Mitosis

Prophase: preparation of the genetic apparatus

The first and most eventful stage is prophase. The cell's main task at this stage is the compaction of genetic material. Under the action of the MPF (mitosis-promoting factor), the condensin protein complex is activated.

Chromatin packing occurs in a strictly sequential manner:

1. Nucleosomal level (initial euchromatin).
2. Superspiral (chromatin fibril).
3. Loop level, where rosettes are formed and the chromonema is produced.
4. Chromatid level (fully condensed structure).

At the end of the phase, chromosomes consist of two sister chromatids held together by cohesins. Tight DNA packaging makes enzyme activity impossible: transcription stops completely, rRNA genes are inactivated, causing nucleoli to disappear without a trace in the nucleus.

Membrane destruction and spindle formation

Concurrently in prophase, MPF phosphorylates nuclear lamina proteins. This leads to the depolymerization of the internal nuclear support framework, and the nuclear envelope breaks down into microvesicles. The endoplasmic reticulum and Golgi apparatus also fragment. The biological significance of this destruction is to remove mechanical obstacles to chromosome movement and prevent extra membranes from entering future nuclei.

Simultaneously, diplosomes (centrioles duplicated in the S-phase) migrate to the poles. Microtubules polymerize from previously stockpiled tubulin protein, forming the mitotic spindle apparatus.

Metaphase and spindle architecture

In metaphase, the nuclear envelope is already absent, and chromosomes reach the peak of their condensation. They align in the equatorial plane, forming the metaphase plate ("mother star"). By this point, cohesins remain exclusively in the centromeric regions. Functionally, the cell carries a tetraploid set of single-chromatid chromosomes.

Spindle microtubules are divided into three functional types:

• Kinetochore — connect the pole to the kinetochore on the chromosome centromere.
• Polar — directed toward the center, where they overlap with microtubules from the opposite pole.
• Astral — anchor the poles by abutting the cell surface.

Anaphase: molecular trigger and chromosome separation

The transition to anaphase is triggered by the APC (anaphase-promoting complex), which is activated by the MPF factor. APC attaches a marker — the ubiquitin protein — to cohesins. The marked cohesins are instantly cleaved in proteasomes. The loss of connection between chromatids initiates their movement.

Chromosomes move toward opposite poles with their centromeres leading. Movement is ensured by the shortening of kinetochore microtubules and the elongation of polar microtubules, as well as the action of specialized motor transport proteins. At the end of anaphase, APC marks the MPF factor itself, and its destruction serves as a signal to transition to the next stage.

Telophase and cytokinesis

In telophase, protein phosphatases that reverse MPF effects predominate. Chromosomes decondense. Membrane vesicles bind to chromosomes, forming separate mini-nuclei — karyomeres. After the insertion of nuclear pores and lamin proteins, the karyomeres of one pole fuse into a single daughter nucleus.

In late telophase, cytotomy (cytokinesis) occurs. An actomyosin ring assembles along the cell equator. It constricts the plasmalemma, dividing the cytoplasm. Only after successful division are the membranes of the ER and Golgi apparatus restored in the daughter cells.

Mitotic cycle control and pathology

Division quality is monitored at specialized checkpoints in the G1, S, G2, and metaphase phases. The system checks DNA integrity (absence of double-strand breaks), spindle assembly, and the correctness of the preceding separation. In the event of a failure, the cycle pauses for repair, and upon unresolvable defects, apoptosis is triggered.

Disruptions in the control system lead to tumor formation (in somatic tissues) or offspring anomalies. If mitosis occurs without cytokinesis, multinucleated cells are formed. In the absence of both nuclear and cytoplasmic division (e.g., as a result of endoreduplication), polyploid cells with an excess set of chromosomes arise.

Briefly: answers to topical questions

Class and exam questions

What else to study on this topic

• Molecular mechanism of the APC (anaphase-promoting complex) operation
• Ubiquitin-proteasome protein degradation system
• Structure and functions of the actomyosin ring during cytokinesis
• Causes and mechanisms of polyploidy development (endoreduplication and endomitosis)
• Function of checkpoints at cell cycle phase boundaries

Other topics in the section "Cell nucleus. Cell division"

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