The SMC1 and SMC3 head domains, which contain ATPase motifs, interact with the C- and N-termini of the REC8 or RAD21 or RAD21L kleisin subunit, effectively closing the ring. SMC1 and SMC3 form a heterodimer, interacting through their hinge regions. These cohesin complexes are important for chromosome segregation, DNA repair, gene expression, development, and genome integrity.Ĭohesin subunits form a ring-like structure. Because these subunits are quite diverse, a wide variety of cohesin complexes with different subunit compositions exists in mitotic and in meiotic cells. In mammals these are the following: two structural maintenance of chromosomes (SMC) subunits (SMC1α or SMC1β and SMC3) one stromalin, HEAT-repeat domain subunit (STAG1or STAG2 or STAG3 also called SA1 or SA2 or SA3, respectively) and one kleisin subunit protein (RAD21 or REC8 or RAD21L) ( Figure 1). Although the subunits differ slightly in mitosis and meiosis, the canonical cohesin complex is composed of four subunits. Cohesion is established during DNA replication before both mitosis and meiosis by multiprotein subunit complexes called cohesins. However, the end result is four haploid spermatids or one haploid oocyte (and two or three polar bodies) that are not identical to each other or to the parent cell.īoth mitosis and meiosis require cohesion to keep the sister chromatids together until separation is imminent at anaphase. The second meiotic division is exactly like the division in mitosis, with separation of the sister chromatids. Unlike in mitosis, the sister chromatids remain attached at their centromeres by cohesion, and only the homologous chromosomes segregate during anaphase I. The bivalents, which are attached to microtubules through their kinetochores and centromeres, align on the metaphase plate during metaphase I. Prophase I is divided into five distinct sub-stages: leptonema, zygonema, pachynema, diplonema, and diakinesis. This involves chromosomal synapsis and formation of a tripartite protein complex, the synaptonemal complex (SC), as well as formation of chiasmata. In prophase I, however, DNA crossovers form between paired homologous chromosomes, called bivalents. During meiosis I, the chromatin condenses as in mitosis and the sister chromatids are held together through a process called cohesion. The key difference between meiosis and mitosis is that meiotic cells undergo two cell divisions, meiosis I and meiosis II, without an intervening S phase. Meiosis occurs strictly in germ cells and differs between males and females. Separation of sister chromatids in mitosis is the most important event during the cell cycle and this process must be monitored effectively. The end result is two daughter cells that are identical to each other and to the parent cell.

The separation is completed in anaphase owing to the loss of cohesion between the sister chromatids. The sister chromatids must be physically connected through the G2 phase and will only begin to separate during the transition from metaphase to anaphase during mitosis. Growth, repair, and maintenance of organism/cellsįor creation of cells used in sexual reproductionĤ haploid cells (chromosome number halved).During the S phase of the cell cycle, DNA replication generates a pair of sister chromatids with identical genetic content. DNA replication is a result of the one-two punch recombination of and independent assortment.ĭiploid parent/mother cell divides into two identical diploid daughter cellsĭiploid parent/mother cell undergoes two separateĭivision events to create 4 haploid daughter cells Also, neither phase of meiosis involves the replication of any new genetic material. It helps to organize these schemes if you keep in mind that meiosis II is simply a mitotic division. Also, as noted, mitosis has one round of phases that gives rise to two daughter cells, whereas meiosis has two phases and gives rise to four daughter cells.

Prophase of mitosis vs meiosis