A-overhang

Adenylation of blunt-ended DNA fragments enables you to create a binding site for a specific adapter to any DNA strand. Adenylation of a blunt-ended DNA results in a 3' end Adenine (A) overhang, and as you remember, Adenine pairs up specifically with Thymine.

Adapters are short, double stranded DNA sequences with a Thymine (T) overhang at the 3' end that will specifically bind to the 3' Adenine of the DNA sample.

A non-proof reading Polymerase such as Taq can be used to catalyze the Adenylation, thanks to their terminal-transferase activity.