Heterochromatin, or packaging of DNA, is a way to control gene expression. Gene expression is a fundamental question on which lies the basis of all cell-based life. Mammalian studies are time consuming, difficult to handle, and lack the ancestral, conserved mechanism which is not understood. Drosophila embryo exist during early development as one giant cytoplasm thus injection is possible and easily monitored. A fertilized Drosophila embryo's diploid genome replicates and divides 14 times before initiating cell membrane formation. This poses an interesting question of how DNA is packaged since it needs to be unpacked to replicate.
How is the packaging inherited by future generations? DNA packaging involves proteins called nucleosomes and heterochromatin protein (HP1) whose binding lead naive chromatin to heterochromatin. We expected that injecting exogenous nucleosomes might dilute the endogenous histone marks. To begin probing this question, we need to know when are nucleosomes exchanged during the rapid cell cycle. My data suggests that it is during S phase.