We have analyzed the influence of constant mutational pressure and selectio
n on die nucleotide composition of DNA sequences of various size, which wer
e represented by the genes of the Borrelia burgdorferi genome. With the hel
p of MC simulations we have found that longer DNA sequences accumulate much
less base substitutions per sequence length than short sequences. This lea
ds us to the conclusion that the accuracy of replication may determine the
size of genome. (C) 1999 Elsevier Science B.V. All rights reserved.