Cell Division
Dr. Mahmood S. Choudhery, PhD
Postdoc: College of Medicine, University of
Arizona, USA
Assistant Professor,
Incharge Tissue Engineering and
Regenerative Medicine Laboratory
King Edward Medical University/Mayo
Hospital Lahore
Cell Division
Unicellular organisms: Reproduction
Multicellular Organisms: Reproduction, growth, and repair
Four Events Must Occur for Cell Division
1. Reproductive signal
2. Replication of DNA
3. Segregation
4. Cytokinesis
Cell Division in Prokaryotes
Cellular DNA is a single molecule, or chromosome.
Prokaryotes reproduce by cell fission.
Cell Division in Eukaryotes
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
Eukaryotic cells divide by mitosis or meiosis
Cell Division in Eukaryotes
The Cell
Cycle
The process of Cell division in eukaryotes
occurs in a series of events collectively called
as a cell cycle.
6
PHASES OF THE CELL
CYCLE



G1---Primary growth phase
S – synthesis; DNA replicated
G2 - secondary growth phase


M – mitosis (karyokinesis)
C – cytokinesis
Interphase
Mitotic phase
During interphase cells are not dividing
while during M phase the cells actually
divide.
7
Cell Cycle
Original
DNA
Two identical
copies of DNA
Search and capture mechanism
Mitosis (Karyokinesis)
Metaphase
Metaphase
Spindle Checkpoint?
Anaphase
Two mechanisms that propels chromosomes
towards poles
1. Cytoplasmic dynein
2. Kinetochore Microtubules shortening
from poles
Pole separation
Telophase
Cytokinesis: The Division of the Cytoplasm
Cytokinesis
Cleavage furrow
in animal cell
Cell plate in Plant
cell
14
Karyotyping
The Number, Size and Shape of Chromosomes
Eukaryotic Chromosome
Chromosome:
Nucleus:
2 meters
5um or 0.000005 meter
 5 families of Histones
 Histones have positive charge: Lysine and
Arginine
Nucleosomes
•
•
•
Eight histones
146 bp or 1.65 turns
of DNA
H1 clamps DNA to
Histone core
Nucleosomes
Regulation of Cell Cycle
Normal cells obey strict rules. They divide
only when told. They die rather than
misbehave.
Activation of CDKs
Phosphorylation
Different Combinations of CDK and Cyclin
1. Cyclin D-Cdk4 acts during the middle of G1.
2. Cyclin E-Cdk2 also acts in the middle of G1.
3. Cyclin A-Cdk2 acts during S
4. Cyclin B-Cdk1 acts at the G2–M boundary
RB/E2F
Regulation of cell Cycle
Growth Factors Can Stimulate Cells to
Divide
Blood clotting
Platelets
Platelet derived growth factor
Meiosis
Formation of Gametes
(Eggs & Sperm)
27
Meiosis: Two Part Cell
Division
Sister Chromatids
Separate
Homologs Separate
Meiosis I
Diploid
Meiosis II
Diploid
Haploid
28
Overall Functions of Meiosis
• To reduce chromosome number from diploid
to haploid. Fertilization then restores 2n
number
• To ensure that each new haploid product has
a complete set of chromosomes
• To promote genetic diversity
Meiosis 1
Meiosis 1