CELL DIVISION

CELL DIVISION
The cell cycle Cell division in plants occurs in meristems and involves two parts: mitosis in which the chromosomes are replicated and sorted into two nuclei, and cytokinesis in which the cell wall, cytoplasm and organelles divide. In dormant meristems, the cells rest in G0 phase. When conditions are correct, the cell begins the processes leading to division. The entire cycle may be considered as four phases, G1 , S, G2 and M (Fig. 1).

In G1 phase the cell doubles in size and new organelles and materials needed for two cells are formed. During this phase, the nucleus migrates to the center of the cell and is surrounded by a sheet of cytoplasmic strands called the phragmosome that bisects the center of the cell at the plane across which it will divide. The phase ends with the G1/S checkpoint. The process can stop at this point (see the figure), or proceed to S phase in which DNA and associated nuclear proteins are replicated. At the end of S phase the cell contains two full copies of its genetic information. It proceeds to G2 phase when the chromosomes begin to condense and structures required for division form. A distinct band of microtubules , the pre-prophase band, forms around the cytoplasm in a ring where the edge of the phragmosome lay, again predicting the plane of cell division. At the end of G2, the cell has to pass another checkpoint (G2/M) at which stage, if conditions are suitable, it enters M phase in which the cell divides. Stages G1 to G 2 are