A-Level Biology: Gene Expression Essentials | A-Level 生物：基因表达 考点精讲

📚 A-Level Biology: Gene Expression Essentials | A-Level 生物：基因表达 考点精讲

Gene expression is the fundamental process by which the genetic code in DNA is used to synthesise functional gene products, primarily proteins. In A-Level Biology, mastering transcription, translation, and the regulation of gene expression is essential for understanding how cells control their activities and respond to the environment. This article provides a comprehensive, section-by-section revision guide covering all the core topics, complete with bilingual explanations to support your learning.

基因表达是遗传信息从DNA流向功能性产物（主要是蛋白质）的基本过程。在A-Level生物学中，掌握转录、翻译以及基因表达的调控，对于理解细胞如何控制自身活动并响应环境至关重要。本文提供逐节全面的复习指南，涵盖所有核心主题，并配有中英双语解释以辅助学习。

1. The Central Dogma and the Flow of Genetic Information | 中心法则与遗传信息流

The central dogma of molecular biology states that genetic information flows from DNA to RNA to protein. DNA is transcribed into messenger RNA (mRNA), which is then translated into a polypeptide chain at the ribosome. Some viruses use reverse transcription, where RNA is copied into DNA, but the fundamental direction in cells is DNA → RNA → protein. Understanding this unidirectional flow helps predict how mutations in DNA can affect protein structure and function.

分子生物学的中心法则指出，遗传信息从DNA流向RNA，再到蛋白质。DNA被转录为信使RNA（mRNA），随后在核糖体上翻译为多肽链。某些病毒利用逆转录将RNA拷贝为DNA，但细胞中的基本方向是DNA → RNA → 蛋白质。理解这一单向流动有助于预测DNA中的突变如何影响蛋白质的结构和功能。

2. Transcription in Prokaryotes | 原核生物的转录

In prokaryotes, transcription occurs in the cytoplasm because there is no nucleus. RNA polymerase binds to the promoter region of DNA, unwinds the double helix, and synthesises a complementary RNA strand from the template strand in the 5′ to 3′ direction. Prokaryotic genes are often organised into operons, so a single mRNA may code for multiple proteins (polycistronic mRNA). The process stops when RNA polymerase reaches a terminator sequence. No post-transcriptional processing is required; translation can begin while transcription is still underway.

在原核生物中，转录发生在细胞质中，因为没有细胞核。RNA聚合酶结合到DNA的启动子区域，解开双螺旋，并以模板链为模板从5’到3’方向合成互补的RNA链。原核基因通常组织为操纵子，因此一条mRNA可以编码多种蛋白质（多顺反子mRNA）。当RNA聚合酶到达终止子序列时过程停止。不需要转录后加工；转录仍在进行时翻译就可以开始。

3. Transcription in Eukaryotes | 真核生物的转录

Eukaryotic transcription takes place inside the nucleus. RNA polymerase II is responsible for transcribing protein-coding genes. It requires the assistance of transcription factors that bind to the promoter (TATA box) to form the transcription initiation complex. The enzyme unwinds DNA and synthesises a primary transcript (pre-mRNA) that is complementary to the template strand. Unlike prokaryotes, the primary transcript undergoes extensive processing before it can be used for translation. Each eukaryotic gene is transcribed individually, producing monocistronic mRNA.

真核生物的转录发生在细胞核内。RNA聚合酶II负责转录编码蛋白质的基因。它需要转录因子的辅助，这些转录因子结合到启动子（TATA框）上形成转录起始复合物。RNA聚合酶解开DNA并合成初级转录本（前体mRNA），该转录本与模板链互补。与原核生物不同，初级转录本在用于翻译之前需要经过广泛的加工。每个真核基因单独转录，产生单顺反子mRNA。

4. Post-Transcriptional Modifications | 转录后修饰

Before eukaryotic mRNA can leave the nucleus, it is modified in three main ways: a 5′ cap (7-methylguanosine) is added for ribosome recognition and stability; a poly-A tail (about 200 adenine nucleotides) is added to the 3′ end to protect against degradation; and splicing removes non-coding introns and joins coding exons together. Alternative splicing allows a single gene to produce multiple different protein isoforms, greatly increasing proteomic diversity. These processed mRNAs are then exported through nuclear pores to the cytoplasm.

