Chromosomes are the physical carriers of genetic information, consisting of long DNA molecules tightly wrapped around histone proteins. The hierarchical organization of DNA into chromosomes is essential for fitting the genome into the nucleus, regulating gene expression, and ensuring accurate chromosome segregation during cell division.
DNA Packaging and the Nucleosome
The basic unit of chromatin is the nucleosome, composed of approximately 147 base pairs of DNA wrapped around an octamer of core histone proteins — two copies each of H2A, H2B, H3, and H4. Nucleosomes are connected by linker DNA segments of 20 to 90 base pairs, with histone H1 binding at the entry and exit points of the nucleosome to stabilize higher-order folding. This beads-on-a-string structure represents the 10 nm fiber, the first level of DNA compaction, which reduces the length of DNA approximately sevenfold. Histone proteins are rich in basic amino acids (lysine and arginine) that neutralize the negative charge of the DNA backbone, and their N-terminal tails protrude from the nucleosome to undergo post-translational modifications that regulate chromatin structure.
Higher-Order Chromatin Structure
The 10 nm fiber is further coiled into a 30 nm fiber, stabilized by interactions between histone H1 molecules and the tail domains of core histones. This structure is organized into topologically associated domains (TADs), genomic regions of approximately 100 kb to 1 Mb within which DNA sequences interact more frequently with each other than with sequences outside the domain. TAD boundaries are established by CTCF (CCCTC-binding factor) and the cohesin complex, which together form loop structures that facilitate enhancer-promoter interactions while preventing inappropriate cross-talk between neighboring domains. Beyond TADs, chromatin is compartmentalized into active A compartments (euchromatin) and inactive B compartments (heterochromatin), each with distinct biochemical properties and nuclear positions.
Euchromatin and Heterochromatin
Euchromatin is less condensed, gene-rich, and transcriptionally active, characterized by histone modifications such as H3 lysine 4 trimethylation (H3K4me3) at promoter regions and H3 lysine 36 trimethylation (H3K36me3) along gene bodies. Heterochromatin is densely packed, transcriptionally silenced, and can be divided into two forms. Constitutive heterochromatin is permanently condensed at regions such as centromeres and telomeres, enriched in H3 lysine 9 trimethylation (H3K9me3) and bound by heterochromatin protein 1 (HP1). Facultative heterochromatin is conditionally silenced, such as the inactive X chromosome in female mammals, and is marked by H3 lysine 27 trimethylation (H3K27me3) deposited by the Polycomb repressive complex 2 (PRC2).
Centromeres and Kinetochores
The centromere is the constricted region of the chromosome where sister chromatids are held together and where the kinetochore assembles. In most eukaryotes, centromeres are defined by the presence of the histone H3 variant CENP-A, which replaces conventional H3 in centromeric nucleosomes and serves as the epigenetic mark for centromere identity. The kinetochore is a large protein complex that assembles on the centromere and mediates attachment to spindle microtubules during mitosis and meiosis, generating the force for chromosome movement and activating the spindle assembly checkpoint.
Telomeres and Replicative Senescence
Telomeres are repetitive DNA sequences (TTAGGG in vertebrates) at the ends of linear chromosomes that protect against degradation, fusion, and recognition as DNA damage. Telomere length is maintained by telomerase, a ribonucleoprotein enzyme that adds telomeric repeats to chromosome ends using its intrinsic RNA template. Telomerase is active in germ cells, stem cells, and most cancer cells but is repressed in somatic cells, leading to progressive telomere shortening with each cell division. When telomeres become critically short, the cell enters replicative senescence or undergoes apoptosis, linking telomere biology to aging and cancer.
Chromosome Banding and Karyotyping
Chromosomes are visualized during metaphase when they are maximally condensed. Staining techniques such as Giemsa banding (G-banding) produce characteristic light and dark bands that reflect regional variation in base composition and gene density: G-light bands are GC-rich and gene-rich, while G-dark bands are AT-rich and gene-poor. A karyotype is an ordered display of the complete chromosome complement, arranged by size, centromere position, and banding pattern, used to detect chromosomal abnormalities such as aneuploidies (trisomy 21, monosomy X), translocations (Philadelphia chromosome in chronic myeloid leukemia), and large deletions or duplications.
Chromosomal Abnormalities
Numerical abnormalities include polyploidy (entire extra sets of chromosomes, usually lethal in humans) and aneuploidy (gain or loss of individual chromosomes), which most often arises from nondisjunction during meiosis and increases in frequency with maternal age. Structural abnormalities include deletions, duplications, inversions, and translocations. Reciprocal translocations between chromosomes 9 and 22 produce the Philadelphia chromosome, which fuses the BCR and ABL genes and drives chronic myeloid leukemia. Chromosomal abnormalities are detected through karyotyping, fluorescence in situ hybridization (FISH), and chromosomal microarray analysis.
