Cellular Control Exam Practice | 细胞调控真题精练

📚 Cellular Control Exam Practice | 细胞调控真题精练

Cellular control mechanisms ensure that genes are expressed at the right time, in the right cell, and in the right amount. This article provides targeted exam practice on key topics such as the lac operon, transcription factors, epigenetics, and post-transcriptional regulation, helping you master the skills needed to tackle A-level Biology questions.

细胞调控机制确保基因在合适的时间、合适的细胞中以合适的量表达。本文针对乳糖操纵子、转录因子、表观遗传学和转录后调控等关键主题提供定向真题练习，帮助你掌握应对 A-level 生物学问题的能力。

1. Overview of Cellular Control | 细胞调控概述

Cellular control allows organisms to respond to environmental changes and to differentiate cells. Gene expression is regulated at multiple levels: transcriptional, post-transcriptional, translational, and post-translational.

细胞调控使生物体能够对环境变化作出反应并实现细胞分化。基因表达在多个层面受到调控：转录水平、转录后水平、翻译水平和翻译后水平。

In prokaryotes, genes are often arranged in operons, which are transcribed into a single polycistronic mRNA. In eukaryotes, chromatin structure, transcription factors, and extensive RNA processing provide more complex control.

在原核生物中，基因通常排列成操纵子，转录为一条多顺反子mRNA。在真核生物中，染色质结构、转录因子以及复杂的RNA加工提供了更精细的调控。

2. Lac Operon: Structure and Regulation | 乳糖操纵子：结构与调控

The lac operon of E. coli consists of a promoter (P), operator (O), and three structural genes: lacZ (β-galactosidase), lacY (permease), and lacA (transacetylase). A separate regulatory gene, lacI, codes for the lac repressor.

大肠杆菌的乳糖操纵子由启动子（P）、操纵基因（O）和三个结构基因组成：lacZ（β-半乳糖苷酶）、lacY（透性酶）和 lacA（转乙酰酶）。另一个调节基因 lacI 编码 lac 阻遏蛋白。

When lactose is absent, the repressor protein binds to the operator, blocking RNA polymerase from transcribing the structural genes. When lactose is present, it is converted to allolactose, which binds to the repressor and causes a conformational change, preventing it from binding to the operator. This allows transcription to proceed.

当乳糖不存在时，阻遏蛋白与操纵基因结合，阻止RNA聚合酶转录结构基因。当乳糖存在时，它被转化为别乳糖，别乳糖与阻遏蛋白结合并引起构象改变，使其无法与操纵基因结合，从而使转录得以进行。

However, RNA polymerase binds weakly to the lac promoter. For high levels of transcription, the cAMP-CAP complex (catabolite activator protein) must bind near the promoter. Glucose inhibits adenylate cyclase, so when glucose is scarce, cAMP levels rise, CAP binds, and transcription is activated.

然而，RNA聚合酶与lac启动子的结合较弱。要实现高水平转录，cAMP-CAP复合物（分解代谢物激活蛋白）必须结合在启动子附近。葡萄糖会抑制腺苷酸环化酶，因此当葡萄糖缺乏时，cAMP水平升高，CAP结合，从而激活转录。

Exam-style question: Explain how the lac operon ensures that the enzymes for lactose metabolism are produced only when needed. (5 marks)

真题练习：解释乳糖操纵子如何确保乳糖代谢酶仅在需要时才产生。（5分）

Model answer point 1: The lac repressor protein binds to the operator in the absence of lactose, blocking transcription.

答案要点1：在缺乏乳糖时，lac阻遏蛋白与操纵基因结合，阻断转录。

Point 2: When lactose is present, allolactose binds to the repressor, inactivating it, so RNA polymerase can transcribe lacZ, lacY, and lacA.

要点2：当乳糖存在时，别乳糖与阻遏蛋白结合使其失活，于是RNA聚合酶可以转录lacZ、lacY和lacA。

Point 3: The positive control by cAMP-CAP ensures that transcription only occurs when glucose is low, as cAMP levels increase and CAP binds to the promoter, enhancing RNA polymerase binding.

要点3：cAMP-CAP的正向调控确保转录仅在葡萄糖水平低时发生——此时cAMP水平升高，CAP与启动子结合，增强RNA聚合酶的结合。

Published by TutorHao | Biology Revision Series | aleveler.com

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