DNA 复制与蛋白质合成：从基因到蛋白质的分子之旅

DNA Replication & Protein Synthesis: The Molecular Journey from Gene to Protein

中文摘要：本文深入探讨 A-Level 生物学中的核心主题——DNA 复制与蛋白质合成。我们将从 DNA 的双螺旋结构出发，详细解析半保留复制的分子机制，包括解旋酶、DNA 聚合酶、引物酶和连接酶等关键酶的协同作用。随后，我们将追踪遗传信息从 DNA 到蛋白质的完整流动路径：转录——在细胞核中将 DNA 编码的信息转化为 mRNA；翻译——在核糖体上将 mRNA 的密码子序列解码为氨基酸链，最终折叠成功能性蛋白质。文章还将涵盖前导链与滞后链的差异、冈崎片段、RNA 剪接（内含子与外显子）、以及翻译的起始、延伸与终止阶段。通过理解这些精密的分子机制，你将掌握 A-Level 考试中的关键知识点，并领会生命最基本过程的优雅与复杂。

English Summary: This article provides an in-depth exploration of DNA replication and protein synthesis, a cornerstone topic in A-Level Biology. We begin with the double-helical structure of DNA and unpack the molecular machinery behind semi-conservative replication — including the coordinated action of helicase, DNA polymerase, primase, and ligase. We then trace the complete flow of genetic information from DNA to protein: transcription, where DNA-encoded instructions are converted into mRNA inside the nucleus; and translation, where the codon sequence of mRNA is decoded into a chain of amino acids at the ribosome, ultimately folding into a functional protein. The article also covers the distinction between leading and lagging strands, Okazaki fragments, RNA splicing (introns and exons), and the initiation, elongation, and termination phases of translation. By understanding these intricate molecular mechanisms, you will master key examination concepts and appreciate the elegance and complexity of life’s most fundamental processes.

第一部分：DNA 的结构 — 遗传信息的蓝图

Part 1: DNA Structure — The Blueprint of Genetic Information

要理解 DNA 复制和蛋白质合成，我们首先必须掌握 DNA 本身的结构。DNA（脱氧核糖核酸）是由两条多核苷酸链组成的双螺旋结构。每条链由交替的脱氧核糖和磷酸基团构成骨架，而含氮碱基（腺嘌呤 A、胸腺嘧啶 T、胞嘧啶 C、鸟嘌呤 G）则像梯子的横档一样向内突出。两条链通过碱基之间的氢键连接在一起：腺嘌呤（A）总是与胸腺嘧啶（T）配对（形成两个氢键），胞嘧啶（C）总是与鸟嘌呤（G）配对（形成三个氢键）。这种互补碱基配对原则是 DNA 复制和转录的分子基础。

To understand DNA replication and protein synthesis, we must first grasp the structure of DNA itself. DNA (deoxyribonucleic acid) is a double helix composed of two polynucleotide chains. Each chain consists of alternating deoxyribose sugar and phosphate groups forming the backbone, while nitrogenous bases — adenine (A), thymine (T), cytosine (C), and guanine (G) — project inward like the rungs of a ladder. The two strands are held together by hydrogen bonds between complementary bases: adenine always pairs with thymine (forming two hydrogen bonds), and cytosine always pairs with guanine (forming three hydrogen bonds). This complementary base-pairing rule is the molecular foundation of both DNA replication and transcription.

DNA 的两条链是反平行的：一条链从 5′ 端到 3′ 端，另一条从 3′ 端到 5′ 端。这个方向性非常重要，因为 DNA 聚合酶只能沿 5′ → 3′ 方向合成新的 DNA 链。这个看似简单的限制对复制机制产生了深远的影响，导致了前导链和滞后链的显著差异，我们将在下文详细讨论。

The two strands of DNA are antiparallel: one strand runs from the 5′ end to the 3′ end, and the other runs from the 3′ end to the 5′ end. This directionality is critically important because DNA polymerase can only synthesise new DNA strands in the 5′ → 3′ direction. This seemingly simple constraint has profound consequences for the replication mechanism, leading to the distinct differences between the leading and lagging strands, which we will discuss in detail below.

