2. What is mitosis? Mitosis is a process of nuclear division in eukaryotic cells conventionally divided into five stages. Mitosis conserves chromosome number by allocating replicated chromosomes equally to each of the daughter nuclei. Mitosis produced the somatic cells that now make up your body and is also the means by which your body continues to generate new cells to replace dead and damaged ones.
3. Interphase A nuclear envelope bounds the nucleus. The nucleus contains one or more nucleoli. Two centrosomes have formed by replication of a single centrosome. In animal cells, each centrosome features two centrioles. Chromosomes that were duplicated during S phase cannot be seen individually because they have not yet condensed.
4. Prophase The chromatin fibers become more tightly coiled. The nucleoli disappear. Each duplicated chromosome appears as two identical sister chromatids joined together at their centromeres and along their arms by cohesins. The mitotic spindle begins to form. The spindle is composed of the microtubules and centrosomes that extend from them. Shorter microtubules that extend from the centrosomes are called asters. The centrosomes move away from each other, pushed away by the lengthening microtubules between them.
5. Prometaphase The nuclear envelope fragments. The microtubules extending from each centrosome can now invade the nuclear area. The chromosomes have become even more condensed. Each of the two chromatids of each chromosome now has a protein structure located at the centromere called a kinetochore. Some of the microtubules attach to the kinetochores, becoming “kinetochore microtubules” that push and pull the chromosomes back and forth. Nonkinetochore microtubules interact with those from the opposite pole of the spindle.
6. Metaphase Metaphase is the longest stage of mitosis, and lasts about 20 minutes. The centrosomes are now at opposite poles of the cell. The chromosomes line up on the metaphase plate, an imaginary plane that is equal distance between the spindle’s two poles. The chromosome’s centromeres lie on the metaphase plate. The kinetochores of the sister chromatids are attached to kinetochore microtubules reaching from opposite poles.
7. Anaphase Anaphase is the shortest stage of mitosis, and lasts only a few minutes. When the cohesin proteins are cleaved, anaphase begins. This allows the two sister chromatids of each pair to part. The two daughter chromosomes begin moving toward opposite ends of the cell as the kinetochore microtubules begin to shorten. The chromosomes move centromere first because the microtubules are attached to the centromeres. They travel at about 1 micrometer per minute. The cell elongates as the nonkinetochore microtubules lengthen. By the end of anaphase, the two ends of the cell have complete and equal collections of chromosomes.
