Nucleotides are joined into a polynucleotide chain by phosphodiester bonds. A phosphodiester bond forms in a condensation reaction between the phosphate group attached to carbon 5 of one nucleotide's sugar and the hydroxyl (OH) group on carbon 3 of the sugar of the next nucleotide. One molecule of water is released for each bond formed.
The name describes the bond: one phosphate group linked by two ester bonds ("di-ester") to two sugars.Repeated phosphodiester bonds produce the sugar-phosphate backbone. The chain therefore has two different ends: a 5' end with a free phosphate and a 3' end with a free OH group. New nucleotides are always added to the 3' end.In cells the reaction is catalysed by DNA polymerase (when making DNA) and RNA polymerase (when making RNA).Phosphodiester bonds are strong covalent bonds, so the base sequence is not easily broken up. They can be broken by hydrolysis, which uses water; nucleases in the gut break down nucleic acids from food in this way.Worked example: a chain of n nucleotides contains n − 1 phosphodiester bonds, so making it releases n − 1 water molecules. A single strand of 100 nucleotides has 99 bonds. A double-stranded molecule of 100 base pairs has 2 × 99 = 198 bonds.
The name describes the bond: one phosphate group linked by two ester bonds ("di-ester") to two sugars.Repeated phosphodiester bonds produce the sugar-phosphate backbone. The chain therefore has two different ends: a 5' end with a free phosphate and a 3' end with a free OH group. New nucleotides are always added to the 3' end.In cells the reaction is catalysed by DNA polymerase (when making DNA) and RNA polymerase (when making RNA).Phosphodiester bonds are strong covalent bonds, so the base sequence is not easily broken up. They can be broken by hydrolysis, which uses water; nucleases in the gut break down nucleic acids from food in this way.Worked example: a chain of n nucleotides contains n − 1 phosphodiester bonds, so making it releases n − 1 water molecules. A single strand of 100 nucleotides has 99 bonds. A double-stranded molecule of 100 base pairs has 2 × 99 = 198 bonds.