Antiparallel Strands

Each backbone in a double helix consists of
deoxyribose sugars alternating with phosphate
groups that link the 3' carbon atom of
one sugar to the 5' carbon of the next in
line.The two polynucleotide
strands of the double helix have opposite
polarity in the sense that the 5' end of one
strand is paired with the 3' end of the other
strand. Strands with such an arrangement
are said to be antiparallel. One implication
of antiparallel strands in duplex DNA is
that in each pair of bases, one base is attached
to a sugar that lies above the plane
of pairing, and the other base is attached to
a sugar that lies below the plane of pairing