DNA polynucleotide chain

    Succesive nucleotides in a polynucleotide chain are joined by 3'-5' phosphodiester bonds. [Recall that an ester bond is R - O - R]. 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.

Figure © 2012 TA Brown, Introduction to Genetics (1st ed.); additional text © 2015 Steven M. Carr