Gene Expression | 基因表达考点精讲

📚 Gene Expression | 基因表达考点精讲

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. Understanding gene expression is fundamental to biology, linking genotype to phenotype. This article covers key concepts relevant to IB and CCEA syllabi, including transcription, translation, regulation (such as the lac operon and epigenetics), and mutations. Let us explore these mechanisms step by step.

基因表达是基因编码信息用于指导合成功能性基因产物（通常是蛋白质）的过程。理解基因表达是生物学的基础，它将基因型与表型联系起来。本文涵盖IB和CCEA教学大纲相关的重要概念，包括转录、翻译、调控（如乳糖操纵子和表观遗传学）以及突变。让我们逐步探索这些机制。

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

The central dogma describes the flow of genetic information: DNA → RNA → protein.

中心法则描述了遗传信息的流向：DNA → RNA → 蛋白质。

In transcription, a DNA template is used to synthesize messenger RNA (mRNA).

在转录中，以DNA为模板合成信使RNA（mRNA）。

In translation, the mRNA sequence is decoded to build a polypeptide chain.

在翻译中，mRNA序列被解码以构建多肽链。

Exceptions occur, such as reverse transcription in retroviruses, but the core principle remains key.

也存在例外，比如逆转录病毒中的反转录，但核心原则仍是关键。

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2. Transcription: DNA-Directed RNA Synthesis | 转录：DNA指导的RNA合成

Transcription begins when RNA polymerase binds to the promoter region, such as the TATA box in eukaryotes.

当RNA聚合酶结合到启动子区域（如真核生物中的TATA盒）时，转录就开始。

The enzyme unwinds the DNA double helix and uses one strand as the template (antisense strand) to assemble complementary RNA nucleotides.

该酶解开DNA双螺旋，并以一条链为模板（反义链）组装互补的RNA核苷酸。

RNA is synthesized in the 5′ → 3′ direction, meaning nucleotides are added to the 3′ end of the growing chain.

RNA沿5′ → 3’方向合成，即核苷酸添加到生长链的3’末端。

In prokaryotes, transcription occurs in the cytoplasm and often produces polycistronic mRNA.

在原核生物中，转录发生在细胞质中，并且常产生多顺反子mRNA。

In eukaryotes, transcription occurs in the nucleus and produces monocistronic mRNA that requires processing.

在真核生物中，转录发生在细胞核内，产生需要加工的单顺反子mRNA。

Termination occurs when RNA polymerase reaches a terminator sequence, causing the enzyme and nascent RNA to dissociate.

当RNA聚合酶到达终止子序列时，转录终止，酶与新生的RNA分离。

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3. Post-Transcriptional Modifications in Eukaryotes | 真核生物的转录后修饰

In eukaryotes, the primary transcript (pre-mRNA) undergoes three major modifications before leaving the nucleus.

在真核细胞中，初级转录物（前体mRNA）在离开细胞核前经历三种主要修饰。

A 5′ cap (a modified guanine nucleotide) is added, protecting the mRNA from degradation and aiding in ribosome binding.

添加5’帽（一种修饰的鸟嘌呤核苷酸），保护mRNA免于降解并帮助核糖体结合。

A poly-A tail (a string of adenine nucleotides) is added to the 3′ end, enhancing stability and export.

在3’末端添加poly-A尾（一串腺嘌呤核苷酸），增强稳定性和输出。

Splicing removes introns (non-coding regions) and joins exons (coding regions) together, carried out by the spliceosome.

剪接去除内含子（非编码区）并将外显子（编码区）连接在一起，由剪接体执行。

Alternative splicing allows a single gene to produce multiple protein variants by combining different exons.

可变剪接通过组合不同的外显子，使一个基因能产生多种蛋白质变体。

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4. The Genetic Code: Triplets & Degeneracy | 遗传密码：三联体与简并性

The genetic code is a set of rules by which information encoded in mRNA is translated into amino acid sequences.

遗传密码是一套将mRNA中编码的信息翻译为氨基酸序列的规则。

A codon consists of three consecutive nucleotides, each specifying one amino acid or a stop signal.

一个密码子由三个连续的核苷酸组成，每一个指定一个氨基酸或终止信号。

The code is degenerate: several codons can encode the same amino acid, minimizing the effect of some mutations.

密码子具有简并性：数个密码子可编码同一种氨基酸，从而减少某些突变的影响。

The code is also universal (across nearly all organisms), unambiguous, and non-overlapping.

密码子还具有通用性（几乎适用于所有生物）、无歧义性和非重叠性。

Start codon AUG codes for methionine and signals the beginning of translation. Stop codons (UAA, UAG, UGA) terminate translation.

起始密码子AUG编码甲硫氨酸并标志翻译的开始。终止密码子（UAA、UAG、UGA）终止翻译。

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5. Translation: From mRNA to Protein | 翻译：从mRNA到蛋白质

Translation occurs on ribosomes, which consist of a small and a large subunit made of rRNA and proteins.

翻译在核糖体上进行，核糖体由rRNA和蛋白质组成的大小亚基构成。

The process has three stages: initiation, elongation, and termination.

