Mitosis: Prophase to Telophase

Mitosis is the process of nuclear division in which duplicated chromosomes are separated into two identical nuclei. It ensures each daughter cell receives an exact copy of the parent cell's genetic material, maintaining the chromosome number across generations of somatic cells. The process was originally described by the German biologist Walther Flemming in 1882, who coined the term from the Greek word mitos (thread), referring to the thread-like appearance of chromosomes during division.

Prophase

Prophase marks the beginning of mitosis. During this phase, the loosely organized chromatin fibres condense into tightly coiled, visible chromosomes. Each chromosome consists of two identical sister chromatids joined at the centromere. The two centrosomes — each containing a pair of centrioles in animal cells — begin migrating to opposite poles of the cell and nucleate the formation of the mitotic spindle, an array of microtubules that will later pull the chromosomes apart. The nucleolus, the site of ribosomal RNA synthesis, disappears as the cell redirects its resources toward division.

Prometaphase

Prometaphase is defined by the breakdown of the nuclear envelope, an event known as nuclear envelope breakdown (NEBD). The double membrane fragments into small vesicles, exposing the chromosomes to the cytoplasm. Specialized protein complexes called kinetochores assemble on the centromere of each sister chromatid. Spindle microtubules emanating from the centrosomes attach to these kinetochores, and the chromosomes begin a series of jerky movements toward the center of the cell as they are captured and oriented by opposing spindle fibers.

Metaphase

During metaphase, all chromosomes are aligned along the metaphase plate — an imaginary plane equidistant between the two spindle poles. Each chromosome is bi-oriented, meaning its two kinetochores are attached to spindle fibers from opposite poles. This bipolar attachment is critical for accurate chromosome segregation.

The spindle assembly checkpoint (SAC) operates at this stage. This surveillance mechanism monitors kinetochore–microtubule attachments and prevents the cell from progressing to anaphase until every chromosome is properly attached. The SAC involves proteins such as Mad1, Mad2, BubR1, and Bub3, which inhibit the anaphase-promoting complex/cyclosome (APC/C) until all attachments are satisfied.

Anaphase

Anaphase begins when the cohesin protein complex holding sister chromatids together is cleaved by the protease enzyme separase. This allows the sister chromatids — now considered individual chromosomes — to be pulled toward opposite poles. Anaphase occurs in two overlapping sub-stages:

Anaphase A

Kinetochore microtubules shorten by depolymerization at both the kinetochore and the spindle pole, pulling the separated chromatids poleward. Motor proteins at the kinetochore, including dynein and CENP-E, help drive this movement.

Anaphase B

The spindle poles themselves move apart. Polar (interpolar) microtubules from opposite poles slide against each other, pushed by the kinesin-5 motor protein, elongating the spindle. Astral microtubules anchored at the cell cortex pull the poles outward via cytoplasmic dynein. Anaphase is the shortest phase of mitosis, typically lasting only a few minutes.

“The abrupt separation of sister chromatids at the onset of anaphase is one of the most dramatic events in cell biology, triggered by the sudden destruction of the cohesin protein complex that holds them together.”

Telophase

Telophase is essentially the reverse of prophase. Nuclear envelopes reform around each set of separated chromosomes as nuclear membrane vesicles reassemble on the chromosome surfaces. The chromosomes begin to decondense back into the diffuse chromatin state, allowing gene transcription to resume. Nucleoli reappear as ribosomal RNA genes become active again. The mitotic spindle disassembles as its microtubules are depolymerized. Cytokinesis — the physical division of the cytoplasm — typically begins during late telophase, although it is technically a separate process from mitosis.

Summary of Mitotic Phases