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
DNA=> Adenine:Thymine --- Guanine:CytosineRNA=> Adenine:Uracil --- Guanine:Cytosine
Substitution
Substitution.
The nitrogenous base is what causes variation. The deoxyribose and phosphate group are the same on all nucleotides.