The document discusses the cell cycle's phases, including interphase (G1, S, G2) and mitosis, highlighting the importance of DNA replication and cell division. It describes the various phases in detail, including the physiological activities during G1, S, and G2, as well as the checkpoints that ensure proper cell division. Additionally, it explains the stages of mitosis and the structural changes that occur as cells prepare for division.

1. Cell Cycle- its regulation
and importance
- Shabana Godme

2. Cell Cycle
• The cell cycle is a cycle of events that takes
place in a cell which leads to the division of cell ,
formation of 2 daughter cells.
• The Cycle consists of 5 major phases:
G1, S, G2, mitosis, cytokinesis.
• Completion of the Cycle requires varying
periods of time from a few hours to several days,
depending on the type of cell and external factors
such as temperature and nutrients available.

3. Points we will be discussing today:
•The preparatory
phase i.e. the
interphase
(association of G1, S
and G2 phases)
•The checkpoints
• The cell division
phase ( mitosis and
cytokinesis)

4. INTERPHASE
Before a cell can begin mitosis and actually
divide, it must replicate its DNA, synthesize more
the proteins to associate with DNA in the
chromosome and produce an efficient amount of
organelles for two daughter cells.
These preparatory processes occur during the
G1, S and G2 phases of the cell cycle,
collectively called as interphase.

5. The key process of DNA
replication occurs during
the S (synthesis) phase
of the cell cycle.
G ( gap) phase precedes
and follows the S phase.
During the G phases, no
DNA synthesis can be
detected in the cell.

6. Some cell types pass through
successive cell cycles throughout the
life of the organism.
An example is the cells in the human
bone marrow that gives rise to red
blood cells.

7. The average RBCs lives only about 120 days,
and there are about 25 trillion of them in an
adult. To maintain this number, about 2.5 million
RBCs must be produced by cell division each
second.
At the other extreme, some highly specialised
cells, such as nerve cells, lose their capacity
to replicate once they mature.

8. G1 phase
The first phase of interphase is G1 phase, from
the end of the previous Mitosis phase until the
beginning of DNA replication is called G1 (G
indicating gap). It is also called the growth phase.
During this phase the biosynthetic activities of the
cell, which had been considerably slowed down
during M phase, resume at a high rate.

9. This phase is marked by synthesis of various
enzymes that are required in S phase, mainly
those needed for DNA replication. Duration of
G1 is highly variable, even among different
cells of the same species.

10. S phase
Initiation of DNA replication is indication of
S phase; when it is complete, all of the
chromosomes have been replicated, at this
time each chromosome has two (sister)
chromatids. Thus, during this phase, the
amount of DNA in the cell has effectively
doubled, though the ploidy of the cell
remains the same.

11. Rates of RNA transcription and protein
synthesis are very low during this phase. An
exception to this is production of histone
protein, which mostly occurs during the S
phase.

12. G2 phase
After S phase or replication cell then enters
the G2 phase, which lasts until the cell enters
mitosis. Again, significant biosynthesis occurs
during this phase, mainly involving the
production of microtubules, which are
required during the process of mitosis.
Inhibition of protein synthesis during G2
phase prevents the cell from undergoing
mitosis.

13. G0 phase
The G0 phase is a period in the cell cycle in
which cells exist in a state. G0 phase is viewed
as either an extended G1 phase, where the cell is
neither dividing nor preparing to divide. G0 is
sometimes referred to as a "post-mitotic" state,
since cells in G0 are in a non-dividing phase
outside of the cell cycle.

14. Some types of cells, such as nerve and heart
muscle cells, become post-mitotic when they
reach maturity but continue to perform their
main functions for the rest of the organism's
life. Multinucleated muscle cells that do not
undergo cytokinesis are also often considered
to be in the G0 stage.

15. checkpoints
● Two important checkpoints occur in G1 and
G2.
● The G1 checkpoint allows the cell to confirm
that the environment is favourable for cell
proliferation and its DNA is intact before
committing to S phase.
● G2 checkpoint ensures that cells do not
enter mitosis until damaged DNA is repaired
and DNA replication is complete.

16. ● There are 3
checkpoints
which the cells
have to cross
for entering the
next phase.
● Cdk- cyclin
dependent
kinase

19. MITOSIS
When cells are in the interphase portions of
the cell cycle, the chromosomal material is
dispersed and is visible, if at all, only as
threadlike strands, the chromatin. As
mitosis begins, the chromatin slowly coils
and condenses into a compact form; this
condensation appears to be necessary for
the complex movements and separation of
the chromosomes during mitosis.

20. When the condensed chromosomes become
visible under the light microscope, each
consists of 2 replicas, called chromatids ,
and the 2 identical chromatids remain joined
together. Their point of attachment is a
constricted area common to both
chromatids, the centromere . Within this
constricted area are disc-shaped protein
structures, the kinetochores.

21. The phases of mitosis
The process of mitosis is conventionally
divided into 4 phases:
•Prophase
•Metaphase
•Anaphase
•Telophase.

22. Time consumed by each phase of the Cycle
Prophase is
usually by far the
longest. If a
mitotic division
takes 10 min.,
about 6 of these
minutes the cell
will be in
prophase.

23. Prophase
•The chromatin condenses, and the individual
chromosomes begin to become visible.
• Each chromosome consists of 2 duplicate
chromatids pressed closely together longitudinally
and connected at the centromere.
• In the cells of most organisms ( higher plants are
the principal exception), 2 centriole pairs can be
seen at one side of the nucleus, outside the nuclear
envelope.

24. • Each pair consists of
1 mature centriole
lying at a right angle to
the first.The cell
becomes more
spheroid and the
cytoplasm more
viscous at this stage.

25. • During prophase, the centriole pairs move
apart. Between the centriole pairs, forming as
they separate, are spindle fibres made up of
microtubules and other proteins.
• In those cells that have centrioles, additional
fibres, collectively known as the aster, radiate
outward from the centrioles.
• By this time the nuclear envelope breaks down
as the chromosomes condense.

26. • By the end of prophase:
1. The chromosomes are fully condensed
and are no longer separated from the
cytoplasm.
2. The centriole pairs are at opposite ends
of the cell, and the members of each
pair are of equal size.

27. 3.The spindle is fully formed. It consists
of 3 groups of microtubules: astral rays ,
continuous fibres reaching from pole to
pole, shorter fibres that are attached to
the kinetochores of each sister chromatid.

28. Metaphase
● The chromatid pairs move back and forth
within the spindle fibres, as if they were
being tugged first toward one pole and then
the other.
● Finally they become arranged precisely at
the midplane (equator) of the cell.
● This marks the end of metaphase.

30. Anaphase
•The centromeres separate simultaneously in all
the chromatid pairs. The chromatids of each pair
then move apart, each chromatid becoming a
separate chromosome, each apparently drawn
toward the opposite pole by the spindle fibres.
• The centromeres move first, while the arms of the
chromosomes seem to drag behind.

31. • As anaphase
continues, the two
identical sets of newly
separated
chromosomes move
toward the opposite
poles of the spindle.
• Anaphase is the
most rapid portion of
mitosis.

32. Telophase
By the beginning of telophase, the chromosomes have reached
the opposite poles and the spindle disperses.
During late telophase, nuclear envelopes from around the two
sets of chromosomes, which once more become diffuse. in each
nucleus, the nucleoli reappears.

33. Often, a new centriole begins to form adjacent
to each of the previous ones. Replication of the
centrioles continues during the subsequent cell
cycle, so that each daughter cell has two
centriole pairs by prophase of the next mitotic
division

35. Thank You