Cytosine-guanine bond. Illustration showing the hydrogen bonding between the nucleotides cytosine (left) and guanine (right).
Cytosine and guanine are two of the four nitrogenous bases found in DNA (deoxyribonucleic acid). Cytosine and guanine always pair together.
The two strands of DNA that make up the double-helix structure are held together by hydrogen bonds between the DNA bases. Hydrogen bonding occurs when a hydrogen atom is bound to a highly electronegative element such as nitrogen, chlorine or oxygen. The electronegative element withdraws charge density from the hydrogen atom making it highly positively charged. The hydrogen then forms intermolecular bonds with other electronegative elements.
Hydrogen bonds are weak bonds that form when a hydrogen atom is bound to a highly electronegative element such as nitrogen, chlorine or oxygen. They only occur over short distances and are easily formed and broken.