📚 A-Level OCR Biology: Gene Expression Key Points | A-Level OCR 生物：基因表达 考点精讲
Gene expression is the fundamental biological process by which the genetic information encoded in DNA is used to synthesise functional gene products, typically proteins. In A-Level OCR Biology, understanding gene expression involves examining transcription, translation, and the sophisticated regulatory mechanisms that control when and to what extent genes are expressed. These concepts underpin cell differentiation, adaptation, and disease.
基因表达是指利用DNA中编码的遗传信息合成功能性基因产物（通常是蛋白质）的基本生物学过程。在A-Level OCR生物中，理解基因表达需要学习转录、翻译，以及控制基因何时表达及表达程度的精细调控机制。这些概念是细胞分化、适应与疾病的基础。
1. Central Dogma and Gene Expression Overview | 中心法则与基因表达概述
The central dogma of molecular biology describes the flow of genetic information: DNA → RNA → protein. Gene expression is the entire process of converting a gene's DNA sequence into a functional product. It comprises two main stages: transcription, where a gene's DNA is copied into messenger RNA (mRNA), and translation, where the mRNA is decoded to build a polypeptide chain. Gene expression is tightly regulated to ensure that proteins are produced only when needed.
分子生物学的中心法则描述了遗传信息的流向：DNA → RNA → 蛋白质。基因表达是将基因的DNA序列转化为功能性产物的整个过程，包括两个主要阶段：转录（将基因的DNA拷贝为信使RNA，mRNA）和翻译（将mRNA解码以合成多肽链）。基因表达受到严格调控，以确保仅在需要时产生蛋白质。
Differences in gene expression between cells lead to specialisation; for example, muscle cells express myosin genes while pancreatic β-cells express the insulin gene. External and internal signals modulate which genes are switched on or off.
细胞间基因表达的差异导致了细胞特化，例如肌细胞表达肌球蛋白基因，而胰腺β细胞表达胰岛素基因。外部和内部信号调节哪些基因开启或关闭。
2. Transcription: From DNA to mRNA | 转录：从DNA到mRNA
Transcription is catalysed by RNA polymerase, which binds to the promoter region of a gene. The DNA double helix unwinds, and the enzyme uses one strand (the template or antisense strand) to synthesise a complementary mRNA molecule. RNA polymerase adds ribonucleotides in the 5'→3' direction, replacing thymine (T) with uracil (U). In prokaryotes, transcription occurs in the cytoplasm; in eukaryotes, it occurs in the nucleus and requires post-transcriptional modifications before the mRNA can be used.
转录由RNA聚合酶催化，该酶与基因的启动子区域结合。DNA双螺旋解开，酶以一条链（模板链或反义链）为模板合成互补的mRNA分子。RNA聚合酶按5'→3'方向添加核糖核苷酸，同时用尿嘧啶（U）替代胸腺嘧啶（T）。在原核生物中，转录发生在细胞质；在真核生物中，转录发生在细胞核，且需要经过转录后加工修饰，mRNA才能被利用。
The genetic code is degenerate (most amino acids are specified by more than one codon), universal, and non-overlapping. Codons are triplets of bases on mRNA, with AUG as the start codon and UAA, UAG, UGA as stop codons.
遗传密码是简并的（大多数氨基酸由多个密码子编码）、通用的、非重叠的。密码子是mRNA上的三联碱基，AUG为起始密码子，UAA、UAG和UGA为终止密码子。
3. RNA Processing in Eukaryotes | 真核生物的RNA加工
In eukaryotes, the primary transcript (pre-mRNA) undergoes processing: addition of a 5' cap (7-methylguanosine), addition of a poly-A tail at the 3' end, and splicing to remove introns and join exons. The 5' cap and poly-A tail protect the mRNA from degradation and aid in ribosome binding and export from the nucleus. Splicing is carried out by a spliceosome, a complex of snRNPs. Alternative splicing allows one gene to produce multiple protein isoforms.
