cell biology

1. Molecular & Cell Biology
S. Rahgozar,PhD
University of Isfahan
Faculty of Science
3. Cell Cycle
3. 1. The Eukaryotic Cell Cycle and Regulation
1389-90

2. Phases of the cell cycle
 Mitosis
 Interphase
Nuclear division ending with cytokinesis
Typically 24h for a proliferating human cell in culture
o G1 Phase: The cell is
metabolically active and
continuously grows without
DNA replication
o S Phase: DNA replication
o G2 Phase: Cell growth
continues and proteins are
synthesized.
1 h
4 h
8 h
11h

3. Embryonic cell cycle
Start
Quiescent stage in cell cycle;G0
30m 30m30m
Nerve cell/fibroblast
cell cycle

4. Determination of cellular DNA content

5. Regulation of cell cycle
 Extracellular signals
Saccharomyces cerevisiae

6. Regulation of animal cell cycles
 Growth factors
Vertebrate oocytes can remain in G2 for
long periods of time until their
progression to M phase is triggered by
hormonal stimulation.
 Hormones
The restriction point
can be placed in
different regions

7.  DNA damage checkpoints
 Spindle assembly checkpoints
Monitor
o The integrity of DNA
o The incorporation of correct bases
Monitor
o The alignment of chromosomes on the mitotic spindle
Cell cycle checkpoints
 Intracellular factors

8.  Check point for
once per cycle
DNA replications
ORC: Origin Recognition Complex proteins
The MCM proteins are controlled
by “liscencing factors” to allow
replication to initiate.

9. Origin recognition complex
Green: rich in As and Ts
Blue: Protein that facilitates ORC binding
o A new pre-RC cannot form at the origin
until the cell progress to G1 phase.
o an Origin of Replication can be used only
if a prereplicative complex forms there in
G1 phase.
o Pre-RC disassemble through
phosphorylation, and DNA replication
starts.
Mechanism of DNA replication initiation
Each origin of replication is activated only once per cell cycle

10. Regulators of
cell cycle progression
 MPF (Mitosis Promoting Factor)
Further experiments
showed that MPF is not
restricted to oocytes and
appeared to act as a general
regulator of the transition
from G2 to M .

11.  cdc (cell division cycle) mutant genes
o Studies on genetic analysis of Saccharomyces cerevisiae and the discovery of
cdc28.
o Studies on genetic analysis of Schizosaccharomyces pombe showed a functionally
homologous gene; cdc2 which arrested the fission yeast both in G1 and G2 to M.

12. k1( the protein kinase encoded by cdc2 & cdc28)
A conserved cell regulator in all eukaryotes
Cyclins
In 3 years later, Joan Ruderman and colleagues supported the importance of cyclins
as the cell cycle regulators, by showing that microinjection of cyclin A into frog
oocytes is sufficient to trigger the G2 to M transition.
cdc28 Cd

13. Accumulation and degradation of cyclins in sea urchin embryos
These proteins were suggested that they might function to induce mitosis,
with their periodic accumulation and destruction controlling entry and exit from
M phase.

14. Structure of MPF (the conserved regulator of the cell cycle)
Cyclin B is a regulatory unit required for the catalytic activity of Cdk1.
MPF activity is controlled by the periodic accumulation and degradation
of cyclin B during cell cycle progression.

15. MPF regulation in mammalian cells
Ubiquitin dependentCell exits M, undergoes Cytokinesis and returns to G1
Happens in G2
cdc25

17. MPF regulation in mammalian cells
Ubiquitin dependentCell exits M, undergoes Cytokinesis and returns to G1
Happens in G2
cdc25

18. Complexes of cyclins and
cyclin-dependent kinases