![]() ![]() ![]() To better understand a polymerase slip, imagine you are reading a page of text that is a repeat of a simple sequence. During DNA replication, these repeat regions are often altered because the polymerase can "slip" as it disassociates and reassociates with the DNA strand (Viguera et al., 2001). ![]() Hot spots include areas of the genome with highly repetitive sequences, such as trinucleotide repeats, in which a sequence of three nucleotides is repeated many times. These "hot spots" are often a result of the DNA sequence itself being more accessible to mutagens. In addition, some areas of the genome simply seem to be more prone to mutation than others. Mutations can result from a number of events, including unequal crossing-over during meiosis (Figure 3). The normal number of repeated trinucleotide sequences is expanded The number of tandem copies of a locus is increased One region of a chromosome is flipped and reinsertedĪ region of a chromosome is lost, resulting in the absence of all the genes in that areaĪ region of a chromosome is repeated, resulting in an increase in dosage from the genes in that regionĪ region from one chromosome is aberrantly attached to another chromosome One or more nucleotides is "skipped" during replication or otherwise excised, often resulting in a frameshift One or more extra nucleotides are inserted into replicating DNA, often resulting in a frameshift One base is incorrectly added during replication and replaces the pair in the corresponding position on the complementary strand Table 2: Types of DNA Mutations and Their Impact Class of Mutation On average, SNPs are found every 1,000–2,000 nucleotides in the human genome, and scientists participating in the International HapMap Consortium have mapped millions of these alterations (International Human Genome Sequencing Consortium, 2001). SNPs are important as markers, or signposts, for scientists to use when they look at populations of organisms in an attempt to find genetic changes that predispose individuals to certain traits, including disease. Moreover, the cutoff of at least 1% prevalence for a variation to be classified as a polymorphism is somewhat arbitrary if the frequency is lower than this, the allele is typically regarded as a mutation (Twyman, 2003). Note that the term "polymorphism" is generally used to refer to a normal variation, or one that does not directly cause disease. Specifically, a polymorphism is any genetic location at which at least two different sequences are found, with each sequence present in at least 1% of the population. While a mutation is defined as any alteration in the DNA sequence, biologists use the term "single nucleotide polymorphism" ( SNP) to refer to a single base pair alteration that is common in the population. By way of this process, those mutations that furnish individual organisms with characteristics better adapted to changing environmental conditions are passed on to offspring at an increased rate, thereby influencing the future of the species. This is because mutations provide the "raw material" upon which the mechanisms of natural selection can act. As a result, the sickle-cell mutation persists in populations where malaria is endemic.īeyond the individual level, perhaps the most dramatic effect of mutation relates to its role in evolution indeed, without mutation, evolution would not be possible. For example, the very same mutation that causes sickle-cell anemia in affected individuals (i.e., those people who have inherited two mutant copies of the beta globin gene) can confer a survival advantage to unaffected carriers (i.e., those people who have inherited one mutant copy and one normal copy of the gene, and who generally do not show symptoms of the disease) when these people are challenged with the malaria pathogen. In addition, some mutations are actually beneficial. However, while many mutations are indeed deleterious, others are "silent" that is, they have no discernible effect on the phenotype of an individual and remain undetected unless a molecular biologist takes a DNA sample for sequence analysis. Because so many diseases are associated with mutations, it is common for mutations to have a negative connotation. Sickle-cell anemia is one of hundreds of life-threatening disorders that are known to be caused by a change in just one of those 3 billion A's, T's, C's, or G's. ![]()
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |