Drosophila Genome

How does it differ?

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.

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 don’t 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.

ALT-alternative telomere maintenance

• Suspected through homologous recombination-telomere-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.

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.

Het-A and TART

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.

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.

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.

Odor receptor genes

• Flies have 57

• Fish has 100

• Mice and worms have 1000.

Transcription factors

• Flies have about 700 (4.5%)

• 500 in worm (3.5%)

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

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.