Cell division

1. CELL DIVISION
Dr. Mahmood S. Choudhery, PhD, Postdoc (USA)
Assistant Professor
Tissue Engineering & Regenerative Medicine
King Edward Medical University

2. Cell Division
The key roles of cell division
Unicellular organisms:
Multicellular Organisms:
Reproduction Growth Repair
Reproduction

3. 1. Reproductive signal
2. Replication of DNA
3. Segregation
4. Cytokinesis
Four Events Must Occur
for Cell Division

4. Cell Division in Prokaryotes
Prokaryotes divide by binary fission
External factors such as food,
temperature act as reproductive signals
DNA replication occurs after a cell get a
reproductive signal
Segregation occurs when cell grows in
size
Cytokinesis occurs by pinching in of
plasma membrane

5. Cell Division in Prokaryotes
Single, circular chromosome, attached to membrane at “Ori
point”
Prokaryotes divide by binary
fission

6. Cell Division in Eukaryotes
Eukaryotic cells divide by mitosis or
meiosis
Cell division in Eukaryotes is not
related to the environment of
single cell
More than one chromosomes
Distinct nucleus
Newly replicated chromosomes
remain attached as sister
chromatids

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Cell Division in Eukaryotes………..
Process of cell division in eukaryotes
occurs in a series of events collectively
called as cell cycle
Cell Cycle

8. Phases of Cell Cycle
Interphase
Cell Cycle
Mitotic Phase
Mitosis
Division of Nucleus
Cytokinesis
Division of cytoplasm
During interphase cells do not divide while during
Mitotic phase the cells actually divide.

9. 3
•DNA repair
•Additional growth
•Makes proteins and
organelles
•Beginning of chromosome
condensation G2
1
•Cell grows in size,
•Organelles are duplicated
•Performs normal functions
•Molecular machinery for
DNA replication is made
G1
S
2
4
Mitotic
Phase
Nucleus and cytoplasm
divide
Phases of Cell Cycle
G0
1
11
8
4

10. Mitosis (karyokinesis)
Division of nucleus
1. Prophase
2. Prometaphase
3. Metaphase
4. Anaphase
5. Telophase
Phases of mitosis

11. 1. Prophase
• Chromatids condense and
become visible
• Nucleolus disappears
• Centrosomes move to
opposite ends of the cell
• Spindle fibers form
• Kinetochore spindle fibers
• Polar spindle fibers
Search and capture mechanism
Phases of mitosis cont……..

12. 2. Prometaphase
• Nuclear membrane
disappears
• Kinetochores develop
in centromere region

13. 3. Metaphase
• All the centromeres arrive
at the equatorial plate
• Chromosomes are
maximally condensed
• Sister chromatids line up at
equatorial plate
• At the end of metaphase,
the chromatid pairs
separate

14. How do chromatid pairs separate at end
of metaphase?
Spindle checkpoint?
Separase

15. Pole separation
4. Anaphase
• Begins with separation of
Chromatids
• Sister chromatids move to
opposite poles
Mechanisms that propels
chromosomes towards poles
1. Cytoplasmic dynein
2. Kinetochore Microtubules shortening
from poles
• Poles are pushed farther
apart
• Movement of
chromosomes is very slow

16. 5. Telophase
• Spindle fibers disintegrate
• Nuclear membrane
reforms
• Chromosomes uncoil and
become less visible
• Nucleoli reappear

17. Cytokinesis
Division of cytoplasm
Animal Cells
Plant Cells
Cytokinesis differs in animal and plant cells

18. Cell cycle Video

19. Growth Factors can Stimulate Cells
to Divide
The cells in G0 must be stimulated by external chemical
signals for cell division
1. Platelets release PDGF that stimulate cells to divide
2. Interleukins and erythropoietin stimulate division and
specialization, respectively, of precursor cells of white
blood cells and red blood cells.

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Occurs during formation of gametes (Eggs &
Sperms)
Meiosis
Overall Functions of Meiosis
1. To reduce chromosome
number from diploid to haploid
2. To ensure that each new
haploid product has a complete
set of chromosomes
3. To promote genetic diversity

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Meiosis: Two Part Cell Division
Meiosis 1 Meiosis II
Although
nucleus divides
twice during
meiosis the DNA
is replicated only
once

22. Meiosis
First cell division: Meiosis I
 Prophase I
 Metaphase I
 Anaphase I
 Telophase I
Second cell division: Meiosis II
 Prophase II
 Metaphase II
 Anaphase II
 Telophase II

23. 1. Prophase I
Phases of Meiosis I
Chromatin material
condenses and becomes
visible
Pairing of homologous
chromosomes,
Coiling and shortening
chromosomes continue
Crossing over occurs
Mitotic
prophase

24. 2. Prometaphase 1
Phases of Meiosis I
Nuclear envelop breakdown
Nucleolus disappear
One kinetochore forms per
chromosome
Chromosomes attach to spindle
fibers

25. 3. Metaphase 1
Phases of Meiosis I
Homologous chromosomes
line up on equatorial plate
Mitotic Metaphase

26. 4. Anaphase 1
Phases of Meiosis I
Homologous chromosomes
move to opposite poles of
cell
Mitotic anaphase
Meiotic anaphase

27. 5. Telophase 1
Phases of Meiosis I
Chromosomes reach to
opposite poles
Chromosome stay
condensed
Nuclear envelope may or
may not appear around
chromosomes

28. Interkinesis
 Typically brief
 Chromatin material is partially uncoiled
 No S phase (no DNA replication)
Period between meiosis I and meiosis II

29. Prophase II Metaphase II Anaphase II Telophase II
Meiosis II
Meiosis II is similar to Mitosis
Main differences between Mitosis and Meiosis II?
1. DNA replicates before mitosis, but not before meiosis II.
2. Sister chromatids differ
3. Number of chromosomes is half

30. Video Meiosis I and II

31. Regulation of Cell Cycle
Checkpoints monitor cell for its progression
in cell cycle
Three main checkpoints that control the cell cycle in
eukaryotic cells are
1. G1 checkpoint
2. G2 checkpoint
3. Metaphase checkpoint

33. Activation of CDKs
Phosphorylation
Different Combinations of CDK and Cyclin
1. Cyclin D-Cdk4 acts during G1.
2. Cyclin E-Cdk2 also acts in G1.
3. Cyclin A-Cdk2 acts during S
4. Cyclin B-Cdk1 acts at the G2 boundary

34. Regulation of cell Cycle
RB/E2F

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