Chromosomes contain the DNA that gives each creature its unique characteristics. As the cells in an organism divide, they must keep the DNA replication intact. If there are variations, this can result in physical problems. For example, in humans, some cancers and one strain of leukemia are due to defective and broken chromosomes.
While each species has its own specific number of chromosomes, all animals within that species will have exactly the same number. The shape of the chromosomes, however, is the same for all animals. It is linear, rather than circular, which is found in most types of bacteria. Linear chromosomes are normally found in cells with nuclei, also referred to as eukaryotic cells. Cells that do not have a defined nucleus, known as prokaryotic cells, have circular chromosomes for the most part.
Some examples of the number of chromosomes found in different animals are: