The nucleus holds chromosomes: long molecules of DNA, each carrying many genes. In body cells they come in pairs (humans have 46 chromosomes, 23 pairs).

When a cell divides by mitosis, the two new cells are genetically identical to each other and to the parent, with the same number of chromosomes. Mitosis is for growth, repair, replacing cells and asexual reproduction.

1. Stage 1: the cell grows, makes more ribosomes and mitochondria, and copies its DNA, so each chromosome has two copies.
2. Stage 2 (mitosis): the chromosomes line up across the middle, then one set is pulled to each end; a nucleus forms at each end.
3. Stage 3: the cytoplasm and cell membrane divide: two identical cells.

Every round doubles the number of cells.

number of cells = starting number × 2ⁿ (n = number of divisions)

1. One cell, 6 divisions
2. 1 × 2⁶ = 64 cells

In a root tip slide, cells are frozen at random points of the cycle. The fraction of cells in a stage equals the fraction of the cycle spent in it.

1. 200 cells counted, 18 in mitosis
2. 18 ÷ 200 × 100 = 9.0% of cells
3. Cycle 20 hours = 1200 minutes
4. 0.09 × 1200 = 108 minutes in mitosis

Do not halve the chromosome number in mitosis: halving is meiosis, which makes gametes.