在真核生物中，初级转录物（前体mRNA）经过加工：添加5'帽（7-甲基鸟苷），在3'端添加poly-A尾，以及通过剪接去除内含子、连接外显子。5'帽和poly-A尾保护mRNA免遭降解，并有助于核糖体结合与从细胞核输出。剪接由剪接体（snRNP复合物）执行。可变剪接使一个基因能产生多种蛋白质亚型。
4. Translation: Protein Synthesis | 翻译：蛋白质合成
Translation occurs on ribosomes, which consist of large and small subunits made of rRNA and proteins. mRNA binds to the small subunit, and a specific initiator tRNA carrying methionine (AUG) recognises the start codon. The ribosome has three sites: A (aminoacyl), P (peptidyl), and E (exit). Elongation involves codon recognition, peptide bond formation (catalysed by peptidyl transferase activity of the rRNA), and translocation. When a stop codon reaches the A site, release factors trigger disassembly, releasing the polypeptide.
翻译在核糖体上进行，核糖体由rRNA和蛋白质组成的大、小亚基构成。mRNA与小亚基结合，携带甲硫氨酸（AUG）的特异起始tRNA识别起始密码子。核糖体具有三个位点：A位（氨酰位）、P位（肽酰位）和E位（出口位）。延伸过程包括密码子识别、肽键形成（由rRNA的肽基转移酶活性催化）和移位。当终止密码子进入A位时，释放因子引发核糖体解体，释放多肽链。
The newly formed polypeptide chain folds into its three-dimensional conformation, often with the help of chaperone proteins, to become a functional protein.
新形成的多肽链折叠成三维构象，通常需要分子伴侣的帮助，从而成为有功能的蛋白质。
5. The lac Operon: Prokaryotic Gene Regulation | 乳糖操纵子：原核基因调控
The lac operon in E. coli is a model system for gene regulation. It contains structural genes lacZ (β-galactosidase), lacY (permease), and lacA (transacetylase), controlled by a shared promoter and operator. The regulatory gene lacI produces a repressor protein that binds the operator and blocks transcription. When lactose is present, it is converted to allolactose, which acts as an inducer by binding the repressor and causing it to release the operator. Additionally, when glucose is absent, cAMP levels rise; cAMP binds the CAP protein, which then binds the CAP site near the promoter, enhancing RNA polymerase binding and transcription. Thus, maximum expression occurs only when lactose is present and glucose is absent.
大肠杆菌的乳糖操纵子是基因调控的经典模型。它包含结构基因lacZ（β-半乳糖苷酶）、lacY（通透酶）和lacA（转乙酰酶），由一个共享的启动子和操纵区控制。调节基因lacI产生阻遏蛋白，该蛋白与操纵区结合，阻断转录。当乳糖存在时，它被转化为异乳糖，异乳糖作为诱导物与阻遏蛋白结合，使其脱离操纵区。此外，当葡萄糖缺乏时，cAMP水平升高；cAMP与CAP蛋白结合，CAP蛋白再与启动子附近的CAP位点结合，增强RNA聚合酶结合和转录。因此，只有当乳糖存在且葡萄糖缺乏时，操纵子才能最大程度表达。
| Component | Function (English) | 功能 (中文) |
|---|---|---|
| lacZ | Encodes β-galactosidase, which cleaves lactose into glucose and galactose | 编码β-半乳糖苷酶，将乳糖分解为葡萄糖和半乳糖 |
| lacY | Encodes permease, facilitating lactose uptake | 编码通透酶，促进乳糖摄入 |
| lacA | Encodes transacetylase, detoxifies compounds |
Published by TutorHao | A-Level Biology Revision Series | aleveler.com
广告 / Sponsored 新Alevel生物OCR A复习题卡 New A-Level Biology OCR A Revision Quest… 生物 备考教辅 · 正版现货 · 与本文内容相关 查看详情 / View on Taobao参考价 ¥179.0更多咨询请联系16621398022（同微信） 备考资讯每日更新，关注不错过 考纲变化 · 科目分析 · 学习方法  在微信里长按二维码识别关注 或截图后用「扫一扫 → 相册」选择本图 公众号：tutorhao |