A change in the sequence of bases in DNA or RNA is called a mutation. Does the word mutation make you think of science fiction and bug-eyed monsters?
Think again. Everyone has mutations. In fact, most people have dozens or even hundreds of mutations in their DNA. Mutations are essential for evolution to occur. They are the ultimate source of all new genetic material new alleles in a species.
Although most mutations have no effect on the organisms in which they occur, some mutations are beneficial. Even harmful mutations rarely cause drastic changes in organisms.
The majority of mutations have neither negative nor positive effects on the organism in which they occur. These mutations are called neutral mutations. Examples include silent point mutations. They are neutral because they do not change the amino acids in the proteins they encode. Many other mutations have no effect on the organism because they are repaired before protein synthesis occurs. Cells have multiple repair mechanisms to fix mutations in DNA. If a cell’s DNA is permanently damaged and cannot be repaired, the cell is likely to be prevented from dividing.
Gene mutation and protein function
The amino acid sequence of a polypeptide is determined by the gene coding for it. A mutation in a gene may change the properties of an enzyme in two different ways:
• One of the amino acids at the active site may be replaced by another
• A change in an amino acid distant from the active site may change the three-dimensional structure of the protein, or it could render it less stable to heat. In some cases, the effects of a mutation can be explained in terms of a defective protein.
Sickle-cell anaemia is a genetic disease in which the haemoglobin is abnormal. It is particularly common in West Africa and amongst people of West African descent. In homozygotes, the haemoglobin becomes crystallised at low oxygen concentrations, causing the red cells to become sickle-shaped. As a result, capillaries may become blocked, and in addition the defective red cells are attacked by white corpuscles, causing severe anaemia. Heterozygotes are hardly affected, and are said to have the sickle-cell trait.
The cause is a single amino acid difference in the β-chain, valine replacing the normal glutamic acid at position #6. Glutamic acid is highly polar and thus strongly attracted to water, but valine is non-polar. As a result the haemoglobin molecules are more attracted to each other than to water, causing them to aggregate into crystals. This effect is greatest at the low oxygen concentrations that occur in capillaries.
Some mutations are characterized by abnormal temperature-sensitivity of certain body processes. Himalayan rabbits are white except for the ears, snout, feet and tail, which are black. The cause is the abnormal temperature-sensitivity of an enzyme involved in the synthesis of the black pigment melanin. Even at normal body temperature the enzyme is denatured, but at the extremities of the body where the temperature is several degrees lower, the enzyme remains active and pigment is produced. Siamese cats are another example.
Some mutations have a positive effect on the organism in which they occur. They are called beneficial mutations. They lead to new versions of proteins that help organisms adapt to changes in their environment. Beneficial mutations are essential for evolution to occur. They increase an organism’s changes of surviving or reproducing, so they are likely to become more common over time. There are several well-known examples of beneficial mutations. Here are just two:
1. Mutations in many bacteria that allow them to survive in the presence of antibiotic drugs. The mutations lead to antibiotic-resistant strains of bacteria.
2. A unique mutation is found in people in a small town in Italy. The mutation protects them from developing atherosclerosis, which is the dangerous buildup of fatty materials in blood vessels. The individual in which the mutation first appeared has even been identified.
Imagine making a random change in a complicated machine such as a car engine. The chance that the random change would improve the functioning of the car is very small. The change is far more likely to result in a car that does not run well or perhaps does not run at all.
By the same token, any random change in a gene’s DNA is likely to result in a protein that does not function normally or may not function at all. Such mutations are likely to be harmful. Harmful mutations may cause genetic disorders or cancer.
• A genetic disorder is a disease caused by a mutation in one or a few genes. A human example is cystic fibrosis. A mutation in a single gene causes the body to produce thick, sticky mucus that clogs the lungs and blocks ducts in digestive organs.
• Cancer is a disease in which cells grow out of control and form abnormal masses of cells. It is generally caused by mutations in genes that regulate the cell cycle. Because of the mutations, cells with damaged DNA are allowed to divide without limits. Cancer genes can be inherited.