Cell division: mitosis and meiosis

What cell division is

Cell division is how one cell becomes more than one. A growing body, a healing cut, and the lining of the gut all depend on it. Most of the cells in an adult were produced by division. In animals, plants, and other eukaryotes, the nucleus divides in one of two ways. Mitosis copies a cell and keeps the chromosome set the same. Meiosis halves that set and is how eggs and sperm are made. They are not two names for one process.

Both start only after the cell has copied its DNA. That copying happens in S phase, the middle of interphase, between G1 and G2. After S phase the cell still has the same number of chromosomes. What changed is that each chromosome is now made of two sister chromatids, identical copies joined at the centromere. A human body cell still has 46 chromosomes after S phase. It has 92 chromatids. Exams often count those chromatids as extra chromosomes. They are not extra chromosomes until the sister chromatids actually separate.

Mitosis is a single division. The stages usually taught are prophase, metaphase, anaphase, and telophase, and then cytokinesis splits the cytoplasm. In prophase the chromosomes condense and the spindle forms. The nuclear envelope breaks down around the end of prophase, which some courses call prometaphase. In metaphase each chromosome lines up at the equator, attached to fibers from both poles. In anaphase the sister chromatids separate and move apart. Each one is now a chromosome on its own. In telophase new nuclear envelopes form. Cytokinesis then makes two cells. In animal cells the cell pinches in. In plant cells a new wall is built between the daughters. Starting from a diploid cell, both daughters are diploid and genetically the same as the parent. A human body cell gives two cells with 46 chromosomes each.

Mitosis is the division used for growth, for replacing cells that wear out, and for asexual reproduction in organisms that use it. Skin and the lining of the gut keep dividing. Most neurons do not. Mitosis is not how humans make gametes.

Meiosis is two divisions, and it happens in the germline. In meiosis I, matching chromosomes — one from each parent, called homologues — pair up. In prophase I, non-sister chromatids of a pair can exchange pieces. That exchange is crossing over. In anaphase I the homologues separate. The sister chromatids stay together. The two cells that result are haploid: one chromosome from each pair, still duplicated. Meiosis II looks more like mitosis. There is no second copying of the DNA. The sister chromatids separate, and four haploid nuclei are the usual result. They are not genetically identical. Crossing over mixes alleles along a chromosome, and each pair lines up independently of the others. That second shuffle is independent assortment.

In humans those haploid cells have 23 chromosomes. Sperm production can turn all four products into gametes. Egg production does not. Meiosis in the ovary yields one egg and small polar bodies, so the chromosome set is haploid without becoming four working eggs. Fertilization joins two haploid gametes and restores 46.

When each division happens is a standard question. Mitosis runs through life in tissues that still divide. Meiosis is restricted to the cells that become eggs and sperm. In the human ovary, meiosis begins before birth and pauses in prophase I. It resumes years later. Meiosis I finishes around ovulation, the secondary oocyte stops in metaphase II, and meiosis II finishes only if a sperm fertilizes it. In the testis, meiosis begins at puberty and continues.

Held side by side: mitosis is one division, sister chromatids separate, two identical diploid cells, for growth and repair. Meiosis is two divisions, homologues separate first and sister chromatids second, haploid cells that differ from each other, for sexual reproduction.