阅读理解
The Central Dogma
Though it comes as no surprise that the composition of DNA between different organisms is different, it is not immediately obvious why the muscle cells, blood cells, and brain cells of any one particular vertebrate(脊椎动物) are so different in their structure and composition when the DNA of every one of their cells is identical. This is the key to one of the most exciting areas of modern cell biology. In different cell types, different sets of the total number of genes (genome) (基因组) are expressed. In other words, different regions of the DNA are "active" in the muscle cells, blood cells, and brain cells.
To understand how this difference in DNA activity can lead to differences in cell structure and composition, it is necessary to consider what is often known as the central dogma(法则) of molecular biology: "DNA makes RNA make protein. " In molecular terms, a gene is that portion of DNA that encodes for a single protein. The dictum" one gene makes one protein" has required some modification(改变) with the discovery that some proteins are composed of several different polypeptide(多肽) chains, but the "one gene makes one polypeptide" rule does hold.
DNA Contains the Blueprint for All Cell Proteins.
Messenger RNA is a precise copy (transcript) of the coded sequence of nucleic acid bases in DNA, and this message is translated into a unique protein molecule on specialist organelles (ribo-somes) present in the cytoplasm(细胞质) of all cells. Proteins(蛋白质), which are largely made up of carbon(C) ,hydrogen(H), oxygen(O) ,and nitrogen(N) ,are constructed from 20 different, common amino acids. The versatility of proteins, the workhorse molecules of the cell, stems from the immense variety of molecular shapes that can be created, by linking amino acids together in different sequences. The smaller proteins consist of only a few dozen amino acids, whereas the larger ones may contain in excess of 200 amino acids, all linked together in a linear(线状的) chain by peptide bonds.
As the proteins are released from the ribosome(核糖体) ,they fold into unique shapes, under the influence of chemical forces that depend on the particular sequence of amino acids. So the protein primary sequence , encoded in the gene and faithfully transcribed and translated into an amino acid chain, determines the three-dimensional structure of the emerging molecule. The human body possesses some 30 000 different kinds of proteins and several million copies of many of these. Each plays a specific role——for example, hemoglobin carries oxygen in the blood, actin(肌动蛋白) and myosin(肌球蛋白) interact to generate muscle movement, and acetylcholine (乙酸胆碱) receptor molecules mediate chemical transmission between nerve and muscle cells.
Enzymes—Protein Biocatalysts
An essential group of proteins—the enzymes (酵素)—act as biological catalysts (催化剂) and regulate all aspects of cell metabolism (新陈代谢). They enable breakdown of high-energy food molecules (carbohydrates) to provide energy for biological reactions, and they control the synthetic pathways that result in the generation of lipids(e.g., fats, cholesterol(胆固醇), and other vital membrane(膜) components), carbohydrates (碳水化合物) (sugars, starch (淀粉) , and cellulose (纤维素) , the key components of plant cell walls) ,and many vital small biomolecules essential for cell function.
单选题
The best title of this passage is______.
【正确答案】
A
【答案解析】B)教条、蛋白质和催化剂；C)DNA、蛋白质和催化剂；D)DNA、蛋白质和酶。第1部分谈了“the central dogma”，故C)、D)可以排除。第2部分谈的是DNA，不是谈论蛋白质，故B)可排除，A)为正确选项。
单选题
Which of the following statements about the central dogma is false?
【答案解析】A)从核糖体中释放的蛋白质组成特定的形状，因为它们的特定排列顺序。这句陈述错误，据“As the proteins are released from the ribosome，they fold into unique shapes，under the influence of chemical forces that depend on the particular sequence of amino acids．”不是由于蛋白质自己的顺序，而是由于“amino acides”的顺序。C)几百万份蛋白质执行着几百万种功能，错误。D)基因的顺序被转译成氨基酸链。这句陈述错误，据“So the protein primary sequence，encoded in the gene and faithfully transcribed and translated into an amino acid chain”是蛋白质的顺序，而不是基因的顺序被转译。B)出现的分子的结构为蛋白质最初的排列顺序所决定。正确，据“so the protein primary sequence…determines the three-dimensional structure of the emerging molecule．”
单选题
Which of the following Statements about Biocatalyst is false?
【正确答案】
D
【答案解析】A)细胞的新陈代谢由一组蛋白质调节。这句陈述正确，据“An essential group of proteins—the enzymes—act as biological catalysts and regulate all aspects of cell metabolism．”B)所有的催化酶作为生物催化剂起作用。据与答案A)相同的原文。C)胆固醇是一种类脂。据“lipids (e．g，fats，cholesterol，and other vital membrane Components)．”D)没有酶，化学反应不能发生。这句陈述是错误的，据“Though grouped together for their capacity to speed up chemical reactions that would proceed only very slowly at room temperature．”无酶，反应在室温下发生得很慢，而不是完全停止反应，故D)为正确选项。