ALevel生物 基因技术 PCR 电泳 考点精讲
基因技术是 A-Level 生物学科中跨度最大、考点最密集的板块之一。从工具酶的选择、重组 DNA 的构建，到 PCR 扩增和凝胶电泳的判读，每年大考都会以图文题或实验题的形式出现。本文用中英双语把整条技术路线拆开讲透，帮助你把零散知识点串成一条完整的答题链条。
Gene technology is one of the most heavily examined areas of A-Level Biology. From the choice of restriction enzymes and the construction of recombinant DNA, to PCR amplification and the interpretation of gel electrophoresis results, these topics appear every year as diagram-based or practical questions. This article walks through the whole technical pathway in bilingual detail, linking scattered facts into one logical chain you can use in the exam hall.
1. 基因技术全景: 从目的基因到转基因生物
基因技术的本质是把一段外源 DNA 导入受体细胞，让受体细胞获得新的性状。经典重组 DNA 技术包括四步: 第一，用限制酶和连接酶把目的基因拼入载体; 第二，用质粒等载体携带目的基因进入宿主细胞; 第三，借助标记基因筛选出真正含重组质粒的细胞; 第四，让目的基因在宿主内表达，产生目标蛋白，例如用大肠杆菌生产人胰岛素。考试中常见的第一问，就是让学生按正确顺序排出这四个环节。
At its core, gene technology transfers a foreign DNA fragment into a host cell so that the host gains a new characteristic. Classic recombinant DNA technology follows four stages. First, restriction enzymes and DNA ligase insert the gene of interest into a vector. Second, the vector, usually a plasmid, carries the gene into the host cell. Third, marker genes allow scientists to identify cells that actually contain the recombinant plasmid. Fourth, the gene is expressed in the host to make the desired protein, such as human insulin produced by E. coli. A common first question is simply to place these four stages in the correct order.
为什么偏偏用细菌生产人胰岛素? 传统上胰岛素从猪或牛的胰腺中提取，成本高且可能引起过敏反应。把人的胰岛素基因导入大肠杆菌后，可以在发酵罐中大规模培养细菌，获得的人胰岛素与人体自身分泌的完全一致，副作用更小，产量也不再受动物来源限制。
Why bacteria for human insulin? Insulin used to be extracted from the pancreas of pigs and cattle, which was expensive and could trigger allergic reactions. Once the human insulin gene is placed in E. coli, fermenters can grow the bacteria on an industrial scale, and the product is identical to the insulin our own bodies produce, with fewer side effects and no reliance on animal sources.
2. 限制酶与连接酶: 切割与拼接的分子剪刀和胶水
限制酶(限制性内切酶)的作用是识别 DNA 上特定的碱基序列并在特定位点切断磷酸二酯键。大多数限制酶识别的是 4 到 8 个碱基的回文序列，即两条链从 5 端到 3 端读出的序列完全相同。切割方式有两种: 平末端切割在两条链的同一位置切断，产生无突出末端的平头片段; 错位切割则让每条链各留下几个未配对的碱基，形成粘性末端。粘性末端因碱基互补可以精确配对，因此更常用于重组实验。DNA 连接酶随后催化相邻核苷酸之间形成新的磷酸二酯键，把来自不同来源的 DNA 片段永久缝合起来。
Restriction enzymes recognise specific base sequences on DNA and cut the phosphodiester bonds at precise points. Most recognise palindromic sequences of four to eight bases, meaning the sequence reads identically on both strands from the 5 prime to the 3 prime direction. There are two cutting styles. A blunt cut severs both strands at the same position, leaving flush ends with no overhang. A staggered cut leaves short unpaired bases on each strand, producing sticky ends. Sticky ends are preferred in cloning because complementary bases pair exactly, guiding the fragments together. DNA ligase then forms new phosphodiester bonds between adjacent nucleotides, permanently joining fragments from different sources.
