Mitosis: stages and importance

1Where does a root grow?

Germinate a bean seed on damp paper until its root is about 2 cm long. With a fine pen and waterproof ink, mark lines on the root 1 mm apart, starting at the tip. Two days later the root is longer, but not evenly: the marks near the top are still 1 mm apart, while the marks in the few millimetres behind the tip have spread far apart. New cells are being made in a small region at the tip, and then they grow longer. The cells in that region are dividing by mitosis.

Figure 1: Ink marks on a bean root, drawn from what the experiment shows. Growth happens only near the tip: cells divide at the tip, then lengthen just behind it.

2The four stages of mitosis

Mitosis is one continuous process, but biologists describe it in four stages: prophase, metaphase, anaphase and telophase. Figure 2 follows an animal cell with $2n=42n\; =\; 4$: one long pair and one short pair of chromosomes, with the chromosomes from the father drawn blue and those from the mother orange. Count the chromosomes in every drawing.

Figure 2: Mitosis in an animal cell with 2n = 4. The + signs mark the centrosomes at the two poles, and the dashed line in metaphase is the equator. Each daughter cell ends with one copy of every chromosome, long and short, blue and orange, exactly like the parent cell.

• Prophase. The chromosomes coil up and become short, thick and visible; each has two sister chromatids. The nucleolus disappears and the nuclear membrane breaks down. In animal cells the two centrosomes move to opposite poles, and the spindle forms between them.
• Metaphase. The chromosomes line up singly across the equator of the spindle, each attached to spindle fibres by its centromere.
• Anaphase. The centromeres split. The spindle fibres shorten and pull the sister chromatids apart, centromere first, to opposite poles. Each separated chromatid is now called a chromosome.
• Telophase. The chromosomes reach the poles and uncoil. A nuclear membrane forms round each group, the nucleoli reappear and the spindle disappears.
• Cytokinesis. The cytoplasm divides. An animal cell pinches in from the outside; a plant cell builds a cell plate across the middle, which becomes the new cell wall.

Plant and animal cells divide in the same four stages, with two differences: plant cells have no centrioles in their centrosomes, and they divide their cytoplasm with a cell plate instead of pinching in. In a flowering plant, mitosis happens mainly in the meristems; in an animal it happens in many tissues, such as the skin, the lining of the gut and the bone marrow.

3Seeing mitosis: an onion root tip squash

Root tips are used because, as Figure 1 showed, they hold a meristem where many cells are dividing at any moment.

1. Stand an onion bulb on a jar of water for a few days, until its roots are 1 to 2 cm long.
2. Cut off the last 5 mm of a few roots.
3. Your teacher warms the tips in dilute hydrochloric acid for a few minutes. The acid breaks down the material that holds the cells together, so they can be spread apart.
4. Rinse a tip in water, put it on a slide and add a drop of a stain that colours chromosomes, such as acetic orcein or toluidine blue.
5. Lower a coverslip, cover it with a fold of filter paper and press straight down gently with your thumb, without twisting, so the cells spread into a single layer (the squash).
6. Find the small, square cells near the tip at low power, then look at them at high power ($\times 400\backslash times\; 400$).

4Why mitosis matters

Mitosis gives two cells with exactly the same chromosomes as the parent. That is what an organism needs in four situations:

• Growth. A zygote becomes a body of millions of millions of cells; a bean root lengthens from its meristem.
• Repair and replacement. Cells divide to close a cut, and to replace cells that wear away, such as the outer cells of the skin and the lining of the gut. Red blood cells have no nucleus and cannot divide, so new ones come from dividing stem cells in the bone marrow.
• Asexual reproduction. A new plant grown from one parent is made by mitosis, so it is genetically identical to that parent: a clone. Farmers in Rwanda use this every season when they plant banana suckers, cassava stem cuttings, sweet potato vines and potato tubers.
• Keeping the chromosome number constant. Every body cell that has a nucleus carries the same 46 chromosomes and the same genes, whether it is a skin cell or a liver cell. Each type of cell uses a different selection of those genes.

5When mitosis goes wrong: cancer

Normally, cells divide only when the body needs new cells, and checks during the cell cycle stop a damaged cell from dividing. If the genes that control division are damaged, a cell may divide again and again, out of control. The mass of cells it forms is a tumour. A benign tumour stays in one place; a malignant tumour, a cancer, invades nearby tissue and can spread to other parts of the body. Things that raise the risk of the damage include tobacco smoke, too much ultraviolet light from the sun, and some viruses, such as the HPV you met in Senior 2. Many cancer treatments, such as chemotherapy and radiotherapy, work by stopping dividing cells from dividing.