📚 DNA Replication for IGCSE Edexcel Biology | IGCSE Edexcel 生物：DNA 复制考点精讲
DNA replication is a fundamental process that ensures every new cell receives an exact copy of the genetic material. In the IGCSE Edexcel Biology specification, you need to understand how the double helix unwinds, how complementary nucleotides are added, and the roles of key enzymes such as DNA helicase and DNA polymerase. The process is described as semi‑conservative because each new DNA molecule contains one original strand and one newly synthesised strand.
DNA 复制是一个基础过程，确保每个新细胞都能获得遗传物质的精确拷贝。在 IGCSE Edexcel 生物大纲中，你需要理解双螺旋如何解旋、互补核苷酸如何添加，以及 DNA 解旋酶和 DNA 聚合酶等关键酶的作用。这一过程被描述为半保留复制，因为每个新 DNA 分子都含有一条原有的链和一条新合成的链。
1. The Need for DNA Replication | 为什么需要 DNA 复制
Before a cell divides by mitosis or meiosis, its entire genome must be duplicated so that each daughter cell inherits a complete set of chromosomes. DNA replication takes place during the S phase of interphase in the cell cycle. Without accurate replication, genetic information would be lost or altered, leading to mutations or cell malfunction.
在细胞通过有丝分裂或减数分裂分裂之前，整个基因组必须复制，以便每个子细胞继承一套完整的染色体。DNA 复制发生在细胞周期间期的 S 期。如果没有准确的复制，遗传信息就会丢失或改变，从而导致突变或细胞功能异常。
2. Semiconservative Nature of Replication | 半保留复制的本质
DNA replication is described as semiconservative because each of the two resulting DNA molecules consists of one original (parental) strand and one newly synthesised strand. This was demonstrated by the Meselson–Stahl experiment using isotopes of nitrogen, confirming that the double helix separates and each strand acts as a template for building a new partner.
DNA 复制被描述为半保留复制，因为形成的两个 DNA 分子各自包含一条原有（亲本）链和一条新合成的链。梅塞尔森–斯塔尔实验利用氮的同位素证明了这一点，证实双螺旋会分开，每条链作为合成新互补链的模板。
3. The Replication Fork and Origin of Replication | 复制叉与复制起点
Replication begins at specific sequences called origins of replication. In the circular DNA of prokaryotes, there is often a single origin; in linear eukaryotic chromosomes, replication starts at multiple points to speed up the process. At each origin, the DNA unwinds to form two Y‑shaped structures known as replication forks, which move in opposite directions.
复制始于称为复制起点的特定序列。在原核生物的环状 DNA 中，通常只有一个起点；在线性真核染色体中，复制从多个点开始，以加快进程。在每个起点，DNA 解旋形成两个 Y 形结构，即复制叉，它们向相反方向移动。
4. Unwinding the Double Helix – DNA Helicase | 解旋双螺旋 – DNA 解旋酶
The enzyme DNA helicase binds to the origin and travels along the DNA molecule, breaking the hydrogen bonds between complementary base pairs. This unzips the double helix, exposing the nitrogenous bases on the two separated strands. The exposed strands immediately become templates for the synthesis of new complementary strands.
DNA 解旋酶结合到复制起点并沿 DNA 分子移动，断开互补碱基对之间的氢键。这使双螺旋解链，暴露出两条分开链上的含氮碱基。暴露的链立即成为合成新互补链的模板。
5. Single‑Strand Binding Proteins and Preventing Re‑annealing | 单链结合蛋白与防止重新配对
Once the strands are separated, single‑strand binding proteins (SSB proteins) attach to the exposed DNA strands. Their role is to stabilise the unwound DNA and prevent the two strands from re‑pairing (re‑annealing) before replication is complete. This keeps the templates accessible for incoming nucleotides.
一旦链分开，单链结合蛋白（SSB 蛋白）就附着在暴露的 DNA 链上。它们的作用是稳定解旋后的 DNA，防止两条链在复制完成之前重新配对（复性）。这样就能保持模板对进入的核苷酸的可及性。
6. Primers and the Role of RNA Primase | 引物与 RNA 引物酶的作用
DNA polymerase cannot start a new strand from scratch; it can only add nucleotides to an existing 3′ end. A short RNA primer, synthesised by the enzyme RNA primase, provides the necessary starting point. The primer is complementary to the template and is later replaced by DNA nucleotides.