3. 目的基因的来源: 基因组文库 cDNA 文库与 PCR 扩增
目的基因可以从三条途径获得。第一条是构建基因组文库: 用限制酶把基因组 DNA 切成片段，全部插入载体并导入宿主菌，每个菌落携带一个片段，再用探针筛选含目的基因的菌落。第二条是构建 cDNA 文库: 从表达该基因的细胞中提取 mRNA，用逆转录酶以 mRNA 为模板合成互补 DNA，即 cDNA。这里埋着一个核心考点: 真核生物基因含有内含子，细菌缺乏切除内含子的剪接机制，而 cDNA 不含内含子，因此把真核基因导入原核宿主时必须用 cDNA，而不能用带内含子的基因组片段。第三条是直接用 PCR 从基因组中扩增目的基因，前提是已知目的基因两侧的序列，以便设计特异性引物。
There are three routes to obtain a gene of interest. The first is a genomic library: genomic DNA is cut into fragments with restriction enzymes, every fragment is inserted into a vector and transformed into host bacteria so that each colony carries one fragment, and a DNA probe is then used to screen for the colony containing the target gene. The second is a cDNA library: mRNA is extracted from cells that express the gene, and reverse transcriptase uses that mRNA as a template to synthesise complementary DNA, or cDNA. This hides a core exam point: eukaryotic genes contain introns, but bacteria lack the splicing machinery to remove them, and cDNA contains no introns, so a eukaryotic gene must be transferred into a prokaryotic host as cDNA rather than as a genomic fragment. The third route is direct PCR amplification from genomic DNA, which is only possible when the sequences flanking the gene are known so that specific primers can be designed.
4. 质粒载体与转化: 标记基因筛选的底层逻辑
质粒是细菌中天然存在的环状 DNA，是使用最广的载体，因为它能独立复制且容易进入细菌。一个合格的表达载体至少要有三个要素: 复制起点让质粒能在宿主内自我复制，多克隆位点提供限制酶的插入位置，标记基因帮助筛选。最常见的标记基因是抗生素抗性基因，例如氨苄青霉素抗性基因。转化时，用氯化钙处理并热激，或用电穿孔法，让细菌暂时打开细胞膜吸收质粒。但并非所有细菌都能成功吸收质粒，所以要把菌落接种在含抗生素的培养基上，只有携带质粒的细菌才能存活并形成菌落。
Plasmids are small circular DNA molecules found naturally in bacteria, and they are the most widely used vectors because they replicate independently and enter bacteria easily. A useful expression vector needs three elements: an origin of replication so the plasmid copies itself inside the host, a multiple cloning site providing an insertion point for restriction enzymes, and a marker gene for selection. The commonest marker genes confer antibiotic resistance, for example ampicillin resistance. During transformation, bacteria are treated with calcium chloride and heat-shocked, or subjected to electroporation, so their membranes become temporarily permeable to plasmid DNA. Because not every cell takes up the plasmid, colonies are grown on agar containing the antibiotic; only bacteria carrying the plasmid survive and form colonies.
5. PCR 扩增: 变性 退火 延伸 与指数增长计算
聚合酶链式反应(PCR)能在数小时内把单个 DNA 片段扩增到数百万份，是基因技术最基础的体外扩增工具。每个循环包含三个温度阶段: 95 摄氏度左右变性，氢键断裂，双链解开成单链; 55 到 65 摄氏度退火，引物与单链模板的互补区段配对; 72 摄氏度延伸，耐热的 Taq 聚合酶沿模板从引物 3 端合成新链。PCR 需要一对引物，它们是约 20 个碱基的单链 DNA，决定了扩增片段的边界。每个循环 DNA 数量翻倍，n 个循环后理论上得到 2 的 n 次方份拷贝。考试常问: 30 个循环后有多少份，或从 1 份模板出发扩增 10 个循环能得到多少份产物，答案是 2 的 10 次方等于 1024 份。
The polymerase chain reaction amplifies a single DNA fragment into millions of copies within hours, making it the essential in vitro amplification tool in gene technology. Each cycle has three temperature stages. Denaturation at about 95 degrees Celsius breaks the hydrogen bonds and separates the double helix into single strands. Annealing at 55 to 65 degrees Celsius allows primers to pair with complementary regions of the single-stranded template. Extension at 72 degrees Celsius lets thermostable Taq polymerase add nucleotides from the 3 prime end of each primer. PCR requires a pair of primers, single-stranded DNA molecules about 20 bases long that define the boundaries of the amplified region. Every cycle doubles the DNA, so after n cycles there are theoretically 2 to the power n copies. A classic calculation asks how many copies result from 10 cycles starting with one template: the answer is 2 to the power 10, which is 1024.
