Directionality of DNA replication
DNA polymerase can only assemble a new strand in the 5' to 3' direction. In other words, the enzyme can only add new nucleotides to the 3' end of a strand being assembled.
Because two complementary strands run in opposite directions, the template (or parent) DNA strand can only be read in the 3' to 5' direction.
The directionality arises because the 3' carbon of one deoxyribose is linked to the 5' carbon of another deoxyribose.
In a double strand, the 3' end on one strand is adjacent to the 5' end on the other strand. The two strands must be read in opposite directions.