Detour: Directionality of DNA Strands

The sugar component of a nucleotide has a ring of five carbon atoms, which are labeled as 1′, 2′, 3′, 4′, and 5′ in the figure given below (left). The 5′ atom is joined onto the phosphate group in the nucleotide and eventually to the 3′ end of the neighboring nucleotide.
The 3′ atom is joined onto another neighboring nucleotide in the nucleic acid chain.

As a result, we call the two ends of the nucleotide the 5′-end and the 3′-end (pronounced “five prime end” and “three prime end”, respectively).