Mitosis is the sequence of events by which a eukaryotic cell (progenitor cell) produces two daughter cells with the same genetic information as the original cell. Mitosis is the stage of the cell cycle that follows interphase, where the cell prepares for the mitotic process.
The phases of mitosis are:
- Prophase
- Prometaphase
- Metaphase
- Anaphase
- Telophase
Below, we describe the fundamental characteristics of each of these stages of mitosis.
| Phases of mitosis | Features |
|---|---|
| Prophase |
First stage of mitosis Chromosome condensation Mitotic spindle assembly |
| Prometaphase |
Nuclear envelope breakdown Binding of chromosomes to the mitotic spindle. |
| Metaphase | Chromosome alignment |
| Anaphase |
Chromatid separation Chromosome segregation |
| Telophase |
Nuclear envelope reconstruction Mitotic spindle disassembly End of mitosis |
Prophase
The first phase of the mitosis process is responsible for building the machinery that will allow the daughter cells to possess the genetic material that corresponds to them.
Prior to mitosis, during the S phase of the cell cycle, duplication of the deoxyribonucleic acid DNA (genetic material) occurs, producing two identical strands that are intertwined. Separating and organizing these strands is a complicated task to ensure that each cell has its own copy of DNA.
To do this, the DNA copies are condensed, separated and cut into pieces, known as sister chromatids, which look like X’s under a microscope. This process is called chromosome condensation.
Simultaneously, the cell begins to build up the mitotic spindlea spider-like structure consisting of the centriole and microtubules. The organelle in charge is the centrosome, which duplicates during the cell cycle. So by the time the cell enters mitosis there are two centrosomes.
The two centrosomes move apart along the nuclear envelope, the microtubules between them intercalate and the mitotic spindle is formed.
Prometaphase
The rupture of the nuclear envelope is the key event of prometaphase. The centrosomes and microtubules are located in the cytoplasm, away from the reach of the chromosomes inside the nucleus, separated by the nuclear envelope.
For chromosomes to bind to the mitotic spindle, the nuclear envelope disassembles into small vesicles.
Around the centromere (center of the sister chromatids) a ring called the kinetochore is assembled. This structure engages the microtubules of the mitotic spindle so that they can be pulled toward the poles of the cell.
Metaphase
Once the chromosomes are attached to the mitotic spindle, they begin to arrange and align at the mid-equatorial line of the cell, in what is known as the metaphase plate. The classic microscopic image of this phase shows the chromosomes in the middle of the cell.
This is the phase of mitosis that takes the longest, about half the time, waiting for the signal of the correct alignment of sister chromatids to proceed to the next phase.
Anaphase
Anaphase begins when chromatid separation occurs, allowing each daughter chromosome to move toward opposite poles of the cell, pulled by the mitotic spindle. This step is called chromosome segregation.
Chromosome separation occurs by the mechanical force of microtubules on mitotic spindles, which act as pulleys.
At the end of anaphase, the daughter chromosomes are at opposite poles of the cell.
It is at this stage that a parallel process of the cell cycle is initiated which is cytokinesis, the division of the cytoplasm.
Telophase
The final phase of mitosis is telophase. The main event of this phase is the packing of the daughter chromosomes into two different nuclei after disassembly of the mitotic spindle.
The chromosomes then uncoil and acquire the interphase chromatin state.
Cytokinesis, i.e. the division of the cytoplasm between the two daughter cells, ends shortly after telophase.
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References
Alberts, B. et al. (2008) Molecular biology of the cell. 5th ed. Garland Science. UK.
McIntosh, J. R. (2016). Mitosis. Cold Spring Harbor perspectives in biology, 8(9), a023218.
