📚 Nucleotides and Nucleic Acids: Experimental Design | 核苷酸与核酸：实验设计
This article guides you through the systematic design of experiments to investigate nucleotides and nucleic acids, focusing on DNA extraction and quantification. You will learn how to formulate a hypothesis, identify variables, choose appropriate techniques, and critically evaluate data — all essential skills for A-level Biology practical assessments and scientific inquiry.
本文引导你系统性地设计实验来研究核苷酸与核酸，重点围绕 DNA 的提取与定量分析。你将学习如何建立假设、识别变量、选择适宜技术并批判性地评价数据，这些都是 A-level 生物实验评估与科学探究中不可或缺的能力。
1. Biochemical Context and Relevance | 生物化学背景与现实意义
Nucleotides are the monomers that polymerise to form nucleic acids such as DNA and RNA. Each nucleotide consists of a pentose sugar (deoxyribose in DNA, ribose in RNA), a phosphate group, and a nitrogenous base (adenine, thymine, cytosine, guanine, or uracil). The sequence of nucleotides encodes genetic information, and experimental manipulation of nucleic acids underpins modern biotechnology, forensic science, and medical diagnostics.
核苷酸是聚合形成 DNA 和 RNA 等核酸的单体。每个核苷酸由一个五碳糖（DNA 中为脱氧核糖，RNA 中为核糖）、一个磷酸基团和一个含氮碱基（腺嘌呤、胸腺嘧啶、胞嘧啶、鸟嘌呤或尿嘧啶）组成。核苷酸的序列编码了遗传信息，而核酸的实验操作则是现代生物技术、法医学和医学诊断的基础。
2. Defining Aims and Formulating Hypotheses | 明确实验目的与提出假设
A well-designed experiment starts with a clear aim. For a practical exploration of nucleotides and nucleic acids, a typical aim might be: ‘To investigate how the concentration of sodium chloride in the extraction buffer affects the yield of DNA from strawberry tissue.’ The corresponding hypothesis could be: ‘Increasing NaCl concentration up to an optimum will increase DNA yield because salt neutralises the negative charges on the phosphate backbone, allowing DNA strands to aggregate and precipitate more efficiently.’
精心设计的实验始于清晰的实验目的。对于核苷酸与核酸的实践探索，一个典型目的可以是：“探究提取缓冲液中氯化钠浓度如何影响草莓组织的 DNA 得率”。对应的假设可表述为：“在一定范围内提高 NaCl 浓度将增加 DNA 得率，因为盐中和了磷酸骨架上的负电荷，使 DNA 链更易聚集并沉淀。”
3. Safety and Ethical Considerations | 安全与伦理考量
Ethanol is highly flammable; keep it away from open flames and use a fume hood if available. Detergents and concentrated salts can irritate the skin and eyes, so wear safety goggles, nitrile gloves, and a lab coat throughout. When using a hot water bath (e.g., for deactivation of enzymes), handle with care to avoid scalds. Although this experiment uses commercially available fruit, you must follow local regulations regarding the disposal of biological and chemical waste. Ethical issues are minimal when using plant material, but the same principles of responsible laboratory conduct apply.
乙醇高度易燃，应远离明火并在通风橱内使用。洗涤剂和高浓度盐会刺激皮肤和眼睛，因此实验全程需佩戴护目镜、丁腈手套和实验服。使用热水浴（如用于灭活酶）时需小心操作以防烫伤。虽然本实验使用市售水果，仍需遵守当地的生物与化学废弃物处理规定。使用植物材料时伦理问题较少，但负责任实验室行为的原则同样适用。
4. Equipment, Reagents, and Rationale | 仪器、试剂与选用理由
The following items are required for a standard DNA extraction and quantification: fresh strawberries or kiwi (rich in DNA and soft tissue), extraction buffer (10% detergent, 2–8% NaCl, distilled water), ice-cold 95% ethanol or isopropanol, protease (e.g., meat tenderiser or pineapple juice), a mortar and pestle, a water bath at 60°C, test tubes, a graduated cylinder, filter paper or cheesecloth, a glass stirring rod, and a colorimeter with cuvettes for the diphenylamine assay. These materials are chosen because detergents disrupt cell and nuclear membranes, salt shields negative charges to promote aggregation, proteases digest histones, and cold ethanol precipitates DNA while leaving other soluble components in solution.
