Some base-pair substitutions produce a stop codon. Many of these substitute an incorrect amino acid in the corresponding position in the encoded protein, and of these a large proportion result in altered protein function. The simplest kinds are changes to single base pairs, called base-pair substitutions. Changes within genes are called point mutations. Some silent mutations are in the DNA between genes, or they are of a type that results in no significant amino acid changes. On the other hand, many mutations are silent, showing no obvious effect at the functional level. A change in the DNA sequence of a gene’s regulatory region can adversely affect the timing and availability of the gene’s protein and also lead to serious cellular malfunction. Hence, a mutation that changes DNA sequence can change amino acid sequence and in this way potentially reduce or inactivate a protein’s function. Each amino acid is encoded by a unique sequence, or codon, of three of the four possible base pairs in the DNA (A–T, T–A, G–C, and C–G, the individual letters referring to the four nitrogenous bases adenine, thymine, guanine, and cytosine). Cells make 20 common amino acids, and it is the unique number and sequence of these that give a protein its specific function. A protein is a chain of usually several hundred amino acids. A gene is typically composed of a regulatory region, which is responsible for turning the gene’s transcription on and off at the appropriate times during development, and a coding region, which carries the genetic code for the structure of a functional molecule, generally a protein. A mutated form of a gene is called a mutant allele. The most serious changes take place in the functional units of DNA, the genes. The genome is composed of one to several long molecules of DNA, and mutation can occur potentially anywhere on these molecules at any time. Know how a single change in the DNA nucleotide results in mutation and why some mutations are harmful See all videos for this article In general, mutation is the main source of genetic variation, which is the raw material for evolution by natural selection. Because mutations are random changes, they are expected to be mostly deleterious, but some may be beneficial in certain environments. Mutations result either from accidents during the normal chemical transactions of DNA, often during replication, or from exposure to high-energy electromagnetic radiation (e.g., ultraviolet light or X-rays) or particle radiation or to highly reactive chemicals in the environment. Mutations in egg or sperm cells ( germinal mutations) may result in an individual offspring all of whose cells carry the mutation, which often confers some serious malfunction, as in the case of a human genetic disease such as cystic fibrosis. (The genomes of organisms are all composed of DNA, whereas viral genomes can be of DNA or RNA see heredity: The physical basis of heredity.) Mutation in the DNA of a body cell of a multicellular organism ( somatic mutation) may be transmitted to descendant cells by DNA replication and hence result in a sector or patch of cells having abnormal function, an example being cancer. Mutation, an alteration in the genetic material (the genome) of a cell of a living organism or of a virus that is more or less permanent and that can be transmitted to the cell’s or the virus’s descendants. SpaceNext50 Britannica presents SpaceNext50, From the race to the Moon to space stewardship, we explore a wide range of subjects that feed our curiosity about space!. Learn about the major environmental problems facing our planet and what can be done about them!
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