Recently, I reviewed the different levels of chromatin structure. The primary level is nucleosomes, where DNA is bound to histones, and has structural similarity to "beads on a string." The secondary level is a 30nm fiber, and the tertiary level is formed by radially looping the fibers.
I've also been learning about the histone code and how different modifications to the core histones relate to transcriptional regulation. Are these modifications the primary regulation mechanism for chromatin structure? In other words, does chromatin assume the most compact structure possible until histone modifications are made to enable transcription? Or have other regulatory mechanisms unrelated to transcription been discovered and characterized?