Polynucleotides have directionality !
nucleotides in a single strand of DNA (polynucleotide)
are joined by a phosphodiester bond
between their 3' and 5' carbons. (Recall that
an ester bond is R - O - R: the bond here is
C - O - P - O - C). 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 found
at 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 critical to the first part of the course !!!