Mitosis preserves chromosome number

Explanation

Interphase includes G1 growth, S-phase DNA replication and G2 preparation. After S phase each chromosome consists of two sister chromatids joined at a centromeric region. DNA amount has doubled, but chromosome number counted by centromeres has not yet doubled.

In prophase chromosomes condense and the spindle develops; in the usual animal-cell model centrosomes containing centrioles move apart and the nuclear envelope breaks down. At metaphase chromosomes align individually at the equator, with sister chromatids attached towards opposite poles.

During anaphase sister chromatids separate and move to opposite poles, each now counted as a daughter chromosome. In telophase nuclear envelopes re-form and chromosomes decondense. Cytokinesis divides the cytoplasm; animal cells constrict their membrane, whereas plant cells build a cell plate.

Mitosis supports growth, tissue repair and asexual reproduction by preserving genetic information, apart from mutations. Regulation checks whether events such as DNA copying and chromosome attachment have been completed appropriately. Failure of regulation can permit damaged or incorrectly segregated genomes to propagate.

To see mitosis, make a root-tip squash. Cut the tips of young roots, where cells divide fastest, and warm them in dilute hydrochloric acid to break down the material holding the cells together. Stain the chromosomes, for example with toluidine blue, then squash the tip gently under a coverslip so the cells spread into a single layer. Count the cells in each stage. Mitotic index = number of cells in mitosis / total number of cells counted x 100; if 36 of 400 counted cells are in mitosis, the index is 9.0%.

A flow cytometer measures the DNA content of thousands of single cells, and the result (sometimes called a FACS profile) is a histogram of cell number against DNA content. Cells in G1 have one amount of DNA, written 2C, and form the tallest peak. Cells in G2 and mitosis have twice as much, 4C, and form a second peak. Cells in S phase are between the peaks because they are part-way through replication. A drug that stops cells completing mitosis makes the 4C peak grow and the 2C peak shrink; a drug that blocks DNA replication keeps cells at or near 2C.