第二部分：DNA 复制 — 半保留机制

Part 2: DNA Replication — The Semi-Conservative Mechanism

DNA 复制是细胞分裂前发生的精确过程，确保每个子细胞获得完整且相同的遗传信息副本。Meselson 和 Stahl 在 1958 年的经典实验使用氮同位素（¹⁵N 和 ¹⁴N）证明了 DNA 复制是半保留的——每条新的 DNA 双链包含一条来自原始分子的旧链（模板链）和一条新合成的链。这一发现是分子生物学史上的里程碑。

DNA replication is a precise process that occurs before cell division, ensuring each daughter cell receives a complete and identical copy of the genetic information. The classic experiment by Meselson and Stahl in 1958, using nitrogen isotopes (¹⁵N and ¹⁴N), demonstrated that DNA replication is semi-conservative — each new DNA double helix contains one old strand (the template strand) from the original molecule and one newly synthesised strand. This discovery was a landmark in the history of molecular biology.

2.1 复制的起始阶段

2.1 Initiation of Replication

复制始于 DNA 分子上的特定序列，称为复制起点（origin of replication）。在真核生物中，每条染色体上有多个复制起点，允许同时从多个位置开始复制，从而加快整个过程。首先，DNA 解旋酶（helicase）结合到复制起点，利用 ATP 水解提供的能量解开双螺旋，破坏碱基对之间的氢键。这形成了复制叉（replication fork）——一个 Y 形结构，两条链在此分离。

Replication begins at specific sequences on the DNA molecule called origins of replication. In eukaryotic cells, multiple origins exist on each chromosome, allowing replication to proceed simultaneously from many positions and thus speeding up the entire process. First, DNA helicase binds to the origin of replication and uses energy from ATP hydrolysis to unwind the double helix, breaking the hydrogen bonds between base pairs. This creates the replication fork — a Y-shaped structure where the two strands separate.

解旋后，单链结合蛋白（single-strand binding proteins, SSBs）迅速结合到暴露的单链 DNA 上，防止它们重新退火形成双链。同时，DNA 拓扑异构酶（topoisomerase，也称为 DNA 旋转酶 gyrase）在复制叉前方工作，通过引入暂时的断裂来缓解解旋产生的超螺旋张力，防止 DNA 分子在复制过程中”打结”。

After unwinding, single-strand binding proteins (SSBs) rapidly bind to the exposed single-stranded DNA, preventing the strands from re-annealing into a double helix. Meanwhile, DNA topoisomerase (also known as DNA gyrase) works ahead of the replication fork, relieving the supercoiling tension generated by unwinding through the introduction of temporary breaks, preventing the DNA molecule from becoming “knotted” during replication.

2.2 引物合成与延伸

2.2 Primer Synthesis and Elongation

DNA 聚合酶不能从头开始合成 DNA——它只能向已有的核苷酸链的 3′ 端添加新的核苷酸。因此，在复制开始前，需要一段短 RNA 引物。引物酶（primase，一种 RNA 聚合酶）合成一段约 10 个核苷酸长的 RNA 引物，其序列与模板 DNA 互补。这个引物的 3′-OH 端为 DNA 聚合酶提供了起始点。

DNA polymerase cannot synthesise DNA from scratch — it can only add new nucleotides to the 3′ end of an existing nucleotide chain. Therefore, a short RNA primer is required before replication can begin. Primase (a type of RNA polymerase) synthesises an RNA primer approximately 10 nucleotides long, with a sequence complementary to the template DNA. The 3′-OH end of this primer provides the starting point for DNA polymerase.

