威胁细胞。端粒酶是一种特殊的逆转录酶，含有自身的 RNA 模板，它延伸端粒的 3′ 突出端，使复制机器能够正常合成互补链。端粒酶在生殖细胞、干细胞和大多数癌细胞中活跃，但在正常体细胞中基本上不活跃，这导致了细胞衰老。

DNA Replication vs Transcription — DNA 复制与转录的比较

Students often confuse DNA replication with transcription, but these are fundamentally different processes with distinct purposes, enzymes, and products. DNA replication produces an entire double-stranded copy of the genome; transcription produces single-stranded mRNA copies of specific genes. DNA replication uses DNA polymerase; transcription uses RNA polymerase. In replication, both strands serve as templates; in transcription, only one strand (the template strand) of a gene is transcribed. The key distinction for Cambridge A-Level is that replication is about copying the entire genome for cell division, while transcription is about expressing specific genes to produce proteins.

学生经常将 DNA 复制与转录混淆，但这是两种根本不同的过程，具有不同的目的、酶和产物。DNA 复制产生基因组的完整双链拷贝；转录产生特定基因的单链 mRNA 拷贝。DNA 复制使用 DNA 聚合酶；转录使用 RNA 聚合酶。在复制中，两条链都作为模板；在转录中，只有基因的一条链（模板链）被转录。对于剑桥 A-Level，关键区别在于复制是为了细胞分裂而复制整个基因组，而转录是为了表达特定基因以产生蛋白质。

Aspect / 方面	DNA Replication / DNA复制	Transcription / 转录
Purpose / 目的	Copy entire genome for cell division / 为细胞分裂复制整个基因组	Synthesise mRNA from specific genes / 从特定基因合成mRNA
Enzyme / 酶	DNA polymerase / DNA聚合酶	RNA polymerase / RNA聚合酶
Template / 模板	Both strands of entire chromosome / 整条染色体的两条链	One strand of a specific gene / 特定基因的一条链
Product / 产物	Double-stranded DNA / 双链DNA	Single-stranded mRNA / 单链mRNA
Primer required? / 需要引物？	Yes / 需要	No / 不需要
Nucleotides used / 使用的核苷酸	dATP, dTTP, dCTP, dGTP	ATP, UTP, CTP, GTP
Base pairing / 碱基配对	A-T, C-G	A-U, T-A, C-G, G-C
Proofreading / 校对	Yes (3′ to 5′ exonuclease) / 有	Limited; lower fidelity / 有限；保真度较低

Exam Tips for Cambridge A-Level — 剑桥 A-Level 考试技巧

Cambridge examination questions on DNA replication typically focus on understanding rather than rote recall. Here are the most important points to remember for the exam:

剑桥 A-Level 关于 DNA 复制的考题通常侧重于理解而非死记硬背。以下是为考试需要记住的最重要要点：

Always state the direction of synthesis – marks are frequently awarded for specifying that DNA polymerase synthesises in the 5′ to 3′ direction. This also explains why the lagging strand must be synthesised discontinuously.
Always state DNA聚合酶合成的方向 – 明确指出 DNA 聚合酶沿 5′ 到 3′ 方向合成，这一点经常得分。这也解释了为什么滞后链必须不连续合成。
Learn the enzyme functions precisely – do not just say “helicase unwinds DNA”; say “DNA helicase breaks hydrogen bonds between complementary base pairs to separate the two strands.” Precision with molecular detail earns the highest marks.
精确学习每种酶的功能 – 不要只说”解旋酶解开 DNA”；要说”DNA 解旋酶断裂互补碱基对之间的氢键以分离两条链。”分子细节的精确度能赢得最高分。
Use correct terminology – “leading strand,” “lagging strand,” “Okazaki fragments,” “semi-conservative,” “replication fork.” Examiners look for these specific terms.
使用正确的术语 – “前导链”、”滞后链”、”冈崎片段”、”半保留”、”复制叉”。考官会关注这些特定的术语。
Connect the Meselson-Stahl experiment to the semi-conservative model – be prepared to describe the experiment and explain how the results exclude the conservative and dispersive models.
将 Meselson-Stahl 实验与半保留模型联系起来 – 准备好描述该实验并解释其结果如何排除全保留和分散模型。
Explain Okazaki fragments in context – if asked about lagging-strand synthesis, mention Okazaki fragments, their size, the role of RNA primers, DNA polymerase I, and DNA ligase.
在上下文中解释冈崎片段 – 如果被问到滞后链合成，要提到冈崎片段、其大小、RNA 引物的作用、DNA 聚合酶 I 和 DNA 连接酶。
Distinguish replication from transcription – comparison questions are common. Use a table format if it helps your revision, even if you write prose in the exam.
区分复制和转录 – 比较题很常见。如果有助于复习，可以用表格格式，即使在考试中写文章即可。

