A chromosome is a packaged and organized structure containing most of the DNA of a living organism. Most eukaryotic cells have a set of chromosomes(46 in humans) with the genetic material spread among them.
During most of the duration of the cell cycle, a chromosome consists of one long double-helix DNA molecule(with associated proteins). During S phase, the chromosome gets replicated, resulting in an X-shaped structure called a metaphase chromosome. Both the original and the newly copied DNA are now called chromatids. The two "sister" chromatids are joined together at a protein junction called a centromere(forming the X-shaped structure).
Chromosomes are normally visible under a light microscope only when the cell is undergoing mitosis(cell division). Even then, the full chromosome containing both joined sister chromatids becomes visible only during a sequence of mitosis known as metaphase(when chromosomes align together, attached to the mitotic spindle and prepare to divide).[1] This DNA and its associated proteins and macromolecules is collectively known as chromatin, which is further packaged along with its associated molecules into a discrete structure called a nucleosome. Chromatin is present in most cells, with a few exceptions, for example, red blood cells. Occurring only in the nucleus of eukaryotic cells, chromatin composes the vast majority of all DNA, except for a small amount inherited maternally, which is found in mitochondria.