📚 IB Biology: Gene Technology and Genetic Engineering Key Points | IB 生物：基因工程考点精讲
Genetic engineering revolutionises modern biology by enabling precise manipulation of DNA. From producing human insulin to engineering disease-resistant crops, the techniques involved—restriction enzymes, PCR, gel electrophoresis, gene cloning, and CRISPR—are central to the IB Biology syllabus. This article distils key concepts, common exam traps, and real-world applications to help you master the topic.
基因工程通过精确操控DNA，彻底改变了现代生物学。从生产人胰岛素到改造抗病作物，限制酶、PCR、凝胶电泳、基因克隆和CRISPR等技术是IB生物课程的核心。本文提炼关键概念、常见考试陷阱和实际应用，助你掌握这一专题。
1. Introduction to Genetic Engineering | 基因工程简介
Genetic engineering, also known as genetic modification, involves the direct alteration of an organism's genome using biotechnological tools. It allows scientists to add, delete, or modify specific genes, leading to new traits or enhanced production of desired proteins. The cornerstone is recombinant DNA technology, which combines DNA from different sources.
基因工程，也称遗传修饰，指利用生物技术工具直接改变生物体的基因组。科学家借此增加、删除或修改特定基因，从而产生新性状或提高目标蛋白的产量。其基石是重组DNA技术，将不同来源的DNA组合在一起。
2. Restriction Enzymes: Molecular Scissors | 限制酶：分子剪刀
Restriction enzymes (endonucleases) cut DNA at specific recognition sequences, usually 4–8 base pairs long and often palindromic. They are isolated from bacteria, where they function as a defence against viral DNA. Type II restriction enzymes, the most common in gene technology, cleave within or near the recognition site, generating either sticky or blunt ends.
限制酶（核酸内切酶）在特定的识别序列处切割DNA，这些序列通常长4–8个碱基对且常为回文序列。它们从细菌中分离，在细菌中作为抵御病毒DNA的防御机制。最常用于基因操作的II型限制酶在识别位点内或附近切割，产生粘性末端或平末端。
| Enzyme | Recognition Sequence (5' → 3') | End Type |
|---|---|---|
| EcoRI | G'AATTC CTTAA'G | Sticky (5' overhang AATT) |
| HindIII | A'AGCTT TTCGA'A | Sticky |
| SmaI | CCC'GGG GGG'CCC | Blunt |
Sticky ends are single-stranded overhangs that can base-pair with fragments cut by the same enzyme, facilitating specific ligation. Blunt ends lack overhangs and can be joined to any blunt-ended fragment but with lower efficiency.
粘性末端是单链突出端，能与同一种酶切割的片段碱基配对，利于特异性连接。平末端没有突出端，可与任何平端片段连接，但效率较低。
3. DNA Ligase and Sticky Ends | DNA 连接酶与粘性末端
DNA ligase catalyses phosphodiester bond formation between the 3′ hydroxyl and 5′ phosphate of adjacent nucleotides, sealing the sugar-phosphate backbone. In gene cloning, T4 DNA ligase is routinely used to fuse vector and insert fragments. Before ligation, complementary sticky ends anneal via hydrogen bonds, aligning the fragments for covalent joining.
DNA连接酶催化相邻核苷酸的3′羟基与5′磷酸之间形成磷酸二酯键，封闭糖-磷酸骨架。在基因克隆中，T4 DNA连接酶常用于融合载体与插入片段。在连接前，互补的粘性末端通过氢键退火，使片段对齐以便共价连接。
Using two different restriction enzymes that produce non-compatible sticky ends ensures directional cloning – the insert can only ligate in the desired orientation, thereby preserving the correct reading frame for protein expression.
使用两种产生不互补粘性末端的限制酶可实现定向克隆——插入片段只能以所需方向连接，从而保持正确的阅读框用于蛋白表达。
4. Plasmids as Vectors | 质粒作为载体
Plasmids are extrachromosomal, circular DNA molecules that replicate autonomously in bacteria. An ideal cloning vector must possess: an origin of replication (ori) for independent multiplication; a selectable marker, often an antibiotic resistance gene (e.g., ampR for ampicillin resistance); and a multiple cloning site (MCS) containing unique restriction enzyme recognition sites.
