The two ends of a DNA or RNA strand
Strands grow from 5' to 3'
A strand of DNA or RNA has a direction. Its 5' end carries a phosphate group on the 5' carbon of the sugar, and its 3' end has a free hydroxyl group on the 3' carbon. New nucleotides can only be added to the 3' end, so strands are always built from 5' to 3'. The two strands of a DNA double helix are antiparallel: they run in opposite directions. By convention, sequences are written from 5' to 3'.