chromosome
Imagine the genetic instructions of a cell as an enormous library written in a single, extremely long thread of DNA. To keep that thread from tangling and to fit it inside a microscopic cell, the cell winds and folds it around proteins into a compact package. A chromosome is one of these neatly bundled packages of DNA plus its associated proteins.
Each chromosome is a single, very long DNA molecule wrapped around proteins called histones and folded into higher-order coils. When a cell is about to divide, the chromosomes condense so tightly that they become visible under a light microscope as distinct rod-shaped bodies, each with a pinched waist (the centromere) and capped ends (the telomeres). Most of the time, between divisions, the DNA is far more loosely organized and is referred to as chromatin rather than as discrete chromosomes.
Different species carry different numbers of chromosomes; humans have 46, organized as 23 pairs, with one member of each pair inherited from each parent. Genes occupy fixed positions along the chromosome, so the chromosome is both the physical carrier of heredity and the structural unit that is faithfully copied and distributed during cell division. The word chromosome literally means “colored body,” because these structures readily take up the dyes early microscopists used.
A human skin cell carries 46 chromosomes; when it prepares to divide, each one is copied and the duplicate halves are pulled to opposite ends so that both daughter cells receive a complete set.
One DNA thread, packaged and copied so it can be shared evenly between cells.
A chromosome is not the same thing as a gene or as DNA. DNA is the molecule, a gene is a stretch of DNA that does a job, and a chromosome is the whole organized package that holds many genes.