A signature feature of living systems is their ability to produce copies of
information-carrying molecular templates such as DNA. These copies are made
by assembling a set of monomer molecules into a linear macromolecule with a sequence determined by the template. The copies produced have a finite length –
they are often “oligomers”, or short polymers – and must eventually detach from
their template. We explore the role of the resultant initiation and termination of
the copy process in the thermodynamics of copying. By splitting the free-energy
change of copy formation into informational and chemical terms, we show that,
surprisingly, copy accuracy plays no direct role in the overall thermodynamics. Instead, finite-length templates function as highly-selective engines that interconvert
chemical and information-based free energy stored in the environment; it is thermodynamically costly to produce outputs that are more similar to the oligomers
in the environment than sequences obtained by randomly sampling monomers. In
contrast to previous work that neglects separation, any excess free energy stored in
correlations between copy and template sequences is lost when the copy fully detaches and mixes with the environment; these correlations therefore do not feature
in the overall thermodynamics. Previously-derived constraints on copy accuracy
therefore only manifest as kinetic barriers experienced while the copy is template
attached; these barriers are easily surmounted by shorter oligomers.