标准 DNA 提取与定量需要以下材料：新鲜草莓或猕猴桃（DNA 含量高且组织柔软）、提取缓冲液（10% 洗涤剂、2–8% NaCl、蒸馏水）、冰镇 95% 乙醇或异丙醇、蛋白酶（如嫩肉粉或菠萝汁）、研钵和杵、60°C 水浴、试管、量筒、滤纸或纱布、玻璃搅拌棒以及用于二苯胺检测的分光光度计及比色皿。选用这些材料是因为：洗涤剂破坏细胞膜和核膜，盐屏蔽负电荷促进聚集，蛋白酶水解组蛋白，而冷乙醇可沉淀 DNA 同时使其他可溶性组分留在溶液中。
5. Stepwise DNA Extraction Protocol | 分步 DNA 提取操作
Step 1: Place 10 g of destalked strawberry into a mortar, add 20 mL of extraction buffer, and grind gently for 2–3 minutes to produce a homogenate. Step 2: Filter the homogenate through a double layer of cheesecloth into a clean test tube to remove cellular debris. Step 3: Add a pinch of protease and incubate the filtrate at 60°C for 15 minutes, then cool on ice. Step 4: Carefully pour twice the volume of ice-cold ethanol down the inner wall of the tube so it forms a layer above the aqueous extract. Step 5: Let the tube stand undisturbed for 5–10 minutes; DNA will appear as a white, stringy precipitate at the interface. Spool the DNA onto a glass rod or pipette tip for recovery.
步骤 1：取 10 g 去蒂草莓放入研钵，加入 20 mL 提取缓冲液，轻轻研磨 2–3 分钟制成匀浆。步骤 2：用双层纱布将匀浆过滤至清洁试管中，去除细胞碎片。步骤 3：加入一小撮蛋白酶，在 60°C 水浴中孵育 15 分钟，随后置于冰上冷却。步骤 4：沿试管内壁小心倒入两倍体积的冰镇乙醇，使其在水相提取液上方形成一层。步骤 5：将试管静置 5–10 分钟，DNA 将在两相界面呈现白色丝状沉淀。用玻璃棒或移液器吸头将 DNA 卷出回收。
6. Identifying Variables and Designing Controls | 识别变量并设计对照
Independent variable (IV): sodium chloride concentration (e.g., 0%, 1%, 2%, 4%, 8%). Dependent variable (DV): mass of recovered DNA (measured on a precision balance after drying) or absorbance at 595 nm using the diphenylamine assay. Control variables include the mass of strawberry tissue, volume of extraction buffer, incubation time and temperature, protease amount, ethanol volume and temperature, and filtration method. A control group with 0% NaCl should be included to confirm that salt is required for effective precipitation. Standardising these variables ensures that any change in the DV can be attributed to the IV.
自变量：氯化钠浓度（如 0%、1%、2%、4%、8%）。因变量：回收 DNA 的质量（干燥后用精密天平称量）或使用二苯胺法在 595 nm 处的吸光度。控制变量包括草莓组织质量、提取缓冲液体积、孵育时间与温度、蛋白酶用量、乙醇体积与温度以及过滤方法。应包含一组 0% NaCl 的对照组，以确认盐对于有效沉淀的必要性。标准化这些变量可确保因变量的任何变化均可归因于自变量。
7. Diphenylamine Assay for Quantification | 二苯胺法定量分析
The diphenylamine test is specific for deoxyribose and therefore detects DNA. Under acidic conditions and heating, deoxyribose is converted to a compound that reacts with diphenylamine to give a blue colour. The intensity of the colour is proportional to DNA concentration. To quantify DNA yield, construct a standard curve using known concentrations of herring sperm DNA. Dissolve the spooled DNA in a fixed volume of TE buffer, mix 1 mL of sample with 2 mL of diphenylamine reagent, heat at 100°C for 10 minutes, cool, and measure absorbance at 595 nm. Use the standard curve equation to calculate DNA concentration in the original extract.
