To whom it concerns,
    
       I am am indidvidual who is about to begin some research in the
science field of cellular and molecular biology over the summer.
Before I get started, I wanted to gain a very good foundation on the
knowledge of this field. So i have been doing independent work and
have come across some questions that I have not been able to
understand. The following question is one that I have had extreme
problem in solving. I need to thoroughly have a good explanation of
this problem. The information that I have found on this has not helped
me answer this question so i turn to you to help me out. I thank you
very much of your help and your time. So here is the question:

This question deals with regulation of the eukaryotic cell cycle and
based on knowledge of this cycle, one has to explain each of the
following three experimental observations:

1. When MPF is injected into cells that have just emerged from S phase,
chromosome condensation and nuclear envelope breakdown occur
immediately, rather than after the normal G2 delay of several hours.

2. When an abnormal, indestructible form of the mitotic cyclin is
introduced into cells, they enter into mitosis but cannot emerge from
it and re-enter Gl phase.


3. Mutations that inactivate the main protein phosphatase used to catalyze
protein dephosphorylation cause a long delay in the reconstruction of the
nuclear envelope that normally takes place at the end of mitosis.

 Once again, thank you very much and I hope you can provide the answer ASAP.

Can you provide citations for where you read these results?

1) MPF or maturation-promoting factor, when injected into a cell that
has just emerged from S phase, will phosphorylate (either directly or
through another kinase), the laminin A and B proteins which make up
the nuclear lamina.  that make up the nuclear envelop.  MPF is a
protein kinase composed of Cdc2/28 and cyclin B.  The reason for
immediate effect of nuclear berakdown and chromosome condensation
(another target of MPF is Histon h1), is that you are presenting the
cell with a fully sythesised cdk/cyclin complex that is in its active
state - with a phosphorylated Thr161 and a dephosphorylated Tyr15.

2) Cyclin B degredation is required for exit of mitosis.  Why you
might ask? Well, the MPF initiates spindle assembly and chromosome
alighnment, and is kept around until everything is peachy. Its
actually a really cool pathway; as MPF is activated, it activates
enzymes which activate more MPF etc, at the same time, ubiquinating
enzymes are activated s l o w l y which ubiquinate the cyclin and
target it for degredation.  This is required for cell cycle exit, as
cdk2 can't be left in its active form.

3) I'm assuming you are talking about cdc25.  I'm not sure on the
details of the pathway, but Remember, laminins are phosphorylated at
the disassembly of the nuclear envelope, these phospates must be
removed for laminin AB dimers to form.

Check out the following books
Cell Cycle Control Ed. Hutchinson and Glover
The Cell Cycle Andrew Murray and Tim Hunt
Molecular Biology of the Cell Alberts et al.

Hope this helps.

good luck

- Patrick