为什么 PCR 必须使用 Taq 聚合酶? 因为每个循环的变性步骤都要把温度升到 95 摄氏度左右，普通聚合酶会在此温度下不可逆失活。Taq 酶来自生活在温泉中的水生栖热菌，天然耐高温，因此在每个循环的高温阶段后仍保持活性，实验无需在每个循环后补充新酶，自动化扩增才成为可能。
Why must PCR use Taq polymerase? Because the denaturation step of every cycle heats the mixture to about 95 degrees Celsius, which would irreversibly destroy ordinary polymerases. Taq polymerase comes from Thermus aquaticus, a bacterium that lives in hot springs, so it survives these extreme temperatures. The enzyme remains active after every high-temperature step, no fresh enzyme is needed between cycles, and fully automated amplification becomes possible.
6. 凝胶电泳: 按大小分离 DNA 片段与结果判读
凝胶电泳用来分离和鉴定 DNA 片段，原理是 DNA 带负电荷，在电场中会向正极移动，而琼脂糖凝胶的多孔结构对片段产生阻力。片段越小，穿过凝胶孔洞越容易，移动越快，所以跑得最远; 片段越大，移动越慢，停留在靠近加样孔的位置。判读时注意两点: 第一，加样孔在负极一端，DNA 从负极游向正极，因此条带位置越靠下代表片段越短; 第二，通过与已知大小的 DNA 标准品对比条带位置，可以估算未知片段长度。在法医学和亲子鉴定中，电泳结合卫星 DNA 的重复序列差异形成 DNA 指纹，每个个体(同卵双胞胎除外)的图谱都不同。
Gel electrophoresis separates and identifies DNA fragments by size. DNA carries a negative charge, so it migrates towards the positive electrode in an electric field, while the porous agarose gel resists movement. Smaller fragments pass through the pores more easily, travel faster and end up furthest from the wells; larger fragments move slowly and stay close to the loading wells. Two points matter when reading results. First, the wells sit at the negative end, so a band lower on the gel belongs to a shorter fragment. Second, comparing band positions with a DNA ladder of known sizes lets you estimate the length of unknown fragments. In forensic science and paternity testing, electrophoresis combined with variable numbers of repetitive satellite DNA sequences produces a DNA profile, and every individual except identical twins has a unique pattern.
判读例题: 同一块凝胶的加样孔中依次加入 DNA 标准品和两份样品。标准品出现四条带，分别对应 1000、700、400 和 200 个碱基对。若样品一的条带位于 700 与 400 两条带之间，可估算其长度约为 550 个碱基对; 若样品二出现两条带，说明其中含有两种不同长度的 DNA 片段。注意样品必须与标准品在同一块凝胶、同一电场条件下电泳，条带位置的比较才有意义。
Worked example: the wells of one gel are loaded with a DNA ladder and two samples. The ladder shows four bands corresponding to 1000, 700, 400 and 200 base pairs. If sample one produces a single band between the 700 and 400 bands, estimate its length at roughly 550 base pairs. If sample two shows two bands, it contains two DNA fragments of different sizes. Comparisons are only valid when the samples and the ladder run on the same gel under the same electric field.
