Toxoplasma gondii is haploid except during the sexual division in the intestine of the cat. Sporozoites are the results of meiosis and seem to follow classical Mendelian laws (Pfefferkorn et al. 1977). The total haploid genome contains approximately 8 x 107 bp (Cornelissen et al. 1984), and a 36kb circular mitocondrial DNA (Borst et al. 1984), which has been partly sequenced (Ossorio et al. 1991). Nine chromosomes have been identified by

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pulse-field gel electrophoresis, and a molecular karyotype constructed by using probes from low-copy number genes (Sibley and Boothroyd 1992a). The tubulin genes have been described, and both contain introns (Boothroyd et al. 1987; Nagel and Boothroyd 1988). Only the B1 gene has been found to be tandemly repeated (Burg et al. 1989), but other sequences repeated many times have been identified (McLeod et al. 1991; Blanco et al. 1992). The T. gondii DNA has been characterized and a genetic nomenclature for T. gondii has been proposed (Sibley et al. 1991).

Toxoplasma gondii rRNA has the usual large and small sub-units (Gagnon et al. 1993). Sequence analysis of the small sub-unit rRNA suggests that T. gondii is phylogenetically related to Sarcocystis, but separate from Plasmodium (Guay et al. 1993).

In 1997 the third DNA-containing organelle, the apicoplast, was described in apicomplexan parasites including T. gondii and Plasmodia spp. (Fichera and Roos 1997). The apicoplast contains several novel enzymatic pathways, which may be potential new drug targets (Roos 1999; Soldati 1999).

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