The cell cycle, mitosis and checkpoints

The cell cycle runs from the moment a cell forms to the moment it divides. Most of it is interphase: in G1 the cell grows and does its normal work, in S it copies its DNA, and in G2 it grows more and makes the proteins it needs to divide. Then comes the M phase: mitosis divides the nucleus and cytokinesis divides the cytoplasm.

After S phase, each chromosome is made of two identical sister chromatids joined at the centromere. A human body cell has 46 chromosomes in G1. After S it still has 46 chromosomes, but now as 92 chromatids, with twice the DNA. Mitosis gives each daughter cell one chromatid from every pair, so each daughter has 46 chromosomes and the same DNA as the parent cell had in G1.

Mitosis has four stages. Prophase: the chromosomes condense and become visible, the spindle begins to form, and the nuclear envelope breaks down. Metaphase: spindle fibers line the chromosomes up across the middle of the cell. Anaphase: the sister chromatids separate and move to opposite poles; each is now its own chromosome. Telophase: a nuclear envelope forms around each set and the chromosomes loosen. Cytokinesis usually overlaps telophase. An animal cell is pinched in two by a ring of actin, which forms a cleavage furrow. A plant cell, held in by its wall, builds a cell plate from vesicles across its middle, and the plate becomes a new wall.

Mitosis makes genetically identical cells, which is how a body grows, replaces cells and repairs tissue, and how some organisms reproduce asexually. Some cells leave the cycle for G0, a nondividing state. Most mature nerve cells stay in G0; liver cells in G0 can return to the cycle after an injury.

Checkpoints hold the cycle until conditions are right. The G1 checkpoint checks cell size, nutrients, growth signals and DNA damage; in many cells it is the main go or no-go decision. The G2 checkpoint checks that the DNA was copied completely and is not damaged. The checkpoint in metaphase holds off anaphase until every chromosome is attached to the spindle. A cell stopped at a checkpoint pauses and repairs the problem, or, if the damage is too great, it can destroy itself by apoptosis.

To describe a stage of mitosis, say what the chromosomes are doing: condensing, lining up, separating or loosening. To explain a result, tie it to what that stage needs: for example, an even split of chromosomes needs the DNA copied first.

To count, track chromosomes and chromatids separately, and count chromosomes by centromeres. When cells in a sample divide at random times, the share of cells seen in a phase estimates the share of the cycle that phase takes. Multiply that share by the length of the whole cycle to estimate the phase's length.