质粒是染色体外的环状DNA分子，可在细菌中自主复制。理想的克隆载体必须具备：复制起点(ori)以便独立增殖；选择标记，常为抗生素抗性基因（如氨苄青霉素抗性基因ampR）；以及含有单一限制酶识别位点的多克隆位点(MCS)。
The lacZ gene enables blue-white screening. When a foreign DNA fragment inserts into the lacZ region, β-galactosidase production is disrupted. Recombinant colonies turn white on X-gal medium, while non-recombinants remain blue, allowing easy identification.
lacZ基因可实现蓝白斑筛选。当外源DNA片段插入lacZ区域，β-半乳糖苷酶的生产被破坏。重组菌落在含X-gal的培养基上呈白色，而非重组菌落保持蓝色，便于识别。
5. Polymerase Chain Reaction (PCR) | 聚合酶链式反应
PCR amplifies a specific DNA sequence exponentially in vitro. The reaction mixture includes template DNA, a pair of primers (short oligonucleotides complementary to the target region's flanks), heat-stable Taq DNA polymerase, deoxynucleotide triphosphates (dNTPs), and a buffer.
PCR在体外指数扩增特定DNA序列。反应混合物包括模板DNA、一对引物（与目标区域两端互补的短寡核苷酸）、热稳定的Taq DNA聚合酶、脱氧核苷酸三磷酸(dNTPs)及缓冲液。
The thermocycler runs three steps per cycle: Denaturation at 95°C separates the DNA strands; Annealing at 50–65°C allows primers to bind; Extension at 72°C enables Taq polymerase to synthesise new strands. The cycle repeats 25–35 times, and the number of target copies follows:
热循环仪每个循环运行三步：95°C变性使DNA双链分离；50–65°C退火使引物结合；72°C延伸使Taq聚合酶合成新链。循环重复25–35次，目标拷贝数遵循：
Number of copies = 2n – 2n (approximately 2n), where n = number of cycles
拷贝数 = 2n – 2n (近似为 2n)，n为循环数
6. Gel Electrophoresis | 凝胶电泳
Gel electrophoresis separates DNA fragments by size. An agarose gel is immersed in buffer, and DNA samples are loaded into wells near the cathode. Since DNA carries a negative charge from its phosphate backbone, fragments migrate toward the positive electrode. Shorter fragments move faster through the gel matrix, resulting in distinct bands after staining.
凝胶电泳根据大小分离DNA片段。将琼脂糖凝胶浸入缓冲液中，DNA样品加入靠近阴极的加样孔。由于DNA因其磷酸骨架而带负电，片段朝正极迁移。较短片段通过凝胶基质的速度更快，染色后形成清晰的条带。
A DNA ladder containing fragments of known lengths is run alongside the samples. The distance migrated is inversely proportional to the logarithm of fragment length. Bands are visualised under UV light using dyes such as ethidium bromide or SYBR Safe.
含已知长度片段的DNA梯度与样品同时电泳。迁移距离与片段长度的对数成反比。使用溴化乙锭或SYBR Safe等染料在紫外光下观察条带。
7. DNA Profiling and Forensic Applications | DNA 指纹分析与法医学应用
DNA profiling relies on short tandem repeats (STRs), non-coding regions where 2–6 base pair sequences are repeated. The number of repeats at each locus varies greatly among individuals. After PCR amplification of several STR loci and electrophoresis, the banding pattern forms a unique DNA profile.
DNA指纹分析依赖于短串联重复序列(STR)，即2–6个碱基对序列重复的非编码区。每个基因座的重复数目在个体间差异很大。对若干STR基因座进行PCR扩增和电泳后，条带模式形成独一无二的DNA图谱。
Forensic science uses 13 standard STR loci (CODIS system). The probability of a random match is astronomically low, making it reliable for criminal identification. Paternity testing confirms a biological relationship if half the child's bands match the alleged father.
