The nucleus of plant and animal cells contains chromosomes. A chromosome consists of protein and one molecule of deoxyribonucleic acid. DNA is responsible for passing along specific traits from one living organism to another.
Although the shape and number of chromosomes varies among living organisms, all chromosomes are tightly wound around histones so they fit inside a cell. Complex organisms, such as plants and animals, have a linear arrangement of chromosomes, whereas bacteria usually have a circular arrangement of one or two chromosomes. The cells of complex organisms contain smaller, circular chromosomes. Similar to the makeup of bacteria, the chromosomes reside in the mitochondria outside the nucleus.