FIG 16.

Hypothesis for heterochromatin formation over multiple cell divisions. The 48-bp prototelomere (black triangles) has a low level of H3K9me2 in nucleosomes (blue cylinders) of the adjacent loci. While telomeres are immediately functional for end protection upon I-SceI cleavage, only a small amount of H3K9me2 mark has been established by 1 PD. Spreading gradually increases over 8 PDs. However, the amount of spreading still varies, as cells continue to grow, even though the telomere repeat tract length is constant. These increases may reflect the fact that more cells have both N termini of histone H3 modified by lysine 9 dimethylation in the histone octamer and more extensive spreading of the heterochromatin domain in some cells, or both.