Preprophase

Date: 2015-10-07; view: 437.

Interphase

Phase of cell cycle and mitosis

The mitotic phase is a relatively short period of the cell cycle. It alternates with the much longer interphase, where the cell prepares itself for cell division. Interphase is divided into three phases: G₁ (first gap), S (synthesis), and G₂ (second gap). During all three phases, the cell grows by producing proteinsand cytoplasmic organelles. However, chromosomes are replicated only during the S phase. Thus, a cell grows (G₁), continues to grow as it duplicates its chromosomes (S), grows more and prepares for mitosis (G₂), and finally it divides (M) before restarting the cycle. All these phases in the cell cycle are highly regulated, mainly via proteins. The phases follow one another in strict order and there are "checkpoints" that give the cell the cues to proceed from one phase to another. There is also a fourth section in Interphase where the cell has the option to enter G₀. Cells continue on through this cell cycle until they become too crowded; at that point they will exit the cell cycle and enter G₀. This reaction is called contact inhibition or density-dependent inhibition. Altogether interphase takes up roughly 90% of a cell's lifespan.

In plant cells only, prophase is preceded by a pre-prophase stage. In highly vacuolated plant cells, the nucleus has to migrate into the center of the cell before mitosis can begin. This is achieved through the formation of a phragmosome, a transverse sheet of cytoplasm that bisects the cell along the future plane of cell division. In addition to phragmosome formation, preprophase is characterized by the formation of a ring of microtubules and actin filaments (called preprophase band) underneath the plasma membrane around the equatorial plane of the future mitotic spindle. This band marks the position where the cell will eventually divide. The cells of higher plants (such as the flowering plants) lack centrioles; instead, microtubules form a spindle on the surface of the nucleus and are then organized into a spindle by the chromosomes themselves, after the nuclear membrane dissolves.

The preprophase band disappears during nuclear membrane dissolution and spindle formation in prometaphase.