Stressful environmental conditions may cause some bacteria to increase mutation rates. This can actually be helpful to the organism, since mutations may sometimes give such bacteria new traits, such as the ability to consume a new food source or to resist a poison in the environment.

Mutagens Some mutations arise from mutagens, chemical or physical agents in the environment. Chemical mutagens include certain pesticides, a few natural plant alkaloids, tobacco smoke, and environmental pollutants. Physical mutagens include some forms of electromagnetic radiation, such as X-rays and ultraviolet light. If these agents interact with DNA, they can produce mutations at high rates. Cells can sometimes repair the damage; but when they cannot, the DNA base sequence changes permanently. Some compounds interfere with base-pairing, increasing the error rate of DNA replication. Others weaken the DNA strand, causing breaks and inversions that produce chromosomal mutations.

In Your Notebook Make a table to keep track of both the helpful and harmful results of mutations. As you read, fill it in.

Harmful and Helpful Mutations As you've already seen, some mutations don't even change the amino acid specified by a codon, while others may alter a complete protein or even an entire chromosome. The effects of mutations on genes vary widely. Some have little or no effect; and some produce beneficial variations. Some negatively disrupt gene function. Many if not most mutations are neutral; they have little or no effect on the expression of genes or the function of the proteins for which they code. Whether a mutation is negative or beneficial depends on how its DNA changes relative to the organism's situation. Mutations are often thought of as negative, since they can disrupt the normal function of genes. However, without mutations, organisms could not evolve, because mutations are the source of genetic variability in a species.

▸ Harmful Effects Some of the most harmful mutations are those that dramatically change protein structure or gene activity. The defective proteins produced by these mutations can disrupt normal biological activities, and result in genetic disorders. Some cancers, for example, are the product of mutations that cause the uncontrolled growth of cells. Sickle cell disease is a disorder associated with changes in the shape of red blood cells. You can see its effects in Figure 13–13. It is caused by a point mutation in one of the polypeptides found in hemoglobin, the blood's principal oxygen-carrying protein. Among the symptoms of the disease are anemia, severe pain, frequent infections, and stunted growth.

FIGURE 13–13 Effects of a Point Mutation Sickle cell disease affects the shape of red blood cells. The round cells in this false-colored SEM are normal red blood cells. The crescent and star-shaped cells are sickled cells. (SEM 1700X)

Table of Contents