The Cell - Challenges in Delivery of Therapeutic Genomics and Proteomics - page 26

Figure 1.11 Cell division and cell
cycle.

S

CELL CYCLE

G1 G2

Cytokinesis
Telophase
Anaphase

M phase

Prophase

Prometaphase

Metaphase

1.8.1 Cell Cycle Control System
The cell requires and has a whole set of protein machinery to ensure efficient cell
cycle control. The control system ensures that the cell cycle proceeds according to
the set sequences. Precise control over the events is exerted by molecular brakes at
various checkpoints.
The cell cycle control system is a centrally acting control system that exercises
its control over synthesis of the material and its distribution and usage at the correct
place. The control system works in cyclic fashion by activating and deactivating key
proteins that are involved in DNA synthesis, mitosis, and cytokinesis.
Cyclin-dependent protein kinases (Cdks) are the major proteins governing the
cell cycle. Cyclins do not possess any inherent enzymatic activity but can activate
protein kinases, which in turn phosphorylate and activate other important proteins of
the cell cycle. There are different types of cyclins produced at different phases in the
cell cycle ( Fig. 1.12 ).
A cell has number of inhibitory mechanisms that ensure that the Cdks are not
activated inappropriately. Repressor proteins reduce the gene transcription for
cyclins. Proteolytic activity exhibited by anaphase-promoting complex (APC) also
increases the rate of degradation of cyclins. Apart from these, the presence of spe-
cific Cdk inhibitors in the G1 phase prevents activation of Cdks. With the end of
mitosis, the Cdk activity is almost absent in the cell. Several mechanisms prevent
Cdk reactivation. These give the cell the necessary time to grow before the next cycle
of replication and division. In response to extracellular environments, G1 cyclins
start accumulating in the cell and push the cell from the gap phase into the synthetic
phase [1-6] .