Cell’s Need for Immense Amounts of Information
Cells face enormous problems in growing. We can develop some idea of the situation by considering a totally self-sufficient toolmaking shop. If we provide the shop with coal for energy and crude ores, analogous to a cell’s nutrient medium, then a very large collection of machines and tools is necessary merely to manufacture each of the parts present in the shop. Still greater complexity would be added if we required that the shop be totally self-regulating and that each machine be self-assem-bling. Cells face and solve these types of problems. In addition, each of the chemical reactions necessary for growth of cells is carried out in an aqueous environment at near neutral pH. These are conditions that would cripple ordinary chemists.
By the tool shop analogy, we expect cells to utilize large numbers of “parts,” and, also by analogy to factories, we expect each of these parts to be generated by a specialized machine devoted to production of just one type of part. Indeed, biochemists’ studies of metabolic pathways have revealed that an E. coli cell contains about 1,000 types of parts, or small molecules, and that each is generated by a specialized machine, an enzyme. The information required to specify the structure of even one machine is immense, a fact made apparent by trying to describe an object without pictures and drawings. Thus, it is reasonable, and indeed it has been found that cells function with truly immense amounts of information.
DNA is the cell’s library in which information is stored in its sequence of nucleotides. Evolution has built into this library the information necessary for cells’ growth and division. Because of the great value of the DNA library, it is natural that it be carefully protected and preserved. Except for some of the simplest viruses, cells keep duplicates of the information by using a pair of self-complementary DNA strands. Each strand contains a complete copy of the information, and chemical or physical damage to one strand is recognized by special enzymes and is repaired by making use of information contained on the opposite strand. More complex cells further preserve their information by pos-sessing duplicate DNA duplexes.
Much of the recent activity in molecular biology can be understood in terms of the cell’s library. This library contains the information necessary to construct the different cellular machines. Clearly, such a library contains far too much information for the cell to use at any one time. Therefore mechanisms have developed to recognize the need for particular portions, “books,” of the information and read this out of the library in the form of usable copies. In cellular terms, this is the regulation of gene activity.
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