DNA Replication – CCEA IGCSE Biology | IGCSE CCEA 生物：DNA复制 考点精讲

📚 DNA Replication – CCEA IGCSE Biology | IGCSE CCEA 生物：DNA复制 考点精讲

DNA replication is the process by which a cell makes an identical copy of its DNA. This is essential for cell division, ensuring that each daughter cell receives a complete set of genetic instructions. In CCEA IGCSE Biology, you need to understand the semi-conservative nature of replication, the roles of key enzymes, and the steps involved in copying both the leading and lagging strands.

DNA复制是细胞制造其DNA完全相同副本的过程。这对于细胞分裂至关重要，确保每个子细胞都能获得一套完整的遗传指令。在CCEA IGCSE生物学中，你需要理解复制的半保留性质、关键酶的作用，以及复制前导链与后随链的步骤。

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1. Overview of DNA Replication | DNA复制概述

DNA replication occurs during the S phase of the cell cycle, before mitosis or meiosis. The double helix unwinds, and each original strand serves as a template for building a new complementary strand. By the end, two identical DNA molecules are produced, each consisting of one old (parental) strand and one newly synthesised (daughter) strand.

DNA复制发生在细胞周期的S期，在有丝分裂或减数分裂之前。双螺旋解开，每条原始链作为模板来合成新的互补链。最终产生两个相同的DNA分子，每个分子由一条旧的（亲代）链和一条新合成的（子代）链组成。

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2. Semi-Conservative Replication | 半保留复制

The term ‘semi-conservative’ means that each new DNA molecule conserves one of the original strands. The Meselson–Stahl experiment confirmed this model. In the exam, you may be asked to explain why replication is semi-conservative and how it ensures genetic continuity across generations.

‘半保留’一词意味着每个新的DNA分子保留了一条原始链。梅塞尔森-斯塔尔实验证实了这一模型。考试中可能会要求你解释为什么复制是半保留的，以及它如何确保遗传信息在世代间的连续性。

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3. Key Enzymes Involved | 关键酶

DNA replication relies on several enzymes and proteins. The main ones to remember for CCEA IGCSE are listed in the table below.

DNA复制依赖于几种酶和蛋白质。以下表格列出了CCEA IGCSE需要记住的主要酶。

Enzyme	Function
Helicase	Unwinds and unzips the DNA double helix by breaking hydrogen bonds between base pairs.
DNA primase	Synthesises short RNA primers to provide a starting point for DNA polymerase.
DNA polymerase	Adds free DNA nucleotides to the growing strand in the 5′ to 3′ direction, complementary to the template.
DNA ligase	Joins Okazaki fragments on the lagging strand by forming phosphodiester bonds.

酶	功能
解旋酶	通过断开碱基对之间的氢键来解开DNA双螺旋。
DNA引物酶	合成短的RNA引物，为DNA聚合酶提供起始点。
DNA聚合酶	沿5’到3’方向将游离的DNA核苷酸添加到正在延长的链上，与模板链互补配对。
DNA连接酶	通过形成磷酸二酯键连接后随链上的冈崎片段。

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4. Step 1: Unwinding the Double Helix | 步骤1：解开双螺旋

Helicase binds at the origin of replication and moves along the DNA, breaking the hydrogen bonds between complementary base pairs. This separates the two strands, creating a Y-shaped replication fork. Single-strand binding proteins stabilise the separated strands to prevent them from reannealing.

解旋酶结合在复制起点并沿DNA移动，断裂互补碱基对之间的氢键。这使两条链分开，形成一个Y形的复制叉。单链结合蛋白稳定分开的链，防止它们重新配对。

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5. Step 2: Primer Binding | 步骤2：引物结合

DNA polymerase cannot start synthesis from scratch; it requires a free 3′-OH group. DNA primase synthesises a short RNA primer complementary to the template strand. This primer provides the necessary 3′ end for DNA polymerase to begin adding DNA nucleotides.

DNA聚合酶不能从零开始合成；它需要一个游离的3′-OH基团。DNA引物酶合成一段与模板链互补的短RNA引物。这个引物为DNA聚合酶开始添加DNA核苷酸提供了必要的3’末端。

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6. Step 3: Elongation – Leading Strand | 步骤3：延伸——前导链

The leading strand is synthesised continuously in the 5′ to 3′ direction towards the replication fork. DNA polymerase reads the template strand in the 3′ to 5′ direction and incorporates complementary nucleotides (A opposite T, C opposite G). Only one primer is needed at the start of this strand.

