📚 Gene Expression: GCSE Biology Key Points | GCSE 生物：基因表达 考点精讲
Gene expression is the fundamental process by which the genetic code stored in DNA is decoded to produce proteins, the workhorses of the cell. Understanding gene expression is essential for GCSE Biology as it explains how traits are inherited, why mutations can cause disease, and how organisms develop and function.
基因表达是将储存在 DNA 中的遗传密码解码以产生蛋白质这一细胞重要分子的基本过程。理解基因表达对 GCSE 生物学至关重要，因为它解释了性状如何遗传、突变为何会导致疾病以及生物体如何发育和行使功能。
1. Overview of Gene Expression | 基因表达概述
Gene expression involves two main stages: transcription and translation. In transcription, a gene's DNA sequence is copied into a messenger RNA (mRNA) molecule. In translation, the mRNA is read by ribosomes to assemble amino acids into a polypeptide chain, which folds into a functional protein.
基因表达包括两个主要阶段：转录 和 翻译。在转录中，基因的 DNA 序列被复制成信使 RNA (mRNA) 分子。在翻译中，核糖体读取 mRNA，将氨基酸组装成多肽链，进而折叠成功能性蛋白质。
This flow of genetic information is often described by the central dogma of molecular biology: DNA makes RNA makes protein. It is unidirectional – information cannot flow back from protein to nucleic acids.
遗传信息的这种流动通常被描述为分子生物学的中心法则：DNA 制造 RNA，RNA 制造蛋白质。这是单向的——信息不能从蛋白质逆向流回核酸。
DNA → mRNA → Protein
2. DNA and Genes: The Blueprint | DNA 与基因：生命的蓝图
DNA (deoxyribonucleic acid) is a double-stranded helix made of nucleotides. Each nucleotide consists of a sugar (deoxyribose), a phosphate group, and one of four nitrogenous bases: adenine (A), thymine (T), cytosine (C), and guanine (G). The bases pair specifically – A with T, and C with G – held by hydrogen bonds.
DNA（脱氧核糖核酸）是由核苷酸组成的双螺旋结构。每个核苷酸包含一个糖（脱氧核糖）、一个磷酸基团和四种含氮碱基之一：腺嘌呤（A）、胸腺嘧啶（T）、胞嘧啶（C）和鸟嘌呤（G）。碱基通过氢键特异性配对——A 与 T 配对，C 与 G 配对。
A gene is a specific sequence of DNA bases that codes for a particular protein or functional RNA molecule. In eukaryotes, genes are located on chromosomes inside the nucleus. Not all DNA is coding; some regions are non-coding and play regulatory roles.
基因是编码特定蛋白质或功能性 RNA 分子的特定 DNA 碱基序列。在真核生物中，基因位于细胞核内的染色体上。并非所有 DNA 都是编码区；有些区域是非编码的，起调控作用。
3. Transcription: Copying the Code | 转录：复制密码
Transcription occurs in the nucleus. The enzyme RNA polymerase different: adenine pairs with uracil (U) in RNA instead of thymine; so A→U, T→A, C→G, G→C.
转录发生在细胞核中。酶 RNA 聚合酶 结合到称为启动子的区域，解开 DNA 双螺旋，并利用其中一条链（模板链）合成一条互补的 mRNA 分子。碱基配对规则略有不同：在 RNA 中，腺嘌呤与尿嘧啶（U）配对，而不是胸腺嘧啶；因此 A→U、T→A、C→G、G→C。
Once the mRNA strand is complete, it detaches and undergoes processing in eukaryotic cells: a 5' cap and a poly-A tail are added, and introns (non-coding sections) are removed by splicing, leaving only exons (coding sections). The mature mRNA then exits the nucleus through a nuclear pore into the cytoplasm.
一旦 mRNA 链完成，它会脱离模板，在真核细胞中经过加工：添加 5' 帽和 poly-A 尾，并通过剪接去除内含子（非编码片段），只留下外显子（编码片段）。成熟的 mRNA 随后通过核孔离开细胞核进入细胞质。
4. Translation: Building Proteins | 翻译：构建蛋白质
Translation takes place on ribosomes in the cytoplasm. The ribosome reads the mRNA sequence in groups of three bases called codons. Each codon corresponds to a specific amino acid or a stop signal. Transfer RNA (tRNA) molecules carry amino acids and have an anticodon that is complementary to the mRNA codon.
翻译在细胞质中的核糖体上进行。核糖体以三个碱基为一组（称为密码子）读取 mRNA 序列。每个密码子对应于一个特定的氨基酸或终止信号。转运 RNA（tRNA）分子携带氨基酸，并具有与 mRNA 密码子互补的反密码子。
The ribosome binds to the mRNA at the start codon (AUG). tRNA molecules bring the correct amino acids, and peptide bonds form between adjacent amino acids, creating a polypeptide chain. The process continues until a stop codon (UAA, UAG, or UGA) is reached, at which point the polypeptide is released and folds into its specific three-dimensional shape.
核糖体在起始密码子（AUG）处与 mRNA 结合。tRNA 分子带来正确的氨基酸，相邻氨基酸之间形成肽键，形成多肽链。该过程持续进行，直到遇到终止密码子（UAA、UAG 或 UGA），此时多肽被释放并折叠成特定的三维形状。
5. The Genetic Code and Codons | 遗传密码与密码子
The genetic code is degenerate, meaning that multiple codons can code for the same amino acid. For example, both UUU and UUC code for phenylalanine. The code is nearly universal across all life forms, which is why human genes can be expressed in bacteria. Below is a simplified codon table showing some common amino acids and their mRNA codons.
遗传密码具有简并性，意味着多个密码子可以编码同一种氨基酸。例如，UUU 和 UUC 都编码苯丙氨酸。该密码在几乎所有生命形式中几乎是通用的，这就是人类基因可以在细菌中表达的原因。下表是一个简化的密码子表，显示了一些常见氨基酸及其 mRNA 密码子。
| Codon (mRNA) | Amino Acid | Abbreviation |
|---|---|---|
| AUG | Methionine (Start) | Met |
|
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