The base pairs in a dna strand are held together with hydrogen bonds. if the two nitrogenous bases in each pair were held together with covalent bonds instead of hydrogen bonds, what effect would it have on the cell?

Interesting! Well, hydrogen bonds are said to the weakest but they provide just enough of a bond for attraction. Covalent bonds involve sharing electrons and and stronger than hydrogen bonds. So if you had covalent bonds they would hold on to the strands with greater strength. This would actually be problematic being that DNA need to have the ability to separate in replication.  

They both determine the exact age of rocks.