Now, adenine and guanine, or simply A and g are two types of nitrogenous bases that consist of two rings. And these types of nitrogenous bases that consist of two rings are known as purines. And notice thymine and cytosine or TNC consist of only a single ring structure. And these types of nitrogenous bases are known as pyrimidines. So let's look at the arrangement of our nucleotide and how the nucleotides bond to one another. So remember, in a protein we have amino acids that bond to one another via covalent bonds known as dipeptide or peptide bonds.