Although most chromosomes are found in the nucleus of a cell, some are also found in the cell's mitochondria. These organelles are responsible for changing energy into a form the cell can use.
Chromosomes are structures that carry an organism's DNA, which contains all of its genetic information. The number of chromosomes varies among types of organisms. For instance, a fruit fly has only four pairs of chromosomes while a dog has 39 pairs. A rice plant is in the middle with 12 pairs. A normal human cell has 23 pairs of chromosomes. Half of them were passed along by the mother and half by the father when the egg was fertilized by the sperm.