ABSTRACT
Deoxyribonucleic acid (DNA) tells cells how to create energy, respond to things in the environment, grow, and reproduce. Without DNA, the cell would not be able to use energy. Even common diseases, such as the cold, which people without the virus fight off easily, become life threatening for people with Human Immunodeficiency Virus (HIV). Like parasites, they invade living cells, inserting their own DNA into the DNA of the cell. The virus DNA tells the cell to makes more viruses. Unlike other cells, bacteria have one circular strand of DNA-one chromosome-shaped like a rubber band. Besides their chromosome, bacteria also have small extra pieces of DNA called plasmids. Smaller organisms usually have smaller genomes, but just as with chromosome number, it varies. A plankton species called Aulacantha has only one cell, but has 1,600 chromosomes.
