• The genomic content of the nucleus is constant for a given species where as the extra chromosomal DNA is found to be variable among the members of a population.

• For example, all the human cells have 46 chromosomes. This is constant in all the humans. But mitochondria in different humans contain different amount of extra chromosomal DNA.

• Similarly, in bacteria, the amount of genomic DNA is same but the extra genomic DNA contained in plastids and mitochondria are different.