📚 Manipulating Genomes: Experimental Design | 基因组操纵：实验设计
Designing and executing experiments to manipulate genomes forms a fundamental part of modern biological investigation. Whether the goal is to clone a gene, determine a DNA sequence, or edit a specific locus using CRISPR-Cas9, experimental rigour is essential. This guide walks through the key steps and considerations for each core technique in the A-level Biology 6.3 specification, highlighting practical tips and common pitfalls.
设计和执行基因组操纵实验是现代生物研究的基本组成部分。无论是克隆一个基因、测定DNA序列，还是利用CRISPR-Cas9编辑特定位点，实验的严谨性至关重要。本指南将逐步介绍A-level生物6.3知识点中每个核心技术的关关键步骤和注意事项，强调实用技巧和常见误区。
1. Core Principles of Experimental Design in Genome Manipulation | 基因组操纵实验设计的核心原则
A well-designed experiment starts with a testable hypothesis, such as ‘the insertion of gene X into plasmid pUC19 will confer ampicillin resistance and blue-white screening capability’. Define independent, dependent and controlled variables clearly.
精心设计的实验始于可验证的假设，例如’将基因X插入质粒pUC19将赋予氨苄青霉素抗性和蓝白筛选能力’。清晰地定义自变量、因变量和控制变量。
Include negative controls (e.g., no ligase, no template DNA) to confirm that observed outcomes are specific to the intended reaction. A positive control with a known target ensures the system works.
纳入阴性对照（例如，无连接酶、无模板DNA），以确认观察到的结果对预期反应是特异性的。使用已知靶标的阳性对照确保系统正常工作。
Replicate experiments at least in triplicate to assess variability. Use aseptic technique when handling bacteria to prevent contamination.
至少设置三个重复实验以评估变异性。在处理细菌时使用无菌操作以防污染。
All reagents must be validated; for instance, test restriction enzyme activity with a control substrate. Document every step meticulously for reproducibility.
所有试剂必须经过验证；例如，用对照底物测试限制酶活性。详细记录每一步以保证可重复性。
2. DNA Extraction and Quality Assessment | DNA提取与质量评估
Genomic DNA extraction typically involves cell lysis using detergents (e.g., SDS) and proteinase K to digest proteins, followed by phenol-chloroform extraction or silica-based column purification. For plasmid DNA, alkaline lysis is commonly used.
基因组DNA提取通常使用去垢剂（如SDS）和蛋白酶K裂解细胞以消化蛋白质，随后进行酚-氯仿抽提或硅胶柱纯化。对于质粒DNA，常采用碱裂解法。
Measure DNA concentration and purity using a spectrophotometer. The A260/A280 ratio should be ~1.8 for pure DNA; lower values indicate protein contamination. Integrity can be checked by agarose gel electrophoresis.
使用分光光度计测定DNA浓度和纯度。纯净DNA的A260/A280比值应约为1.8；较低比值表明蛋白质污染。可通过琼脂糖凝胶电泳检查完整性。
For long-read sequencing or sensitive applications, additional clean-up steps and RNAse treatment are necessary. Quantify with a fluorometer if higher accuracy is needed.
对于长读长测序或敏感应用，需要额外的纯化步骤和RNA酶处理。如果需要更高精度，可用荧光计定量。
3. Restriction Enzyme Digestion and Ligation Strategies | 限制性酶切与连接策略
Type II restriction endonucleases recognise specific palindromic sequences and cleave DNA at defined positions, generating blunt or sticky ends. For cloning, the same enzyme(s) are used to cut both the vector and insert to create compatible cohesive ends.
II型限制性内切核酸酶识别特定的回文序列并在确定位置切割DNA，产生平末端或黏性末端。为了克隆，使用相同的酶切割载体和插入片段以产生兼容的黏性末端。
| Enzyme | Recognition Site (5’→3′) | Ends Generated |
|---|---|---|
| EcoRI | G↓AATTC | 5′ sticky (AATT overhang) |
| HindIII | A↓AGCTT | 5′ sticky (AGCT overhang) |
| SmaI | CCC↓GGG | Blunt |
After digestion, DNA fragments are purified to remove enzymes. Ligation is catalysed by T4 DNA ligase, which forms phosphodiester bonds between adjacent 3′-hydroxyl and 5′-phosphate ends in the presence of ATP.
消化后，纯化DNA片段以去除酶。连接由T4 DNA连接酶催化，在ATP存在下，在相邻的3′-羟基和5′-磷酸末端之间形成磷酸二酯键。
A typical ligation uses a vector:insert molar ratio of 1:3 to favour intermolecular joining. A control reaction without insert should be included to estimate background re-ligation.
典型的连接反应使用载体:插入片段摩尔比1:3以有利于分子间连接。应包含一个无插入片段的对照反应以评估背景自连。
4. Agarose Gel Electrophoresis for Fragment Analysis | 用于片段分析的琼脂糖凝胶电泳
Agarose gel electrophoresis separates DNA fragments based on size. The percentage of agarose determines the resolution range: 1% gels are suitable for 0.5–7 kb, while 2% gels resolve 0.1–3 kb fragments.
琼脂糖凝胶电泳依据大小分离DNA片段。琼脂糖百分比决定分辨范围：1%凝胶适用于0.5–7 kb，2%凝胶可分辨0.1–3 kb的片段。
Load samples mixed with loading dye into wells and run at a constant voltage (e.g., 5 V/cm) in TAE or TBE buffer. Include a DNA ladder with known band sizes to estimate fragment lengths.
