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FIGURE 6. Cell viability increases as the average kinetoplast size increases and as the number of minicircle classes decreases (dotted line 70 classes, solid line 17 classes). This is because classes have more copies and hence there is more chance that a daughter cell receives the full complement of classes. Averages were taken over 10 simulations; error bars show ± 2 SEM. Previously published in Savil and Higgs (1999).

In the case of T. brucei, random segregation of the 250+ sequence classes would lead to a predicted cell viability in this model of less than 0.5, and hence population extinction. However, incorporating the information that each minicircle in this species encodes multiple gRNAs and that genetic exchange occurs, it has been shown that the model can produce the observed situation of evolutionary viability and multiple redundant and nonfunctional gRNAs (N.J.S. and P. G. Higgs, unpublished results). Mutation of the gRNA genes and drift in the minicircle copy numbers lead to an ever-increasing number of necessary classes encoding ever fewer functional gRNAs per minicircle.

C TO U EDITING AND THE ORIGIN OF URIDINE-INSERTION EDITING IN TRYPANOSOMES
UGA Codon Reassignment

Kinetoplastids use a nonuniversal genetic code in which the UGA stop codon is read as tryptophan (de la Cruz et al., 1984). The codon capture hypothesis (Inagaki et al., 1998; Osawa et al., 1992) proposes that evolutionary reassignment of a stop codon involves first the disappear-



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