<P> There are, however, many cells that do not follow this organisation . For example, spermatozoa and avian red blood cells have more tightly packed chromatin than most eukaryotic cells, and trypanosomatid protozoa do not condense their chromatin into visible chromosomes for mitosis . </P> <P> Chromatin undergoes various structural changes during a cell cycle . Histone proteins are the basic packer and arranger of chromatin and can be modified by various post-translational modifications to alter chromatin packing (Histone modification). Most of the modifications occur on the histone tail . The consequences in terms of chromatin accessibility and compaction depend both on the amino - acid that is modified and the type of modification . For example, Histone acetylation results in loosening and increased accessibility of chromatin for replication and transcription . Lysine tri-methylation can either be correlated with transcriptional activity (tri-methylation of histone H3 Lysine 4) or transcriptional repression and chromatin compaction (tri-methylation of histone H3 Lysine 9 or 27). Several studies suggested that different modifications could occur simultaneously . For example, it was proposed that a bivalent structure (with tri-methylation of both Lysine 4 and 27 on histone H3) was involved in mammalian early development . </P> <P> Polycomb - group proteins play a role in regulating genes through modulation of chromatin structure . </P> <P> For additional information, see Histone modifications in chromatin regulation and RNA polymerase control by chromatin structure . </P>

When does the chromatin network separate out to form chromosomes