Chromosomes are microscopic units containing organized genetic information, eukaryotic chromosomes are located in the nuclei of diploid and haploid cells (e.g., human somatic and sex cells). Prokaryotic chromosomes are also present in one-cell non-nucleated (unicellular microorganisms) prokaryotic cells (e.g., bacteria). The sum-total of genetic information contained in different chromosomes of a given individual or species are generically referred to as the genome.

In humans, eukaryotic chromosomes are structurally made of roughly equal amounts of proteins and DNA. Each chromosome contains a double-strand DNA molecule, arranged as a double helix, and tightly coiled and neatly packed by a family of proteins called histones. DNA strands are comprised of linked nucleotides. Each nucleotide has a sugar (deoxyribose), a nitrogenous base, plus one to three phosphate groups. Each nucleotide is linked to adjacent nucleotides in the same DNA strand by phosphodiester bonds. Phosphodiester is another sugar, made...