The Cell Cycle, Mitosis and Meiosis — University of Leicester
Two successive nuclear divisions occur, Meiosis I (Reduction) and Meiosis II ( Division). Meiosis produces 4 haploid cells. The division cycle of most eukaryotic cells is divided into four discrete phases: M, G1, S, and G2. M phase (mitosis) is usually followed by cytokinesis. S phase is. In cell biology, mitosis is a part of the cell cycle when replicated chromosomes are separated into two new nuclei. Cell division gives rise to genetically identical cells in which the number of chromosomes is maintained. In general, mitosis ( division of the nucleus) is preceded by the S stage of Mitosis and cytokinesis together define the mitotic (M) phase of an animal.
After crossing over, the spindle begins to capture chromosomes and move them towards the center of the cell metaphase plate. This may seem familiar from mitosis, but there is a twist.
The Cell Cycle, Mitosis and Meiosis
Each chromosome attaches to microtubules from just one pole of the spindle, and the two homologues of a pair bind to microtubules from opposite poles. So, during metaphase I, homologue pairs—not individual chromosomes—line up at the metaphase plate for separation. The phases of meiosis I.
Homologous chromosomes pair up and exchange fragments in the process of crossing over. Homologue pairs line up at the metaphase plate. Homologues separate to opposite ends of the cell. Sister chromatids stay together. Each chromosome still has two sister chromatids, but the chromatids of each chromosome are no longer identical to each other.
When the homologous pairs line up at the metaphase plate, the orientation of each pair is random. For instance, in the diagram above, the pink version of the big chromosome and the purple version of the little chromosome happen to be positioned towards the same pole and go into the same cell. But the orientation could have equally well been flipped, so that both purple chromosomes went into the cell together. This allows for the formation of gametes with different sets of homologues.
Diagram showing the relationship between chromosome configuration at meiosis I and homologue segregation to gametes.
In this case, four different types of gametes may be produced, depending on whether the maternal homologues are positioned on the same side or on opposite sides of the metaphase plate. In anaphase I, the homologues are pulled apart and move apart to opposite ends of the cell.
The sister chromatids of each chromosome, however, remain attached to one another and don't come apart. Finally, in telophase I, the chromosomes arrive at opposite poles of the cell. Cytokinesis usually occurs at the same time as telophase I, forming two haploid daughter cells. These cells are haploid—have just one chromosome from each homologue pair—but their chromosomes still consist of two sister chromatids.
In meiosis II, the sister chromatids separate, making haploid cells with non-duplicated chromosomes. Starting cells are the haploid cells made in meiosis I.
Endoreduplication or endoreplication occurs when chromosomes duplicate but the cell does not subsequently divide. This results in polyploid cells or, if the chromosomes duplicates repeatedly, polytene chromosomes. Instead of being divided into two new daughter nuclei, the replicated chromosomes are retained within the original nucleus.
Platelet -producing megakaryocytes go through endomitosis during cell differentiation.
Phases of the cell cycle (article) | Khan Academy
Karyokinesis without cytokinesis originates multinucleated cells called coenocytes. Related cell processes[ edit ] Cell rounding[ edit ] Cell shape changes through mitosis for a typical animal cell cultured on a flat surface. The cell undergoes mitotic cell rounding during spindle assembly and then divides via cytokinesis. Rounding also occurs in live tissue, as described in the text.
Mitotic cell rounding In animal tissue, most cells round up to a near-spherical shape during mitosis. Generation of pressure is dependent on formin -mediated F-actin nucleation  and Rho kinase ROCK -mediated myosin II contraction,    both of which are governed upstream by signaling pathways RhoA and ECT2   through the activity of Cdk1. Mitotic recombination[ edit ] Mitotic cells irradiated with X-rays in the G1 phase of the cell cycle repair recombinogenic DNA damages primarily by recombination between homologous chromosomes.
Evolution[ edit ] Some types of cell division in prokaryotes and eukaryotes There are prokaryotic homologs of all the key molecules of eukaryotic mitosis e.
CELL DIVISION: Meiosis
Being a universal eukaryotic property, mitosis probably arose at the base of the eukaryotic tree. As mitosis is less complex than meiosismeiosis presumably arose after mitosis. In relation to the forms of mitosis, closed intranuclear pleuromitosis seems to be the most primitive type, as it is the more similar to bacterial division.
Polar microtubules, shown as green strands, have established a matrix around the currently intact nucleus, with the condensing chromosomes in blue.