Based on 152 mitochondrial genomes and 36 bacterial chromosomes that have been 
completely sequenced, as well as three long contigs for human chromosomes 6, 21, 
and 22, we examined skews of mononucleotide frequencies and the relative 
abundance of dinucleotides in one DNA strand. Each group of these genomes has 
its own characteristics. Regarding mitochondrial genomes, both CpG and GpT are 
underrepresented, while either GpG or CpC or both are overrepresented. The 
relative frequency of nucleotide T vs A and of nucleotide G vs C is strongly 
skewed, due presumably to strand asymmetry in replication errors and 
unidirectional DNA replication from single origins. Exceptions are found in the 
plant and yeast mitochondrial genomes, each of which may replicate from multiple 
origins. Regarding bacterial genomes, the "universal" rule of CpG deficiency is 
restricted to archaebacteria and some eubacteria. In other eubacteria, the most 
underrepresented dinucleotide is either TpA or GpT. In general, there are 
significant T vs A and G vs C skews in each half of the bacterial genome, 
although these are almost exactly canceled out over the whole genome. Regarding 
human chromosomes 6, 21, and 22, dinucleotide CpG tends to be avoided. The 
relative frequency of mononucleotides exhibits conspicuous local skews, 
suggesting that each of these chromosomal segments contains more than one DNA 
replication origin. It is concluded that, when there are several replicons in a 
genomic region, not only the number of DNA replication origins but also the 
directionality is important and that the observed patterns of nucleotide 
frequencies in the genome strongly support the hypothesis of strand asymmetry in 
replication errors.