When Genes Go Bad: Mutations and Disease
DNA is constantly subject to mutations, accidental changes in its code. Mutations can lead to missing or malformed proteins, and that can lead to disease.
Sickled blood cells (left) and normal blood cells.
Sickle Cell
These are the sickle-shaped blood cells of someone with sickle cell anemia, a genetic disease common among those of African descent.
Sickle cell anemia is the result of a point mutation, a change in just one nucleotide in the gene for hemoglobin. This mutation causes the hemoglobin in red blood cells to distort to a sickle shape when deoxygenated. The sickle-shaped blood cells clog in the capillaries, cutting off circulation.
Having two copies of the mutated genes cause sickle cell anemia, but having just one copy does not, and can actually protect against malaria - an example of how mutations are sometimes beneficial.
We all start out our lives with some mutations. These mutations inherited from your parents are called germ-line mutations. However, you can also acquire mutations during your lifetime. Some mutations happen during cell division, when DNA gets duplicated. Still other mutations are caused when DNA gets damaged by environmental factors, including UV radiation, chemicals, and viruses.
Few mutations are bad for you. In fact, some mutations can be beneficial. Over time, genetic mutations create genetic diversity, which keeps populations healthy. Many mutations have no effect at all. These are called silent mutations.
But the mutations we hear about most often are the ones that cause disease. Some well-known inherited genetic disorders include cystic fibrosis, sickle cell anemia, Tay-Sachs disease, phenylketonuria and color-blindness, among many others. All of these disorders are caused by the mutation of a single gene.
Most inherited genetic diseases are recessive, which means that a person must inherit two copies of the mutated gene to inherit a disorder. This is one reason that marriage between close relatives is discouraged; two genetically similar adults are more likely to give a child two copies of a defective gene.
Diseases caused by just one copy of a defective gene, such as Huntington's disease, are rare. Thanks to natural selection, these dominant genetic diseases tend to get weeded out of populations over time, because afflicted carriers are more likely to die before reproducing.
Scientists estimate that every one of us has between 5 and 10 potentially deadly mutations in our genes-the good news is that because there's usually only one copy of the bad gene, these diseases don't manifest.
Cancer usually results from a series of mutations within a single cell. Often, a faulty, damaged, or missing p53 gene is to blame. The p53 gene makes a protein that stops mutated cells from dividing. Without this protein, cells divide unchecked and become tumors.
