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what is prophase !?
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Answer:
Mitosis describes the division of one cell into two identical daughter cells. It occurs in several stages, each stage describing a stereotyped set of changes in cell contents and structure. In this article, we will look at the stages of mitosis and a clinical application of mitosis.
Figure 1 - Microscope image of cells in various stages of mitosis
Fig 1 – Microscope image of cells in various stages of mitosis
Stages of Mitosis
Prophase
Each chromosome is made of two genetically identical chromatids, joined by a centromere. During DNA replication, the genetic material is loosely packed as chromatin. For mitosis however, the DNA needs to be more tightly packed to allow for easier separation in anaphase. At the start of prophase, chromatin begins condensing into chromosomes.
In addition, mitotic spindles begin to form. Mitotic spindles are structures made from microtubules that aid in the organisation and arrangement of chromosomes. The spindles originate from an organelle known as the centrosome. Each cell in mitosis has two centrosomes. During prophase, the centrosomes begin to move in opposite directions.
Figure 2 - Prophase
Fig 2 – Prophase
Prometaphase
In this stage the chromosomes finish condensing into their compact state. The nuclear envelope begins to breakdown, allowing spindle fibres to attach to the chromosomes. The mitotic spindles attach at a site called the kinetochore. The kinetochore is an area of the centromere on each sister chromatid. The sister chromatids are attached to spindles that originate from the opposite centrosome.
Figure 3 - Prometaphase
Fig 3 – Prometaphase
Metaphase
At this stage, the chromosomes align upon a theoretical line known as the metaphase plate. Furthermore, the centrosomes have orientated themselves to opposite ends of the cell. At this stage, the cell will check that all the chromosomes are aligned along the metaphase plate, with their kinetochores correctly attached. This helps to ensure sister chromatids are split evenly between the two daughter cells. An error in alignment or in a spindle attachment will result in the cell halting further progress until the problem is fixed.
Figure 4 - Metaphase
Fig 4 – Metaphase
Anaphase
During this stage the sister chromatids are pulled to opposite ends of the cell. The spindle fibres contract, breaking the chromatids at the centromere and moving them to opposite poles of the cell. Spindle fibres not attached to chromatids will elongate the cell to prepare the cell for division.
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Telophase
In this phase the cell has elongated and is nearly finished dividing. Cell-like features begin to reappear such as reformation of two nuclei (one for each cell). The chromosomes decondense and the mitotic spindles fibres are broken down.
