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.
Mutations that occur during DNA replication or recombination, or that are due to an unknown cause, are said to be spontaneous. Most, however, are induced by external agents called mutagens. There are three classes of mutagen.
• Radiation – such as X-rays, γ-rays, α-rays, β-rays and neutrons. These are called ionising radiation because they break chemical bonds, producing ions or other particles that are so reactive that they combine with almost any molecule with which they come into contact. If this happens to be DNA, then one base may be changed into another.
Ionising radiation therefore acts indirectly on DNA.Ultraviolet light on the other hand specifically damages DNA. Where two thymine bases occur next to each other along one of the two DNA strands, they can become linked to form a thymine dimer. The dimer can be replicated as a single base, which results in a frameshift, possibly mutation, possibly resulting in skin cancer.
• Chemicals – there are hundreds of chemical mutagens, which act in different ways. Some change one base into another, e.g. nitrous acid changes cytosine into uracil. Since this pairs with adenine, the effect is to change a C–G pair into an A–T pair. Substances such as 5-bromouracil and 2-amino purine are called base analogues because they are sufficiently like the normal DNA bases to be incorporated into replicating DNA, but have slightly different pairing properties. As a result, the next generation of DNA contains the wrong base. Acridine dyes, which become lodged in the DNA helix and interfere with replication, causing an extra base to be inserted, i.e. a frame-shift mutation.
• Viruses – some viruses insert their DNA into that of the host cell. In doing so they may disrupt host genes, causing mutation. The hepatitis B virus produces liver cancer in some people many years after the original infection.
Types of Gene Mutations
Insertion: An insertion changes the number of DNA bases in a gene by adding a piece of DNA. As a result, the protein made by the gene may not function properly.
Nonsense mutation: A nonsense mutation is also a change in one DNA base pair. Instead of sub – stituting one amino acid for another, however, the altered DNA sequence prematurely signals the cell to stop building a protein. This type of mutation results in a shortened protein that may function improperly or not at all.
Deletion mutation: A deletion changes the number of DNA bases by removing a piece of DNA. Small deletions may remove one or a few base pairs within a gene, while larger deletions can remove an entire gene or several neighboring genes. The deleted DNA may alter the function of the resulting protein(s).
Repeat expansion: Nucleotide repeats are short DNA sequences that are repeated a number of times in a row. For example, a trinucleotide repeat is made up of 3-basepair sequences, and a tetranucleotide repeat is made up of 4-base-pair sequences. A repeat expansion is a mutation that increases the number of times that the short DNA sequence is repeated. This type of mutation can cause the resulting protein to function improperly.
Frameshift mutation: This type of mutation occurs when the addition or loss of DNA bases changes a gene’s reading frame. A reading frame consists of groups of 3 bases that each code for one amino acid. A frameshift mutation shifts the grouping of these bases and changes the code for amino acids. The resulting protein is usually nonfunctional. Insertions, deletions, and duplications can all be frameshift mutations.
Duplication: A duplication consists of a piece of DNA that is abnormally copied one or more times. This type of mutation may alter the function of the resulting protein.
“Silent” mutation: does not change an amino acid, but in some cases can still have a phenotypic effect, e.g., by speeding up or slowing down protein synthesis, or by affecting splicing.
Substitution: base is replaced by one of the other three bases
Inversion: 180° rotation of piece of DNA
Reciprocal translocation: parts of nonhomologous chromosomes change places
Chromosomal rearrangements: affect many genes at one time