LECTURES 36 & 37
20 & 23 November 2009 (P.J.
The Cell Cycle and Its Control
Reading: pp. 609-623; DVD 18.2;
Probs: 93-97; exam IV‘06, #8, 9; ECB Q18-5,-12
The basic scheme of cell cycle control
Necessary events, as we’ve discussed in the past two lectures:
(1) Cells reproduce by duplicating their contents and dividing in two.
organisms, the result is a new organism; for metazoans, many cell divisions give rise to
a whole organism and more are necessary to maintain the organism.
(2) The most fundamental processes in the cell cycle are the replication of DNA and its
faithful segregation into daughter cells; a minimal cell cycle must provide for this.
<There has been a revolution in knowledge of how the cell cycle is controlled, with powerful
implications for cancer and developmental disorders.
The 2001 Nobel Prize for Medicine went
to Tim Hunt, Leland Hartwell, and Sir Paul Nurse for their founding contributions to this field.>
Old view of continuous & discrete processes
(1) The cell cycle used to be viewed as including:
a set of discrete events whose
occurrence could be demonstrated biochemically (e.g. DNA synthesis) or by direct
observation (e.g., chromosome condensation, nuclear envelope breakdown, mitosis,
a set of continuous events that were thought to occur essentially
throughout the cycle (e.g., protein, lipid, and mRNA synthesis).
(2) The traditional view of how the cell cycle was controlled was a “domino effect”
model, in which each event in the cell cycle triggered the next event.
(3) It is now clear that the ordered events of the cell cycle are triggered by a central
control system that is itself discrete from the process.
Basic events of G , S, G , M
[see fig below]
The G phase is characterized by normal cell metabolism and growth, and organelle
DNA replication (and thus chromosome duplication) occur during S phase.
In G the cell grows in preparation for mitosis, and in M phase the characteristic events
of nuclear division (karyokinesis) and cell division
(cytokinesis) occur (lectures 34-35).
New view: a central control system
(1) The cell cycle control system is analogous to a
certain critical points by feedback from the
processes of the cycle.
There are several such
critical points (see fig at top of next page), or
“checkpoints” in the cell cycle - we will discuss
two, and focus most of our attention on one.