Cell Biology

1. Cell Cycle
By
Mrs Sanchita Choubey
(M.Sc., PGDCR, Pursuing Ph. D)
Assistant Professor of Microbiology
Dr. D Y Patil Arts Commerce and
Science College Pimpri, Pune

2. OVERVIEW OF THE CELL CYCLE
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3. INTERPHASE
Interphase includes:
• G1 Phase:
• S Phase:
• G2 Phase:

4. INTERPHASE: G1 PHASE
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• Recovery from previous
division
• Cell doubles its organelles
• Cell grows in size
• Accumulates raw materials
for DNAsynthesis (DNA
replication)

5. INTERPHASE: S PHASE
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• DNA replication
• Proteins associated with DNA
are synthesized

6. INTERPHASE: G2 PHASE
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• Between DNA replication and
onset of mitosis
• Cell synthesizes proteins
necessary for division

7. CELL CYCLE: MITOSIS PHASE
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Mitosis phase includes:
• Mitosis (karyokinesis)
• Nuclear division
• Daughter chromosomes
distributed to two daughter
nuclei
• Cytokinesis
• Cytoplasm division
• Results in two genetically
identical daughter cells

8. CELL CYCLE: MITOSIS PHASE
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9. SIGNIFICANCE OF MITOSIS
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• Permits growth and repair.
• In plants it retains the ability to
divide throughout the life of the
plant
• In mammals, mitosis is necessary:
• Fertilized egg becomes an
embryo
• Embryo becomes a fetus
• Allows a cut to heal or a broken
bone to mend

10. • Chromatin condenses.
• Centrosomes separate,
moving to opposite ends
of the nucleus
• The centrosomes start to
form a framework used to
separate the two sister
chromatids called the
mitotic spindle, that is
made of microtubules
• Nucleolus disappears
• Nuclear envelope
disintegrates
MITOSIS PHASE: PROPHASE
What’s happening? What the cell looks like?
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11. MITOSIS PHASE: PROMETAPHASE
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What’s happening? What the cell looks like?
• Nuclear envelope
fragments
• Chromosomes become
more condensed
• A kinetochore is formed
at the centromere, the
point where the sister
chromatids are attached
• Microtubules attach at
the kinetochores

12. MITOSIS PHASE: METAPHASE
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What’s happening? What the cell looks like?
• Chromosomes align on
an axis called the
metaphase plate
• Note: the spindle
consists of
microtubules, one
attached to each
chromosome

13. MITOSIS PHASE:ANAPHASE
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What’s happening? What the cell looks like?
• Each centromere splits
making two chromatids
free
• Each chromatid moves
toward a pole
• Cell begins to elongate,
caused by microtubules
not associated with the
kinetochore

14. MITOSIS PHASE: TELOPHASE
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What’s happening? What the cell looks like?
• Formation of nuclear
membrane and nucleolus
• Short and thick
chromosomes begin to
elongate to form long and
thin chromatin
• Formation of the cleavage
furrow - a shallow groove in
the cell near the old
metaphase plate
• Cytokinesis = division of the
cytoplasm

15. RESULTS OF MITOSIS
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• Two daughter nuclei
• Each with same
chromosome number as
parent cell ( 2n)
• Genetically identical to
each other and the
parent cell

16. MEIOSIS
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• Formation of Gametes (Eggs & Sperm)
• Called Reduction- division
• Preceded by interphase which includes
chromosome replication
• Two meiotic divisions
• Meiosis I and Meiosis II
• Original cell is diploid (2n)
• Four daughter cells produced that are
haploid (n)

17. SIGNIFICANCE OF MEIOSIS
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• Two haploid (1n) gametes are brought together
through fertilization to form a diploid (2n) zygote

18. MEIOSIS I: PROPHASE I
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Prophase I is further subdivided into
periods known as
•Leptotena
•Zygotena
•Pachytena
•Diplotena
•Diakinesis

19. A physical exchange of
chromosome pieces
PROPHASE I
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20. METAPHASE I
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Homologous pairs
of chromosomes
align along the
equator of the cell

21. ANAPHASE I
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Homologs separate and
move to opposite poles.
Sister chromatids remain
attached at their centromeres.

22. TELOPHASE I
 Nuclear
envelopes
reassemble.
 Spindle disappears.
 Cytokinesis divides
cell into two.
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23. MEIOSIS II: PROPHASE II
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Nuclear envelope
fragments.
Spindle forms.

24. MEIOSIS II: METAPHASE II
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Chromosomes align
along equator of cell.

25. MEIOSIS II: ANAPHASE II
 Sister chromatids separate
and move to opposite poles.
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Equator
Pole

26. MEIOSIS II: TELOPHASE II
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Nuclear envelope
assembles.
Chromosomes
decondense.
Spindle disappears.
Cytokinesis divides cell
into two.

27. RESULTS OF MEIOSIS
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• Four haploid cells with one
copy of each chromosome

28. SUMMARY OF MEIOSIS I
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Nucleus
Spindle
fibers
Nuclear
envelope
EARLY
PROPHASE
I
LATE PROPHASE I METAPHASE I ANAPHASE I TELOPHASE I &
CYTOKINESIS

29. SUMMARY OF MEIOSIS II
Prophase II Metaphase II Anaphase II Telophase II 4 I
Undentical
haploid
cells
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30. Intracellular control of the
cell cycle
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The cell cycle is controlled by
regulator molecules that either:
promote the process (positive)
stop it from progressing (negative)

31. Positive: Cdks & Cyclins
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 Cyclins
◦ The regulatory subunits of the protein
kinases that control the cell cycle
 Cyclin-Dependent Kinases (Cdks)
◦ The catalytic subunits of the protein
kinases
◦ Must be associated with a cyclin in order
to be activated

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33. Negative: Rb & p53
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 Tumor suppressor genes
 Tumor suppressor gene codes for a
signaling protein in an inhibitory
pathway. If a tumor suppressor gene
mutates, the end result can be
active cell division.

34. Negative: Rb & p53
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◦ Retinoblastoma protein(Rb)
◦ prevents cell moving into S phase by
binding to a transcription factor
◦ When Rb is phosphorylated it cannot bind
so cell can move into S phase
◦ p53
◦ prevents damaged from dividing (by
inhibiting Rb pathway)

35. p53
 p53 protein halts cell division if it detects
damaged DNA
 options:
 stimulates repair enzymes to fix DNA
 forces cell into G0 resting stage
 keeps cell in G1 arrest
 causes apoptosis of damaged cell
 ALL cancers have to shut down p53 activity
 Cancer is essentially a failure of cell division
control
p53 is the
Cell Cycle
Enforcer
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37. Generic cell cycle checkpoints
Is environment favorable?
Is environment favorable?
Are all DNA replicated?
Are all chromosomes
attached to the spindle?
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38. G1
S
G2
M
G1 Checkpoint
G2 Checkpoint
M Checkpoint
G1
Cdk
G1
Cyclin
P
Active G1 Cdk-Cyclin
• Growth factors
• Nutritional state of cell
• Size of cell
Degraded G1 Cyclin
Mitotic
Cdk
Mitotic
Cyclin
P
ActiveMitotic
Cdk-Cyclin
(MPF)
• Replication completed
• DNA integrity
APC
Chromosomes attached at
metaphase plate
Degraded Mitotic Cyclin
Control of the
Cell Cycle

39. THANK YOU!
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