一旦引物到位，DNA 聚合酶 III（在原核生物如大肠杆菌中）或相应的真核生物 DNA 聚合酶开始沿模板链移动，按互补碱基配对原则逐一添加脱氧核苷三磷酸（dNTPs）。新核苷酸通过磷酸二酯键（phosphodiester bond）连接到增长中的链上，释放焦磷酸（pyrophosphate）。整个过程沿 5′ → 3′ 方向进行，这意味着 DNA 聚合酶沿着模板链的 3′ → 5′ 方向”读取”。

Once the primer is in place, DNA polymerase III (in prokaryotes such as E. coli) or the equivalent eukaryotic DNA polymerase begins moving along the template strand, adding deoxyribonucleoside triphosphates (dNTPs) one by one according to the complementary base-pairing rules. Each new nucleotide is joined to the growing chain by a phosphodiester bond, with the release of pyrophosphate. The entire process proceeds in the 5′ → 3′ direction, meaning DNA polymerase “reads” the template strand in the 3′ → 5′ direction.

2.3 前导链与滞后链

2.3 The Leading Strand and the Lagging Strand

这是 A-Level 考试中经常出现的重点概念。由于 DNA 双链是反平行的，而 DNA 聚合酶只能沿 5′ → 3′ 方向合成，两条链的复制方式截然不同：

This is a key concept that frequently appears in A-Level examinations. Because the two DNA strands are antiparallel and DNA polymerase can only synthesise in the 5′ → 3′ direction, the two strands replicate in fundamentally different ways:

前导链（Leading Strand）：这条链的模板方向为 3′ → 5’（相对于复制叉的移动方向）。因此，DNA 聚合酶可以沿 5′ → 3′ 方向连续合成，只需要一个 RNA 引物。新链的合成方向与复制叉前进方向一致，因此复制过程是连续的。

Leading Strand: The template of this strand runs 3′ → 5′ relative to the direction of replication fork movement. Consequently, DNA polymerase can synthesise continuously in the 5′ → 3′ direction, requiring only a single RNA primer. The direction of new strand synthesis is the same as the direction of fork progression, so replication proceeds continuously.

滞后链（Lagging Strand）：这条链的模板方向为 5′ → 3’（相对于复制叉的移动方向）。DNA 聚合酶不能沿 3′ → 5′ 方向合成，因此必须采用一种”退行式”策略。复制进行时，新的解旋区域暴露出来，引物酶合成一个新的 RNA 引物，然后 DNA 聚合酶沿 5′ → 3′ 方向合成一小段 DNA，直到遇到前一个引物。这些短片段称为冈崎片段（Okazaki fragments），每个原核生物中的冈崎片段约 1000-2000 个核苷酸长，真核生物中约 100-200 个核苷酸长。

Lagging Strand: The template of this strand runs 5′ → 3′ relative to the direction of replication fork movement. DNA polymerase cannot synthesise in the 3′ → 5′ direction, so a “backstitching” strategy must be employed. As replication proceeds and new unwound regions are exposed, primase synthesises a new RNA primer, and DNA polymerase then synthesises a short segment of DNA in the 5′ → 3′ direction until it reaches the previous primer. These short segments are called Okazaki fragments — approximately 1000–2000 nucleotides long in prokaryotes and about 100–200 nucleotides in eukaryotes.

滞后链的最终成熟需要两个额外的步骤：DNA 聚合酶 I 去除 RNA 引物并用 DNA 替换它们；然后 DNA 连接酶（ligase）催化相邻冈崎片段之间磷酸二酯键的形成，将所有片段连接成一条连续的链。在 A-Level 考试中，你需要能够解释为什么滞后链的复制是不连续的，并描述冈崎片段的作用。

The final maturation of the lagging strand requires two additional steps: DNA polymerase I removes the RNA primers and replaces them with DNA; then DNA ligase catalyses the formation of phosphodiester bonds between adjacent Okazaki fragments, joining all the fragments into a single continuous strand. In A-Level examinations, you need to be able to explain why lagging strand replication is discontinuous and describe the role of Okazaki fragments.

