Nucleotides in both DNA and RNA are bound by phosphate ester bonds. See the following link for a detailed discussion: http://www.ncc.gmu.edu/dna/structur.htm
the nucleotides are jointed to one another by a covalent bond between the sugar of one nucleotide and the phosphate of the next
All nucleotides are similar except for the nitrogen bases, which may either be adenine, cytosine, guanine, thymine, or uracil.
DNA=> Adenine:Thymine --- Guanine:CytosineRNA=> Adenine:Uracil --- Guanine:Cytosine
Substitution