Lecture23S11 - BIS101/Engebrecht Lecture23 Chromosome...

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Lecture23 5/24/11 Chromosome mutations Approximately 8% of all known human mutations are due to large chromosomal alterations. In addition, chromosomal mutations are important tools in genetics as well as provide the driving force for evolution. I showed you a picture of human and mouse chromosomes. We saw that they are very similar and that large regions are derived from different chromosomes that have been reshuffled by inversions and translocations (chromosomal mutations). Those regions that have an identical gene order in different but related species are referred to as syntenic. I would like you to understand 4 types of chromosome mutations: Deletions Deletions or deficiencies remove part of a chromosome. I represented deletions as: AB . CDEFGHIJ AB . CDEIJ The dot represents the centromere. I am indicating the letters not necessarily as genes but regions of the chromosome. The region removed (FGH) will be lost because it does not have a centromere for segregation or telomeres for stability. In the case of large deletions, several genes will be removed when the region is deleted. Deletions are induced by ionizing radiation and chemicals (such as tobacco). Deletions are stable, that is, do not revert, tend to result in loss-of-function mutations and are recessive. However, exceptions to being recessive are in regions that are haploinsufficient, i.e., where there is a phenotype in the heterozygote because of reduced gene product. For example, a deletion at the tip of human chromosome 5 leads to the crit-du-chat syndrome in the heterozygote because of the reduced level of gene products encoded by that region of the chromosome. This syndrome has many phenotypes including mental retardation. Deletions will also lead to a change in linkage. Deletions can be interstitial (internal), as in the example above, or terminal, where an end of the chromosome is removed. In both cases, they are usually a result of two breakage events because even in the terminal case, you still need the telomeres to maintain the stability of the chromosome. Deletions can be examined cytologically and this has been an important tool for geneticists. Drosophila in particular has been an important system for studying chromosomal deletions because of the unusual properties of polytene chromosomes. Polytene chromosomes are found in the salivary gland of Drosophila larve; see powerpoint for image. These are giant chromosomes because they have undergone several rounds of DNA replication without any mitosis or cytokinesis. Thus, lots of identical strands of DNA are perfectly aligned and can be seen in the light microscope. Such gene amplification is probably due to the need for the production of a lot of gene products in these metabolically active cells. Polytene chromosomes have characteristic banding patterns of dark and light bands that correspond to highly condensed (dark) and less condensed (light) regions of the genome. Thus, it is easy to map deletions cytologically because the missing bands in the heterozygote results in the creation of a
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This note was uploaded on 06/05/2011 for the course BIS 101 taught by Professor Simonchan during the Spring '08 term at UC Davis.

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Lecture23S11 - BIS101/Engebrecht Lecture23 Chromosome...

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