chromosome

A chromosome is a structure within cells that carries genetic material in the form of DNA, along with proteins that package and organize it. In eukaryotes, chromosomes are linear molecules located in the nucleus and organized as chromatin by histone proteins. In contrast, bacterial and archaeal chromosomes are typically a single circular DNA molecule in the nucleoid, often accompanied by smaller plasmids.

Each chromosome has key features: a centromere, where sister chromatids attach and kinetochores form during cell division; telomeres at the ends protecting the chromosome from degradation; and origins of replication that enable DNA copying. When a chromosome is replicated, it consists of two sister chromatids held together at the centromere. During mitosis, chromosomes condense and segregate into daughter cells; meiosis produces haploid gametes with recombination.

In humans, somatic cells normally contain 46 chromosomes, arranged as 23 pairs (22 autosomes and 1 pair of sex chromosomes). Chromosome number and structure vary across species. Abnormal chromosome number or structure, such as trisomy 21, can cause developmental disorders. Karyotyping and molecular techniques like fluorescence in situ hybridization and sequencing are used to study chromosome number, structure, and anomalies.

Chromosomes carry genes and other DNA elements that regulate expression and heredity. The linear arrangement of genes along chromosomes and their organization into chromatin influence replication, repair, and transcription. Understanding chromosomal biology is central to genetics, evolution, and medicine.