Mutation In this chapter you learn about n

Mutation In this chapter you learn about n germinal mutation. n somatic mutations n Spontaneous and Induced Mutations n Morphological mutants n Biochemical mutations

Mutaion n n A mutation is any change in the sequence of the DNA encoding a gene. Our studies of mutations has lead to a better understanding about the nature of genes. Originally genes were thought to be beads-on -string, where each bead was a single entity responsible for a phenotype. This theory leads to the concept that only single mutation was possible for a specific gene. Detailed genetic experiments proved that the gene actually consists of many individual units, and specific changes in these units can lead to several mutant phenotypes.

Germinal and Somatic Mutations n n Eukaryotic organisms have two primary cell types--germ and somatic. Mutations can occur in either cell type. If a gene is altered in a germ cell, the mutation is termed a germinal mutation. Because germ cells give rise to gametes, some gametes will carry the mutation and it will be passed on to the next generation when the individual successfully mates. Germinal mutations are not expressed in the individual containing the mutation. The only instance in which it would be expressed is if it negatively (or positively) affected gamete production.

Germinal and Somatic Mutations n n Mutations in somatic cells are called somatic mutations. Because they do not occur in cells that give rise to gametes, the mutation is not passed along to the next generation by sexual means. To maintain this mutation, the individual containing the mutation must be cloned. Two example of somatic clones are navel oranges and red delicious apples. Horticulturists first observed the mutants. They then grafted mutant branches onto the stocks of "normal" trees. After the graft was established, cuttings from that original graft were grafted onto tree stocks. In this way the mutation was maintained and proliferated.

Germinal and Somatic Mutations n n Eukaryotic organisms have two primary cell types- germ and somatic. Mutations can occur in either cell type. If a gene is altered in a germ cell, the mutation is termed a germinal mutation. Because germ cells give rise to gametes, some gametes will carry the mutation and it will be passed on to the next generation when the individual successfully mates. Typically germinal mutations are not expressed in the individual containing the mutation. The only instance in which it would be expressed is if it negatively (or positively) affected gamete production.

Cancer tumors n Cancer tumors are a unique class of somatic mutations. The tumor arises when a gene involved in cell division, a protooncogene, is mutated. All of the daughter cells contain this mutation. The phenotype of all cells containing the mutation is uncontrolled cell division. This results in a tumor that is a collection of undifferentiated cells called tumor cells.

Spontaneous and Induced Mutations n n In general, the appearance of a new mutation is a rare event. Most mutations that were originally studied occurred spontaneously. This class of mutation is termed spontaneous mutations. But these mutations clearly represent only a small number of all possible mutations. New mutations were created by scient ists by treating an organism with a mutagenizing agent. These mutations are called induced mutations.

Spontaneous and Induced Mutations n n n The spontaneous mutation rate varies. Large gene provide a large target and tend to mutate more frequently. A study of the five coat color loci in mice showed that the rate of mutation ranged from 2 x 10 -6 to 40 x 10 -6 mutations per gamete per gene. Data from several studies on eukaryotic organisms shows that in general the spontaneous mutation rate is 2 -12 x 10 -6 mutations per gamete per gene. Given that the human genome contains 100, 000 genes, we can conclude that we would predict that 1 -5 human gametes would contain a mutation in some gene.

Spontaneous and Induced Mutations n n radiation Mutations can be induced by several methods. The three general approaches used to generate mutations are radiation, chemicals and transposon insertion. The first induced mutations were created by treating Drosophila with X-rays. In addition to Xrays, other types of radiation treatments that have proven useful include gamma rays and fast neutron bombardment. These treatments can induce point mutations (changes in a single nucleotide) or deletions (loss of a chromosomal segment).

Spontaneous and Induced Mutations n n n Chemical mutagens work mostly by inducing point mutations. Point mutations occur when a single base pair of a gene is changed. These changes are classified as transitions or transversions. Transitions occur when a purine is converted to a purine (A to G or G to A) or a pyrimide is converted to a pyrimidine (T to C or C to T). A transversion results when a purine is converted to a pyrimidine or a pyrimidine is converted to a purine. A transversion example is adenine being converted to a cytosine. You can determine other examples.

Spontaneous and Induced Mutations n n Chemicals Two major classes of chemical mutagens are routinely used: alkylating agents and base analogs. Alkylating agents [such as ethyl methane sulphonate (EMS), ethyl ethane sulphonate (EES) and mustard gas can mutate both replicating and non-replicating DNA. By contrast, a base analog (5 -bromouracil and 2 aminopurine) only mutate DNA when the analog is incorporated into replicating DNA. Each class of chemical mutagen has specific effects that can lead to transitions, transversions or deletions.

Spontaneous and Induced Mutations n n n Transposable elements Scientists are now using the power of transposable elements to create new mutations. Transposable elements are mobile pieces of DNA that can move from one location in a geneome to another. Often when they move to a new location, the result is a new mutant. The mutant arises because the presence of a piece of DNA in a wild type gene disrupts the normal function of that gene.

