Why Thawed food should never be Refrozen ?

You shouldn't do this because it can increase the chances of spoiling the food due to bacteria. When taking food out of the freezer and thawing it bacteria can leach onto it, partially spoiling the food when leaving it out.

during the process of dna replication, brdu can replace thymidine and incorporate into the newly synthesized dna, cell proliferation thus can be reflected in the extent of brdu incorporation.