Re: Does mitosis in plants and animals occur at a specific time?

Both animal and plant cells reproduce and divide their contents in a 
process called the cell cycle. As you may know, the cell cycle consists of 
four phases: G1 (Gap 1 phase), S phase ( the phase in which DNA synthesis 
occurs), G2 (Gap 2 phase) and M phase (or mitosis). So, mitosis does occur 
at a specific time in the cell cycle. Once a cell has been allowed to grow 
and replicated its DNA content, it can enter mitosis and divide into two 
daughter cells. Since a cell needs ample time to grow before they divide, 
the standard cell cycle can be 12 hours or more. Depending on the cell 
type, mitosis can last anywhere from 30 minutes to several hours.  Mitosis 
is  only a small fraction of the cell cycle. Once mitosis is completed, 
both daughter cells can begin a new cell cycle. 

Both animal and plant cells can undergo mutiple rounds of the cell cycle.
You have already mentioned a major difference between animal and plant 
cells. Animal cells pinch their membranes into two cells in a process 
called cytokinesis. Plant cells partion their cytoplasm through the 
formation of a new  cell wall. First, a cell plate is formed from 
vessicles transported from the Golgi apparatus. The vessicles deposit 
their membranes between the two daughter cells.This occurs during 
telophase and the cell plate grows outward until it reaches the walls of 
the cell. This will become the primary cell wall after each cell deposits 
new material to strengthen the new membrane. 

If you like to see some time lapse pictures of plant mitosis, I would 
suggest you try this link: 
http://www.life.umd.edu/cbmg/faculty/wolniak/wolniakmitosis.html

Good luck with your studies!
Elsa