DNA polymerase is fast and accurate, but it makes mistakes. For example, it might read A and place a C across from it where it should have placed a T. In Escherichia coli, a bacterial species in which DNA replication has been most thoroughly studied, about three errors occur for every 100,000 bases copied. At this rate of error, every generation of cells would have about 1,000 faulty proteins, coded for by DNA that had been miscopied. To help prevent such catastrophic damage to the organism, the DNA is continuously scanned for errors. After DNA polymerase has replicated a strand, a smaller polymerase comes along, "proofreads" it, and makes corrections where needed—for example, removing C and replacing it with T. This improves the accuracy of replication to one error per billion bases—only one faulty protein for every 10 cell divisions (in E. coli).
Changes in DNA structure, called mutations,1 can result from replication errors or environmental factors. Uncorrected mutations can be passed on to the descendants of that cell, but some of them have no adverse effect. One reason is that a new base sequence sometimes codes for the same thing as the old one. For example, ACC and ACG both code for threonine (see table 4.2), so a mutation from C to G in the third place would not change protein structure. Another reason is that a change in protein structure is not always critical to its function. For example, humans and horses differ in 25 of the 146 amino acids that make up their p hemoglobin, yet the hemoglobin is fully functional in both species. Some mutations, however, may kill a cell, turn it cancerous, or cause genetic defects in future generations. When a mutation changes the sixth amino acid of p hemoglobin from glutamic acid to valine, for example, the result is a crippling disorder called sickle-cell disease. Clearly some amino acid substitutions are more critical than others, and this affects the severity of a mutation.
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This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.