Mutation

A change in a cell’s DNA. Many mutations are harmless; however, some are harmful, giving rise to cancers, birth defects, and hereditary diseases. Very rarely, a mutation may be beneficial. A mutation results from a fault in the replication of DNA when a cell divides. A daughter cell inherits some faulty DNA, and the fault is copied each time the new cell divides, creating a cell population containing the altered DNA. Some mutations occur by chance. Any agent that makes mutations more likely is called a mutagen. There are several types of mutation. Point mutations affect only one gene and may lead to the production of defective enzymes or other proteins. In other mutations, chromosomes (or parts of them) are deleted, added, or rearranged. This type may produce greater disruptive effects than point mutations. If a mutated cell is a somatic (body) cell, it can, at worst, multiply to form a group of abnormal cells. These cells often die out, are destroyed by the body’s immune system, or have only a minor effect. Sometimes, however, they may become a tumour. A mutation in a germ cell (immature egg or sperm) may be passed on to a child, who then has the mutation in all of his or her cells. This may cause an obvious birth defect or an abnormality in body chemistry. The mutation may also be passed on to the child’s descendants. Genetic disorders (such as haemophilia and achondroplasia) stem from point mutations that occurred in the germ cell of a parent, grandparent, or more distant ancestor. Chromosomal abnormalities (such as Down’s syndrome) are generally due to mutations in the formation of parental eggs or sperm.