Chromosome behaviour in mitosis

In prophase the chromosomes condense, the nucleolus disappears, the centrosomes move apart and the spindle forms, and the nuclear envelope breaks down at the end of the stage. Division of the centromeres and separation of sister chromatids is anaphase. Nuclear envelopes re-form in telophase, at the end of mitosis. DNA replication happens in S phase of interphase, before mitosis begins; mitosis only shares out DNA that is already copied.

Animal cells have centrosomes, each containing a pair of centrioles, that move to the poles and organise the spindle; plant cells have no centrioles, although they still form a spindle. Spindle fibres attach to the centromeres in both plant and animal cells. A cell plate formed from Golgi vesicles is the plant method of cytokinesis, so it is seen in plant cells only, the reverse of what is asked. The nuclear envelope breaks down at the end of prophase in both.

The chromosomes form two dense groups at opposite ends of the cell, each surrounded by a new nuclear envelope, and a cell plate (the row of vesicles) is forming across the middle: this is telophase, with cytokinesis starting. In anaphase the chromatids are still moving apart as V shapes, with no new nuclear envelopes. In prophase there is a single nucleus with condensing chromosomes in the centre of the cell. In metaphase the chromosomes are in one line across the equator.

After S phase each of the 16 chromosomes consists of two sister chromatids, and in metaphase they are still joined at their centromeres, so there are 16 × 2 = 32 chromatids. 16 is the number of chromosomes (centromeres), not chromatids. 8 halves the number, as if mitosis reduced the chromosome number like meiosis. 64 doubles the DNA twice.

Warm acid hydrolyses the middle lamella that sticks plant cells together, so the tissue softens and the cells separate into a single layer when squashed. Staining is done by a separate stain such as acetic orcein or toluidine blue; the acid itself does not colour the chromosomes. The acid kills the cells, so it cannot make more of them divide; the mitotic index reflects what was happening when the tip was cut. Condensation of chromosomes happens naturally in prophase in living cells; the acid does not cause it.

Fraction in anaphase = 30 ÷ 1000 = 0.030. Duration = 0.030 × 22 h = 0.66 h, and 0.66 × 60 = about 40 minutes. 50 minutes divides by the 800 interphase cells instead of the total (30 ÷ 800 × 22 = 0.825 h). 66 minutes is 0.66 h misread as 66 minutes; there are 60 minutes in an hour, not 100. 3.3 hours divides by the 200 cells in mitosis, which gives the fraction of mitosis spent in anaphase, not the fraction of the whole cycle.

Chromosomes still condense in prophase without a spindle, but the chromatids cannot be attached and pulled to the poles, so cells build up with condensed chromosomes at the metaphase stage and none reach anaphase. Anaphase needs the spindle fibres to shorten and pull the chromatids apart, so it cannot happen. Condensation of the chromosomes does not depend on the spindle, so chromosomes are still visible. Without a spindle the chromatids are not shared out at all, so the cells do not complete mitosis; mitosis never halves the chromosome number in any case.

The mitotic index falls from 0.24 at 1 mm to 0.02 at 8 mm, so the fraction of cells in mitosis falls with distance and most division happens in the meristem just behind the tip. The cells not in mitosis are in interphase, so the proportion in interphase rises (from 0.76 to 0.98), not falls. The mitotic index shows how many cells are dividing, not how long each stage lasts; a higher index near the tip means more cells are dividing there. At 8 mm the index is 0.02, not zero, so a few cells are still dividing beyond 5 mm.