|Index to this page| The period between M and S is called G1;
that between S and M is G2. Their levels in the cell rise and
fall with the stages of the cell cycle. Their levels in the cell
remain fairly stable, but each must bind the appropriate cyclin
(whose levels fluctuate) in order to be activated. They add
phosphate groups to a variety of protein substrates that control
processes in the cell cycle. |Separation of the sister
chromatids depends on the breakdown of the cohesin that has been
holding them together. It works like this. |Link to discussion
of proteasomes.| |This is only one of the mechanisms by which
the cell ensures that every portion of its genome is copied once
— and only once — during S phase. Link to discussion.| |Some
cells deliberately cut the cell cycle short allowing repeated S
phases without completing mitosis and/or cytokinesis. This is
called endoreplication and is described on a separate page. Link
to it.| |Discussion of DNA replication.| |Link to discussion of
the roles of the spi