kromatiini

Kromatiini, or chromatin, is the complex of DNA and proteins that packages the eukaryotic genome inside the cell nucleus. Its primary components are DNA and histone proteins, with non-histone proteins and RNA contributing to regulation and structure. The basic unit is the nucleosome, in which about 147 base pairs of DNA wrap around a histone octamer (two each of H2A, H2B, H3, and H4). The linker DNA and the histone H1 help stabilize the structure. This “beads-on-a-string” arrangement can fold further into higher-order fibers, forming looped domains anchored to a scaffold in the nucleus; however, the exact higher-order organization is still a topic of study and may vary by cell type and condition.

Chromatin exists in different functional states. Euchromatin is relatively open and enriched in actively transcribed genes, while heterochromatin is more condensed and generally transcriptionally silent. Heterochromatin can be constitutive (always compact) or facultative (reversibly decondenses to permit expression). Gene activity and accessibility are modulated by chemical modifications to DNA (e.g., methylation) and to histones (e.g., acetylation, methylation, phosphorylation). Chromatin remodeling complexes use energy from ATP to reposition, eject, or restructure nucleosomes, altering accessibility to transcriptional machinery.

During the cell cycle, chromatin condenses further to ensure accurate chromosome segregation during mitosis and meiosis, then decondenses to resume transcription in daughter cells. Abnormal chromatin states and epigenetic marks are linked to development, differentiation, and disease. Techniques such as ATAC-seq, DNase-seq, and ChIP-seq are used to study chromatin accessibility and histone modifications.