Mitosis

Core Concept

Mitosis is the division of the nucleus that distributes the duplicated chromosomes equally into two daughter nuclei. Because each duplicated chromosome was copied during S phase, mitosis separates the sister chromatids so each daughter cell receives one complete, identical set of chromosomes. Mitosis is conventionally described in five stages — prophase, prometaphase, metaphase, anaphase, telophase — followed by cytokinesis, which divides the cytoplasm.

Key Concepts

The mitotic spindle and centrosomes

The mitotic spindle is the machine that moves chromosomes, built from microtubules. In animal cells, spindle microtubules grow from centrosomes (microtubule-organizing centers, each containing a pair of centrioles) that migrate to opposite poles during prophase. The spindle has three classes of microtubules: kinetochore microtubules (attach to chromosomes), polar microtubules (overlap at the middle and push the poles apart), and astral microtubules (anchor the spindle to the cell cortex).

Kinetochores

The kinetochore is a protein structure assembled on the centromere of each sister chromatid. Spindle microtubules attach to kinetochores and use them as handles to pull chromosomes. Each sister chromatid has its own kinetochore facing opposite poles, which is what guarantees the two sisters go to opposite ends.

Phases of mitosis

• Prophase: Chromatin condenses into visible chromosomes (each with two sister chromatids); the nucleolus disappears; the mitotic spindle begins to form; centrosomes move apart. In animal cells, the nuclear envelope is still intact at the end of prophase.
• Prometaphase: The nuclear envelope fragments; spindle microtubules invade the nuclear region and attach to kinetochores. Chromosomes begin to move.
• Metaphase: Chromosomes line up at the metaphase plate (the cell's equator), each held by kinetochore microtubules from both poles. This is the classic "chromosomes in a row" picture.
• Anaphase: Cohesin is cleaved, allowing sister chromatids to separate. Each chromatid — now an independent chromosome — is pulled toward opposite poles as kinetochore microtubules shorten. This is the shortest phase and the moment chromosome number transiently doubles.
• Telophase: Two daughter nuclei re-form. Chromosomes decondense, nuclear envelopes reassemble around each set, nucleoli reappear, and the spindle disassembles.

Cytokinesis: animal vs. plant

Cytokinesis overlaps with telophase and divides the cytoplasm. In animal cells, a contractile ring of actin and myosin filaments pinches the cell inward, forming a cleavage furrow that deepens until the cell splits in two. In plant cells, a rigid cell wall prevents pinching; instead, Golgi-derived vesicles carrying wall materials line up at the metaphase plate and fuse into a cell plate that grows outward until it fuses with the plasma membrane, forming two daughter cells and new cell walls.

How It Works

Start with one duplicated genome: every chromosome consists of two identical sister chromatids held together by cohesin rings. During prophase and prometaphase, the cell builds a bipolar spindle and attaches each sister's kinetochore to opposite poles (a state called biorientation). At metaphase, opposing forces align every chromosome at the equator. At anaphase, the enzyme separase cleaves cohesin, the sisters part, and motor proteins plus depolymerizing microtubules pull each set of chromosomes to a pole. Telophase rebuilds two nuclei; cytokinesis splits the cell. Because each chromosome was duplicated precisely in S phase and separated faithfully, the two daughters are genetically identical to each other and to the parent.

How it works

Start with one duplicated genome: every chromosome consists of two identical sister chromatids held together by cohesin rings. During prophase and prometaphase, the cell builds a bipolar spindle and attaches each sister's kinetochore to opposite poles (a state called biorientation). At metaphase, opposing forces align every chromosome at the equator. At anaphase, the enzyme separase cleaves cohesin, the sisters part, and motor proteins plus depolymerizing microtubules pull each set of chromosomes to a pole. Telophase rebuilds two nuclei; cytokinesis splits the cell. Because each chromosome was duplicated precisely in S phase and separated faithfully, the two daughters are genetically identical to each other and to the parent.

Common confusions

• "Anaphase separates homologous chromosomes." Wrong in mitosis — anaphase separates sister chromatids (identical copies). Separating homologs happens in meiosis I.
• "Chromosome number doubles in anaphase." Misleading — it briefly doubles as chromatids become separate chromosomes at the poles, but the final daughter cells each have the original number.
• "Centromere and kinetochore are the same thing." Wrong — the centromere is the DNA region where sisters are joined; the kinetochore is the protein complex built on it that microtubules grab.
• "Plant cells divide with a cleavage furrow." Wrong — the rigid cell wall prevents pinching; plants build a cell plate from fused vesicles.
• "Prophase and prometaphase are interchangeable." Wrong — prometaphase is defined by nuclear envelope breakdown and kinetochore attachment, distinct events from prophase condensation.

Quick review

• Five phases: prophase, prometaphase, metaphase, anaphase, telophase (+ cytokinesis).
• Spindle microtubules, centrosomes, and kinetochores move chromosomes.
• Metaphase plate = equator; biorientation ensures one sister to each pole.
• Anaphase: cohesin cleavage → sister chromatids separate.
• Telophase: two nuclei re-form.
• Animal cells: cleavage furrow; plant cells: cell plate.
• Result: two identical daughter cells.