CHROMOSOMAL MUTATIONS

=Chromosome Mutations:= Chromosome mutations or ‘block’ mutations occur as a result of errors in crossing over during meiosis. Certain mutagens may also induce Chromosomal mutations. Chromosome mutations affect large segments of DNA containing many genes. There are four different types of chromosomal mutations: Deletions, Translocations, Duplications and Inversions (pictured below). [image:http://i.imgur.com/bBJYlLs.png] Note that any chromosome mutation resulting in a significant loss of genetic material (Deletion) is most likely to be lethal. While many chromosome mutations do not result in a loss of genetic material, the position of a gene on a chromosome can affect its expression. Moving genes from one location to another can affect their expression especially during early developmental stages. In humans significant changes to the position of many genes can prevent proper foetal development (lethal). As with gene mutations, chromosome mutations can be neutral, deleterious, lethal or even beneficial. However, because chromosome mutations affect much larger regions of DNA potentially carrying hundreds or even thousands of genes, they are much more likely to be deleterious or lethal. =EXAMPLE: Translocations Can Cause Down Syndrome= [image:http://i.imgur.com/8mAi5YH.png?1] Down syndrome is associated with some impairment of cognitive ability and physical growth as well as characteristic body / facial features. Down syndrome is most commonly the result of Aneuploidy. However, some cases (2-3%) are caused by a translocation during meiosis that transfers most of chromosome 21 onto chromosome 14. The resulting recombinant chromosome 14 effectively now carries a copy of chromosome 21. If a gamete receives this new recombinant chromosome 14 as well as the normal chromosome 21 the resulting zygote will effectively inherit an additional copy of chromosome 21. =EXAMPLE: Chronic Myeloid Leukaemia= Chromic myeloid leukaemia is a type of leukaemia (cancer of the blood) caused by a '''translocation between chromosomes 9 and 22'''. [image:http://i.imgur.com/AQD0lia.png] It can clearly be seen here that crossing over has occurred between chromosomes 9 and 22. As a result the two chromosomes have exchanged chromosome tips altering the position and thus expression of many different genes. Extra for Experts: Chronic myeloid leukaemia is not actually the result of altered gene expression. Rather the point at which the two chromosomes cross over happens to be within coding regions (genes) on both chromosomes. The result is a gene which is an abnormal fusion of the two existing genes. The resulting protein contains a region or domain capable of stimulating cell division, but no longer requires to be activated by other cell signals. Instead it is constantly switched on, continuously stimulating cell division resulting in a loss of the ability to regulate cellular growth