Genetic Engineering: Principles, Tools and Applications | 基因工程：原理、工具与应用

📚 Genetic Engineering: Principles, Tools and Applications | 基因工程：原理、工具与应用

Genetic engineering is the direct manipulation of an organism’s DNA using biotechnology. In Cambridge A-Level terms, it combines molecular biology techniques with an engineering mindset: define a genetic target, design a DNA construct, build it in a vector, and test the phenotype in a host cell.

基因工程是利用生物技术对生物体 DNA 进行直接操作。在剑桥 A-Level 考试中，它把分子生物学技术与工程思维结合起来：先确定基因靶点，设计 DNA 构建体，在载体中构建，再在宿主细胞中检测表型。

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1. From Selective Breeding to Precision Engineering | 从选择育种到精准工程

Traditional selective breeding changes genomes indirectly by crossing organisms with desirable traits over many generations. It is slow, limited by sexual compatibility, and cannot easily transfer single genes between unrelated species.

传统的选择育种通过多代杂交具有优良性状的生物来间接改变基因组。这种方法速度慢，受物种间可交配性的限制，很难在无关物种之间转移单个基因。

Genetic engineering bypasses reproductive barriers by isolating a specific gene and inserting it into a target genome. This makes the process more predictable, faster, and measurable, which is why it is often described as biological engineering.

基因工程绕过了生殖屏障，可分离特定基因并将其插入目标基因组。这使得过程更可预测、更快速且可测量，因此常被称为生物工程。

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2. Core Tools: Restriction Enzymes and DNA Ligase | 核心工具：限制性内切酶与 DNA 连接酶

Restriction endonucleases are enzymes that cut DNA at specific recognition sequences. Many recognition sites are palindromic, such as EcoRI, which cuts 5′-GAATTC-3′ to produce sticky ends with overhanging single strands.

限制性内切酶是在特定识别序列处切割 DNA 的酶。许多识别位点是回文序列，例如 EcoRI 切割 5′-GAATTC-3′ 产生带有单链突出的粘性末端。

DNA ligase joins the sugar-phosphate backbones of two DNA fragments by forming phosphodiester bonds. Sticky ends cut with the same restriction enzyme can base-pair by complementary hydrogen bonding before ligase seals the backbone.

DNA 连接酶通过形成磷酸二酯键连接两个 DNA 片段的糖-磷酸骨架。用同一种限制酶切出的粘性末端可通过互补氢键配对，再由连接酶封闭骨架。

Key enzymes and their functions are summarised below:

关键酶及其功能总结如下：

Enzyme | 酶	Function | 功能
Restriction endonuclease | 限制性内切酶	Cuts DNA at specific sequences | 在特定序列处切割 DNA
DNA ligase | DNA 连接酶	Joins DNA backbones | 连接 DNA 骨架
DNA polymerase | DNA 聚合酶	Synthesises new DNA strands | 合成新 DNA 链
Reverse transcriptase | 逆转录酶	Makes cDNA from mRNA | 从 mRNA 合成 cDNA

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3. Vectors and Plasmid Design | 载体与质粒设计

A vector is a DNA molecule used to carry foreign DNA into a host cell. Plasmids are the most common vectors for bacterial work because they are small, circular, and replicate independently of the chromosomal DNA.

载体是用于将外源 DNA 带入宿主细胞的 DNA 分子。质粒是细菌实验中最常用的载体，因为它们小、呈环状，并能独立于染色体 DNA 进行复制。

An engineered plasmid typically contains an origin of replication, a selectable marker such as an antibiotic resistance gene, and a multiple cloning site. The cloning site contains several unique restriction sites so the insert can be ligated in a known orientation.

工程质粒通常含有一个复制起点、一个选择标记（如抗生素抗性基因）和一个多克隆位点。多克隆位点含有多个单一限制酶切点，使插入片段能够按已知方向连接。

A strong promoter upstream of the insert, such as the lac promoter, allows transcription to be switched on in the host. Terminator sequences stop transcription and improve RNA stability.

插入片段上游的强启动子（如 lac 启动子）可在宿主中启动转录。终止子序列则终止转录并提高 RNA 的稳定性。

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4. Building a Recombinant DNA Molecule | 构建重组 DNA 分子

The target gene and the plasmid are cut with the same restriction enzyme to generate compatible sticky ends. The fragments are mixed, and complementary overhangs align, allowing DNA ligase to join them into a recombinant plasmid.

用同一种限制酶切割目的基因和质粒，产生兼容的粘性末端。混合后，互补突出的单链对齐，DNA 连接酶将它们连接成重组质粒。

To reduce self-ligation of the vector, alkaline phosphatase may be used to remove phosphate groups from the cut vector ends. Successful recombinants are then introduced into competent host cells for propagation.

为减少载体自身连接，可用碱性磷酸酶去除载体切口末端的磷酸基团。随后将成功重组的质粒导入感受态宿主细胞中进行扩增。

Recombinant DNA construction follows the sequence:

重组 DNA 构建遵循以下流程：

Vector + Insert → Restriction digest → Ligation → Recombinant plasmid

This sequence is central to Cambridge questions on genetic modification.

该顺序是剑桥考试中关于基因修饰的核心内容。

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5. Gene Transfer Methods | 基因转移方法

Bacterial cells can be made competent by treating them with cold calcium chloride followed by a brief heat shock. This creates temporary pores in the membrane, allowing plasmid DNA to enter.

可用冷氯化钙处理细菌细胞，

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