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Definition of CHROMOSOME
: any of the rod-shaped or threadlike DNA-containing structures of cellular organisms that are located in the nucleus of eukaryotes, are usually ring-shaped in prokaryotes (as bacteria), and contain all or most of the genes of the organism; also: the genetic material of a virus — compare chromatin
: any of the usually linear bodies of the cell nucleus of eukaryotic organisms, the usually circular bodies of prokaryotic organisms (as bacteria), or especially in some schools of molecular biology the genomes of DNA viruses (as bacteriophages) that take up basophilic stains and contain most or all of the genes of the organism <both the chromosomes of cells and those of viruses can duplicate only in the complex environment of a living cell—J. D. Watson><an episome, an element that may exist as a free circular plasmid, or that may become integrated into the bacterial chromosome as a linear sequence—Benjamin Lewin>
Microscopic, threadlike part of a cell that carries hereditary information in the form of genes. The structure and location of chromosomes differentiate prokaryotic cells from eukaryotic cells (seeprokaryote, eukaryote). Every species has a characteristic number of chromosomes; humans have 23 pairs (22 pairs of autosomal, or nonsex, chromosomes and one pair of sex chromosomes). Human chromosomes consist primarily of DNA. During cell division (seemeiosis, mitosis), chromosomes are distributed evenly among daughter cells. In sexually reproducing organisms, the number of chromosomes in somatic (nonsex) cells is diploid, while gametes or sex cells (egg and sperm) produced by meiosis are haploid (seeploidy). Fertilization restores the diploid set of chromosomes in the zygote.