At the end of G2 a cell has two copies of every chromosome, joined as sister chromatids (The Cell Cycle). Mitosis is the careful dance that gives one copy of each to each new cell.
Key idea
In prophase chromosomes condense and the spindle forms; in metaphase they line up in the middle; in anaphase sister chromatids pull apart to opposite ends; in telophase two nuclei form. Cytokinesis splits the cell into two identical cells.
| Stage | What happens | How to spot it |
|---|---|---|
| Prophase | Chromosomes coil up; the nuclear envelope breaks down; spindle fibers form | Thick, visible X-shaped chromosomes scattered in the cell |
| Metaphase | Chromosomes line up across the middle, each attached to spindle fibers from both poles | A neat line across the center |
| Anaphase | Sister chromatids separate and move to opposite poles | Two V-shaped groups moving apart |
| Telophase | Nuclear envelopes form around each group; chromosomes loosen | Two nuclei at opposite ends |
| Cytokinesis | The cytoplasm divides (a furrow in animal cells, a cell plate in plant cells) | Two separate cells |
Why it works
The key moment is anaphase. In metaphase, each chromosome is attached to spindle fibers from both poles. When the sister chromatids separate, one goes each way. Since every chromosome is lined up and split the same way, each pole gets exactly one copy of every chromosome. The two new cells have the same number of chromosomes as the parent, and the same DNA. That is what growth and repair need: new skin cells that are the same as the old ones.
Worked example: Count through mitosis
A cell with $2n = 8$ divides by mitosis. How many chromosomes does it have in metaphase, in anaphase (whole cell), and in each new cell?
- Metaphase: 8 chromosomes (each two chromatids).
- Anaphase: the 16 chromatids separate and each now counts as a chromosome, so the cell briefly holds 16.
- Each new cell gets half of those: 8 chromosomes, the same as the parent.
Answer: 8, 16, and 8 in each new cell
Common mistake
Thinking the chromosome number halves in mitosis, or that homologous pairs separate. In mitosis the sister chromatids separate; homologs never pair up at all. So each new cell has the same number of chromosomes as the parent: 46 in, 46 in each of the two cells out. Halving belongs to Meiosis.