真核mRNA在离开细胞核之前，会经过三种主要修饰：添加5’帽（7-甲基鸟苷）用于核糖体识别和稳定性；在3’端添加多聚A尾（约200个腺苷酸）以防降解；以及剪接去除非编码的内含子并将编码的外显子连接起来。可变剪接允许单个基因产生多种不同的蛋白质亚型，极大地增加了蛋白质组的多样性。加工后的mRNA随后通过核孔被运送到细胞质。

5. The Genetic Code and its Features | 遗传密码及其特征

The genetic code is the set of rules by which the nucleotide sequence in mRNA is translated into the amino acid sequence of a polypeptide. Codons are triplets of bases (e.g. AUG, UUU). The code is degenerate (most amino acids are specified by more than one codon), non-overlapping, and universal across almost all organisms. AUG is the start codon and codes for methionine. Three stop codons (UAA, UAG, UGA) signal termination. Reading frames must be correct to produce a functional protein; frameshift mutations are often devastating.

遗传密码是mRNA中的核苷酸序列翻译为多肽氨基酸序列的一套规则。密码子是三个碱基为一组（如AUG、UUU）。遗传密码具有简并性（多数氨基酸由多个密码子编码）、不重叠性和几乎通用的特性。AUG是起始密码子，编码甲硫氨酸。三个终止密码子（UAA、UAG、UGA）发出终止信号。阅读框必须正确才能产生功能性蛋白质；移码突变通常具有破坏性。

6. Translation – Initiation | 翻译 – 起始

Translation begins when the small ribosomal subunit binds to the mRNA near the 5′ cap. In eukaryotes, it scans for the start codon AUG in a favourable Kozak sequence. The initiator tRNA carrying methionine pairs with AUG, and then the large ribosomal subunit joins to form a functional ribosome. The initiator tRNA occupies the P site. This process requires GTP and initiation factors. In prokaryotes, the ribosome recognises the Shine-Dalgarno sequence upstream of AUG.

当核糖体小亚基结合到靠近5’帽的mRNA上时，翻译开始。在真核生物中，它会扫描寻找处于有利的Kozak序列中的起始密码子AUG。携带甲硫氨酸的起始tRNA与AUG配对，然后核糖体大亚基加入形成功能性的核糖体。起始tRNA占据P位。该过程需要GTP和起始因子。在原核生物中，核糖体识别AUG上游的Shine-Dalgarno序列。

7. Translation – Elongation and Termination | 翻译 – 延伸与终止

During elongation, the next aminoacyl-tRNA binds to the A site, complementary to the codon. A peptide bond forms between the amino acids in the P and A sites, catalysed by peptidyl transferase (a ribosomal enzyme). The ribosome translocates three bases along the mRNA, moving the empty tRNA to the E site for exit and shifting the peptidyl-tRNA from A to P site. Elongation continues until a stop codon enters the A site. Release factors bind, causing the polypeptide chain to be released and the ribosomal complex to dissociate.

延伸过程中，下一个氨酰tRNA与A位上的密码子互补结合。在肽基转移酶（核糖体酶）的催化下，P位和A位上的氨基酸之间形成肽键。核糖体沿mRNA移位三个碱基，空载tRNA移到E位离开，肽基tRNA从A位移到P位。延伸持续进行，直到终止密码子进入A位。释放因子结合，导致多肽链释放，核糖体复合物解体。

8. Regulation of Gene Expression in Prokaryotes: The lac Operon | 原核生物基因表达调控：乳糖操纵子

The lac operon in E. coli is a classic example of transcriptional regulation. It consists of a promoter, operator, and three structural genes (lacZ, lacY, lacA) required for lactose metabolism. In the absence of lactose, a repressor protein binds to the operator, blocking RNA polymerase. When lactose is present, it is converted to allolactose, which binds to the repressor, inactivating it. This allows transcription. Additionally, when glucose is scarce, cAMP levels rise and the CAP-cAMP complex binds near the promoter, enhancing RNA polymerase binding. This dual control ensures the operon is only fully active when lactose is available and glucose is absent.

大肠杆菌中的乳糖操纵子是转录调控的经典例子。它由启动子、操纵基因以及乳糖代谢所需的三个结构基因（lacZ、lacY、lacA）组成。无乳糖时，阻遏蛋白结合到操纵基因上，阻断RNA聚合酶。有乳糖时，乳糖转变为别乳糖，与阻遏蛋白结合并使其失活，从而允许转录。此外，当葡萄糖匮乏时，cAMP水平升高，CAP-cAMP复合物结合到启动子附近，增强RNA聚合酶的结合。这种双重控制确保操纵子仅在乳糖存在且葡萄糖缺失时才完全活跃。

9. Regulation of Gene Expression in Eukaryotes: Transcription Factors and Epigenetics | 真核生物基因表达调控：转录因子与表观遗传

Eukaryotic gene expression is controlled at multiple levels. Transcription factors (TFs) are proteins that bind to specific DNA sequences (enhancers and silencers) to activate or repress transcription. TFs interact with the mediator complex and RNA polymerase to influence the initiation rate. Epigenetic modifications, such as DNA methylation (usually silencing) and histone acetylation (usually activating), alter chromatin structure without changing the DNA sequence. Tightly packed heterochromatin is transcriptionally inactive, while open euchromatin allows access to transcription machinery.

