Key Concepts of Section 8.5

• In eukaryotic cells, DNA is associated with about an equal mass of histone proteins in a highly condensed nucleoprotein complex called chromatin. The building block of chromatin is the nucleosome, consisting of a histone octamer around which is wrapped about 147 bp of DNA (see Figure 8-24).

• The chromatin in transcriptionally inactive regions of DNA within cells is thought to exist in a condensed, 30-nm fiber form and higher-order structures built from it (see Figure 8-25 and 8-36).

• The chromatin in transcriptionally active regions of DNA within cells is thought to exist in an open, extended form.

• The flexible, intrinsically disordered N-terminal tails of histones, particularly H4 lysine 16, are required for beads-on-a-string chromatin (the 10-nm chromatin fiber) to fold into a 30-nm fiber.

• Histone tails can be modified by acetylation, methylation, phosphorylation, and ubiquitinylation (see Figure 8-26). These modifications influence chromatin structure by regulating the binding of histone tails to other, less abundant chromatin-associated proteins.

• The reversible acetylation and deacetylation of lysine residues in the N-terminal tails of the core histones regulate chromatin condensation. Proteins involved in transcription, replication, and repair, and enzymes such as DNase I, can more easily access chromatin with hyperacetylated histone tails (euchromatin) than chromatin with hypoacetylated histone tails (heterochromatin).

• When metaphase chromosomes decondense during interphase, areas of heterochromatin remain much more condensed than regions of euchromatin.

• Heterochromatin protein 1 (HP1) uses a chromodomain to bind to histone H3 trimethylated at lysine 9. The chromoshadow domain of HP1 associates with itself and with the histone methyl transferase that methylates H3 lysine 9. These interactions cause condensation of the 30-nm chromatin fiber and spreading of the heterochromatic structure along the chromosome until a boundary element is encountered (see Figure 8-29).

• One X chromosome in nearly every cell of mammalian females consists of highly condensed heterochromatin, resulting in repression of expression of nearly all genes on that inactive chromosome. This inactivation results in dosage compensation so that genes on the X chromosome are expressed at the same level in both males and females.

• Each eukaryotic chromosome contains a single DNA molecule packaged into nucleosomes and folded into a 30-nm chromatin fiber, which is associated with structural maintenance of chromosome (SMC) proteins thought to organize it into the megabase loops observed by hybridization to fluorescently labeled DNA probes and in lampbrush chromosomes observed in oocytes (see Figures 8-30, 8-31, and 8-32c). Additional folding of the chromosomes further compacts the structure into the highly condensed form of metaphase chromosomes (see Figure 8-36).

• In interphase cells, chromosomes are localized to largely non-overlapping “territories” in the nucleus (see Figure 8-33).

• Chromosome conformation capture methods indicate that chromatin is organized into topological domains separated by boundary elements (see Figure 8-34c). These topological domains may correspond to the loops in lampbrush chromosomes observed in the giant nuclei of oocytes (see Figure 8-31) and inferred by studies of fluorescently labeled DNA probes hybridized to interphase nuclei (see Figure 8-30).

• During mitosis, chromosomes condense greatly, decreasing their lengths severalfold and increasing their diameter to generate metaphase chromosomes visible by light microscopy. The geometry of the 30-nm chromatin fiber in metaphase chromosomes is not well understood, but intermediates of increasing diameter and decreasing length have been observed during prophase.