cromozom

A chromosome is a DNA-protein complex that carries hereditary information. In eukaryotes it is contained in the nucleus, while prokaryotes have a single circular chromosome in the cytoplasm. The chromosome is a long molecule of deoxyribonucleic acid bound to proteins called histones, which package DNA into a compact structure.

In eukaryotes, chromosomes are linear and have distinctive features: a centromere, from which the two arms (p and q) extend, and telomeres at the ends that protect the ends from degradation. After DNA replication, each chromosome consists of two sister chromatids held together at the centromere. Genes are arranged along the chromosome and are expressed under cellular control; the amount of coding sequence varies widely.

Chromosome number is species-specific. In humans, somatic cells have 46 chromosomes, organized into 23 pairs, including 22 autosomes and 1 pair of sex chromosomes (XX in females and XY in males). Karyotyping is used to study chromosome number and structure. During meiosis, homologous chromosomes pair and exchange segments, producing haploid gametes and increasing genetic diversity.

Abnormalities of chromosome number or structure can cause disease. Aneuploidies such as trisomy 21 result in Down syndrome; sex-chromosome aneuploidies include Turner syndrome (X) and Klinefelter syndrome (XXY). Structural changes like deletions, duplications, inversions, and translocations can disrupt gene function. Evolutionarily, chromosome structure and number vary among organisms but share core roles in heredity; bacteria carry a single circular chromosome, and organelles such as mitochondria and chloroplasts have their own circular genomes.