二苯胺检测对脱氧核糖具有专一性，因而可检测 DNA。在酸性加热条件下，脱氧核糖转化为可与二苯胺反应生成蓝色的化合物，颜色深浅与 DNA 浓度成正比。为定量 DNA 得率，使用已知浓度的鲱鱼精子 DNA 绘制标准曲线。将卷出的 DNA 溶解在固定体积的 TE 缓冲液中，取 1 mL 样品与 2 mL 二苯胺试剂混合，于 100°C 加热 10 分钟，冷却后在 595 nm 处测吸光度。利用标准曲线方程计算原始提取液中的 DNA 浓度。
8. Investigating Factors That Influence DNA Yield | 探究影响 DNA 得率的因素
Beyond salt concentration, you can design variations to test the effect of temperature (e.g., extraction buffer preheated to 40°C, 60°C, or 80°C before homogenisation), pH of the extraction buffer (adjusted with dilute HCl or NaOH), type of detergent (SDS vs. Triton X-100), or source of tissue (banana, onion, liver). For each factor, keep all other variables constant and repeat each trial at least three times to ensure reliability. For instance, to study the effect of pH, prepare buffers at pH 5, 7, and 9, extract DNA following the standard protocol, and quantify the yield. Such systematic investigations deepen understanding of the biochemical principles underlying nucleotide interactions.
除盐浓度外，还可设计实验测试温度（如匀浆前将提取缓冲液预热至 40°C、60°C 或 80°C）、提取缓冲液的 pH（用稀 HCl 或 NaOH 调节）、洗涤剂类型（SDS 对比 Triton X-100）或组织来源（香蕉、洋葱、肝脏）的影响。对于每种因素，保持其他变量不变，每个条件至少重复三次以确保可靠性。例如，要研究 pH 的影响，配制 pH 5、7、9 的缓冲液，按标准步骤提取 DNA 并定量产量。此类系统性探究可加深对核苷酸相互作用背后的生化学原理的理解。
9. Data Recording, Presentation, and Statistical Analysis | 数据记录、展示与统计分析
Record raw data in a table with columns for independent variable values, trial number, DNA mass (g), and absorbance readings. Calculate mean values and standard deviations. Present the relationship between IV and DV using a scatter plot, adding error bars to show spread. If the data suggest a linear or curvilinear trend, perform a regression analysis and calculate the correlation coefficient (r). A table of sample data might look like this:
在表格中记录原始数据，列标题包括自变量取值、试验编号、DNA 质量（g）和吸光度读数。计算平均值和标准差。使用散点图展示自变量与因变量的关系，并添加误差线以显示数据分布。若数据呈现线性或曲线趋势，可进行回归分析并计算相关系数（r）。示例数据表如下：
| NaCl conc. (%) | Mean DNA mass (g) | Std Dev |
|---|---|---|
| 0 | 0.01 | 0.002 |
| 2 | 0.18 | 0.015 |
| 4 | 0.25 | 0.020 |
| 8 | 0.22 | 0.018 |
Statistical tests such as the t-test (for two groups) or ANOVA (for multiple groups) can determine whether differences in DNA yield are significant. Always report the p-value and state whether the null hypothesis is rejected.
