In DNA and RNA, a phosphodiester linkage connects the 3' carbon of one nucleotide and the 5' carbon of another.
Nucleotide are joined together by covalent bonds between the phosphate group of one nucleotide and the third carbon atom of the pentose sugar in the next nucleotide. This produces an alternating backbone of sugar - phosphate - sugar - phosphate all along the polynucleotide chain.