Common Mistakes to Avoid — 需要避免的常见错误

Saying DNA polymerase adds nucleotides to the 5′ end – it always adds to the 3′ end. This is the single most common error in exam answers.
说 DNA 聚合酶向 5′ 端添加核苷酸 – 它总是向 3′ 端添加。这是考试答案中最常见的错误。
Confusing DNA helicase with topoisomerase – helicase breaks hydrogen bonds; topoisomerase (DNA gyrase in prokaryotes) relieves the torsional stress (supercoiling) ahead of the replication fork. They have different functions and both are often required in exam answers.
混淆 DNA 解旋酶与拓扑异构酶 – 解旋酶断裂氢键；拓扑异构酶（原核生物中的 DNA 旋转酶）缓解复制叉前方的扭转应力（超螺旋）。它们具有不同的功能，在考试答案中两者经常都需要提及。
Forgetting that primase makes RNA primers, not DNA primers – the primers are RNA, which is why they must later be removed and replaced with DNA.
忘记引物酶制造的是 RNA 引物而非 DNA 引物 – 引物是 RNA，这就是为什么它们之后必须被移除并用 DNA 替换。
Thinking that only one replication fork forms – replication is bidirectional from each origin, producing two forks moving in opposite directions.
认为只形成一个复制叉 – 复制从每个起点双向进行，产生两个向相反方向移动的复制叉。
Omitting DNA ligase when describing Okazaki fragment joining – marks are specifically allocated for stating that DNA ligase seals the phosphodiester backbone between fragments.
描述冈崎片段连接时遗漏 DNA 连接酶 – 明确指出 DNA 连接酶封闭片段之间的磷酸二酯骨架，这有专门的分数分配。
Stating that the lagging strand is synthesised 3′ to 5′ – all DNA synthesis is 5′ to 3′. The lagging strand is synthesised 5′ to 3′ in short segments away from the fork. Remember: the strand is called “lagging” because synthesis lags behind fork movement, not because synthesis runs in the reverse direction.
声称滞后链是 3′ 到 5′ 合成的 – 所有 DNA 合成都是 5′ 到 3’。滞后链是以远离复制叉的短片段形式 5′ 到 3′ 合成的。请记住：该链被称为”滞后”是因为合成落后于复制叉的移动，而不是因为合成沿相反方向进行。

Practice Questions — 练习题

Question 1 / 问题 1

Describe the role of DNA helicase in DNA replication and explain why its activity is essential for the process. (3 marks / 3分)

Model Answer / 标准答案: DNA helicase breaks the hydrogen bonds between complementary base pairs (A-T and C-G) on the two strands of the DNA double helix (1). This unwinds and separates the two parental strands (1), exposing the bases so that each strand can act as a template for the synthesis of a new complementary strand (1). Without helicase activity, the strands would remain paired and DNA polymerase could not access the template.

Question 2 / 问题 2

Explain why the synthesis of the lagging strand during DNA replication differs from that of the leading strand. (4 marks / 4分)

Model Answer / 标准答案: DNA polymerase can only synthesise DNA in the 5′ to 3′ direction (1). On the leading strand, the template runs 3′ to 5′ towards the replication fork, allowing continuous synthesis (1). On the lagging strand, the template runs 5′ to 3′ towards the fork, so the polymerase must synthesise short, discontinuous Okazaki fragments away from the fork (1). Each fragment requires a new RNA primer, and the fragments are later joined by DNA ligase (1).

Question 3 / 问题 3

Outline the Meselson-Stahl experiment and explain how the results support the semi-conservative model of DNA replication. (5 marks / 5分)

Model Answer / 标准答案: E. coli were grown for many generations in a medium containing the heavy nitrogen isotope ¹⁵N, so all their DNA contained ¹⁵N (1). The bacteria were transferred to a medium containing the normal ¹⁴N isotope and allowed to divide once (1). DNA was extracted and separated by density-gradient centrifugation (1). After one generation, a single hybrid band of intermediate density was observed, indicating each DNA molecule contained one ¹⁵N strand and one ¹⁴N strand (1). This result is predicted only by the semi-conservative model – the conservative model would produce separate heavy and light bands, while the dispersive model would produce a single band of intermediate density after the first generation but no separate light band after the second generation (1).

Summary — 总结

DNA replication is a precisely orchestrated molecular process that ensures the faithful duplication of the genome before cell division. The semi-conservative mechanism, demonstrated by the Meselson-Stahl experiment, is universally conserved across prokaryotes and eukaryotes. Key enzymes – helicase, primase, DNA polymerase, and ligase – work in a coordinated manner at the replication fork to synthesise new DNA strands. The antiparallel nature of the double helix means that the leading strand is synthesised continuously while the lagging strand is produced as Okazaki fragments. Proofreading by DNA polymerase and post-replicative repair mechanisms together achieve remarkable fidelity, with errors occurring at a rate of only about one per billion nucleotides. For Cambridge A-Level Biology, mastering these concepts – with precise terminology and a clear understanding of the experimental evidence – is essential for success.

DNA 复制是一个精确编排的分子过程，确保基因组在细胞分裂前忠实地复制。由 Meselson-Stahl 实验证明的半保留机制在原核生物和真核生物中普遍保守。关键酶 – 解旋酶、引物酶、DNA 聚合酶和连接酶 – 在复制叉处以协调的方式工作，合成新的 DNA 链。双螺旋的反向平行性质意味着前导链连续合成，而滞后链以冈崎片段的形式产生。DNA 聚合酶的校对和复制后修复机制共同实现了非凡的保真度，错误率仅为每十亿个核苷酸约一个。对于剑桥 A-Level 生物学，掌握这些概念 – 使用精确的术语并对实验证据有清晰的理解 – 是成功的关键。

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