  Living systems produce copies of information-carrying molecules such as DNA
by assembling monomer units into finite-length oligomer (short polymer) copies.
We explore the role of 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 copy accuracy
plays no direct role in the overall thermodynamics. Instead, it is
thermodynamically costly to produce outputs that are more similar to the
oligomers in the environment than sequences obtained by randomly sampling
monomers. Copy accuracy can be thermodynamically neutral, or even favoured,
depending on the surroundings. Oligomer copying mechanisms can thus function as
information engines that interconvert chemical and information-based free
energy. Hard thermodynamic constraints on accuracy derived for infinite-length
polymers instead manifest as kinetic barriers experienced while the copy is
template-attached. These barriers are easily surmounted by shorter oligomers.