可采用 t 检验（两组间比较）或方差分析 ANOVA（多组间比较）等统计检验，判断 DNA 得率的差异是否显著。始终报告 p 值并说明是否拒绝原假设。
10. Evaluating Sources of Error and Suggesting Improvements | 评估误差来源并提出改进
Common errors include incomplete cell lysis due to insufficient grinding, loss of DNA during filtration if the filter pore size is too small, and incomplete precipitation if ethanol is not cold enough. Systematic errors can arise from a poorly calibrated balance or colorimeter. Random errors may come from variations in pipetting or temperature fluctuations. To improve reliability, pre-cool ethanol on ice, increase the number of replicates, use a vortex mixer for resuspension, and spin samples in a centrifuge (if available) to pellet DNA more consistently. Additionally, confirm DNA purity by measuring the A260/A280 ratio using a UV spectrophotometer; pure DNA has a ratio of about 1.8.
常见误差包括研磨不充分导致细胞裂解不完全、滤纸孔径过小造成 DNA 在过滤过程中损失、以及乙醇温度不够低引起沉淀不充分。系统误差可能源自未校准的天平或分光光度计。随机误差可能来自移液差异或温度波动。为提高可靠性，可在冰上预冷乙醇、增加重复次数、使用涡旋混合器重悬 DNA，并在条件允许时用离心机沉淀 DNA 以提高一致性。此外，使用紫外分光光度计测定 A260/A280 比值可确认 DNA 纯度；纯 DNA 的比值约为 1.8。
11. Extensions: Enzyme Digestion and Gel Electrophoresis | 拓展实验：酶切消化与凝胶电泳
Once DNA is extracted, you can design further experiments to explore its nucleotide-level properties. Digest a portion of the DNA with specific restriction endonucleases such as EcoRI or HindIII, which cut at defined palindromic sequences. Run the fragments on an agarose gel electrophoresis, stain with ethidium bromide (or safer alternatives such as SYBR Safe), and visualise under UV light. By comparing band patterns with a DNA ladder, you can estimate fragment sizes and infer the positions of restriction sites. This links the practical work directly to the concept of nucleotide sequence specificity and demonstrates how nucleic acid biochemistry translates into analytical tools.
提取 DNA 后，可设计进一步实验以探索其在核苷酸层面的性质。用特定的限制性内切酶（如 EcoRI 或 HindIII）消化部分 DNA，这些酶在特定的回文序列处切割。将酶切片段进行琼脂糖凝胶电泳，用溴化乙锭（或 SYBR Safe 等更安全的替代品）染色，并在紫外光下观察。通过与 DNA ladder 比较条带模式，可以估算片段大小并推断酶切位点的位置。这将实验工作直接与核苷酸序列特异性概念联系起来，并展示了核酸生物化学如何转化为分析工具。
12. Conclusion and Connections to Syllabus | 结论与考纲衔接
The experimental design process described here bridges the gap between the theoretical frameworks of nucleotide structure and the practical skills required in the A-level Biology syllabus. By manipulating extraction variables and quantifying DNA, you gain evidence of how chemical environments affect nucleic acid behaviour. This reinforces key concepts such as hydrogen bonding in base pairing, the role of phosphate charges, and the importance of controls. Whether you are preparing for a written paper or a practical endorsement, mastering such experimental design will enhance your ability to think like a scientist.
此处描述的实验设计过程架起了核苷酸结构理论框架与 A-level 生物大纲所需操作技能之间的桥梁。通过调控提取变量并定量 DNA，你获得了化学环境影响核酸行为的证据。这巩固了碱基配对中的氢键、磷酸电荷的作用以及对照重要性等核心概念。无论你是在准备笔试还是实践考核，掌握此类实验设计将提升你像科学家一样思考的能力。
Published by TutorHao | Biology Revision Series | aleveler.com
更多咨询请联系16621398022（同微信）
屏轩国际教育cambridge primary/secondary checkpoint, cat4, ukiset,ukcat,igcse,alevel,PAT,STEP,MAT, ibdp,ap,ssat,sat,sat2课程辅导，国外大学本科硕士研究生博士课程论文辅导Cancel reply