Re: why do stages in mitosis vary in duration

Hi James,
This is a great question and one many Cell Biologists have been working 
on for years.  As you may know there are many proteins which regulate the 
process of cell division and the cell cycle as a whole. The two main 
class of proteins are the cyclins and the Cyclin-Dependent Kinases or 
CDK's. The regulation of these proteins are well conserved but the timing 
of the cell cycle vary mainly because of individual differences in each 
cell type.

In general, there are two GAP phases where the cell determines it stage 
and fate. One GAP before it commits to duplicate it's chromosomes (called 
S phase) and another after DNA duplication but before cell division (also 
called mitosis, M phase). These GAP phases are called G1 and G2 
respectively.  The GAP phases can vary for a number of reasons. Some 
cells, like frog oocytes, have GAP phases that are very small to no time 
at all. They go to and from S phase to M phase so quickly that the two 
GAP phases are not seen.

Factors that control cell division are mainly external. If an organism 
needs rapid growth the the signals to induce a longer GAP phase will not 
be present and these cells would divide faster. The opposite would also 
be true.

So one answer to your question is in how each organism or cell decides to 
dictate the time it needs to spend in each GAP phase.

Studies in many cell types, but first shown in yeast, have shown the 
presence of checkpoints.  These are regulatory proteins that monitor if a 
cell is capable of entering S phase and/or M phase. If a cell gets into 
troube in these phases they can arrest, or stop growing, which allows the 
cell either time to fix the problem or commit cell suicide (apoptosis).

Timing of S-phase and M-phase are also cell specific.  Many scientists 
have formulated that the timing is equivalent to the complexity, amount 
of DNA and specificity of each cell in determining why one grows faster 
than another.

As for mitosis, each separate phase within mitosis are classified 
primariyl on how the DNA is arranged and organized.  There is prophase, 
which is where you will see the DNA condensing. Therefore the amount of 
time it takes to condense will equal how long this process is.  There is 
no clear timeframe for each phase. Metaphase is the time is takes for the 
newly condensed DNA, now called chromosomes to align in the center of the 
cell.  This timing will also vary as this is the site of the major 
mitotic checkpoint.  During this checkpoint all of the chromosomes 
must be connected to the spindle pole.  Studies have shown that if you 
inhibit this, either chemically or mechanically, the cell will just sit 
there and wait.  Once the cell has given the signal that all is ok, the 
spindles will pull the chromosomes to each pole initializing the next 
step in mitosis, anaphase.  Telophase is next were the cell begins to 
form an invagination of its membrane to form two cells. The DNA 
decondenses and cytokinesis starts. Cytokinesis is seen as the new 
membranes form and two new cells are seen.

Since all these events take a number of regulatory proteins, why one is 
timed faster than another is more speculation than scientific proof. One 
thing that can be said is that the outside or external signals will 
dictate when cell division takes place. Internal signals will monitor 
this process and slow the cell down or arrest it if any problems arise.

These internal cellular clocks are rather interesting and still a topic 
of research and debate.

I hope this answer helps you out and I encourage you to Google your 
questions as there is load of information out there that may help you see 
it better.

Good luck,
Matt