第三部分：转录 — 从 DNA 到 mRNA

Part 3: Transcription — From DNA to mRNA

转录是遗传信息从 DNA 流向蛋白质的第一步。这个过程发生在真核细胞的细胞核中（在原核生物中发生在细胞质中），由 RNA 聚合酶执行。转录的结果是生成一条信使 RNA（mRNA）分子，其序列与基因的编码链互补。

Transcription is the first step in the flow of genetic information from DNA to protein. This process occurs in the nucleus of eukaryotic cells (and in the cytoplasm of prokaryotes) and is carried out by RNA polymerase. The result of transcription is a messenger RNA (mRNA) molecule whose sequence is complementary to the coding strand of the gene.

3.1 转录的起始

3.1 Initiation of Transcription

转录起始于基因上游的启动子（promoter）区域。在真核生物中，启动子通常包含一个 TATA 盒（TATA box），这是一个富含胸腺嘧啶和腺嘌呤的序列。转录因子（transcription factors）首先识别并结合到 TATA 盒上，然后招募 RNA 聚合酶 II（负责 mRNA 合成的 RNA 聚合酶类型）形成转录起始复合物。RNA 聚合酶随后解开 DNA 双螺旋的一小段区域（约 10-20 个碱基对），暴露模板链。与原核生物不同，真核生物中每个基因都有独立的启动子，基因表达受到严格的调控。

Transcription begins at the promoter region located upstream of the gene. In eukaryotes, the promoter typically contains a TATA box, a sequence rich in thymine and adenine. Transcription factors first recognise and bind to the TATA box, then recruit RNA polymerase II (the type of RNA polymerase responsible for mRNA synthesis) to form the transcription initiation complex. RNA polymerase then unwinds a short region of the DNA double helix (approximately 10–20 base pairs), exposing the template strand. Unlike prokaryotes, each gene in eukaryotes has its own promoter, and gene expression is tightly regulated.

3.2 延伸阶段

3.2 The Elongation Phase

一旦转录起始复合物形成，RNA 聚合酶开始沿模板 DNA 链的 3′ → 5′ 方向移动，按互补碱基配对原则添加核糖核苷三磷酸（NTPs）。关键区别是：在 RNA 中，尿嘧啶（U）取代了胸腺嘧啶（T）。因此，当模板链上是腺嘌呤（A）时，RNA 中添加尿嘧啶（U）；当模板链上是胞嘧啶（C）时，RNA 中添加鸟嘌呤（G），依此类推。

Once the transcription initiation complex is formed, RNA polymerase begins moving along the template DNA strand in the 3′ → 5′ direction, adding ribonucleoside triphosphates (NTPs) according to the complementary base-pairing rules. The key difference is that in RNA, uracil (U) replaces thymine (T). Therefore, when the template strand has adenine (A), uracil (U) is added to the RNA; when the template strand has cytosine (C), guanine (G) is added, and so on.

合成的 mRNA 链沿 5′ → 3′ 方向增长，与 DNA 复制类似。随着 RNA 聚合酶向前移动，DNA 双链在被转录后立即重新退火。值得注意的是，转录不像 DNA 复制那样需要引物——RNA 聚合酶可以从头开始合成 RNA。

The synthesised mRNA strand grows in the 5′ → 3′ direction, similar to DNA replication. As RNA polymerase moves forward, the DNA double helix re-anneals immediately after being transcribed. Notably, transcription does not require a primer — RNA polymerase can initiate RNA synthesis de novo.

3.3 终止与 mRNA 加工（真核生物特有）

3.3 Termination and mRNA Processing (Eukaryote-Specific)

当 RNA 聚合酶到达终止信号序列时，转录结束。在原核生物中，终止子序列可以直接导致 RNA 聚合酶脱离；而在真核生物中，机制更为复杂。但真正重要且常考的是真核生物中转录后的 mRNA 加工过程：

Transcription ends when RNA polymerase reaches a termination signal sequence. In prokaryotes, terminator sequences can directly cause RNA polymerase to dissociate; in eukaryotes, the mechanism is more complex. What is truly important and frequently examined, however, is the post-transcriptional processing of mRNA in eukaryotes:

1. 5′ 端加帽（5′ Capping）：转录开始后不久，一个经过修饰的鸟嘌呤核苷酸（7-甲基鸟苷）通过特殊的三磷酸键添加到 mRNA 的 5′ 端。这个 “帽子” 结构保护 mRNA 不被核酸外切酶降解，并帮助核糖体在翻译过程中识别 mRNA。

1. 5′ Capping: Shortly after transcription begins, a modified guanine nucleotide (7-methylguanosine) is added to the 5′ end of the mRNA via a unique triphosphate linkage. This “cap” structure protects the mRNA from degradation by exonucleases and helps the ribosome recognise the mRNA during translation.

