sugar is called a nucleoside and a base linked to a sugar and one or more phosphate groups is called a nucleotide . A polymer comprising multiple linked nucleotides (as in DNA) is called a polynucleotide . [ 11 ] The backbone of the DNA strand is made from alternating phosphate and sugar residues. [ 12 ] The sugar in DNA is 2-deoxyribose , which is a pentose (five- carbon ) sugar. The sugars are joined together by phosphate groups that form phosphodiester bonds between the third and fifth carbon atoms of adjacent sugar rings. These asymmetric bonds mean a strand of DNA has a direction. In a double helix the direction of the nucleotides in one strand is opposite to their direction in the other strand: the strands are antiparallel . The asymmetric ends of DNA strands are called the 5′ ( five prime ) and 3′ ( three prime ) ends, with the 5′ end having a terminal phosphate group and the 3′ end a terminal hydroxyl group. One major difference between DNA and RNA is the sugar, with the 2-deoxyribose in DNA being replaced by the alternative pentose sugar ribose in RNA. [ 10 ] A section of DNA. The bases lie horizontally between the two spiraling strands. [ 13 ] ( animated version ). Nucleobase classification The nucleobases are classified into two types: the purines , A and G, being fused five- and six-membered heterocyclic compounds , and the pyrimidines , the six-membered rings C and