Section 16 Variation in Chromosome Structure

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This presentation was originally prepared by C.
William Birky, Jr. Department of Ecology and
Evolutionary Biology The University of
Arizona It may be used with or without
modification for educational purposes but not
commercially or for profit. The author does not
guarantee accuracy and will not update the
lectures, which were written when the course was
given during the Spring 2007 semester.
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Section 16Variation in Chromosome Nunber and
Structure
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Indian Muntjac 2N 6/7 Chinese Muntjac 2N 46
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Polytene Chromosomes
Some insects, including Drosophila During
differentiation of some tissues (salivary glands,
Malpighian tubules, etc.) cells go through
repeated S phases (e.g. 10) without mitosis --gt
polyploid or polytene nuclei. Polytene
chromosomes all copies held tightly together and
in alignment.
Stain, see bands where DNA is more
concentrated. PHYSICAL LANDMARKS!
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Bar
Double-Bar
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Deletion of 8p23.1 Died age 3
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Note that most rearrangements may take place by
crossing-over between repeats or other homologous
DNA segments (segments with similar
sequence). Duplicative transposition is a major
source of such duplicated regions. If a
transposable element invades a genome, it can
spread and facilitate rearrangements. Thus it can
increase variation in chromosome structure and
eventually lead to evolution of new gene
arrangements and new phenotypes. Transpsable
elements can contribute to, or even give rise to,
promoters and other regulatory sequences. Transpo
sable elements can spread from one organism to
another. e.g. P element was transferred from
Drosophila willistoni (or a close relative) to
Drosophila melanogaster, probably by a parasitic
mite that sucks cytoplasm from fly eggs. The
evolutionary origin(s) of transposable elements
are unknown (last I heard).
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Physical and Genetic Maps Agree in Gene Order,
Not Distances Reason recombination frequencies
arent uniform along chromosome.
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Other Methods of Relating Physical and Genetic
Maps

• FISH
• Finding genes in complete genome sequences.