The Cell Cycle
Mitosis
How one cell becomes two genetically identical cells. The whole process takes about 1 hour.
What is mitosis?
Mitosis is the process by which a eukaryotic cell divides its replicated chromosomes into two identical daughter nuclei. It is the fundamental mechanism of asexual reproduction in single-celled organisms and the basis for growth, repair, and tissue maintenance in multicellular ones.
The result: two daughter cells, each with the same number of chromosomes as the parent — typically 46 in humans (2n = 46).
The four phases
Mitosis is divided into four sequential phases. Each is defined by the position and behavior of the chromosomes.
Prophase
Chromatin condenses into visible chromosomes. Each chromosome consists of two identical sister chromatids joined at the centromere. The nuclear envelope begins to break down, and the mitotic spindle — composed of microtubules — starts forming from the centrosomes at opposite poles.
Metaphase
Chromosomes align along the metaphase plate, a plane equidistant from the two spindle poles. Spindle fibers attach to the kinetochore of each sister chromatid. This is the longest phase of mitosis and serves as a critical checkpoint: the cell verifies that every chromosome is properly attached before proceeding.
Anaphase
The centromeres split, and sister chromatids separate, becoming individual chromosomes. Spindle fibers shorten, pulling the chromatids toward opposite poles of the cell. By the end of anaphase, each pole has a complete set of chromosomes.
Telophase
Chromosomes arrive at the poles and begin to decondense. New nuclear envelopes form around each set of chromosomes, creating two distinct nuclei. The spindle apparatus disassembles. Mitosis is technically complete here — but the cell isn't yet two cells.
Cytokinesis (the encore)
Cytokinesis isn't part of mitosis itself, but it always follows. The cytoplasm physically splits, producing two genetically identical daughter cells. In animal cells, a contractile ring of actin pinches the cell in two. In plant cells, a cell plate forms across the middle.
The simple version
Imagine your cell is a chef who needs to make an exact copy of themselves. Mitosis is the recipe for "one chef → two identical chefs."
Why bother? Because every time you grow taller, heal a cut, or replace dead skin, your cells are doing this. Roughly 25 million of your cells finished mitosis just while you were reading this paragraph.
Four phases, easy way to remember: P-MAT
Phase 1: Prophase ("prep")
The cell tidies up. The DNA — usually a tangled mess like spaghetti in a bowl — coils up into neat, visible chromosomes. Picture them as X-shapes, because they've already been copied. The nucleus's wall starts dissolving like a curtain coming down.
Phase 2: Metaphase ("middle")
All the chromosomes line up at the equator of the cell, perfectly straight, like dancers at a curtain call. This is also the safety check: the cell makes sure every chromosome is correctly attached before allowing the next step. If something's wrong, it pauses here.
Phase 3: Anaphase ("away")
Each X-shape gets pulled apart at the middle, and the two halves get yanked toward opposite ends of the cell — like two people in a tug-of-war winning at the same time. Now each end of the cell has a complete set of chromosomes.
Phase 4: Telophase ("two")
Each end of the cell wraps a fresh nucleus around its chromosomes. The cell is technically two nuclei now, sharing one body. Then cytokinesis kicks in: the cell pinches in the middle and physically splits. Two cells. Both identical to the original. Done.
The whole thing in 12 words
Cell copies its DNA, lines it up, splits it, divides in half.
Mitosis mastered!
You crushed it. Two cells, both identical, both yours.