DNA can be broken down into small segments called genes, the basic units of heredity in living things. Within their unique sequence of nucleotides is the information needed to make proteins; because each gene has a different sequence, each protein is also ultimately different. Before proteins can be made, however, the gene must be copied into messenger RNA. When it is time to transcribe the gene, the two strands of the DNA molecule separate and an enzyme called RNA polymerase attaches to one of the strands and makes a copy.
