Mitosis: Two Cells That Match the One They Came From

Copy first, divide second

The question worth answering is how one cell becomes two without either of them ending up short. The answer is that all the copying is done in advance. During interphase, before mitosis starts, the cell grows, makes more ribosomes and mitochondria, and replicates every chromosome so that each exists as two identical copies joined together.

Mitosis itself is the separation. It pulls one copy of each chromosome to each end of the cell, and only then does the cytoplasm and cell membrane divide, in a step called cytokinesis. Strictly, mitosis is the nuclear division alone, and the whole sequence of growth, replication, mitosis and cytokinesis is the cell cycle.

That order explains the result stated in every definition. The daughter cells are genetically identical because they received copies of the same molecules, and the chromosome number is unchanged because the doubling had already happened.

The four stages, in order

The classical stages are prophase, metaphase, anaphase and telophase, remembered as PMAT. Not every syllabus asks for them by name — some describe the cell cycle in three broader stages instead — so check what your course requires before memorising them.

What happens in each is short enough to state in a line:

• Prophase — chromosomes condense and become visible, and the membrane around the nucleus breaks down
• Metaphase — chromosomes line up across the middle of the cell
• Anaphase — the copies are pulled apart to opposite ends by spindle fibres
• Telophase — a nuclear membrane forms around each new set of chromosomes
• Cytokinesis — the cytoplasm and cell membrane divide, producing two separate cells

What mitosis is actually for

Three jobs, and identical daughter cells are what all three need. Growth: a whole organism is built from one fertilised egg by repeated mitosis. Repair and replacement: skin, gut lining and blood cells wear out constantly and are replaced with cells that match. Asexual reproduction: organisms that reproduce without gametes, such as a strawberry plant putting out runners, produce offspring genetically identical to the parent.

The same process is behind two things that sound unrelated. Stem cells divide by mitosis before differentiating into specialised cells, which is the basis of every proposal to grow replacement tissue. And when the controls on mitosis fail, cells divide when they should not, forming a tumour.