Deoxyribonucleic acid, or DNA, is what genes are made of. Within a DNA molecule, four different nucleotide building blocks occur. Each contains a five carbon sugar and a phosphate group, but differs depending on what organic base is attached. The four bases found in a DNA molecule are adenine, thymine, cytosine and guanine.
A DNA molecule is made up of two strands of nucleotides that spiral around each other to form a double helix. The nucleotide backbone is created by the sugar of one nucleotide bonding with the phosphate group of the next. The two strands are held together by hydrogen bonds between the bases of the opposite nucleotides. This hydrogen bonding is very specific and only occurs between complementary base pairs.
The structure of each base determines the exact base it will pair with. All four bases have a ring structure that contains both carbon and nitrogen atoms, so they are often referred to as nitrogenous bases. Although each has a different chemical structure, they are grouped into two categories based on the number of rings they contain. Adenine and guanine are purine bases and have a double ring structure. Cytosine and thymine have a single ring structure and are pyrimidine bases.
Two important constraints are placed on how the cross rungs between the DNA strands can be formed in order for the hydrogen bonds to form and the regular coiling of the double helix to occur. First, purine bases only bond with pyrimidine bases. By only having purine bases bond with pyrimidine bases, the length of the cross rung between the DNA strands will remain constant. If purine bases could bond with purine bases or pyrimidine bases with pyrimidine bases, the length of the cross rung would change causing the DNA molecule to bow in and out.
Secondly, and more specifically, adenine only bonds with thymine and cytosine only bonds with guanine. When adenine bonds with thymine, two hydrogen bonds are formed. Three hydrogen bonds are formed between cytosine and guanine. Only these two pairs are capable of forming the necessary hydrogen bonds to maintain the stability of the DNA molecule.
What is unimportant in the DNA molecule is the order that the bases appear. This means that there can be four different cross rungs – adenine with thymine, thymine with adenine, cytosine with guanine, and guanine with cytosine. This is biologically significant as it means that the base sequence of one strand of a DNA molecule specifies the base sequence of the other strand. In other words, the two strands can be separated and exact copies made each time a cell divides.
Thymine is unique amongst the four bases as it only occurs in DNA molecules. Adenine, cytosine and guanine are also found in nucleotides that make up ribonucleic acid, or RNA. Within an RNA molecule, thymine is replaced by uracil.