Lecture 23 Regulation of the Cell Cycle
Campbell, Chap. 12.
Ed.; pp. 224-227.
Ed.; pp. 228-230.
Ed.; pp 238-241.
The Cell Cycle
The eukaryotic cell cycle is divided into 4 phases; G1, S, G2, and M.
Together, G1, S, and G2
make up interphase, while M is the mitotic phase we discussed last time.
DNA is synthesized (that
is, chromosomes are replicated) in S phase.
Note that because S occurs between G1 and G2, there
is twice as much DNA per cell in G2 as in G1.
The cell has its “normal” amount of DNA in G1,
while it contains 2 identical copies of its DNA in G2.
Cells don’t always divide continually.
Even bacteria and unicellular eukaryotes can stop dividing
when conditions get too bad for growth.
In this case, they form inert structures called spores.
These remain in suspended animation for long periods of time.
When conditions improve, the
spores start growing again.
In multicellular organisms like us, many cells divide only rarely.
we reach our adult size, most cells divide only under special circumstances.
(Some cells, like skin
cells, are exceptions, and divide continually to replace skin that’s sloughed off.
Other cells like
neurons in our brains, never divide.)
Cells that do not divide are said to be in G0 phase, and to have
left the cell cycle.
For cells that do divide, progress through the cell cycle is highly regulated.
experiment provided the first hint that something in the cytoplasm regulates the cell cycle.
Scientists took 2 cells, one in M phase and the other in G1 phase, and artificially fused them.
course, this wouldn’t happen normally, but was induced to happen using chemicals).
After the cells
were fused, molecules in the cytoplasm of the 2 cells could diffuse freely throughout the new single
Before the cells were fused, the condensed chromosomes and mitotic spindle in the cell
in M phase were clearly visible, as were the nuclear envelope and chromatin of the cell in G1 phase.
The scientists saw that after the cells were fused, the chromosomes from the G1 cell condensed, as
they would during mitosis.
In a second experiment, a cell in S phase was fused with one in G1
In this case, the cell in G1 phase now entered S phase, as determined by synthesis of DNA.
These experiments showed that after the cells were fused, some molecule in the cytoplasm of the
cell in M or S phase induced the changes characteristic of mitosis in the G1 cell.
Further work led
to the identification of the molecules and the elucidation of their mechanism of action, as discussed
Cell Cycle Checkpoints