| The genomes are
often
asymmetric: the composition of the strand being
continuously
replicated (leading strand) and its complementary (lagging strand) show
different G+C content. We have generated GC- and
AT-skew images
to show this behaviour for all sequenced bacterial chromosomes and
plasmids. To
compute GC-skew, sequenced prokaryotic genomes were scanned for
(G-C)/(G+C) ratio with a sliding window along the genome. In some genomes the distribution of positive and negative values for the (G-C)/(G+C) ratio is closely related to leading strand (positive) and lagging strand (negative). It has been suggested G-C substitution to be responsible for this effect. In the example bellow, E.coli K12 genome switched polarity in the vicinity of the terminus (ter) and origin (ori) of replication, with the leading strand manifesting a positive skew. Interesting genomic features can be deduced from these graphs, such as genome reduction, multiple replication origins, genomic inversions or horizontal gene transfer events. For further information check Mackiewicz et al., 2004 Example of GC- and AT skews for E. coli K12 genomeCG-skew
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