Polynucleotides have directionality !
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, and 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 !!!