Mutations are dened as changes to the genetic material (DNA for most organisms, RNA for many types of viruses) that are inherited by the progeny. There are many correction systems for DNA and, therefore, a large percentage of the DNA changes are corrected before they have any effects on the cells or are passed to progeny. Therefore, there is an inheritance lter that removes any changes in the DNA prior to mitosis and cell division (Figure 8.1). For RNA, the situation differs in that there are few mechanisms to correct errors. This is the primary reason that RNA viruses usually have higher rates of mutation than do DNA viruses or, for that matter, all DNA-based organisms. It also is the reason that no large genome RNA organisms exist because mutation rates are too high to allow a large set of genes to be accurately maintained. Once the mutation is set, selective pressures begin to act. If the mutation occurs in the promoter region of a gene, or changes the ability of RNA polymerase to attach to the promoter, this can affect transcription of the gene. This can be a change that increases or decreases transcription. When either of these occurs, the difference for the cell or the organism can range from no detectable change to a lethal change. Therefore, there is a selective screen that acts at this point (Figure 8.1). If there is a sufcient amount of transcription, then the RNA that is produced can proceed to the next steps in gene expression. If the amounts of RNA are insufcient to proceed through the next steps of expression, expression of that gene ends at that point. If the gene normally produces a vital gene product, then the cell dies and, therefore, the mutation disappears.