CELL CYCLEIRAWAN YUSUFDepartment of Physiology Faculty of
MedicineHasanuddin University
CELL CYCLESThe repeating sequence of growth and division through
which cells pass each generation The purpose of the cell cycle is
to allow cells to reproduce without alteration of genetic
materialThe stages are well preserved between species
Why Do Cells Divide?GrowthDevelopmentMaintenanceReproduction
Events of Cell CycleG1 Phase -Period of growth; for many
organisms this occupies a major portion of the cells life spanS
Phase DNA replicates, resulting in duplicate chromosomeG2 Phase
Cell prepares for division (mitosis), building necessary materials;
centriole and other organelles doubleM Phase Cell division,
consisting of nuclear division and cytoplasmic division
(cytokinesis)
Cell Cycle ProgressionHighly regulated process controlled by a
large number of factors include:Growth stimulating factorsGrowth
inhibiting factorsEnzymes that alter functional state of other
proteins by adding (kinases) or removing (phosphatases) phosphate
groupsFailure in the mechanisms by which the cell cycle is
regulated can lead to uncontrolled cell growth and the development
of cancer
The Cell Cycle ControlThere are important checkpoints during the
cell cycle:at the G1 S transitionat the S G2 transitionat the G2 M
transitionRegulation of the cell cycle occurs as a consequence of
cyclin Cdk interactionOther proteins such as P53, Rb and oncogene
play a role in regulating the cell cycleP27 : can block entry into
S (synthesis) phase.
Cell Cycle Is Regulated By Opposing Effects of Positive and
Negative FactorsSG2G1MCyclinCdkP53Rb+-
Cell Cycle Is Regulated By Opposing Effects of Positive and
Negative FactorsTumor Suppressor(P53, Rb)Oncogene(BCl2,BRCA1)Tumor
Suppressor(P53, Rb)Oncogene(BCl2,BRCA1)NORMALCANCERCell
proliferationCell proliferationCell deathCell death
Cyclins and Cyclin Dependent Kinases (Cdk)CyclinCdkProcess
regulatedCyclin DCyclin ECyclin ACyclin ACyclin BCdk 4,6Cdk 2Cdk
2Cdk 1(Cdc2)Cdk 1G1 phase progressionG1 to S phaseS phase
progressionS through G2M phase
Waves of Cyclin-Cdk Activity Control The Cell
CycleG1SG1G2MCyclin D-Cdk 4/6Cyclin E-Cdk 2CyclinA-Cdk1Cyclin
B-Cdk1GFKinase ActivityTime
CHECKPOINTSThe cell cycle control system has checkpoints during
G1, G2, S and MCheckpoint signals report cells status:Is the cell
big enough?Is environment favorable?Is DNA replicated?Are
chromosomes attached to opposite poles?
