Mitosis is the nuclear division process that separates the sister chromatids of replicated chromosomes into two daughter nuclei.

Its purpose is not to replicate DNA—that occurred earlier—but to distribute one copy of each replicated chromosome to each side of the cell.

Mitosis is often divided into prophase, prometaphase, metaphase, anaphase and telophase. These names describe a continuous process rather than perfectly discrete molecular switches.

Prophase and prometaphase: prepare chromosomes for segregation

During prophase, replicated chromosomes condense into compact structures that can be moved without extensive entanglement. The mitotic spindle begins to form.

During prometaphase, the nuclear envelope is opened or reorganized sufficiently for spindle microtubules to reach chromosome kinetochores. Dynamic capture and error correction establish spindle attachments.

Metaphase: establish biorientation

By metaphase, replicated chromosomes are typically aligned near the spindle equator with sister kinetochores attached toward opposite poles.

The central accomplishment is not the visual line of chromosomes itself but biorientation: each future daughter chromosome copy is mechanically connected to a different pole.

Anaphase: convert sisters into independent chromosomes

At the metaphase-to-anaphase transition, cohesion holding sister chromatids together is removed. The sisters can then move apart.

Once separated, each former sister chromatid is an independent chromosome.

Two coordinated motions contribute to segregation:

• chromosomes move toward spindle poles as kinetochore-associated microtubules and motors generate motion;
• spindle poles can move farther apart as interpolar and cortical forces elongate the spindle.

The result is two spatially separated, equivalent sets of chromosome copies.

Telophase: rebuild daughter nuclei

During telophase, chromosomes arrive near opposite poles and begin to decondense. Nuclear envelopes re-form around the separated chromosome sets, and the mitotic spindle is dismantled or reorganized.

Schematically:

replicated chromosomes
        ↓ spindle attachment
bioriented sister pairs
        ↓ cohesion release
separated chromosomes
        ↓ movement to opposite poles
one copy of each chromosome type per daughter nucleus

Mitosis is therefore an equal-copy segregation process: after DNA has been replicated, sister chromatids are separated so that the two daughter nuclei receive corresponding chromosome complements. It should not be confused with DNA replication itself or with cytokinesis, which physically separates the cell body.