单项选择题

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. The best title of this passage is______.

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