Mitosis

Mitosis is the part of the eukaryotic cell cycle in which the nucleus divides. It follows interphase, a preparatory period that includes the S phase, when the cell replicates its DNA. After that replication, each chromosome consists of two sister chromatids joined near a centromere. Mitosis does not copy the DNA; it ensures that the replicated chromosome material is apportioned between two new nuclei.

In textbooks, the larger cell-cycle phase is often called the mitotic phase or M phase. It includes mitosis and usually cytokinesis, the physical separation of the cytoplasm. The terms are related but not interchangeable: mitosis refers specifically to nuclear division, while cytokinesis completes the division of the cell body. These processes may overlap, and in some cells nuclear division can occur without immediate cytokinesis.

As mitosis begins, chromosomes become more compact and the mitotic spindle forms. In a detailed five-stage account, prophase is followed by prometaphase, when the nuclear envelope breaks down and spindle microtubules attach to protein structures called kinetochores on the chromosomes. During metaphase, chromosomes line up near the cell’s middle. In anaphase, sister chromatids separate and move toward opposite poles; each separated chromatid is then counted as a chromosome. In telophase, chromosomes begin to uncoil and new nuclear envelopes form around the two sets.

Some introductory accounts list four stages—prophase, metaphase, anaphase and telophase—by including prometaphase within the broader description of prophase. The five-stage terminology makes an additional transition explicit; both describe the same general sequence of chromosome condensation, alignment, separation and nuclear re-formation.

Cytokinesis usually follows or overlaps the later stages of mitosis. In many animal cells, a contractile ring draws the cell membrane inward to form a cleavage furrow. Plant cells build a cell plate between the new nuclei; it develops into a partition and, ultimately, a new cell wall. The two daughter cells ordinarily receive equivalent sets of chromosomes, although copying or segregation errors can occur.

Mitosis provides a way to increase cell number during growth and replace worn cells in multicellular organisms. It is also a means of asexual reproduction in some eukaryotes. Unlike meiosis, which involves two successive nuclear divisions and reduces chromosome number in the resulting cells, mitosis ordinarily maintains the parent cell’s chromosome number. Mitosis therefore produces daughter nuclei with the same chromosome complement as the parent nucleus, rather than the chromosome-number reduction associated with meiosis.