Figure 1. Landmark events of sporulation.
Sporulation involves chromosome distribution and spore morphogenesis. In meiosis, chromosomes first replicate, and then homologous chromosomes align and undergo recombination during prophase. Two consecutive nuclear divisions follow, in which, first, homologous chromosomes segregate apart (meiosis I), and then sister chromatids separate (meiosis II). These divisions are preceded by duplication and separation of the spindle pole bodies (SPBs) and formation of the spindle. One of the first discernible steps of spore morphogenesis occurs after the SPBs have duplicated and formed the meiosis II spindles. At this point, each of their outer plaques becomes modified and nucleates the formation of a flattened membrane sac which grows to become a prospore membrane. Maturation of the spore walls then results in an ascus containing a tetrad of four haploid spores. The gene expression program of sporulation has previously been characterized as a transcriptional cascade involving four temporal classes, indicated on the diagram. Ime1 is a transcriptional activator for some known early genes; Ndt80 is a transcriptional activator for some known middle genes.