Drosophila Genome

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Drosophila Genome

• How does it differ?

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Differences

• Drosophila lacks canonical telomeres and the
  ortholog of vertebrate telomerase.
• Instead it has a unique transposition mechanism.
• Two non-LTR (long terminal repeat)
  retrotransposable elements, HeT-A and TART
  telomere-associated retrotransposons are attached
  to the chromosome ends.

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Telomere function

• Prevent the end of the chromosome from being
  treated as dsDNA break.
• In mammals, loss of telomeres results in
  cell-cycle arrest and eventual apoptosis.
• In Drosophila, terminal deletions can be
  recovered and maintained in Drosophila. These
  ends dont contain HeT-A or TART sequences thus
  erode over time, but no cell cycle arrest.
• End protection in Drosophila may be sequence
  independently mediated by heterochromatin protein
  HP1.

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ALT-alternative telomere maintenance

• Suspected through homologous recombination-telomer
  e-repeat elongation to lengths longer than
  telomerase creates larger repeated sequences may
  be interpersed within telomeres.
• Drosophila-an occational transposition event
  drives the extension of the telomeric sequences.

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Evolution of Transposition

• Loss of sequence dependence of capping
• Loss of telomerase
• Use of an ALT
• Transposable element recruitment to maintain
  telomeric activity.
• OR
• Telomerase diverged from transposable elements.

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Het-A and TART
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How does the retrotransposition take place?

• The colocalization suggests that these two
  telomeric transposons may have coevolved into
  symbiotes, with TART supplying the reverse
  transcriptase and HeT-A the nuclear targeting.

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How does the retrotransposition take place?

• For most non-LTR elements, the reverse
  transcription is primed by a 3' hydroxyl exposed
  at a nick in chromosomal DNA. Reverse
  transcription of HeT-A and TART is hypothesized
  to be primed by the 3' hydroxyl on the extreme
  end of the chromosome.

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Drosophila, worm, mammals

• Size of organism is not correlated with size of
  genome.
• Smaller worm has 35 more genes 62 more paralogs
  than flies.
• Half of fly genes have orthologs in mammals, only
  1/3 of worm genes has.

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Odor receptor genes

• Flies have 57
• Fish has 100
• Mice and worms have 1000.

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Transcription factors

• Flies have about 700 (4.5)
• 500 in worm (3.5)

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Polytene Chromosomes

• Polytene chromosomes are giant chromosomes common
  to many dipteran (two-winged) flies. They begin
  as normal chromosomes, but through repeated
  rounds of DNA replication without any cell
  division (called endoreplication), they become
  large, banded chromosomes

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Polytene Chromosomes

• Size of each band is an average length of 26.2
  kb.
• X chromosome puff- a series of 3.5 (each 350 bp)
  inverted repeats flaking 154 kb region-these
  repeats alters the chromosomes macrostructure.