2. 3′ 端聚腺苷酸化（Polyadenylation）：转录结束后，一种酶在 mRNA 的 3′ 端切割序列，然后 poly-A 聚合酶添加一条由约 200 个腺嘌呤核苷酸组成的 poly-A 尾巴。这条尾巴同样保护 mRNA 不被降解，并协助其从细胞核输出到细胞质。

2. 3′ Polyadenylation: After transcription ends, an enzyme cleaves the sequence at the 3′ end of the mRNA, and then poly-A polymerase adds a poly-A tail consisting of approximately 200 adenine nucleotides. This tail also protects the mRNA from degradation and assists in its export from the nucleus to the cytoplasm.

3. RNA 剪接（RNA Splicing）：这是 A-Level 生物学中的核心考点。在真核生物中，初始转录产物（称为 pre-mRNA 或初级转录本）包含编码区（外显子，exons）和非编码区（内含子，introns）。在剪接过程中，剪接体（spliceosome）——一个由小核核糖核蛋白（snRNPs）组成的大型 RNA-蛋白质复合物——精确地切除内含子并将外显子连接在一起，形成成熟的 mRNA。选择性剪接（alternative splicing）使一个基因可以通过不同的外显子组合产生多种不同的蛋白质，这解释了为什么人类约 20,000 个基因可以产生远多于 20,000 种蛋白质。

3. RNA Splicing: This is a core examination topic in A-Level Biology. In eukaryotes, the initial transcription product (called pre-mRNA or the primary transcript) contains coding regions (exons) and non-coding regions (introns). During splicing, the spliceosome — a large RNA-protein complex composed of small nuclear ribonucleoproteins (snRNPs) — precisely excises the introns and joins the exons together to form mature mRNA. Alternative splicing allows a single gene to produce multiple different proteins through different exon combinations, explaining why the ~20,000 genes in the human genome can produce far more than 20,000 different proteins.

只有当这些加工步骤全部完成后，成熟的 mRNA 才通过核孔复合物从细胞核运输到细胞质中，为下一步——翻译——做好准备。

Only after all these processing steps are completed does the mature mRNA exit the nucleus through nuclear pore complexes and enter the cytoplasm, ready for the next step — translation.

第四部分：翻译 — 从 mRNA 到蛋白质

Part 4: Translation — From mRNA to Protein

翻译是蛋白质合成的过程，发生在细胞质中的核糖体上。这个过程将 mRNA 中的核苷酸序列”翻译”为氨基酸序列，最终折叠成功能性的三维蛋白质。翻译涉及三种主要类型的 RNA：mRNA（携带遗传信息）、tRNA（转运 RNA，将正确的氨基酸运送到核糖体）和 rRNA（核糖体 RNA，构成核糖体的结构和催化核心）。

Translation is the process of protein synthesis that occurs at ribosomes in the cytoplasm. This process “translates” the nucleotide sequence in mRNA into an amino acid sequence, which ultimately folds into a functional three-dimensional protein. Translation involves three main types of RNA: mRNA (carrying the genetic information), tRNA (transfer RNA, delivering the correct amino acids to the ribosome), and rRNA (ribosomal RNA, forming the structural and catalytic core of the ribosome).

