Cell Division: Mitosis and Meiosis

Every cell comes from an existing cell dividing. The cell cycle is the repeating series of stages a cell goes through between one division and the next. Cells divide for a few clear reasons: an organism needs to grow by adding new cells, damaged or worn-out tissue needs to be repaired, and in sexually reproducing organisms, special cells divide to make gametes (sperm and egg) for reproduction. There are two kinds of cell division covered here. Mitosis produces two new cells that are genetically identical to the parent cell, used for growth and repair. Meiosis produces reproductive cells with half the usual chromosome number, used only to make gametes. Both start from the same kind of cell and both begin with the cell copying its DNA, but they end very differently.

A chromosome is a single, tightly coiled strand of DNA. Most body cells are diploid, meaning they carry two full sets of chromosomes, written 2n, one set inherited from each parent. A human somatic cell is diploid with 2n = 46 chromosomes, arranged as 23 matching pairs. Each pair is a set of homologous chromosomes, two chromosomes that carry genes for the same traits in the same order, one from the mother and one from the father. A cell with only one set of chromosomes, half the usual number, is haploid, written n. In humans, n = 23. Only gametes (eggs and sperm) are haploid.

Before any division, a cell spends most of its life in interphase, which has three parts. In G1 (first gap), the cell grows and carries out its normal functions. In S phase (synthesis), the cell copies every chromosome, so each one now consists of two identical halves called sister chromatids, joined at a point called the centromere. In G2 (second gap), the cell grows further and makes the proteins needed for division. Copying the DNA in S phase does NOT change the chromosome number. A cell with 2n = 46 chromosomes still has 46 chromosomes after S phase. Each of those 46 chromosomes is simply doubled, now made of two sister chromatids instead of one strand.

Mitosis is the division of the nucleus into two identical nuclei. It happens in four phases, easy to remember with the letters PMAT.

After telophase, cytokinesis splits the cytoplasm into two separate cells. In animal cells the membrane pinches inward, a cleavage furrow. In plant cells, which have a rigid cell wall, a cell plate forms down the middle and grows outward into a new wall. Example: A skin cell with 2n = 46 enters mitosis. Because DNA was already copied in interphase, each chromosome is doubled going into prophase. When anaphase separates the sister chromatids, 46 chromatids move to each pole. Each resulting daughter cell ends up with 46 single chromosomes, so both daughter cells are 2n = 46, identical to the parent.

Meiosis happens only in the ovaries and testes, and it produces gametes. If gametes were diploid like every other cell, fertilization would double the chromosome number every generation. Meiosis solves this by cutting the chromosome number in half, from diploid (2n) to haploid (n), so that when a haploid sperm fertilizes a haploid egg, the resulting cell is diploid again. Meiosis starts the same way mitosis does, a diploid cell copies its DNA once in interphase. But instead of one division, meiosis carries out two divisions in a row, called meiosis I and meiosis II, without copying the DNA again in between. One diploid cell going through meiosis ends with four haploid cells, and none of the four are genetically identical to each other.

Meiosis I is called the reductional division because it is the step that actually cuts the chromosome number in half. In prophase I, homologous chromosomes pair up and can swap segments in a process called crossing over. In metaphase I, homologous PAIRS (not single chromosomes) line up at the middle of the cell, and which pair faces which pole is random, called independent assortment. In anaphase I, whole homologous chromosomes separate from their partner and move to opposite poles, sister chromatids stay joined. The result is two cells, each haploid (n), but each chromosome is still doubled into two sister chromatids. Meiosis II looks just like mitosis, but starting from a haploid cell instead of a diploid one. In each of the two cells from meiosis I, the sister chromatids line up, separate in anaphase II, and are pulled to opposite poles. The end result of meiosis II is four haploid cells total, each with single, unduplicated chromosomes.

Mitosis produces genetically identical cells on purpose, a body needs its new skin or muscle cells to match. Meiosis does the opposite: it is built to shuffle genetic material so that every gamete is unique. Three things happen during meiosis that create this variation. Crossing over, in prophase I, homologous chromosomes exchange small segments of DNA, mixing maternal and paternal genes onto the same chromosome. Independent assortment, in metaphase I, each homologous pair lines up at random with respect to the other pairs, so which maternal and paternal chromosomes end up together in a gamete is random. Random fertilization, at the end, which sperm fertilizes which egg is also random, multiplying the variation further. Together, these three sources of shuffling are why full siblings (other than identical twins) are never genetically identical, even though they share the same two parents.

Example: Starting from one diploid human cell (2n = 46), mitosis produces 2 daughter cells, each 2n = 46, identical to the parent. Starting from the same cell, meiosis produces 4 daughter cells, each n = 23, and each one genetically distinct from the others because of crossing over and independent assortment.

Using onion-root-tip-cell-counts.csv: 1. How many total cells were counted across all 5 fields? 2. What percent of the total cells were in interphase? Round to the nearest tenth of a percent. 3. What percent of cells were in prophase, and about how many minutes of the 1440-minute cell cycle does that percent represent? 4. Which phase has the fewest cells counted? What percent of the cycle, and how many minutes, does that phase represent? 5. Compare metaphase and anaphase: how many minutes of the cycle does each represent, and which one is longer?