If PCR is performed using a proofreading DNA polymerase, such as Pfu DNA polymerase, the product will have blunt ends. Taq DNA polymerase catalyzes the non-template directed addition of an adenine residue to the 3´-end of both strands of DNA molecules to make it suitable for TA cloning.

Details: http://openwetware.org/wiki/Addition_of_3'_A_overhangs_to_PCR_products

Despite remarkable variation in morphology and function,
all neurons possess a set of attributes that allow their
assignment to this cell type (Kandel et al., 2000). These
features are likely determined by genes, called pan-neuronal
genes, uniquely expressed in all (or nearly all) neurons, but not
in other cell types. However, in many instances broadly
expressed neuronal genes are also expressed in other cell types
(Iwasaki et al., 1997; Nonet et al., 1999; Rajaram et al., 1999;
Sieburth et al., 2005). Characterization of such broadly
expressed neuronal genes (a term we will use interchangeably
with pan-neuronal) has a potential to uncover the genetic
modules that give neurons their distinctive features. Identification
of these genes has lagged behind the progress made toward
understanding the mechanisms of neuronal subtype specification
(Melkman and Sengupta, 2004; Shirasaki and Pfaff, 2002;
Thor and Thomas, 2002). The identification of these genes may
also provide clues to the molecular mechanisms that regulate
their expression. If subsets of pan-neuronal genes are controlled
by a common set of transcription factors, they are likely to share
transcription factor binding sites in their cis-regulatory regions
(Zhang, 1999). We investigated this possibility by searching for
sequence motifs overrepresented in the promoters of known
pan-neuronal genes.

From:
http://ruvinskylab.uchicago.edu/files/ruvinsky_2007.pdf

Detection of broadly expressed neuronal genes in C. elegans