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our work is supported by the state committee for scientific research , grant numbers 6 p04a 025 - 18 and 6 p04a 016 20 .
the continuous line is the dna strand replicated as the leading strand ; the dashed line is the dna strand replicated as the lagging strand ; ori , the origin of replication ; ter , the terminus of replication .
ori and ter divide the chromosome into two replichores , arbitrarily called left and right .
plots of the relative positions of orthologs in the helicobacter pylori j99 and h. pylori 26695 genomes .
the values on the x and y axes represent the positions of genes on chromosomes , in base pairs .
( a ) the closest orthologs ( best matches ) that have not switched their positions between the leading and the lagging dna strands .
( b ) all orthologs that have switched positions between the leading and the lagging dna strands .
the genome sequences and orthologs , extracted from the database of clusters of orthologous groups ( ' cogs ' ) , were obtained from the national center for biotechnology information .
note that if the sense strand is lying on the leading dna strand , the direction of transcription of the gene is the same as the direction of replication - fork movement .
( a ) a symmetrical inversion encompassing the origin of replication . after the inversion , the distances to the origin and the locations of the genes do not change with respect to the leading and lagging dna strands .
( b ) the inverted region encompasses the origin but the origin is not located in the center of this region . as a result , the lengths of replichores change and the distances of the noninverted genes to the origin change , although the locations of the genes do not change with respect to the leading and lagging dna strands .
the locations of genes within the inverted sequence change with respect to the leading and lagging dna strands .
furthermore , genes located away from the center of the inverted region change their distance from the origin .
a response to   evidence for symmetric chromosomal inversions around the replication origin in bacteria by ja eisen , jf heidelberg , o white , sl salzberg . 
 genome biology 2000 , 1:research0011.1 - 0011.9 .