Kromosomes

Kromosomes are long, thread-like molecules that carry genetic information in living cells. In most organisms they are made of DNA wrapped around proteins and, in eukaryotes, reside in the cell nucleus; in prokaryotes the chromosome is typically a single circular DNA molecule in the cytoplasm. The term kromosome is an alternate transliteration used in some languages for chromosome; in English, the standard term is chromosome.

Chromosomes occur in pairs in diploid organisms, with autosomes and sex chromosomes. They are replicated during the S phase of the cell cycle, and during cell division their sister chromatids are held together at the centromere and separated during mitosis or meiosis. In eukaryotes, chromosomal DNA contains genes and noncoding regions; the order and structure of genes along a chromosome influence regulation, expression, and inheritance.

Beyond the nucleus, mitochondria and chloroplasts contain their own circular genomes that resemble chromosomes. The number of chromosomes is characteristic of a species; humans typically have 46 in most cells, while plants, fungi, and other organisms show wide variation. Chromosome structure and number can influence development and evolution.

Chromosomes participate in inheritance, gene expression, and evolution. They are the substrate of genetic recombination, mutation, and chromosomal rearrangements such as deletions, duplications, inversions, and translocations. Abnormal chromosome number or structure can lead to developmental disorders or reduced viability.

A karyotype is the complete set of chromosomes of an organism and is used in diagnostics to detect aneuploidy and structural rearrangements. The study of chromosomes underpins much of genetics, cytogenetics, and evolutionary biology.