Basic concepts concerning the cell cycle

1. JG NEL
DEPT OF HAEMATOLOGY AND CELL BIOLOGY
UFS
SEPTEMBER 2008
The Cell Cycle, basic concepts

2. Lecture outline
Historical perspective
Phases of the cell cycle
Regulation of the cycle
The importance of the cycle

3. Introduction
The cell is the basic structural and functional unit of
all living organisms.
Understanding how the cell cycle operates, and is
controlled, is therefore an important problem in
biology with implications for medicine

4. The cell cycle
Universal process by which cells reproduce
Underlies the growth and development of all living
organisms –central to their heredity and evolution
Results in
 Duplication
 Transmission of genetic information
The precision with which the cell cycle is executed
ensures survival of all living organisms

5. Historical perspective
What we know today is the result of work done over
the past +/- 150 years
Nägeli(1844)+Remak(1852) -described the division
of plant and animal cells
Study of the cell cycle -began with the discovery of
cell division

6. Phases of the cell cycle

7. Phases of the cell Cycle
M phase
 Karyokinesis
 Cell’s chromosomes divided between two daughter cells
 Cytokinesis
 Cytoplasm divides –forming distinct cells
G1 phase
 Between M phase and the beginning of DNA synthesis
 Cellular biosynthetic activities resume at a high rate
 Marked by the synthesis of various enzymes required in S
phase

8. Phases of the cell Cycle
S phase
 Starts when DNA synthesis commences
 Complete when all the chromosomes have been replicated
 Rates of RNA transcription and prot synth low during this
phase -with the exception of histone production

9. Phases of the cell Cycle
G2
 Lasts until the cell enters mitosis
 Significant protein synthesis occurs –mainly involving the
production of microtubules
 Inhibition of prot synth prevents cell from entering mitosis
G0
 Quiescent and senescent cells

11. Cell cycle regulation:
 Ensures the orderly progression of events so that the
nuclear cycle is coordinated with cell growth and
physical separation.
Replication must occur once per cell cycle and
precede chromosome segregation; segregation must
be complete before cytokinesis.

12. Regulation of the cell cycle

13. 2001 Nobel prize in Physiology or Medicine

14. 2001 Nobel prize in Physiology or Medicine
Leland Hartwell
 60s
 Budding yeasts, identified specific mutants that blocked specific
stages of the cell cycle
 Cell division cycle genes
 “start” gene in yeasts –CDC28
 Checkpoints
Paul Nurse
 70s
 Isolated fission yeast mutants that could speed up the cell cycle
 CDK- cdc2, CDK-1
 1987 discovers the human “start” gene
Tim Hunt
 Discovered “cyclins”

15. Regulation of the cell cycle
Crucial to the cell
Involves the detection and repair of genetic damage
Provision of various checks to prevent uncontrolled
cell division
Molecular events that control cell cycle –ordered and
directional

16. Regulation of the cell cycle
Role players:
Cyclins + Cyclin-dependent kinases
Restriction points
P53
CDK inhibitors

17. Regulation of the cell cycle:CDKs

18. CDK-2

19. Regulation of the cell cycle:CDKs
Exert their effects on cell-cycle events by
phosphorylating a large number of proteins in the
cell
Two lobed structure
Structure modified so that the active site of the CDK
is blocked in the absence of cyclin

20. Regulation of the cell cycle: Cyclins and CDKs
Cyclins -regulatory subunits
CDKs -catalytic subunits
Cyclins have no catalytic activity on their own
Cyclins with their bound and activated CDK
functions during distinct stages of the cell cycle
Activated heterodimer

21. Regulation of the cell cycle:Cyclins
Periodicity of Cyclins
 ensure well-delineated transitions between cell cycle stages
Degradation
 by ubiquination and proteasomal degradation
 Details of conjugation to ubiquitins differ
 G1 cyclins -by SCF complex
 Mitotic cyclins -by APC

22. Regulation of the cell cycle: Restriction points

23. Regulation of the cell cycle: Restriction points
Cell cycle proceeds by a defined sequence of events
 Later events depend on completion of earlier events
Regulation necessary to ensure complete and
accurate replication of genome
Checkpoints set at various stages during the cell
cycle
 Latter 1/3 of G1
 In S phase
 G2/M
 Spindle checkpoint

24. Regulation of the cell cycle: Restriction points
Checkpoints prevent cell cycle progression
-allowing :
 Verification of necesary phase processes
 Repair of DNA damage

25. The location of checkpoints in the cell cycle

26. P53 protein

27. Regulation of the cell cycle: P53
Monitors the integrity of the genome
In the presence of genomic damage –interrupts
cycling to allow time for genomic repair
Levels in normal dividing cells – low/undetectable
Negatively regulated by MDM2

28. Regulation of the cell cycle: P53
MDM2
Negatively regulates p53
 Functions at two sites
 Level of gene –down regulates p53 transcription
 Binds to P53 protein –inhibiting activity, mediating export from
the nucleus, ubiquination and proteasomal degradation

29. Regulation of the cell cycle: P53
In presence of DNA damage
 P53 binds to sequence specific DNA site –inducing increased P53
protein synthesis
 Phosphorylation of p53
 Activates the protein
 Renders the protein resistant to binding and inactivation by MDM2 –
resulting in doubling of the half life of P53 and increasing activity of the
protein
Mechanism of P53 action
Interrupts cell cycle by inhibition of Rb phosphorylation
If damage too extensive for repair induces the expression of
Bax

30. Regulation of the cell cycle: P53
Interrupts cell cycle by
 Transcriptional upregulation of P21, preventing
phosphorylation of Rb by active inhibitiob of CDK 4,6,2

31. Regulation of the cell cycle: P53

32. The Rb protein

33. Regulation of the cell cycle: Rb proteins
Pocket proteins
Sequester E2F transcription proteins
Release of E2F dependant upon the phosphorylation
state of Rb protein
 Unphosphorylated/hypophosphorylated tightly binds E2F
 Phosphorylation
 By CDK4/6
 E2F dissiociates from Rb –free to transcribe responder
genes
 Cyclin E –required for progression through restriction point

34. Regulation of the cell cycle: Rb proteins
Hypophosphorylated Rb guards restriction point –
preventing cell cycle progression

35. Regulation of the cell cycle: CDK inhibitors
Two families involved in cell cycle regulation
 Cip/Kip
 P27 +p21
 Functions at several sites in the cell cycle
 Targets –CDK 4,6,2
 INK4a
 P16INK4a
 Inhibits CDK4/6
 P19ARF
 Binds to MDM2 and blocks P53 degradation
Actions of CKI inhibits Rb phosphorylation and
subsequently keeps the cell in G1

36. The cell cycle

37. The importance of the cell cycle
Dynamic field
Study of the cell cycle is providing insights into
physiology and pathophysiology
After 150 years of study , there is still a lot of things
to learn, understand and prove.