Practice question

DNA polymerase synthesizes DNA strictly in 5' to 3' direction by adding nucleotides to the free 3' hydroxyl group of the growing chain. In Sanger sequencing, labeled primer anneals to template and polymerase extends it; each termination product represents a strand made 5' to 3'. Consequently, when fragments are ordered by size from smallest to largest after electrophoresis, sequence read corresponds to 5' to 3' synthesis product, complementary to template strand oriented 3' to 5'. Synthesis never proceeds 3' to 5' enzymatically, and reading is directional not random.