Deoxyribonucleic acid (DNA) is a molecule composed of two polynucleotide chains that interact together in order to form a double helix. This molecule (DNA) carries the genetic instructions that make each species unique. In DNA, each polynucleotide chain is composed of nucleotide monomers: a nucleotide is composed of a deoxyribose sugar attached to a phosphate group and one nitrogen-containing base (i.e., adenine, thymine, guanine and cytosine). This basic structure is the same among different species, and, therefore, genetic differences between different groups (in this case, animals, plants, and bacteria) are caused by differences in the nucleotide-base sequences of their DNA molecules.
