DNA Structure and Function
A double strand is a helix, as the basepairs move with a horizontal angle with each step to the next basepair. Identifying the sides is important for DNA-replication and RNA synthesis as the way DNA is read is from 3' to 5' so that the complementary bases can be ''glued'' together from 5' to 3' in which the phosphate groups bind to the OH groups of the other molecule by a polymerase reaction. 
Disregard first answer.

There.