"Because ss is not continually expressed after the initial pulse in R7 precursors, we hypothesized that the absence of transcription during the transition phase was allowing repressive H3K27me3 to spread back over the locus."

[From A MECHANISM FOR STOCHASTIC CELL FATE SPECIFICATION by Caitlin Anderson, PhD., 2019]

Caity Anderson, PhD
Research Associate in Genetics
Curriculum Fellow in Online Learning
Harvard Medical School
[onlinelearning.hms.harvard.edu/hmx]

------- Explanation ---------

H3K27me3 is an epigenetic modification to the DNA packaging protein Histone H3. It is a mark that indicates the tri-methylation of lysine 27 on histone H3 protein. This tri-methylation is associated with the downregulation of nearby genes via the formation of heterochromatic regions.

The genomic DNA of eukaryotic cells is wrapped around special protein molecules known as Histones. The complexes formed by the looping of the DNA are known as Chromatin. The basic structural unit of chromatin is the Nucleosome: this consists of the core octamer of histones (H2A, H2B, H3 and H4) as well as a linker histone and about 180 base pairs of DNA. These core histones are rich in lysine and arginine residues. The carboxyl (C) terminal end of these histones contribute to histone-histone interactions, as well as histone-DNA interactions. The amino (N) terminal charged tails are the site of the post-translational modifications, such as the one seen in H3K27me3.

[Wikipedia H3K27me3, 2022]