法医学使用13个标准STR基因座（CODIS系统）。随机匹配的概率极低，使其成为可靠的犯罪鉴定工具。亲子鉴定中，若孩子的一半条带与假设父亲匹配，则确认生物学关系。
8. Gene Cloning and Transformation | 基因克隆与转化
Gene cloning inserts a target gene into a plasmid vector, introducing the recombinant molecule into host bacteria (typically E. coli) via transformation. Inside the host, the plasmid replicates and the gene may be expressed to produce protein. The key steps are: digestion with restriction enzymes, ligation, transformation, and selection using antibiotics or colour markers.
基因克隆将目标基因插入质粒载体，通过转化将重组分子引入宿主细菌（通常为大肠杆菌）。在宿主体内，质粒复制，基因可能被表达以生产蛋白质。关键步骤是：限制酶消化、连接、转化以及使用抗生素或颜色标记进行筛选。
Transformation can be achieved by heat shock—treating cells with ice-cold CaCl₂ followed by a brief 42°C pulse—or by electroporation, which temporarily disrupts the cell membrane, allowing plasmid uptake.
转化可通过热休克——用冰冷的氯化钙处理细胞后短暂42°C热冲击——或电穿孔实现，后者暂时破坏细胞膜使质粒进入。
9. Genetically Modified Organisms (GMOs) | 转基因生物
A GMO contains a transgene from a different species. Bt corn expresses the Cry toxin gene from Bacillus thuringiensis, conferring resistance to insect larvae. Golden rice is engineered with genes from daffodil and a bacterium to synthesise beta-carotene in the endosperm, addressing vitamin A deficiency.
转基因生物含有来自不同物种的转基因。Bt玉米表达苏云金芽孢杆菌的Cry毒素基因，赋予对昆虫幼虫的抗性。黄金大米通过导入黄水仙和一种细菌的基因，在胚乳中合成β-胡萝卜素，解决维生素A缺乏症。
Recombinant human insulin is produced by E. coli carrying the human insulin gene. The product is chemically identical to human insulin, safer, and free from animal contaminants. This replaced animal-derived insulin for diabetic therapy worldwide.
重组人胰岛素由携带人胰岛素基因的大肠杆菌生产。其化学结构与人胰岛素相同，更安全且无动物污染物，已在全球取代动物源胰岛素用于糖尿病治疗。
10. CRISPR-Cas9 and Gene Editing | CRISPR-Cas9 与基因编辑
CRISPR-Cas9 is an RNA-guided genome editing tool adapted from a bacterial adaptive immune system. A single guide RNA (sgRNA) is designed to be complementary to the target DNA sequence, directing the Cas9 endonuclease to introduce a double-strand break (DSB) at the precise locus.
CRISPR-Cas9是一种RNA引导的基因组编辑工具，改编自细菌的适应性免疫系统。设计的单链向导RNA (sgRNA) 与目标DNA序列互补，引导Cas9核酸内切酶在精确位点引入双链断裂(DSB)。
The cell repairs the DSB via non-homologous end joining (NHEJ), often resulting in small insertions or deletions that knock out gene function. Alternatively, if a repair template is provided, homology-directed repair (HDR) can introduce a specific new sequence, enabling gene correction or insertion.
细胞通过非同源末端连接(NHEJ)修复DSB，常导致小片段插入或缺失，从而敲除基因功能。如果提供修复模板，同源定向修复(HDR)可引入特定的新序列，实现基因修正或插入。
11. Ethical Considerations and Risks | 伦理考量与风险
Genetic engineering stirs ethical debates. GMO release may cause ecological disruption, such as gene flow to wild relatives or harm to non-target organisms. There are concerns about allergenicity of novel proteins in food and corporate control over patented seeds. The Cartagena Protocol on Biosafety provides international guidelines.
基因工程引发伦理争议。转基因生物的释放可能导致生态破坏，如基因流向野生近缘种或对非目标生物造成伤害。人们担忧食物中新型蛋白的致敏性，以及企业对专利种子的控制。《卡塔赫纳生物安全议定书》提供了国际
Published by TutorHao | IB Biology Revision Series | aleveler.com
更多咨询请联系16621398022（同微信）
或截图后用「扫一扫 → 相册」选择本图