E. coli DNA Polymerases
The earliest studies of DNA polymerases began in the 1950s with Arthur Kornberg's discovery of DNA polymerase from E. coli. Three DNA polymerases are known in E. coli:
DNA polymerases
use deoxyribonulceoside triphosphates (dNTPs) to synthesize DNA
sequences. dNTPs are activated compounds which are cleaved to
generate a pyrophosphate and a deoxyribonucleoside monophosphate
(dNMP) covalent linked into a DNA chain. dNMPs are added into
the growing chain exclusively in the 5' to 3' direction. DNA polymerases
require a "primer", which is a preexisting nucleic acid
annealed at the site where replication is to begin (Figure
24.15). Primers of DNA, as well as RNA, can be extended
by a DNA polymerase. In addition to the catalytic activity
for polymerizing DNA, many DNA polymerases also contain
one or more exonuclease activities. The function of these is described
with each enzyme above. DNA polymerases can copy around a circular
single-strand template, such as the DNA extracted from small bacteriophages
(e.g., 
X174 or M13), as long as a primer is present. DNA
polymerases cannot, however, join the ends. When the template
is linear, polymerase copies only to the 5´ end of the template
and then it dissociates.