prophase

Prophase, from the ancient Greek πρό (before) and φάσις (stage), is a stage of mitosis in which the chromatin condenses into a highly ordered structure called a chromosome in which the chromatin becomes visible. This process, called chromatin condensation, is mediated by the condensin complex. Since the genetic material has been duplicated in an earlier phase of the cell cycle, there are two identical copies of each chromosome in the cell. Identical chromosomes, called sister chromatids, are attached to each other at a DNA element present on every chromosome called the centromere. During prophase, giemsa staining can be applied to elicit G-banding in chromosomes. Prophase accounts for approximately 3% of the cell cycle's duration.

An important organelle in mitosis is the centrosome, the microtubule organizing center in metazoans. During prophase, the two centrosomes, which replicate independently of mitosis, have their microtubule-nucleating activity increased due to the recruitment of γ-tubulin. The centrosomes will be pushed apart to opposite ends of the cell nucleus by the action of molecular motors acting on the microtubules. The nuclear envelope breaks down to allow the microtubules to reach the kinetochores on the chromosomes, marking the end of prophase. Prometaphase, the next step of mitosis, will see the chromosome being captured by the microtubules.

Prophase in plant cells

In this first phase of mitosis, plant cells undergo a series of events preparing them for mitosis before the onset of the prometaphase. In highly vacuolated plant cells, the contractile vacuole has to migrate into the center of the cell before mitosis can begin. This is achieved during the G2 phase of the cell cycle. A transverse sheet of cytoplasm bisects the cell along the future plane of cell division.

Prophase in plant cells is preceded by a stage only found in plants, the formation of a ring of microtubules and actin filaments underneath the plasma membrane around the equatorial plane of the future mitotic spindle and predicting the position of cell plate fusion during telophase. The preprophase band disappears during nuclear envelope disassembly and spindle formation in prometaphase despite contrary belief.

The cells of higher plants lack centrioles. Instead, the nuclear envelope serves as a microtubule organising center. Spindle microtubules aggregate on the surface of the nuclear envelope during preprophase and prophase, forming the prophase spindle.^[1]

References

1. ^ Raven, Peter H.; Ray F. Evert, Susan E. Eichhorn (2005). Biology of Plants, 7th Edition. New York: W.H. Freeman and Company Publishers. pp. 58–67. ISBN 0-7167-1007-2. 

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