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.
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.