5.7 Packaging of DNA helix · Part 2

C- Chromatin fiber, D- Coiled chromatin fiber, E- Coiled coil, F- metaphase chromatid XII Std Biology-Zoology Chapter- XII Std Biology-Zoology Chapter- Molecular Genetics octameres and the two turns are sealed off by an H1 molecule. Chromatin lacking H1 has a beads-on-a-string appearance in which DNA enters and leaves the nucleosomes at random places. H1 of one nucleosome can interact with H1 of the neighbouring nucleosomes resulting in the further folding of the fibre. The chromatin fiber in interphase nuclei have a diameter of nm and represents inactive chromatin. nm fibre arises from the folding of nucleosome, chains into a solenoid structure having six nucleosomes per turn. This structure is stabilized by interaction between different H1 molecules. DNA is a solenoid and packed about folds. The hierarchical nature of chromosome structure is illustrated in (Fig. . ) . Additional set of proteins are required for packing of chromatin at higher level and are referred to as non-histone chromosomal proteins (NHC). In a typical nucleus, some regions of chromatin are loosely packed (lightly stained) and are referred to as euchromatin. The chromatin that is tightly packed (stained darkly) is called heterochromatin. Euchromatin is transcriptionally active and heterochromatin is transcriptionally inactive.