Final answer:
The phosphate group is connected to the sugar molecule at the 5' carbon.
Step-by-step explanation:
The phosphate group is connected to the sugar molecule at the 5' carbon. In DNA, the sugar is deoxyribose, and in RNA, the sugar is ribose. The carbon atoms of the sugar molecule are numbered as 1', 2', 3', 4', and 5' (1' is read as "one prime"). The phosphate group is attached to the hydroxyl group of the 5' carbon of one sugar and the hydroxyl group of the 3' carbon of the sugar of the next nucleotide, which forms a 5'-3' phosphodiester linkage.