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.