Polynucleotides
have directionality !
Adjacent nucleotides in a
single
strand of DNA
(polynucleotide) are joined by a phospho - diester
bond
between their 3' and 5'
carbons. (Recall that an ester bond is R - O - R).
This
means that
the respective 5' and 3' carbons are exposed
at either
end of the
polynucleotide, which are therefore called the
5' end
and the 3' end. These
are also
called the
phosphoryl and hydroxyl ends,
respectively, because of
the chemical groups typically attachd to
those ends. A polynucleotide therefore has
directionality:
the
two ends of the molecule have very different biochemical
properties,
and behave very differently in molecular genetic
processes. It is
therefore critical to recognize which end is which.
A
detailed understanding of this figure is key to the first
part of
the course !!!