DNA polynucleotide chain
The two ends of the
polynucleotide chain have different chemical properties,
and are designated the 5' end [shown at the top
left] and the 3' end [shown at the bottom
right], respectively, according to the deoxyribose
carbon at either end. The 5' end is also called
the phosphoryl end, because a PO4 group
is usually attached to it. Likewise, the 3' end is also called
the hydroxyl end, becaue of the -OH group attached to
the 3' carbon.
Note that designation of the ends as "right" and "left" , or "top"
and "bottom" is arbitrary, according to how the
figure is drawn. Do not assume that DNA strands will
always be drawn from left to right in the 5' to 3' or PO4 to OH directions.