kromosom

A kromosom is a thread-like structure located in the nucleus of a cell that carries genetic material in the form of DNA. The term originates from the Greek words for color and body, reflecting the early ability to visualize chromosomes after staining. In most cells, chromosomes are not visible except during cell division, when they condense and become distinguishable under a light microscope.

In eukaryotes, chromosomes are highly organized into chromatin, a complex of DNA with proteins called histones that help package the long DNA molecules. Each chromosome has a centromere, which constrains the sister chromatids together during cell division, and telomeres, which protect the ends of the DNA from deterioration. The genes are arranged along the DNA sequence, and their expression can be influenced by chromatin structure and higher-order folding.

Humans typically have 46 chromosomes in somatic cells, arranged as 23 pairs: 22 autosome pairs and one sex chromosome pair (XX in females, XY in males). Gametes (sperm and egg) contain 23 unpaired chromosomes. During cell division, mitosis distributes an identical set of chromosomes to two daughter cells, while meiosis reduces the chromosome number by half to form haploid gametes, with genetic recombination and independent assortment contributing to variation.

Chromosomes can vary in number and structure across species. Abnormal chromosome number or structure can lead to developmental disorders or disease. Common examples include aneuploidies such as trisomy 21 (Down syndrome) or monosomy X (Turner syndrome), and structural changes like deletions, duplications, inversions, or translocations. Karyotyping and other cytogenetic methods are used to study chromosome composition and diagnose related conditions.