EUKARYOTIC CELL CYCLE.ppt (pharmacotherapeutics)

EUKARYOTIC CELL CYCLE

1. EUKARYOTIC CELL CYCLE
BY:
DR SARITA SHARMA
ASSOCIATE PROFESSOR
DEPARTMENT OF PHARMACOLOGY
MMCP, MMDU

2.  Division cycle of most cells consists of four (4)
co-ordinated processes-
1. Cell growth
2. DNA replication
3. Distribution of duplicated chromosomes to
daughter cells
4. Cell division

3.  Eukaryotic cell cycle is more complex than
prokaryotic.
Consists of overall 2 stages – mitosis (dividing stage) &
interphase (non-dividing or resting stage).
 Cells spend most of their life span in interphase  a
period of intense biosynthetic activity  cells double
in size & duplicate their chromosome.
 For some cells interphase is extremely prolonged
(circulating lymphocytes) or permanent (skeletal
muscle & nerve cells).

4. PHASES OF CELL CYCLE
 Four (4) discrete phases – M, G
1
, S, and G
2
.
 M - phase is usually followed by cytokinesis.
- M phase - mitosis
- G
1
phase - interval (gap) between mitosis &
initiation of DNA replication
- S phase – synthesis of DNA
- G
2 phase – 2nd gap between DNA synthesis &
initiation of mitosis
For rapidly proliferating human cell of cycle time 24h-
* G
1
– 11hrs, S – 8hrs, G
2
– 4hrs, & M – 1hr.

6.  During G
2
a cell contains two times (4n) the amount
of DNA present in the original diploid cell (2n).
After mitosis the daughter cells again enter the G
1
period and have a DNA content equivalent to 2n.

7.  Some cells divide occasionally to replace the dead
and injured cells, e.g, skin fibroblasts & cells of
some internal organs, such as liver.
 These cells exit G
1
phase to enter a quiescent stage
of the cell cycle called G
0
.
They remain metabolically active but no longer
proliferate unless called on to do so by appropriate
extracellular signals.

8. DETERMINATION OF CELLULAR DNA CONTENT
More cells exist in the G
1
phase with 2n DNA content
followed by cells in G
2 phase with 4n DNA content.
Cells in S phase have DNA contents between 2n & 4n
and are distributed between these two peaks.

9. CONDENSATION-DECONDENSATION
CYCLE OF CHROMOSOMES
Eukaryotic chromosomes undergo condensation-
decondensation cycles at cell division.
During interphase the chromosomes are
decondensed and cannot be distinguished under the
microscope.
G
1 chromosomes are completely dispersed; S-phase
duplication occurs; G
2
-condensation starts.
At metaphase (M) and anaphase (A), the
condensation is maximal and the two centromeres
are clearly visible.

10. Condensation-decondensation cycle of chromosomes