A base pair is made up of 2 complemetary bases joined together by Hydrogen bonds. The complementary bases in Deoxyribonucleic Acid (DNA) include: Adenine (A) with Thymine (T) together and Cytosine (C) with Guanine (G) together. Within Ribonucleic Acid (RNA) and Messenger Ribonucleic Acid (mRNA) there is no Thymine base present, so therefore Adenine complementary base pairs with Uracil (U). There are 2 Hydrogen bonds between bases Adenine and Thymine and 3 Hydrogen bonds between bases Cytosine and Guanine (can be remembered as there are 3 letters between C and G in the alphabet). This means that more energy is required and a higher temperature to overcome the Hydrogen bonding in C-G due to more bonds present, so therefore is more stable and stronger than A-T. Additional to this, base pairing occurs between a purine with a pyrimidine. The purine's are Adenine and Guanine and the pyrimidine's are Cytosine and Thymine. A purine has a double ring structure in comparison to a pyramidine which has a single ring structure only. In the case of RNA and mRNA, Adenine is a purine and Uracil is a pyramidine which complementary base pair to form 2 Hydrogen bonds.