4.1 遗传密码 — 生命的语言

4.1 The Genetic Code — The Language of Life

遗传密码（genetic code）是将 mRNA 中的核苷酸序列”翻译”为蛋白质中氨基酸序列的规则集。mRNA 上的每三个连续的核苷酸组成一个密码子（codon）。由于有 4 种碱基（A、U、C、G），三个一组的排列方式共有 4³ = 64 种可能的密码子。其中 61 个密码子编码 20 种标准氨基酸（因此大多数氨基酸由多个密码子编码——遗传密码是简并的），另外 3 个（UAA、UAG、UGA）是终止密码子（stop codons），信号翻译结束。AUG 密码子既编码甲硫氨酸（methionine），也是起始密码子（start codon），标识翻译的起点。

The genetic code is the set of rules by which the nucleotide sequence in mRNA is “translated” into the amino acid sequence of a protein. Every three consecutive nucleotides on the mRNA constitute a codon. With four bases (A, U, C, G), there are 4³ = 64 possible codons. Of these, 61 codons encode the 20 standard amino acids (hence most amino acids are specified by multiple codons — the genetic code is degenerate), while three (UAA, UAG, UGA) are stop codons, signalling the end of translation. The AUG codon both encodes methionine and serves as the start codon, marking where translation begins.

遗传密码的几个重要特征在考试中经常出现：(1) 它是通用的——几乎所有生物使用相同的遗传密码（极少数例外，如线粒体和某些原生动物）；(2) 它是简并的——多个密码子可以编码同一种氨基酸，这提供了对突变的缓冲能力；(3) 它是非重叠的——每个核苷酸只属于一个密码子，以线性方式连续读取。

Several important features of the genetic code frequently appear in examinations: (1) It is universal — virtually all organisms use the same genetic code (with very rare exceptions such as mitochondria and certain protozoa); (2) It is degenerate — multiple codons can encode the same amino acid, providing a buffer against mutations; (3) It is non-overlapping — each nucleotide belongs to only one codon and is read consecutively in a linear manner.

4.2 tRNA 与氨基酸的活化

4.2 tRNA and Amino Acid Activation

转运 RNA（tRNA）是翻译过程中关键的适配分子。每个 tRNA 分子具有三叶草形的二级结构和一个特征性的 L 形三级结构。tRNA 有两个关键功能位点：(1) 反密码子环（anticodon loop）含有一个与 mRNA 上特定密码子互补的三碱基反密码子；(2) 3′ 端接受臂（acceptor stem）携带着与反密码子对应的特定氨基酸。

Transfer RNA (tRNA) is the key adapter molecule in translation. Each tRNA molecule has a cloverleaf secondary structure and a characteristic L-shaped tertiary structure. The tRNA has two key functional sites: (1) the anticodon loop contains a three-base anticodon complementary to a specific codon on the mRNA; (2) the 3′ acceptor stem carries the specific amino acid that corresponds to the anticodon.

在 tRNA 能够参与翻译之前，必须先将正确的氨基酸连接到其 3′ 端。这个活化步骤由氨酰-tRNA 合成酶（aminoacyl-tRNA synthetase）催化，这是一个高度特异性的酶——每种氨基酸有自己专门的合成酶。该反应需要 ATP，分为两步：首先氨基酸与 ATP 反应形成氨酰-AMP（活化中间体），然后活化的氨基酸转移到 tRNA 的 3′ 端，形成氨酰-tRNA（aminoacyl-tRNA 或 charged tRNA）。这个过程的精确性至关重要，因为一旦氨基酸被连接到 tRNA 上，核糖体只会根据反密码子-密码子配对来识别 tRNA，而不再检查所携带的氨基酸是否正确。

Before tRNA can participate in translation, the correct amino acid must first be attached to its 3′ end. This activation step is catalysed by aminoacyl-tRNA synthetase, a highly specific enzyme — each amino acid has its own dedicated synthetase. The reaction requires ATP and occurs in two steps: first, the amino acid reacts with ATP to form aminoacyl-AMP (an activated intermediate); then the activated amino acid is transferred to the 3′ end of the tRNA, forming aminoacyl-tRNA (or charged tRNA). The accuracy of this process is absolutely critical because once an amino acid is attached to a tRNA, the ribosome only checks the anticodon-codon pairing and does not verify whether the attached amino acid is correct.

