Gene Expression | 基因表达

📚 Gene Expression | 基因表达

Gene expression is the process by which the information encoded in a gene is used to direct the synthesis of a functional gene product, typically a protein. It involves two major steps: transcription, where DNA is copied into messenger RNA (mRNA), and translation, where the mRNA sequence is decoded to build a polypeptide. In both IB Biology and AQA A-level Biology, a deep understanding of the molecular mechanisms, regulation, and consequences of mutations is essential. This article unpacks these concepts with clarity, providing a robust revision guide for exam success.

基因表达是基因中编码的信息被用来指导合成功能性基因产物（通常是蛋白质）的过程。它包括两个主要步骤：转录（DNA 被拷贝成信使 RNA）和翻译（根据 mRNA 序列解码合成多肽）。在 IB 生物学和 AQA A-level 生物学中，深入了解分子机制、调控以及突变的影响至关重要。本文将清晰地解析这些概念，为考试成功提供坚实的复习指南。

1. Central Dogma of Molecular Biology | 分子生物学中心法则

The central dogma describes the flow of genetic information within a biological system: DNA → RNA → protein. During transcription, an RNA polymerase enzyme synthesises a complementary RNA strand from a DNA template. Translation then reads the mRNA in triplets (codons) to assemble a specific sequence of amino acids. Note that in retroviruses, reverse transcription (RNA → DNA) occurs, which is an exception covered in both syllabuses.

中心法则描述了生物系统中遗传信息的流动：DNA → RNA → 蛋白质。在转录过程中，RNA 聚合酶从 DNA 模板合成一条互补的 RNA 链。随后，翻译以三个碱基为一组（密码子）读取 mRNA，组装特定的氨基酸序列。注意在逆转录病毒中，会发生逆转录（RNA → DNA），这是两个课程大纲都涉及的一个例外情况。

Key enzymes: RNA polymerase for transcription, ribosomes (containing rRNA and proteins) for translation. The process is highly conserved across all domains of life.

关键酶：转录所需的 RNA 聚合酶，翻译所需的核糖体（含 rRNA 和蛋白质）。该过程在所有生物域中高度保守。

2. Transcription: From DNA to mRNA | 转录：从 DNA 到 mRNA

Transcription begins when RNA polymerase binds to a promoter region upstream of the gene. In prokaryotes, a sigma factor assists binding, while in eukaryotes, basal transcription factors (e.g., TFIID) are required. RNA polymerase unwinds the DNA double helix and reads the template strand in the 3′ → 5′ direction, synthesising a complementary mRNA strand in the 5′ → 3′ direction. The nucleotide sequence is identical to the coding strand (with uracil replacing thymine).

转录起始于 RNA 聚合酶与基因上游启动子区域的结合。在原核生物中，σ 因子协助结合；而在真核生物中，需要基础转录因子（如 TFIID）。RNA 聚合酶解开 DNA 双螺旋，以 3′ → 5′ 方向读取模板链，并以 5′ → 3′ 方向合成互补的 mRNA 链。其核苷酸序列与编码链相同（但尿嘧啶取代了胸腺嘧啶）。

Termination occurs either via a hairpin loop (prokaryotes) or cleavage and polyadenylation signal (eukaryotes). Remember that AQA emphasises the role of DNA–RNA hybrid duplex formation, while IB expects students to draw and label transcription stages.

终止通过发夹环（原核生物）或切割与多聚腺苷酸化信号（真核生物）实现。记住，AQA 强调 DNA-RNA 杂交双链的形成，而 IB 要求学生绘制并标注转录阶段。

3. RNA Processing in Eukaryotes | 真核生物的 RNA 加工

Primary transcripts (pre-mRNA) in eukaryotes undergo processing before leaving the nucleus. A 5′ cap (7-methylguanosine) is added, protecting the mRNA from degradation and aiding ribosome binding. A poly-A tail (around 200 adenines) is added to the 3′ end. Most importantly, splicing removes introns (non-coding sequences) and joins exons (coding sequences) together. The spliceosome (a complex of snRNPs) catalyses this process.

真核生物的初级转录本（前体 mRNA）在离开细胞核前需要经过加工。5′ 端添加帽子结构（7-甲基鸟苷），保护 mRNA 免受降解并协助核糖体结合；3′ 端添加 poly-A 尾巴（约 200 个腺嘌呤）。最为重要的是，剪接过程去除内含子（非编码序列），并将外显子（编码序列）连接在一起。剪接体（由 snRNP 组成的复合物）催化这一过程。

Alternative splicing allows a single gene to produce multiple protein variants by joining different combinations of exons. This significantly increases proteomic diversity, a high-yield concept for both IB and AQA exams.

选择性剪接通过不同的外显子组合方式，使一个基因能够产生多种蛋白质变体。这极大增加了蛋白质组的多样性，是 IB 和 AQA 考试中的高频考点。

4. The Genetic Code and tRNA Activation | 遗传密码与 tRNA 活化

The genetic code is degenerate, universal, and non-overlapping. Each codon (three mRNA nucleotides) specifies one amino acid. Start codon AUG codes for methionine. Stop codons (UAA, UAG, UGA) signal termination. Wobble hypothesis explains how some tRNAs can recognise more than one codon due to flexibility in the third base pair.

遗传密码具有简并性、通用性和非重叠性。每个密码子（mRNA 上的三个核苷酸）编码一个氨基酸。起始密码子 AUG 编码甲硫氨酸；终止密码子（UAA、UAG、UGA）发出终止信号。摆动假说解释了由于第三个碱基配对的灵活性，某些 tRNA 能够识别多个密码子。

tRNA molecules must be charged with their specific amino acid by aminoacyl-tRNA synthetases in an ATP-dependent reaction. This ‘tRNA activation’ is a key step highlighted in both syllabuses; students should be able to outline the process and its importance for translation fidelity.

tRNA 分子必须通过氨酰 tRNA 合成酶在 ATP 供能的反应中，装载上特定的氨基酸。这一“tRNA 活化”步骤是两个大纲强调的关键环节；学生应能描述该过程及其对翻译保真度的重要性。

5. Translation: Polypeptide Synthesis | 翻译：多肽合成

Translation occurs on ribosomes, consisting of initiation, elongation, and termination. The small ribosomal subunit binds to the mRNA, and the initiator tRNA^Met pairs with the start codon. The large subunit joins, forming the translation complex. During elongation, incoming aminoacyl tRNA binds to the A site, a peptide bond forms between the amino acids in the P and A sites (catalysed by peptidyl transferase), and the ribosome translocates. Termination occurs when a stop codon enters the A site, recognised by release factors, causing the polypeptide to be released.

翻译在核糖体上进行，包括起始、延长和终止阶段。核糖体小亚基与 mRNA 结合，起始 tRNA^Met 与起始密码子配对；大亚基加入后形成翻译复合物。延长阶段中，进位的氨酰 tRNA 结合到 A 位，P 位和 A 位氨基酸之间形成肽键（由肽基转移酶催化），随后核糖体移位。当终止密码子进入 A 位，被释放因子识别，多肽链即被释放。Published by TutorHao | IB Biology Revision Series | aleveler.com

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