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 left],
respectively, according to the reactive deoxyribose carbon
at either end. The 5'
end is also called the phosphoryl end, because a
single 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" or "left" is arbitrary: do not
assume that DNA
strands will always be drawn from left to right in
the 5' to 3' or PO4 to OH directions.