Spontaneous and Induced Mutations n n Stocks of transposable elements are created in which a specific type of element is present. This stock is then crossed to a genetic stock that does not contain the element. Once that element enters the virgin stock, it can begin to move around that genome. By carefully observing the offspring, new mutants can be discovered. This approach to developing mutants is termed insertional mutagenesis.

Types of Mutations n n n Morphological mutants affect the outward appearance of an individual. Plant height mutations could changes a tall plant to a short one. Biochemical mutations have a lesion in one specific step of an enzymatic pathway. For bacteria, biochemical mutants need to be grown on a media supplemented with a specific nutrient. Such mutants are called auxotrophs. Often though, morphological mutants are the direct result of a mutation in a biochemical pathway. In mouse, albinism, is the result of a mutation in the pathway; converts the amino acid tyrosine to the skin pigment melanin. Therefore, in a strict genetic sense, if appropriate experiments are performed, a morphological mutation can be explained at the biochemical level.

Types of Mutations n n For some mutations to be expressed, the individual needs to be placed in a specific environment. This is called the restrictive condition. But if the individual grow in any other environment (permissive condition), the wild type phenotype is expressed. These are called conditional mutations. Mutations that only expressed at a specific temperature (temperature sensitive mutants), usually elevated, can be considered to be conditional mutations.

Types of Mutations n Lethal mutations are also possible. As the term implies, the mutations lead to the death of the individual. Death does not have to occur immediately, it may take several months or even years.

Types of Mutations n n n Wild type alleles typically encode a product necessary for a specific biological function. If a mutation occurs in that allele, the function for which it encodes is also lost. The general term for these mutations is loss-of-function mutations. The degree to which the function is lost can vary. If the function is entirely lost, the mutation is called a null mutation. If is also possible that some function may remain, but not at the level of the wild type allele. These are called leaky mutations.

Types of Mutations n n Loss of function mutations are typically recessive. When a heterozygote consists of the wild-type allele and the loss-of-function allele, the level of expression of the wild type allele is often sufficient to produce the wild type phenotype. Genetically this would define the loss-of-function mutation as recessive. Alternatively, the wild type allele may not compensate for the loss-of-function allele. In those cases, the phenotype of the heterozygote will be equal to that of the loss-of-function mutant, and the mutant allele will act as a dominant.

Complementation Testing n Occasionally, multiple mutations of a single wild type phenotype are observed. The appropriate genetic question to ask is whether any of the mutations are in a single gene, or whether each mutations represents one of the several genes necessary for a phenotype to be expressed. The simplest to distinguish between the two possibilities is the complementation test. The test is simple to perform two mutants are crossed, and the F 1 is analyzed. If the F 1 expresses the wild type phenotype, we conclude each mutation is in one of two possible genes necessary for the wild type phenotype. When it is shown that shown genetically that two (or more) genes control a phenotype, the genes are said to form a complementation group. Alternatively, if the F 1 does not express the wild type phenotype, but rather a mutant phenotype, we conclude that both mutations occur in the same gene.

Complementation Testing n n These two results can be explained by considering the importance of genes to phenotypic function. If two separate genes are involved, each mutant will have a lesion in one gene while maintaining a wild type copy of the second gene. When the F 1 is produc ed, it will expresses the mutant allele of gene A and the wild type allele of gene B (each contributed by one of the mutant parents). The F 1 will also express the wild type allele for gene A and the mutant allele for gene B (contributed by the other mutant parent). Because the F 1 is expressing both of the necessary wild type alleles, the wild type phenotype is observed. Conversely, if the mutations are in the same gene, each homolog will express a mutant version of the gene in the F 1. Without a normal functioning gene product in the individual, a mutant phenotype occurs

n n Conversely, if the mutations are in the same gene, each homolog will express a mutant version of the gene in the F 1. Without a normal functioning gene product in the individual, a mutant phenotype occurs. Eye color in Drosphila (fig 7. 2) is a good model to demonstrate the complementation test. A wide array of spontaneous mutations have been studied. These experiments demonstrated that five genes (white, ruby, vermillion, garnet and carnation) controlling ey e color reside within 60 c. M of each other on the X chromosome. The dominant wild type allele for each gene produces the deep red eyes. The mutant alleles produce a different color. If mutants from any of these five genes are crossed, the F 1 would expre ss deep red eye color (wild type phenotype).

n Eye color in Drosphila (fig 7. 2) is a good model to demonstrate the complementation test. A wide array of spontaneous mutations have been studied. These experiments demonstrated that five genes (white, ruby, vermillion, garnet and carnation) controlling eye color reside within 60 c. M of each other on the X chromosome. The dominant wild type allele for each gene produces the deep red eyes. The mutant alleles produce a different color. If mutants from any of these five genes are crossed, the F 1 would express deep red eye color (wild type phenotype).