Protein synthesis is one of the main metabolic processes. Information about the structure of a protein, that is, about the order in which amino acid residues are joined in its molecule, is stored in DNA. DNA molecules are mainly found in the nuclei of cells (nuclear DNA), a small amount of DNA is found in mitochondria and plastids (extra-nuclear DNA).
DNA is a polynucleotide. Each nucleotide (monomer) of DNA contains:
- five-carbon sugar – deoxyribose,
- the remainder of phosphoric acid,
- one of four nitrogenous bases: adenine, guanine, cytosine, and thymine.
The deoxyribonucleic acid (DNA) molecule consists of two helically twisted chains. The chains in the DNA molecule are oppositely directed. The backbone of DNA chains is formed by sugar-phosphate residues, and the nitrogenous bases of one chain are located in a strictly defined order opposite the nitrogenous bases of the other (the rule of complementarity).
Opposite the adenine of one chain, the thymine of the other chain is always located, opposite the guanine – cytosine.
There are always two hydrogen bonds between adenine and thymine, and three hydrogen bonds between guanine and cytosine.
The A – T pair is connected by two hydrogen bonds, and the G – C pair – by three.
Thus, the nucleotide pairs adenine and thymine, as well as guanine and cytosine, strictly correspond to each other and are complementary to each other. Knowing the sequence of the arrangement of nucleotides in one DNA strand, the nucleotides of the other (second) strand can be determined by the principle of complementarity.
The ratio of the number of nucleotides of different types and nitrogenous bases in the DNA molecule determines the Chargaff rule (the rule of complementarity).
In a DNA molecule, the amount of adenine is equal to the amount of thymine, and the amount of guanine is equal to the amount of cytosine: A = T, G = C.