The Polynucleotide Chain Contains Mononucleotides Linked by Phosphodiester Bonds

The Polynucleotide Chain Contains Mononucleotides Linked by Phosphodiester Bonds 

Nucleotides are the building blocks of nucleic acids their structures and biochemistry were discussed in chapter 23. When a 5 -phosphomononucleotide is joined by a phosphodiester bond to the 3 -OH group of another mononucleotide, a dinucleotide is formed. The 3 -5 -linked phosphodiester intemucleotide structure of nucleic acids was firmly established by Lord Alexander Todd in 1951. Repetition of this linkage leads to the formation of polydeoxyribonucleotides in DNA or polyribonucleotides in RNA. The structure of a short polydeoxyribonucleotide is shown in figure 25.3. The polymeric structure consists of a sugar phosphate diester backbone with bases attached as distinctive side chains to the sugars. 

The polynucleotide chain has a directional sense with 5 and 3 ends. Either of these ends may contain a free hydroxyl group or a phosphorylated hydroxyl group. The structure shown in figure 25.3 contains a phosphate group on the 5 end but none on the 3 end. By convention, one writes a nucleic acid sequence from the 5 to the 3 end so that a comparable structure is written pTpApCpG. With no 

The structure of a deoxyribonucleotide. Drawn in abbreviated form at lower left. The illustrated structure is written pTpApCpG. 

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