Cell Division

Mitosis is how one cell becomes two with the same chromosomes. Before it starts, each chromosome has been copied into two sister chromatids. Tap and the chromatin threads coil into compact chromosomes (prophase), the nuclear envelope breaks into pieces and spindle fibers from the two centrosomes reach the chromosomes (prometaphase), which line up across the middle (metaphase). The sisters are pulled to opposite poles (anaphase), new nuclei form round each set (telophase) and a ring pinches the cell in two (cytokinesis).

A human cell has 46 chromosomes; four are shown, two pairs. One daughter then grows back into a whole cell.

The splash shows an animal cell dividing by mitosis, phase by phase, in the order and with the events a biology textbook gives. The cell is a diagram: four chromosomes stand for a human cell's 46, and the parts are drawn clear and simple.

1. Correct

   Prophase: the chromatin condenses into chromosomes of two sister chromatids, the nucleolus disappears and the centrosomes move toward opposite poles.¹

   Thin chromatin threads draw into eight solid chromatids (four chromosomes, sisters side by side); the nucleolus shrinks away; the two centrosomes travel round the nucleus to the poles.

2. Simplified

   Prometaphase: the nuclear envelope breaks into pieces and spindle microtubules reach the kinetochores.¹

   The envelope comes apart in twelve pieces; a fiber grows from each pole to each chromatid's kinetochore.

3. Correct

   Metaphase: the chromosomes line up on the metaphase plate midway between the poles.¹

   The chromosomes move to the plane between the poles, each sister attached to its own pole.

4. Correct

   Anaphase: the sister chromatids separate and are pulled to opposite poles, and the cell elongates.¹

   Each pair of sisters parts; each moves as a solid piece toward its pole as its fiber shortens; the poles move apart and the cell stretches.

5. Simplified

   Telophase and cytokinesis: new nuclear envelopes form round each set of chromosomes, which decondense, and a contractile ring of actin pinches the cell in two at the cleavage furrow.¹

   New nuclei fade in round the chromosomes at each pole; a red ring at the equator closes as the membrane pinches into two cells.

6. Simplified

   A human cell has 46 chromosomes; the splash shows four, two pairs of homologs (each a slightly different shade).²

   Four chromosomes, two long and two short, in two shades of pink and two of blue.

1. 1Biology 2e, 10.2 The Cell Cycle, OpenStax
2. 2How many chromosomes do people have?, MedlinePlus Genetics (U.S. National Library of Medicine)

Simplified

Four chromosomes stand for 46; their banding is decorative, not a real karyotype.

Simplified

Each daughter gets one centrosome; the copy that appears beside it at the end stands for the duplication in the next S phase.

Simplified

Spindle fibers are drawn as a few lines (a real spindle has thousands of microtubules) and astral microtubules are left out.

Simplified

Only one daughter is followed: it grows back into the starting cell (G1), so the splash can play again; the other drifts away.

Simplified

The whole division takes about 18 seconds; in a human cell mitosis takes about an hour.

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