<P> Deoxyribonucleic acid (/ diˈɒksɪraɪboʊnjuːkliːɪk, - kleɪ - / (listen); DNA) is a molecule composed of two chains (made of nucleotides) that coil around each other to form a double helix carrying the genetic instructions used in the growth, development, functioning and reproduction of all known living organisms and many viruses . DNA and ribonucleic acid (RNA) are nucleic acids; alongside proteins, lipids and complex carbohydrates (polysaccharides), nucleic acids are one of the four major types of macromolecules that are essential for all known forms of life . </P> <P> The two DNA strands are also known as polynucleotides since they are composed of simpler monomeric units called nucleotides . Each nucleotide is composed of one of four nitrogen - containing nucleobases (cytosine (C), guanine (G), adenine (A) or thymine (T)), a sugar called deoxyribose, and a phosphate group . The nucleotides are joined to one another in a chain by covalent bonds between the sugar of one nucleotide and the phosphate of the next, resulting in an alternating sugar - phosphate backbone . The nitrogenous bases of the two separate polynucleotide strands are bound together, according to base pairing rules (A with T and C with G), with hydrogen bonds to make double - stranded DNA . </P> <P> The complementary nitrogenous bases are divided into two groups, pyrimidines and purines . In DNA, the pyrimidines are thymine and cytosine; the purines are adenine and guanine . </P>

What are the four elements present in dna