4.3 翻译的三个阶段

4.3 The Three Phases of Translation

起始（Initiation）：在真核生物中，小核糖体亚基（40S）与起始因子（eIFs）一起结合到 mRNA 的 5′ 帽结构上。然后，40S 亚基沿 mRNA 移动（扫描），寻找起始密码子 AUG。一旦找到 AUG，携带甲硫氨酸的起始 tRNA（tRNAⁱᴹᵉᵗ）与其反密码子配对，大核糖体亚基（60S）结合，形成完整的 80S 核糖体。在原核生物中，起始机制略有不同，涉及 Shine-Dalgarno 序列，但基本原理相似。

Initiation: In eukaryotes, the small ribosomal subunit (40S) binds to the 5′ cap structure of the mRNA along with initiation factors (eIFs). The 40S subunit then moves along the mRNA (scanning), searching for the start codon AUG. Once AUG is found, the initiator tRNA carrying methionine (tRNAⁱᴹᵉᵗ) pairs with its anticodon, and the large ribosomal subunit (60S) joins, forming the complete 80S ribosome. In prokaryotes, the initiation mechanism is slightly different, involving the Shine-Dalgarno sequence, but the fundamental principle is the same.

延伸（Elongation）：核糖体有三个 tRNA 结合位点：A 位（氨酰位）、P 位（肽位）和 E 位（出口位）。延伸是一个循环过程：(a) 一个新的氨酰-tRNA，其反密码子与 mRNA 上 A 位中的下一个密码子互补，进入 A 位；(b) 肽基转移酶（peptidyl transferase，在真核生物中由大亚基 rRNA 的催化活性提供）在 P 位上氨基酸链的羧基端与 A 位上氨基酸的氨基之间形成肽键，将多肽链从 P 位的 tRNA 转移到 A 位的 tRNA；(c) 核糖体沿 mRNA 向 3′ 端移动三个核苷酸（易位，translocation）——P 位的空 tRNA 移到 E 位后离开，A 位的 tRNA（现在带有延伸多肽链）移到 P 位。这个循环以惊人的速度重复进行，每个氨基酸的添加仅需约 0.05 秒。

Elongation: The ribosome has three tRNA binding sites: the A site (aminoacyl site), the P site (peptidyl site), and the E site (exit site). Elongation is a cyclic process: (a) A new aminoacyl-tRNA, whose anticodon is complementary to the next codon in the A site of the mRNA, enters the A site; (b) Peptidyl transferase (provided by the catalytic activity of the large subunit rRNA in eukaryotes) forms a peptide bond between the carboxyl end of the amino acid chain in the P site and the amino group of the amino acid in the A site, transferring the polypeptide chain from the tRNA in the P site to the tRNA in the A site; (c) The ribosome moves three nucleotides along the mRNA toward the 3′ end (translocation) — the empty tRNA in the P site moves to the E site and exits, and the tRNA in the A site (now carrying the extended polypeptide chain) moves to the P site. This cycle repeats at an astonishing speed, with each amino acid addition taking only about 0.05 seconds.

终止（Termination）：当核糖体的 A 位遇到终止密码子（UAA、UAG 或 UGA）时，没有 tRNA 能够识别这些密码子。取而代之的是释放因子（release factors）结合到 A 位上。释放因子触发肽基转移酶将多肽链转移到水分子（而不是另一个氨基酸），从而将多肽链从最后的 tRNA 上水解释放。核糖体亚基随后解离，释放出 mRNA、tRNA 和新合成的多肽链。

Termination: When a stop codon (UAA, UAG, or UGA) reaches the A site of the ribosome, no tRNA can recognise these codons. Instead, release factors bind to the A site. The release factors trigger peptidyl transferase to transfer the polypeptide chain to a water molecule (rather than another amino acid), thereby hydrolysing the polypeptide chain from the final tRNA. The ribosomal subunits then dissociate, releasing the mRNA, the tRNA, and the newly synthesised polypeptide chain.