前导链是沿5’到3’方向朝复制叉连续合成的。DNA聚合酶以3’到5’方向读取模板链，并掺入互补的核苷酸（A与T配对，C与G配对）。这条链在起始时只需要一个引物。

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7. Step 4: Elongation – Lagging Strand | 步骤4：延伸——后随链

The lagging strand is synthesised discontinuously in short fragments called Okazaki fragments. Because DNA polymerase can only add nucleotides in the 5′ to 3′ direction, synthesis on this strand proceeds away from the replication fork. Multiple RNA primers are laid down, and DNA polymerase extends each primer to create fragments.

后随链是不连续合成的，形成被称为冈崎片段的短片段。由于DNA聚合酶只能沿5’到3’方向添加核苷酸，这一条链的合成方向是远离复制叉的。需要多个RNA引物，DNA聚合酶延伸每个引物以产生片段。

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8. Step 5: Joining of Okazaki Fragments | 步骤5：冈崎片段的连接

Once the fragments are synthesised, the RNA primers are removed and replaced with DNA by a different DNA polymerase. DNA ligase then seals the gaps between fragments by forming phosphodiester bonds, creating a continuous strand. This step is essential for completing the new lagging strand.

一旦片段合成完毕，RNA引物被移除并由另一种DNA聚合酶替换为DNA。然后DNA连接酶通过形成磷酸二酯键封闭片段之间的缺口，形成一条连续的链。这一步对于完成新的后随链至关重要。

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9. Proofreading and Error Correction | 校对与纠错

DNA polymerase has a proofreading function. It checks the newly added nucleotide against the template. If an incorrect base is inserted, the enzyme removes it and replaces it with the correct one. This reduces the mutation rate to about one error per billion base pairs replicated.

DNA聚合酶具有校对功能。它会根据模板检查新添加的核苷酸。如果插入了错误的碱基，该酶会将其切除并换上正确的碱基。这使突变率降低到每复制十亿个碱基对大约只出现一个错误。

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10. Importance of DNA Replication | DNA复制的重要性

Accurate DNA replication ensures that genetic information is faithfully transmitted from one generation of cells to the next. It underpins growth, tissue repair, and asexual reproduction. Without precise replication, mutations could accumulate, leading to genetic disorders or cancer.

准确的DNA复制确保遗传信息能够忠实地从一代细胞传到下一代。它是生长、组织修复和无性繁殖的基础。没有精确的复制，突变就可能累积，导致遗传疾病或癌症。

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11. Common Exam Questions and Misconceptions | 常见考题与误区

Students often confuse the direction of synthesis for the two strands or think that DNA polymerase works from 3′ to 5′. Remember: synthesis always occurs in the 5′ to 3′ direction. Another common mistake is forgetting that the lagging strand requires multiple primers and DNA ligase. Exam questions might also ask you to explain why replication is semi-conservative or to describe the roles of specific enzymes using correct terminology.

学生常混淆两条链的合成方向，或认为DNA聚合酶是从3’到5’工作的。请记住：合成总是沿5’到3’方向进行。另一个常见错误是忘记后随链需要多个引物以及DNA连接酶。考题也可能要求你解释为什么复制是半保留的，或使用正确术语描述特定酶的作用。

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12. Summary Table: Leading vs Lagging Strand | 总结表：前导链与后随链对比

Feature	Leading Strand	Lagging Strand
Direction of synthesis	5′ to 3′ towards fork	5′ to 3′ away from fork
Continuity	Continuous	Discontinuous (Okazaki fragments)
Number of primers	One	Many
Enzymes required	Helicase, primase, DNA polymerase	Helicase, primase, DNA polymerase, ligase

特征	前导链	后随链
合成方向	5’到3’朝向复制叉	5’到3’远离复制叉
连续性	连续	不连续（冈崎片段）
引物数量	一个	多个
所需酶	解旋酶、引物酶、DNA聚合酶	解旋酶、引物酶、DNA聚合酶、连接酶

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