Prophase

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An illustration of prophase in Gray's Anatomy. The chromatin has condensed.

Prophase is a stage of mitosis in which the chromatin condenses into a highly ordered structure called a chromosome (it is at this stage giemsa staining can be applied to elicit G-banding in chromosomes). This process, called chromatin condensation, is mediated by the condensin complex. Since the genetic material has been duplicated, 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.

When chromosomes are paired up and attached, each individual chromosome in the pair is called a chromatid, while the whole unit is called a chromosome. When the chromatids separate, they are no longer called chromatids, but are called chromosomes again. The task of mitosis is to assure that only one copy of each sister chromatid goes to each daughter cell after cell division.

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 recruitement 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. The nuclear envelope break down marks the end of prophase. Prometaphase, the next step of mitosis will see the chromosome being captured by the microtubules.

Prophase in plant cells

The cells of lower plants (such as the flowering plants) undergo a series of events preparing them for mitosis before the onset of prophase. In highly vacuolated plant cells, the nucleus has to migrate into the center of the cell before mitosis can begin. This is achieved during the G2 phase of the cell cycle through the formation of a phragmosome, a transverse sheet of cytoplasm that bisects the cell along the future plane of cell division.

Prophase in plant cells is preceded by a preprophase stage only found in plants. Preprophase is characterized by the formation of a ring of microtubules and actin filaments (called preprophase band) underneath the plasmamembrane 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.

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

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