将样品与上样缓冲液混合加入孔中，在TAE或TBE缓冲液中恒定电压（如5 V/cm）电泳。加入已知条带大小的DNA梯状标记以估算片段长度。
After electrophoresis, stain the gel with ethidium bromide or a safer alternative (SYBR Safe) and visualise under UV light. Document the gel image and measure migration distance for each band.
电泳后，用溴化乙锭或更安全的替代物（SYBR Safe）染色，在紫外灯下观察。记录凝胶图像并测量每条带的迁移距离。
A semi-log graph of log(size) vs. migration distance can be plotted to determine unknown fragment sizes accurately.
可以绘制log(大小)与迁移距离的半对数图，以准确确定未知片段大小。
5. PCR Primer Design and Amplification | PCR引物设计与扩增
Polymerase chain reaction (PCR) amplifies a specific DNA region exponentially. Primer design is critical: each primer should be 18–25 nucleotides long, have a GC content of 40–60%, and terminate with a G or C at the 3′ end for stable annealing.
聚合酶链式反应(PCR)以指数方式扩增特定DNA区域。引物设计至关重要：每条引物应为18–25个核苷酸，GC含量40–60%，3’端以G或C结尾以确保稳定退火。
Calculate the melting temperature (Tm) approximately using:
近似计算解链温度(Tm)的公式：
Tm = 4(G + C) + 2(A + T) °C
The annealing temperature is usually set 3–5 °C below the lower Tm of the primer pair. Avoid primer dimers and secondary structures by using software such as Primer-BLAST.
退火温度通常设为引物对中较低Tm的3–5 °C以下。利用Primer-BLAST等软件避免引物二聚体和二级结构。
A typical PCR mix contains template DNA, forward and reverse primers, dNTPs, Taq DNA polymerase, Mg²⁺, and buffer. The thermal cycling includes initial denaturation at 95 °C, then 30–35 cycles of denaturation (95 °C, 30 s), annealing (55–65 °C, 30 s), and extension (72 °C, 1 min per kb), followed by final extension.
典型的PCR混合液包含模板DNA、正向和反向引物、dNTPs、Taq DNA聚合酶、Mg²⁺和缓冲液。热循环包括95 °C初始变性，然后30–35个循环的变性(95 °C, 30 s)、退火(55–65 °C, 30 s)、延伸(72 °C, 每kb 1 min)，最后延伸。
Copy number = 2n (n = number of cycles)
Always include a no-template control (NTC) to detect contamination. The product can be verified by gel electrophoresis and purified for downstream use.
始终设置无模板对照(NTC)以检测污染。产物可通过凝胶电泳验证并纯化供下游使用。
6. Sanger Sequencing: Principle and Experimental Setup | 桑格测序：原理与实验设置
Sanger sequencing uses dideoxynucleotides (ddNTPs) labelled with different fluorophores to terminate DNA synthesis. Each of the four ddNTPs emits a distinct fluorescent signal, allowing the sequence to be read by capillary electrophoresis.
桑格测序使用标记不同荧光基团的双脱氧核苷酸(ddNTPs)终止DNA合成。四种ddNTP各自发出独特荧光信号，通过毛细管电泳读取序列。
The reaction mix includes a single primer, DNA template, DNA polymerase, dNTPs, and a low concentration of fluorescent ddNTPs. The products are separated by size, and the order of fluorescent peaks yields the DNA sequence.
反应混合液包含单条引物、DNA模板、DNA聚合酶、dNTPs和低浓度的荧光ddNTPs。产物按大小分离，荧光峰的顺序即得DNA序列。
For experimental design, ensure the template is pure and the primer is located 50–100 bp upstream of the region to be sequenced. Poor template quality or secondary structure in the DNA can cause premature termination.
实验设计中，确保模板纯净且引物位于待测区域上游50–100 bp处。模板质量差或DNA二级结构会导致提前终止。
Modern sequencing services require 10–50 ng of purified PCR product per reaction. The resulting chromatogram should be inspected for double peaks or high background, which indicate mixed templates or primer problems.
现代测序服务每个反应需要10–50 ng纯化的PCR产物。应检查所得的色谱图是否存在双峰或高背景，这可能表明模板混杂或引物问题。
7. Gene Cloning and Plasmid Vector Construction | 基因克隆与质粒载体构建
A successful cloning experiment requires a suitable plasmid vector containing an origin of replication (ori), a selectable marker (e.g., ampicillin resistance gene), and a multiple cloning site (MCS) within a reporter gene such as lacZ for blue-white screening.
成功的克隆实验需要合适的质粒载体，包含复制起点(ori)、选择标记（如氨苄青霉素抗性基因）以及位于报告基因（如lacZ）内的多克隆位点(MCS)，用于蓝白筛选。
Digest both the vector and the DNA insert with the same restriction enzymes to generate compatible ends. Dephosphorylate the vector with alkaline phosphatase to prevent self-ligation if using a single enzyme.
用相同的限制酶消化载体和DNA插入片段以产生兼容末端。如果使用单酶切，可用碱性磷酸酶对载体进行去磷酸化，防止自连。
Set up ligation with a vector:insert ratio of 1:3, using T4 DNA ligase overnight at 16 °C or at room temperature for 1 hour. Include a vector-only ligation control to check for background colonies.
设置连接反应，载体:插入片段比例为1:3，使用T4 DNA连接酶，16 °C过夜或室温1小时。设置仅载体连接对照以检查背景菌落。
After ligation, the recombinant plasmids can be introduced into competent E. coli cells for propagation and identification.
连接后，重组质粒可导入感受态大肠杆菌细胞中，进行扩增和鉴定。
8. Transformation and Selection of Recombinant Bacteria | 转化与重组菌筛选
Bacterial transformation involves making cells competent by chemical treatment (e.g., Ca
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