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Suspension Cultures
-Suspension cultures can be maintained in either of the following two forms:

(i) Batch cultures are initiated as single cells in 100 - 250 ml flasks and are propagated by transferring regularly small aliquots of suspension to a fresh medium.

(ii) Continuous cultures, are maintained in a steady state for long period by draining out the used medium and adding fresh medium; in this process either the cells separated from drained medium are added back to suspension culture (these are called 'close continuous cultures') or the addition of medium is accompanied by the harvest of an equal volume of suspension culture (these are called 'open continuous cultures').

In the second case, a steady state of suspension culture is maintained for a long period. Despite advantages associated with continuous cultures, they are rarely used, since a lot of attention is needed and the required equipment may not be easily available.

The medium used for suspension, cultures is usuallly the same as for callus growth except that agar is omitted. A manipulation of auxin/ cytokinin ratio may be desirable to achieve cell dispersion. The suspension culture needs to be regularly agitated to avoid clumping of cells and to provide aeration. A shaking speed of 30 - 150 rpm is satisfactory.

Cell suspensions are often asynchronous, different cells being in different stages of cell cycle (Gl, S, G2, M). Therefore, efforts have also been made to obtain synchrony in suspension cultures. Synchrony can be achieved by one of the following two methods:

(i) Starvation: cells are starved of a nutrient (e.g. a growth hormone) necessary for cell division, so that cells are arrested at G 1 or G2. These arrested cells are supplied with the nutrient (after sometime) to obtain synchronous cell division.

(ii) Inhibition inhibitors of DNA synthesis (e.g. 5- aminouracil, Fudr = fluorodeoxyuracil, hydroxyurea and thymidine) have also been used for achieving synchrony of cell division.

Due to inhibition of DNA synthesis, cells are arrested at G1, so that the removal of inhibitors is followed by synchronous division of cells.
   
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