DNA 聚合酶不能从头开始合成新链；它只能在已有 3′ 端添加核苷酸。由 RNA 引物酶合成的一段短 RNA 引物提供了必要的起点。引物与模板互补，随后会被 DNA 核苷酸替代。
7. Building the New Strand – DNA Polymerase | 构建新链 – DNA 聚合酶
DNA polymerase attaches to the primer and moves along the template strand in the 3′ to 5′ direction, adding free DNA nucleotides to the 3′ end of the growing chain. It selects the correct nucleotide by complementary base pairing: adenine (A) pairs with thymine (T), and cytosine (C) pairs with guanine (G). The new strand is therefore synthesised in the 5′ to 3′ direction.
DNA 聚合酶附着到引物上，沿模板链 3′ 至 5′ 方向移动，将游离的 DNA 核苷酸添加到生长链的 3′ 端。它通过互补碱基配对选择正确的核苷酸：腺嘌呤（A）与胸腺嘧啶（T）配对，胞嘧啶（C）与鸟嘌呤（G）配对。因此新链沿 5′ 至 3′ 方向合成。
8. Leading and Lagging Strand Synthesis | 前导链与滞后链的合成
Because the two template strands run antiparallel, replication proceeds differently on each strand. The strand that runs 3′ to 5′ towards the replication fork can be copied continuously; this is called the leading strand. The opposite strand runs 5′ to 3′ towards the fork and must be copied in short, discontinuous segments known as Okazaki fragments; this is called the lagging strand.
由于两条模板链是反向平行的，每条链上的复制进行方式不同。朝向复制叉方向为 3′ 至 5′ 的链可以连续复制，称为前导链。另一条链朝向复制叉为 5′ 至 3′，必须以不连续的小片段——冈崎片段——的形式复制，称为滞后链。
9. Okazaki Fragments and DNA Ligase | 冈崎片段与 DNA 连接酶
On the lagging strand, primase repeatedly synthesises short RNA primers, and DNA polymerase extends each primer to form an Okazaki fragment. Once all fragments are synthesised, the primers are removed and replaced with DNA by a different polymerase. The enzyme DNA ligase then seals the gaps between adjacent fragments by forming phosphodiester bonds, creating a continuous sugar‑phosphate backbone.
在滞后链上，引物酶反复合成短 RNA 引物，DNA 聚合酶延伸每个引物形成冈崎片段。所有片段合成后，引物被移除并由另一种聚合酶替换为 DNA。然后 DNA 连接酶通过形成磷酸二酯键将相邻片段之间的缺口密封，形成连续的糖‑磷酸骨架。
10. The Role of Free Nucleotides and Energy Supply | 游离核苷酸与能量供应的作用
Free deoxyribonucleoside triphosphates (dATP, dTTP, dCTP, dGTP) are present in the nucleus. When a nucleotide is added to the growing chain, two phosphate groups are cleaved off, releasing energy that drives the formation of the phosphodiester bond. This hydrolysis makes the polymerisation reaction energetically favourable.
细胞核中存在游离的脱氧核苷三磷酸（dATP、dTTP、dCTP、dGTP）。当一个核苷酸添加到生长链上时，两个磷酸基团被切除，释放出能量推动磷酸二酯键的形成。这种水解作用使聚合反应在能量上有利于进行。
11. Proofreading and Error Correction | 校对与错误修正
DNA polymerase also has a proofreading function. As it adds nucleotides, it checks that each new base pair is correct. If a mismatch is detected, the enzyme removes the incorrect nucleotide and replaces it with the correct one. This dramatically reduces the error rate, making DNA replication a highly accurate process.
DNA 聚合酶还具有校对功能。在添加核苷酸时，它会检查每个新碱基对是否正确。如果检测到错配，该酶会移除错误的核苷酸并用正确的替换。这大大降低了错误率，使 DNA 复制成为一个高度精确的过程。
12. Comparison of DNA Replication in Prokaryotes and Eukaryotes | 原核与真核生物 DNA 复制的比较
Prokaryotes have a single circular chromosome and a single origin of replication; replication proceeds bidirectionally around the circle. Eukaryotes possess multiple linear chromosomes, each with many replication origins. The fundamental enzymes and semiconservative mechanism are conserved, but eukaryotic replication is more complex, involving more types of polymerases and regulatory proteins.
原核生物有一条环状染色体和一个复制起点；复制围绕环状 DNA 双向进行。真核生物拥有多条线性染色体，每条染色体有多个复制起点。基本的酶和半保留机制是保守的，但真核生物的复制更复杂，涉及更多类型的聚合酶和调控蛋白。
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