Cells "multiply" by dividing into two. One of the main ways of cell division is mitosis, in which two daughter cells appear from one mother cell, carrying the same genetic set. The biological significance of mitosis is represented by the growth of the multicellular organism, replacement of the worn-out cells (for example upper epidermis, blood cells, and gut cells), and restoration of the nucleo-cytoplasmic ratio after the cell growth. Mitosis is a simple and relatively quick way to increase the number of cells. In this topic, we will discuss the stages of mitosis, its significance, and the organelles involved.
Stages of mitosis
Each cell in the life cycle goes through several stages (to learn more about this read the topic Cell cycle). The period of growth and duplication of DNA as a whole is called interphase and it separates mitotic divisions from each other. The cell has the first period of rest – G1, after which the cell begins to prepare for division – the S1 phase. In this phase, the cell duplicates its DNA so that it can then pass each copy onto two daughter cells. In order to make the division more convenient, the cell compacts the DNA, forming chromosomes. The chromosomes are made up of 2 sister chromatids which are attached by a centromere (pic. 1). Before mitosis begins, in most organisms, each chromosome consists of two identical chromatids. A human individual has two copies of each chromosome (this is a diploid set), and this number doubles before mitosis so that 4 copies of each chromosome "enter" mitosis. Centromeres have a disc-shaped structure known as kinetochores, which help in the attachment of mitotic spindle fibers to the chromosomes. After the S phase, another phase of rest follows – G2. Finally, the cell goes into the stage of division – mitosis, after which each daughter cell goes into interphase and repeats the cycle again.
Before mitosis, the cell prepares not only a double set of DNA but also special organelles – centrioles. Centrioles are microtubules that are involved in the formation of an organelle important for mitosis – the mitosis spindle, which is located at the poles of the cell and helps to "pull" DNA along the edges to form new cells.
Mitosis may take minutes or hours, depending upon the kind of cells and species of organisms. It is influenced by time of day, temperature, and chemicals. It is divided into 5 phases, namely, prophase, prometaphase, metaphase, anaphase, and telophase. In prophase, the spindle is formed – a roadmap for future division. The nuclear membrane dissolves in it, and the chromosomes enter the cytoplasm. In prometaphase chromosomes "connect" to microtubules. In metaphase, the chromosomes line up at the equator of the cell. In anaphase, the chromosomes separate into two halves, each of which is pulled to opposite poles. Telophase completes mitosis and new nuclei begin to form.
Prophase
Before the start of mitosis, the cell already has a double supply of chromosomes and all the proteins necessary for division. The signal for the beginning of mitosis is the condensation of chromosomes – the DNA strands become thicker, more tightly packed to each other, and the chromosome acquires its "usual" form in the form of the X. The nuclear membrane bursts to release the chromosomes. The process of chromosome condensation will continue until metaphase.
During this phase, the mitotic spindle also develops – a special structure made of microtubules, which will later help the cell separate the chromosomes at its different ends.
Prometaphase
The cell cycle's prometaphase is one of its most active phases. Microtubules extend from both poles of the mitotic spindle and pull chromosomes in opposite directions during prometaphase. Sister chromatids are still held together. Chromosomes are bioriented at the end of prometaphase, meaning that sister chromatid kinetochores are attached by microtubules to opposite spindle poles.
Metaphase
In metaphase, the chromosomes reach their most condensed state (it is at this moment that they become similar to the "classical" chromosomes in the form of the letter X). The chromosomes align along the nuclear equator, which is the midline of the cell. The collapse of the nuclear membrane causes the condensed chromosomes to travel to the center or equator of the cell, where they align into a plane known as the metaphase plate.
Anaphase
The sudden separation of sister chromatids indicates that cells are transitioning from metaphase into anaphase. Each chromatid pair splits into two identical chromosomes during anaphase, and the spindle fibers drag these chromosomes to the opposite ends of the cell to form a daughter cell nucleus. Centromeres, which up until this point have been at the poles, start to direct chromatids toward the poles and start to separate.
The chromosomes travel toward the spindle poles when the kinetochore microtubules shrink during the initial phase of anaphase. The spindle poles split during the second phase of anaphase as the non-kinetochore microtubules pass one another. The motor proteins that link microtubules with opposing polarity and then "walk" toward the end of the microtubules are assumed to be the catalysts for these later motions.
Telophase and cytokinesis
As the telophase progresses, nuclear membranes resurface. The endoplasmic reticulum, nucleoli, and Golgi complexes are also reconstituted, the spindle however disappears. When chromosomes reach their respective poles, they begin to decondense.
In the final step, cytokinesis, the mitotic cell splits into two daughter cells during this stage. On the plasma membrane of animal cells, a ridge forms that gradually thickens and unites in the middle, splitting the cytoplasm of the cell into two. A major protein in this process is dynamin. The two daughter cells have the same mitochondria and other cell organelles and are complete to be viable by themselves.
Conclusion
Mitosis involves five phases, based on the physical state of the chromosomes and spindle. These phases are prophase, prometaphase, metaphase, anaphase, and telophase. Cytokinesis is the final physical cell division that follows telophase. Mitosis is an essential biological process, which provides new cells for growth and replacement of worn-out cells.