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Unformatted text preview: Chromosome Structure, outline Lecture 11 2007 Chromosome Structure [Essential reading: SS 114-124, 148-151] Chromosome features One chromatid = one double helix of DNA + proteins Homologous chromosomes are the same length Nonhomologous chromosomes of any organism vary in length The centromere separates each chromosome into two arms Relative chromosome length and ratio of arm length are measures used to identify individual chromosomes One (or sometimes more) chromosome is associated with the nucleolus at the nucleolar organizing region (NOR) NOR is associated with ribosome formation Tolerance to changes in chromosome structure Diploid organisms tolerate only small changes in the euploid chromosome set Addition or removal of one gene can disrupt normal function, in many instances Sex chromosomes generally are more tolerant to changes Polyploids are more tolerant to changes in chromosome structure Three categories of chromosomal structural changes = chromosomal mutations Duplications/deletions small, but cytologically visible Inversions chromosome rearrangement where a section of a chromosome is excised, inverted, and reinserted Translocation chromosome rearrangememt where a section of chromosome is excised and inserted into a nonhomologous chromosome Occurrence: natural or induced by X-irradiation Duplications and deletions Deletions are often lethal when homozygous For example, `waltzing' mice Small deletion heterozygote (i.e. deletion in one chromosome but not its homologue) causes animals to walk in circles Homozygous deletions lethal, so inherited as 2 waltzing : 1 nonwaltzing progeny in cross between 2 waltzing mice Cri du chat in humans is small deletion in Chromosome 5 Duplications are better tolerated than deletions For example, Bar eye of Drosophila Duplication of one band of X chromosome, relative to wild type Double bar is a triplication Dominant mutations reduces eye size Double Bar moreso than Bar Both duplications and deletions result in a loop during synteny in Prophase I of meiosis Duplications of genes play a major role in genome evolution Chromosome Structure, outline Lecture 11 2007 Single copy of a gene minimal tolerance for major mutations Duplication flexibility for one member of the duplication to mutate to a new function New function potential for more developmental complexity Inversions Cause loops during synteny in Prophase I of meiosis Two types, relative to the centromere Pericentric inversions include the centromere Paracentric inversions do not include the centromere Translocations Many translocations are reciprocal translocations Excision of two chromosome sections and reinsertion in new locations Cause cross-shaped associations of two bivalents in Prophase I of meiosis Disjunction from translocations in meiosis usually yields few balanced gametes reduced fertility in plants and reduced progeny viability in animals Inversions and translocations in nature Speciation among higher plants and animals often involves inversions and translocations For example Graminaceaous crop (wheat, rice, maize, etc.) map relationships mentioned in Linkage Maps and the Genome lecture Cause, effect, or unrelated ? ...
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- Fall '07