DNA polynucleotide chain

The two ends of the polynucleotide chain have different chemical properties, and are designated the 5' end [shown at right] and the 3' end [shown at the bottom], 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 completely 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.

Figure 2012 TA Brown, Introduction to Genetics (1st ed.); additional text 2013 by Steven M. Carr