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Lecture 13

Lecture 13: "Chromosomal Rearrangements"

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Western University
Biology 2581B
Jim Karagiannis

Genetics Lecture No. 13: Chromosomal Rearrangements th Wednesday February 27 , 2013 The Paradox: -At the sequence level, there are many similarities (same types of genes, genes with similar sequences) between the mouse and human genomes. However when comparing the 20 mouse and the 23 human chromosomes, we see that the staining patterns of the chromosomes suggested no conservation. After both genomes were sequenced, each mouse chromosome could be “pieced” together from different human chromosomes using syntenic segments (the identity, order, and transcriptional direction of the genes are almost exactly the same in the two genomes). When comparing both genomes as a whole, the mouse genome is broken into hundreds of fragments, but they can be aligned with the human genome according to package size and package content. Rearrangements Of Chromosome Sequences & Lack Of Diversity: -Rearrangements of chromosome sequences are not necessarily always deleterious. For example, they 9 are required for the generation of antibodies (10 different antibodies can be encoded by only 20,000- 30,000 genes), helping the immune system respond to new challenges? Segments of the chromosome 14 recombine with each other (D segments, J segments and V segments) and then get transcribed and translated with the constant region to give rise to many different types of antibodies, leading to B cell development. If there is a lack of diversity in making antibodies, this can lead to a shift in the formation of T and B cell clones and is quite often observed in lymphoma cancers. We use assays to examine clonal diversity and fewer arrangements, in this case, will lead to cancer. Deletions, Their Detection, Effect On Recombination & Use To Map Mutations: -Deletions are characterized by the loss of sequences: small deletions affect a single gene, while large deletions lead to the loss of tens or hundreds of genes. Deletions of the chromosome can be caused by X-rays or other chromosome damaging agents that break the DNA backbone. One way to detect deletions is by PCR. The two PCR primers will amplify a larger PCR product from wild-type DNA than from DNA with a deletion. Very large deletions are visible at the relatively low resolution of a karyotype, showing up as the loss of one or more bands from a chromosome. Most homozygous deletions are lethal (even most heterozygous deletions are lethal), but exceptions include: Wolf–Hirschhorn syndrome and cri du chat syndrome, which typically lead to severe phenotypes in humans. Most deletions have an effect (known as gene dosage) and humans cannot survive if more than 3% of their genome is deleted. - During prophase of meiosis I, the undeleted region of the normal chromosome has nothing with which to pair and forms a deletion loop. No recombination can occur within the deletion loop (because the genes in the loop cannot be separated and the genetic distance between loci on either side will be underestimated). One of the most notable examples of using deletions to map mutations are the polytene chromosomes (giant chromosomes consisting of many identical chromatids lying in parallel register) in the salivary glands of Drosophila. Duplications, Their Causes, Their Detection & Duplication Loops: -In tandem duplications, the repeated regions lie adjacent to each other in the same or in reverse order. In nontandem duplications, the two copies of the same region are separated. In one scenario for duplication formation, X-rays break one chromosome twice and its homolog once. A fragment of the first chromosome inserts elsewhere on its homolog to produce a
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