Mitosis is the sequence of events by which a eukaryotic cell (parent 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
Next, we describe each of these stages of mitosis.
Phases of mitosis Characteristics Prophase
first stage of mitosis
chromosome condensation
Assembly of the mitotic spindle
prometaphase
rupture of the nuclear envelope
Attachment of chromosomes to the mitotic spindle
Metaphase Alignment of chromosomes Anaphase
chromatid separation
chromosome segregation
telophase
Nuclear envelope reconstruction
Disassembly of the mitotic spindle
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.
Before mitosis, during the S phase of the cell cycle, duplication of 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.
For this, the DNA copies are condensed, separated and cut into pieces, in what is known as sister chromatids, which look like Xs under the microscope. This process is called chromosome condensation.
Simultaneously, the cell begins to build the mitotic spindle, a spider-like structure made up of the centriole and microtubules. The organelle in charge is the centrosome, which duplicates itself 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 forms.
prometaphase
The breakdown of the nuclear envelope is the key event of prometaphase. Centrosomes and microtubules are found in the cytoplasm, far from the reach of chromosomes that are within the nucleus, separated by the nuclear envelope.
In order for the chromosomes to attach to the mitotic spindle, the nuclear envelope breaks down into small vesicles.
Around the centromere (center of the sister chromatids) a ring called the kinetochore assembles. This structure hooks the microtubules of the mitotic spindle so that they can drag them towards the poles of the cell.
metaphase
Once the chromosomes are attached to the mitotic spindle, they begin to arrange and align on the cell’s mid-equator line, 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, approximately half the time, waiting for the signal of the correct alignment of the sister chromatids to move on to the next phase.
Anaphase
Anaphase begins when the chromatids separate, allowing each daughter chromosome to move to opposite poles of the cell, pulled along by the mitotic spindle. This step is called chromosome segregation.
The separation of the chromosomes is produced by the mechanical force of the microtubules in the mitotic spindles, which act as pulleys.
At the end of anaphase, the daughter chromosomes meet at opposite poles of the cell.
It is in this phase that a parallel process of the cell cycle begins, 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 disassembling the mitotic spindle.
The chromosomes then uncoil and acquire the chromatin state of interphase.
Cytokinesis, that is, 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, JR (2016). Mitosis. Cold Spring Harbor perspectives in biology, 8(9), a023218.