The evolutionary origin of ori sequences

The extremely high homology of the eight canonical ori sequences indicates that they arose as the result of duplication and translocation events. More precisely, we proposed (see Fig. 2.20) that the canonical ori sequences derive from a primitive ori sequence (probably made of only a monomeric cluster C and its flanking sequences r and r) through (i) a series of duplications and inversions generating clusters A and B and (ii) an expansion process producing the AT stretches of ori sequences. It is possible that the ori sequences arc folded in a tertiary structure, as in the hypothetical model of Fig, 2.21. 

E- igurc 2,20, Hypothetical scheme for the evolutionary construction of ori sequences. (Irom dc Zamaroezy 

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