Maize Mitochondrial Elements, Chromosomal DNA, Mitochondria, Cytoplasm, DNA elements, Plasmid, Homology, Genome, Linear element

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	Maize Mitochondrial Elements - Studies of cytoplasmic male sterility (CMS) have revealed several of these extrachromosomal DNA elements. Unlike transposable elements, these appear to replicate autonomously in the mitochondria and may integrate into the mitochondrial genome. The cytoplasm of plants exhibiting, CMS may be classified into three types, T, C and S, based on their response to nuclear restorer genes. This division correlates well with features of the mitochondrial DNA and the presence of DNA elements.
Wild type mitochondrial CMS cytoplasms contain a 1.95 kbp circular element, a small cryptic self-replicating plasmid; all except T-CMS contain in addition a 2.4 kbp linear element. C-CMS cytoplasm has two small circular elements of 1.55 and 1.42 kbp; S-SMS cytoplasm contains two linear molecules of 6.2 kbp and 5.2 kbp called S₁ and S₂ respectively. These two elements have an inverted terminal repeat of 200 bp, sharing a region of homology of 1.3 kbp near to one end and, although they are linear and nonintegrated, appear to be capable of transposition within the mitochondrial genome. Indeed, they may have been derived from this genome as regions of homology exist between S₁ and S₂ and normal mitochondrial DNA.