7. CRISPR-Cas9 基因编辑与基因治疗
CRISPR-Cas9 是目前最受关注的基因编辑技术，它的系统由两部分组成: 一段向导 RNA 负责识别并定位目标 DNA 序列，Cas9 蛋白则像一把分子剪刀在该位置切断 DNA 双链。细胞修复断口时可能发生碱基缺失或插入，从而敲除目标基因; 如果同时提供一段修复模板，则可以精确替换或插入新基因。与传统转基因相比，CRISPR 更精准、更快捷、成本更低，已被用于改良作物性状、构建疾病模型，并探索治疗镰状细胞贫血等遗传病。2020 年诺贝尔化学奖授予了 CRISPR 技术的两位奠基人，这一知识点常以拓展题形式出现。
CRISPR-Cas9 is currently the most prominent gene editing tool. The system has two components: a guide RNA that recognises and locates the target DNA sequence, and the Cas9 protein, which acts like molecular scissors and cuts both strands of the DNA at that site. When the cell repairs the break, bases are often deleted or inserted, knocking out the target gene. If a repair template is supplied at the same time, a new gene can be inserted or an existing one precisely replaced. Compared with traditional genetic modification, CRISPR is more accurate, faster and cheaper. It has been used to improve crop traits, build disease models and explore treatments for genetic disorders such as sickle cell anaemia. The 2020 Nobel Prize in Chemistry honoured two pioneers of CRISPR, and this fact often appears in extension questions.
基因治疗分为体细胞治疗与生殖系编辑两条路线。体细胞基因治疗只修改患者自身的体细胞，修正的性状不会遗传给后代，目前临床上的尝试多属此类; 对生殖细胞或早期胚胎的编辑会改变后代乃至后代的基因库，多数国家立法禁止。2023 年，针对镰状细胞贫血的 CRISPR 疗法在英国获批上市，标志着基因编辑正式进入常规临床治疗。
Gene therapy follows two broad routes. Somatic cell therapy modifies only the patient's own body cells, so the correction is not inherited by children, and most clinical attempts belong to this category. Germline editing, which changes gametes or early embryos, would alter the gene pool of future generations and is banned by law in most countries. In 2023 a CRISPR-based treatment for sickle cell disease was approved in the UK, marking the arrival of gene editing in routine clinical care.
8. 考点速记: 五类高频陷阱与答题要点
第一，分清酶的作用对象: 限制酶切磷酸二酯键，解旋酶断氢键，两者不可混淆，这是选择题最高频的干扰项。第二，粘性末端靠互补碱基配对结合，但真正把片段连牢的是连接酶形成的磷酸二酯键，不是氢键。第三，PCR 退火温度取决于引物，延伸方向永远从引物 3 端开始，所以引物序列决定了扩增区域。第四，电泳判读时先确认加样孔位置，片段越小跑得越远，答题时描述为向正极移动而不是向下移动。第五，计算题先看清起点模板数，若从 1 份开始，n 个循环后是 2 的 n 次方; 若从 m 份开始，则是 m 乘以 2 的 n 次方。把这几条写进错题本，基因技术部分的送分题就不会再丢。
Memorise five high-frequency traps. First, know which enzyme acts on which bond: restriction enzymes cut phosphodiester bonds, while helicase breaks hydrogen bonds, and mixing them up is the most common distractor in multiple choice questions. Second, sticky ends associate through complementary base pairing, but the fragments are truly sealed by phosphodiester bonds formed by ligase, not by hydrogen bonds. Third, the annealing temperature in PCR depends on the primers, and extension always starts from the 3 prime end of each primer, so the primers define the amplified region. Fourth, when reading a gel, locate the wells first; smaller fragments travel furthest, so describe movement towards the positive electrode rather than simply downwards. Fifth, read the starting template number carefully in calculations: starting from one template, n cycles give 2 to the power n copies, but starting from m templates the answer is m times 2 to the power n. Add these rules to your error log and the easy marks in gene technology are yours.
基因技术考查的核心是理解每一步操作的原理和目的，而不是死记流程名称。建议你把本文的流程图自己重画一遍，标出每步的酶、温度和作用对象，再对照真题检验。掌握了工具酶、目的基因来源、载体筛选、PCR 计数和电泳判读这几根支柱，无论试卷如何变化，你都能稳住这一模块的分数。
The key to gene technology questions is understanding why each step is done, not memorising names. Redraw the flow diagram yourself, labelling the enzymes, temperatures and molecules at every stage, then check your version against past papers. Once you master the pillars of restriction tools, gene sources, vector selection, PCR counting and gel interpretation, this module will stay solid no matter how the questions change.
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