Finally, we probed the extent to which the selective binding of trimers to fiber sides can lead to the correction of errors in replication. The nature of the hexamer exposed at a fiber end plays an important role in replication, which causes that mutants formed at the fiber end tend to propagate. However, the selective binding of trimers to the fiber side may reduce this tendency for an error in replication to propagate. We probed the degree of error correction by determining the fraction of cases where, starting from a seed with a specific building block composition, after a mutation occurred (i.e. a hexamer with a different building block composition formed at the fiber end), it was followed by elongation by a second hexamer with the correct composition (i.e. a composition identical to that of the original seed). In case (i) this fraction of errors corrected was independent of whether replication took place from fiber-bound trimers and appears to be governed only by chance (SI section S7). In case (ii), the fraction of errors corrected for A6B0 upon replication from surfacebound trimers was more than double of that in the control without surface binding (Figure ). In case (iii), the fraction of errors corrected (for A6B0 or A0B6) was about 25 times higher than for the control, but of low absolute value (Figure ). The strong self-interactions did indeed increase the chance the correct hexamer will stack (Figure ), and they also decrease the chance an error will propagate. In case (iv), as expected, the degree of error correction was not significantly affected by the precursors binding to fiber sides (Figure ). (B) case iii (E AA = E BB = -2, and E AB = 0 growing on equal amounts of A6B0 and A0B6 fibers); and (C) case iv (E AA = E BB = 0, and E AB = -2; seeded with A3B3 fibers).