真核生物的基因表达在多个层面受到调控。转录因子是结合到特定DNA序列（增强子和沉默子）以激活或抑制转录的蛋白质。转录因子与中介体复合物及RNA聚合酶相互作用，影响起始速率。表观遗传修饰，如DNA甲基化（通常沉默基因）和组蛋白乙酰化（通常激活基因），可以在不改变DNA序列的情况下改变染色质结构。紧密包装的异染色质在转录上不活跃，而开放的常染色质允许转录机器接近。

10. RNA Interference and Post-Transcriptional Regulation | RNA干扰与转录后调控

Small non-coding RNAs play a crucial role in regulating gene expression after transcription. Small interfering RNA (siRNA) and microRNA (miRNA) are processed by the enzyme Dicer and incorporated into the RNA-induced silencing complex (RISC). siRNA typically binds to perfectly complementary mRNAs, causing their cleavage and degradation. miRNA often binds with partial complementarity, leading to translational repression or mRNA destabilisation. This mechanism is widely used in gene knockdown experiments and is a natural defence against viruses.

小非编码RNA在转录后调控基因表达中起关键作用。小干扰RNA（siRNA）和微RNA（miRNA）由Dicer酶加工，并被整合到RNA诱导的沉默复合物（RISC）中。siRNA通常与完全互补的mRNA结合，导致其切割和降解。miRNA常以部分互补的方式结合，导致翻译抑制或mRNA不稳定。这一机制广泛用于基因敲低实验，也是抵御病毒的自然防御方式。

11. Comparing Prokaryotic and Eukaryotic Gene Expression | 原核与真核基因表达的比较

Key differences include: prokaryotic transcription and translation are coupled in the cytoplasm, while in eukaryotes they are separated by the nuclear envelope. Prokaryotes have polycistronic mRNA and no introns; eukaryotes have monocistronic mRNA with introns that require splicing. Prokaryotic control often uses operons and simple repressor/activator systems; eukaryotic control is more complex, involving chromatin remodelling, multiple transcription factors, and RNA processing. However, the fundamental chemistry of RNA synthesis, the genetic code, and the ribosomal translation mechanism are highly conserved across domains.

关键区别包括：原核生物的转录和翻译在细胞质中偶联进行，而真核生物这两步被核膜分隔。原核生物拥有多顺反子mRNA且无内含子；真核生物则为单顺反子mRNA，带有需要剪接的内含子。原核生物调控常使用操纵子和简单的阻遏/激活系统；真核生物调控更为复杂，涉及染色质重塑、多种转录因子和RNA加工。然而，RNA合成的基本化学过程、遗传密码和核糖体翻译机制在不同域中高度保守。

12. Exam Tips and Common Misconceptions | 考试提示与常见误区

Students often confuse the template and coding strands of DNA – remember that the mRNA sequence matches the coding strand (with U replacing T) except that the template strand is used for synthesis. Another common error is mixing up start and stop codons with promoter and terminator sequences. Start and stop codons are on mRNA; promoters and terminators are DNA signals for transcription. Be precise with terminology: ‘transcription factors’ vs ‘translation factors’, ‘introns’ vs ‘exons’. Practice drawing and annotating labelled diagrams of transcription and translation for high mark allocation questions. Always link structure to function in extended answers.

考生常混淆DNA的模板链和编码链——记住，mRNA序列与编码链匹配（用U替代T），但合成时实际以模板链为模板。另一个常见错误是把起始和终止密码子与启动子和终止子序列混淆。起始和终止密码子位于mRNA上；启动子和终止子是DNA上转录的信号。术语要准确区分：“转录因子”与“翻译因子”，“内含子”与“外显子”。对于高分值题目，要练习绘制并标注转录和翻译的图解。在扩展作答中要始终将结构与功能联系起来。

---

Published by TutorHao | Biology Revision Series | aleveler.com

更多咨询请联系16621398022（同微信）