Structure of DNA Nucleotides

DNA (deoxyribonucleic acid) is a polymer made of monomers called nucleotides. Each nucleotide has three parts:
• a phosphate group
• the sugar 2-deoxyribose, a pentose (five-carbon) sugar with a five-membered ring containing an oxygen atom
• one of four nitrogen-containing bases: adenine (A), cytosine (C), guanine (G) or thymine (T)The structures of these parts are given in the AQA data booklet, so you do not need to memorise them, but you should be able to recognise them and show how they join.In a nucleotide, the phosphate is bonded to the CH₂ carbon (carbon 5) of 2-deoxyribose, and the base is bonded through one of its nitrogen atoms to carbon 1 of the sugar.Nucleotides join by condensation reactions. The phosphate group of one nucleotide forms a covalent bond with the -OH on carbon 3 of the sugar of the next nucleotide, and water is eliminated. Repeating this builds a single strand with a sugar-phosphate backbone: sugar-phosphate-sugar-phosphate, with a base attached to each sugar. The order of bases along the strand carries the genetic code.The name 2-deoxyribose tells you that this sugar is ribose without the oxygen atom on carbon 2 (it has H instead of OH there).Worked example. A single strand contains 120 nucleotides. How many phosphate-sugar links join them, and how many water molecules were eliminated? A chain of n nucleotides has (n − 1) links, so 119 links and 119 water molecules.