a microscopic, generally threadlike or rodlike body consisting of linear segments of deoxyribonucleic acid (DNA), carrying the genes, and forming the primary constituent of the cell nucleus —note chromosomes are individually distinguishable only during nuclear division, and their number, size, and form are generally constant for each species —see cytogenetics, cytology, meiosis, mitosis, mutation, nucleusThis definition last updated 07/18/2008