CELL CYCLEIRAWAN YUSUFDepartment of Physiology Faculty of
Medicine Hasanuddin University
CELL CYCLES The 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
material The stages are well preserved between species
Why Do Cells Divide? Growth Development Maintenance
Reproduction
Events of Cell Cycle G1 Phase - Period of growth; for many
organisms this occupies a major portion of the cells life span S
Phase DNA replicates, resulting in duplicate chromosome G2 Phase
Cell prepares for division (mitosis), building necessary materials;
centriole and other organelles double M Phase Cell division,
consisting of nuclear division and cytoplasmic division
(cytokinesis)
Cell Cycle Progression Highly regulated process controlled by a
large number of factors include: Growth stimulating factors Growth
inhibiting factors Enzymes that alter functional state of other
proteins by adding (kinases) or removing (phosphatases) phosphate
groups
Failure in the mechanisms by which the cell cycle is regulated
can lead to uncontrolled cell growth and the development of
cancer
The Cell Cycle Control There are important checkpoints during
the cell cycle: at the G1 S transition at the S G2 transition at
the G2 M transition
Regulation of the cell cycle occurs as a consequence of cyclin
Cdk interaction Other proteins such as P53, Rb and oncogene play a
role in regulating the cell cycle P27 : can block entry into S
(synthesis) phase.
Cell Cycle Is Regulated By Opposing Effects of Positive and
Negative Factors
SCyclin + Cdk P53
G2 M
G1
Rb
Cell Cycle Is Regulated By Opposing Effects of Positive and
Negative FactorsTumor Suppressor (P53, Rb) Cell death NORMAL Cell
death Tumor Suppressor (P53, Rb) Oncogene (BCl2,BRCA1) Cell
proliferation CANCER Oncogene (BCl2,BRCA1) Cell proliferation
Cyclins and Cyclin Dependent Kinases (Cdk)
Cyclin Cyclin D Cyclin E Cyclin A Cyclin A Cyclin B
Cdk Cdk 4,6 Cdk 2 Cdk 2 Cdk 1(Cdc2) Cdk 1
Process regulated G1 phase progression G1 to S phase S phase
progression S through G2 M phase
Waves of Cyclin-Cdk Activity Control The Cell CycleG1 S G2 M
G1
Cyclin B-Cdk1
Kinase Activity
CyclinA-Cdk1 Cyclin E-Cdk 2 Cyclin D-Cdk 4/6
Time GF
CHECKPOINTS The cell cycle control system has checkpoints during
G1, G2, S and M Checkpoint signals report cells status: Is the cell
big enough? Is environment favorable? Is DNA replicated? Are
chromosomes attached to opposite poles?
G1
S
G2
prophase
metaphase
anaphase
Cytokinesis
Mitosis
