At one end of the strand, a free
(meaning that it’s unattached on one side) phosphate group
is attached to the 5′-carbon atom of the sugar in the nucleotide. This end of the strand is therefore referred to as the 5′ end
. The other end of the strand, referred to as the 3′ end
, has a free OH group
attached to the 3′-carbon atom of the sugar.