Mutations and Genetic Variation

Mutations and Genetic Variation Getting the Idea An organism's traits, or characteristics, are determined by the genetic information in its DNA. If something changes the nucleotide sequence in the DMA passed from one generation to the next, new traits may appear in the offspring. In some cases, these new traits affect the health or fitness of the offspring. Mutations Recall that before a cell divides, its chromosomes and DNA replicate, or copy themselves. DNA usually replicates without error, producing new cells whose genes are identical to those of the parent cell. However, DNA does not always replicate correctly. Incorrect replication results in a mutation, a change in a gene or chromosome. Mutations can occur for many reasons. Some mutations are caused by spontaneous errors during replication. For example, cytosine may incorrectly pair with thymine, or adenine with cytosine. Cells have enzymes that fix errors during DNA replication. However, these enzymes are imperfect. They can miss errors and can sometimes change a correct nucleotide to an incorrect one. Environmental factors can damage chromosomes. For example, ultraviolet radiation from the sun, X-rays, and ionizing radiation can cause mutations. Certain chemicals, including some substances in tobacco, can also cause mutations. A mutagen is any agent that changes the DNA of an organism. In addition to chemicals and radiation, viruses can be mutagens. A virus is a tiny particle of genetic material that is surrounded by a protein coat. Viruses become mutagens when they enter a cell and insert their genes into its DNA. Mutation is the source of variation and new traits in organisms that reproduce asexually. However, recall that many organisms, including most plants and nearly all animals, reproduce sexually. In these organisms, variation results from both meiosis and mutation. Kinds of Mutations There are different kinds of mutations, but they all change a cell's DNA. Recall that sequences of three bases in DNA code for specific amino acids. In mRNA, the complementary sequence is called a codon. A mutation in DNA changes the base sequence. As a result, a different amino acid or series of amino acids is incorporated into the protein made after transcription and translation. A mutation can affect a single nucleotide or a large segment of DNA. Three common types of mutations are base-pair substitutions, base insertions, and base deletions. A base-pair substitution is a mutation in which one nucleotide base replaces another. In the diagram below, a DNA segment with the bases CTT should form the mRNA codon GAA, which produces glutamic acid. However, in this example, adenine replaces one thymine in the DNA segment. The altered DNA forms the mRNA codon QUA, which produces valine. Base-pair substitutions affect only one codon, so they change only one amino acid in a peptide chain. That change may or may not have serious effects on the organism. The effect depends on where the mutation is and how it affects the protein for which it codes. A base-pair substitution changes only one amino acid. By contrast, base insertions and deletions can change many amino acids in a peptide chain by shifting the entire sequence of codons. A base insertion is the addition of one or more bases into a nucleotide sequence. Consider two codons in sequence: GAC and UAC. GAC codes for aspartic acid. UAC codes for tyrosine. However, if a G is inserted between the two original codons, the new codon sequence will be GAC GUA. Because QUA codes for leucine, leucine rather than tyrosine will be inserted into the protein. This insertion mutation will alter the rest of the peptide chain because the C will be attached to the next codon, and so on. The diagram below shows a base insertion in DNA that would change the codon UAC to GUA. Base deletion removes one or more bases from a nucleotide sequence. Consider again the codons GAC and UAC. If the first G is deleted, the first codon becomes ACU. That codon produces threonine. Again, a different amino acid is put into the peptide chain. As with insertion, this change affects the rest of the peptide chain. The diagram below shows this base deletion in a DNA sequence. Base-pair substitutions, base insertions, and base deletions affect only a single gene or segment of DNA. A chromosomal mutation affects an entire chromosome. Chromosomal mutations change the number of genes in a chromosome or the organization of the chromosome. Nondisjunction is a chromosomal mutation that produces an offspring with an incorrect number of chromosomes. Nondisjunction occurs when a pair of chromosomes is not pulled apart during gamete formation in a parent cell. This causes the resulting zygote to have too many or too few of the affected chromosome. Nondisjunction of sex chromosomes can result in Kleinfelter syndrome, in which an individual has a genotype of XXY. It can also produce Turner syndrome, in which an individual has a genotype of XO, meaning that the individual has only one sex chromosome, an X. People with either of these syndromes are usually infertile—that is, unable to produce offspring. Recall from Lesson 22 that a disorder called Down syndrome results when someone has three copies of chromosome 21. Down syndrome is an example of an autosomal disorder resulting from nondisjunction. An autosome is a chromosome other than a sex chromosome. Effects of Mutations Many organisms have two types of cells: body cells and gametes (sex cells). Mutations in body cells are called somatic mutations. Somatic mutations are not passed from parent to offspring. They can, however, dramatically affect the individual in which they occur. Some cancers, such as skin and lung cancers, are caused by mutations to the genes that control the rate of cell division. Most skin cancer is caused by damage from ultraviolet radiation in sunlight. In most cases of lung cancer, genes were damaged by chemicals in tobacco. Asbestos, radiation, and some air pollutants can also alter cells and cause lung cancer. Recall that a virus acts as a mutagen when it injects its DNA or RNA into the host cell's DNA. The viral DNA or RNA can direct the synthesis of new viruses in the cell. Eventually the new viruses cause the host cell to burst, killing the cell and releasing viruses that can infect and kill other cells. Depending on the virus, this process can lead to a short-term illness, severe disease, or even death. Some viruses can change a cell's DNA in ways that can cause cancer. For example, the human papillomavirus (HPV) causes cervical cancer.