4.4 多核糖体与翻译后修饰

4.4 Polyribosomes and Post-Translational Modifications

一条 mRNA 分子可以同时被多个核糖体翻译，形成多核糖体（polyribosome 或 polysome）。这大大提高了蛋白质合成的效率——单个 mRNA 可以在短时间内产生大量蛋白质拷贝。翻译完成后，新生的多肽链通常需要经过折叠和翻译后修饰（post-translational modifications）才能成为完全功能的蛋白质。这些修饰包括磷酸化（phosphate groups added）、糖基化（sugar chains added）、乙酰化（acetyl groups added）、蛋白水解切割（proteolytic cleavage）等，它们可以改变蛋白质的活性、定位、稳定性以及与其他分子的相互作用。

A single mRNA molecule can be translated simultaneously by multiple ribosomes, forming a polyribosome (or polysome). This greatly increases the efficiency of protein synthesis — a single mRNA can produce many copies of a protein in a short period. After translation is complete, the nascent polypeptide chain typically requires folding and post-translational modifications to become a fully functional protein. These modifications include phosphorylation, glycosylation, acetylation, proteolytic cleavage, and more, which can alter protein activity, localisation, stability, and interactions with other molecules.

第五部分：关键知识总结与考试技巧

Part 5: Key Summary and Examination Tips

DNA 复制要点：

DNA 复制是半保留的（Meselson-Stahl 实验）
关键酶：解旋酶（解开双链）、DNA 聚合酶（合成新链，5’→3’）、引物酶（合成 RNA 引物）、连接酶（连接冈崎片段）
前导链连续合成，滞后链不连续合成（冈崎片段）
DNA 聚合酶的校对功能（3’→5′ 外切核酸酶活性）确保高保真度

DNA Replication Key Points:

DNA replication is semi-conservative (Meselson-Stahl experiment)
Key enzymes: helicase (unwinds the double helix), DNA polymerase (synthesises new strands, 5’→3′), primase (synthesises RNA primers), ligase (joins Okazaki fragments)
Leading strand is synthesised continuously; lagging strand is synthesised discontinuously (Okazaki fragments)
DNA polymerase’s proofreading function (3’→5′ exonuclease activity) ensures high fidelity

蛋白质合成要点：

转录发生在细胞核（真核），由 RNA 聚合酶催化
mRNA 加工：5′ 加帽、3′ poly-A 尾巴、RNA 剪接（切除内含子）
翻译发生在细胞质核糖体上
tRNA 的反密码子与 mRNA 密码子互补配对
三个翻译阶段：起始（AUG）、延伸（A位→P位→E位循环）、终止（UAA/UAG/UGA + 释放因子）
遗传密码是通用的、简并的、非重叠的

Protein Synthesis Key Points:

Transcription occurs in the nucleus (eukaryotes), catalysed by RNA polymerase
mRNA processing: 5′ capping, 3′ poly-A tail, RNA splicing (intron removal)
Translation occurs at ribosomes in the cytoplasm
tRNA anticodon pairs complementarily with mRNA codon
Three translation phases: initiation (AUG), elongation (A→P→E site cycle), termination (UAA/UAG/UGA + release factors)
The genetic code is universal, degenerate, and non-overlapping

本文涵盖了 A-Level 生物学中 DNA 复制与蛋白质合成的核心知识。建议同学们结合教材中的图表和过往试卷进行巩固练习，特别注意那些要求”描述并解释”的长答题——考官希望看到你对分子机制的精确描述和对关键概念（如半保留复制、冈崎片段、mRNA 加工等）的清晰解释。祝大家学习顺利！

This article covers the core knowledge of DNA replication and protein synthesis for A-Level Biology. Students are encouraged to consolidate their understanding using textbook diagrams and past paper practice. Pay particular attention to long-answer questions that ask you to “describe and explain” — examiners expect precise descriptions of molecular mechanisms and clear explanations of key concepts such as semi-conservative replication, Okazaki fragments, and mRNA processing. Good luck with your studies!

DNA 复制与蛋白质合成：从基因到蛋白质的分子之旅 | DNA Replication & Protein Synthesis: The Molecular Journey from Gene to Protein