Chromatin and chromosomes

The basic packing unit is the nucleosome: 147 bp of DNA wound ≈1.7 times around a histone octamer (two each of H2A, H2B, H3 and H4 — positively charged, lysine/arginine-rich proteins), joined by linker DNA bound by histone H1. This "beads-on-a-string" 10-nm fibre is folded further into loops anchored by cohesin and CTCF (topologically associating domains), and each chromosome occupies its own territory in the interphase nucleus. Maximal compaction (≈10 000-fold) is reached in the mitotic chromosome, assisted by condensins.

Euchromatin is loosely packed, stains lightly and contains most actively transcribed genes. Heterochromatin is condensed, stains darkly and is transcriptionally silent; it lines the nuclear envelope and the nucleolus. Constitutive heterochromatin (centromeres, telomeres, repetitive satellite DNA) is always condensed; facultative heterochromatin is silenced in a cell-type-specific way — the best example is the inactive X chromosome in female cells, visible as the Barr body (and as a "drumstick" appendage on some neutrophil nuclei).

Human somatic cells have 46 chromosomes: 22 pairs of autosomes and one pair of sex chromosomes (XX or XY). After replication each chromosome consists of two identical sister chromatids held by cohesin and joined at the centromere, where the kinetochore attaches spindle microtubules. Chromosome ends are capped by telomeres (TTAGGG repeats). By centromere position chromosomes are metacentric, submetacentric or acrocentric (13, 14, 15, 21, 22, Y). A karyotype is the ordered display of an individual's metaphase chromosomes, usually G-banded (Giemsa), described as e.g. 46,XX or 47,XY,+21; p denotes the short arm and q the long arm.