Meiosis: an overview of key differences from mitosis

Abstract

Meiosis is the specialized cell division that generates gametes. In contrast to mitosis, molecular mechanisms and regulation of meiosis are much less understood. Meiosis shares mechanisms and regulation with mitosis in many aspects, but also has critical differences from mitosis. This review highlights these differences between meiosis and mitosis. Recent studies using various model systems revealed differences in a surprisingly wide range of aspects, including cell-cycle regulation, recombination, postrecombination events, spindle assembly, chromosome-spindle interaction, and chromosome segregation. Although a great degree of diversity can be found among organisms, meiosis-specific processes, and regulation are generally conserved.

Figure 1.

Bouquet formation and oscillatory nuclear movement in fission yeast meiosis. Clustering of telomeres and their linkage to the cytoskeleton enable oscillatory movement of the prophase nucleus and facilitate paring of homologous chromosomes. SPB, spindle pole body.

Figure 2.

Formation of the karyosome in Drosophila oocytes. The conserved protein kinase NHK-1 phosphorylates barrier-to-autointegration factor (BAF), a linker between the nuclear envelope and chromatin, to release meiotic chromosomes from the nuclear envelope. The meiotic recombination checkpoint suppresses NHK-1 activity to keep the nucleus in the recombination state when DNA double-strand breaks (DSBs) are still present.

Figure 3.

Reductional and equational chromosome segregation. Cohesin connects sister chromatids. In mitosis, sister kinetochores are attached to microtubules from the opposite poles. Cohesin connects sister chromatids and the removal of cohesin along chromosomes triggers sister chromatid separation. Homologous chromosomes behave independently. In meiosis I, sister kinetochores are attached to microtubules from the same pole. Homologous chromosomes are attached to the opposite poles and connected by chiasma. Destruction of cohesin from chromosome arms triggers homologous chromosome separation. Cohesin at centromeres is protected to provide a linkage among sister chromatids. In meiosis II, sister kinetochores are attached to microtubules from the opposite poles. Destruction of the centromeric cohesin triggers sister chromatid separation.

Figure 4.

Chromosome-mediated spindle assembly through Ran. Ran-GDP is converted into Ran-GTP by chromosome-associated RCC1 to generate a Ran-GTP gradient. Near chromosomes, Ran-GTP activates spindle-assembly factors by removing importin from them. Microtubules and the spindle can be assembled only in proximity to the chromosomes.