该过程分为三个阶段：起始、延伸和终止。

During initiation, the small ribosomal subunit binds to the mRNA and a specific initiator tRNA carrying methionine pairs with the start codon AUG.

在起始阶段，小亚基结合到mRNA上，携带甲硫氨酸的特定起始tRNA与起始密码子AUG配对。

The large subunit then joins, forming a functional ribosome with three sites: A (aminoacyl), P (peptidyl), and E (exit).

然后大亚基加入，形成有功能的核糖体，包含三个位点：A位（氨酰位）、P位（肽酰位）和E位（退出位）。

In elongation, a tRNA carrying the next amino acid enters the A site; a peptide bond forms between the amino acids in the P and A sites.

在延伸阶段，携带下一个氨基酸的tRNA进入A位；P位和A位上的氨基酸之间形成肽键。

The ribosome translocates along the mRNA, moving the empty tRNA to the E site for exit, and the peptidyl-tRNA to the P site.

核糖体沿mRNA移位，将空载tRNA移至E位排出，将肽酰-tRNA移至P位。

Termination occurs when a stop codon enters the A site; release factors bind, causing the polypeptide to be released.

当终止密码子进入A位时，翻译终止；释放因子结合，促使多肽释放。

Multiple ribosomes can translate a single mRNA simultaneously, forming a polysome.

多个核糖体可同时翻译同一条mRNA，形成多聚核糖体。

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6. Regulation of Gene Expression in Prokaryotes: The lac Operon | 原核生物基因表达调控：乳糖操纵子

Prokaryotes often control gene expression through operons—clusters of genes under the control of a single promoter.

原核生物常通过操纵子——由单个启动子控制的一组基因——来调控基因表达。

The lac operon in E. coli contains three structural genes: lacZ (β-galactosidase), lacY (permease), and lacA (transacetylase), alongside regulatory elements.

大肠杆菌的乳糖操纵子包含三个结构基因：lacZ（β-半乳糖苷酶）、lacY（通透酶）和lacA（转乙酰酶），以及调控元件。

In the absence of lactose, the lac repressor protein binds to the operator, blocking RNA polymerase from transcribing the structural genes.

在缺乏乳糖时，乳糖阻遏蛋白结合到操纵基因，阻止RNA聚合酶转录结构基因。

When lactose is present, it is converted to allolactose, which binds to the repressor and inactivates it, allowing transcription.

当乳糖存在时，它转变为别乳糖，别乳糖与阻遏蛋白结合并使其失活，从而允许转录。

The operon is also positively regulated by the catabolite activator protein (CAP). When glucose is scarce, cAMP levels rise; cAMP binds CAP, and the complex promotes RNA polymerase binding to the promoter.

该操纵子还受到分解代谢物激活蛋白（CAP）的正调控。当葡萄糖稀缺时，cAMP水平升高；cAMP与CAP结合，此复合物促进RNA聚合酶结合启动子。

Thus, maximum transcription occurs only when lactose is present and glucose is absent.

因此，只有当乳糖存在且葡萄糖缺乏时，转录水平才达到最高。

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7. Regulation of Gene Expression in Eukaryotes | 真核生物基因表达调控

Eukaryotic gene regulation is more complex, involving multiple levels: chromatin structure, transcription, post-transcription, translation, and post-translation.

真核基因调控更为复杂，涉及多个层面：染色质结构、转录、转录后、翻译和翻译后。

Transcription factors are proteins that bind to specific DNA sequences (enhancers, silencers, or promoter-proximal elements) to activate or repress transcription.

转录因子是结合特定DNA序列（增强子、沉默子或启动子邻近元件）以激活或抑制转录的蛋白质。

Chromatin remodeling, involving histone acetylation and methylation, can make DNA more or less accessible to the transcriptional machinery.

染色质重塑，包括组蛋白乙酰化和甲基化，可使DNA对转录装置的可接近性增加或减少。

DNA methylation (adding methyl groups to cytosine bases) is often associated with gene silencing.

DNA甲基化（在胞嘧啶碱基上添加甲基）通常与基因沉默相关。

Enhancers can be located far from the promoter and loop to interact with the transcription initiation complex via mediator proteins.

增强子可位于远离启动子的位置，并通过中介蛋白与转录起始复合物发生环状相互作用。

Post-transcriptional regulation includes alternative splicing, mRNA stability, and small regulatory RNAs (e.g., miRNA) that can degrade mRNA or block translation.

转录后调控包括可变剪接、mRNA稳定性以及能降解mRNA或阻断翻译的小调控RNA（如miRNA）。

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8. Epigenetics: Beyond DNA Sequence | 表观遗传学：超越DNA序列

Epigenetics refers to heritable changes in gene expression that do not involve alterations to the DNA sequence itself.

表观遗传学是指不涉及DNA序列本身改变的基因表达的可遗传变化。

Mechanisms include DNA methylation, histone modification, and non-coding RNAs, all of which influence chromatin structure and gene accessibility.

机制包括DNA甲基化、组蛋白修饰和非编码RNA，它们都影响染色质结构和基因可接近性。

Epigenetic marks can be influenced by environmental factors such as diet, stress, and toxins, and may be passed to offspring.

表观遗传标记可受环境因素（

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