Adenine and Thymine are named after Thymus. They were isolated from the thymus gland. Adeno means gond. Cytosine is named after cyto. Cyto means cell. Guanine comes from the Spanish word guano. Guano means the droppings of bats. Alkalies are usually denoted by the alphabet AGTCU. DNA molecules have the letters AGTC and RNA molecules have the letters AGCU.