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CELL DIVISION
Dr. Mahmood S. Choudhery, PhD, Postdoc (USA)Assistant Professor
Tissue Engineering & Regenerative Medicine King Edward Medical University
Learning Objectives
1. Cell division in prokaryotes
2. Cell division in Eukaryotes
• Mitosis
• Meiosis
3. Cell Cycle regulation
Cell Division
The key roles of cell divisionUnicellular organisms:
Multicellular Organisms:
Reproduction growth repair
Reproduction
1. Reproductive signal
2. Replication of DNA
3. Segregation
4. Cytokinesis
Four Events Must Occur
for Cell Division
Cell Division in Prokaryotes
Single, circular chromosome, attached to membrane at “Ori
point”
Prokaryotes divide by Binary fission Cytokinesis
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
7
Cell Division in Eukaryotes
Process of cell division in eukaryotes
occurs in a series of events collectively
called as a cell cycle
Cell Cycle
Phases of Cell Cycle
Interphase
Cell Cycle
Mitotic Phase
MitosisDivision of Nucleus
CytokinesisDivision of cytoplasm
During interphase cells are not dividing while during
Mitotic phase the cells actually divide.
3
DNA repairAdditional growth
Beginning of chromosome condensation
G21
Cell grows in size,Performs normal functionsOrganelles are duplicated
Molecular machinery for DNA replication is made
G1
S
2
4
Mitotic Phase
Nucleus and cytoplasm divide
Phases of Cell Cycle
Phases of Cell Cycle
Mitosis (karyokinesis)Division of nucleus
1. Prophase
2. Metaphase
3. Anaphase
4. Telophase
Phases of mitosis
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……..
2. Prometaphase
• Nuclear membrane disappears
• Kinetochore develop in centromere region
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 centromeres divide
How does chromatids separate at the
end of metaphase
Spindle checkpoint?
4. Anaphase
• Begins with separation of
centromeres
• Sister chromatids move to
opposite poles
Mechanisms that propels
chromatids towards poles
1. Cytoplasmic dynein (75%)
2. Kinetochore Microtubules
shortening from poles (25%)
Poles are pushed farther
apart1. Astral microtubules pull poles apart
2. Polar microtubules have kinesisns
that exert additional pull on
chromosome
Anaphase cont…..
5. Telophase• Spindle fibers disintegrate
• Nuclear membrane
reforms
• Nucleoli reappear
• Chromosomes uncoil and
become less visible
CytokinesisDivision of cytoplasm
Animal Cells
Plant Cells
Cytokinesis differs in animal and plant cells
Original Cell Division
Cell cycle Video
Regulation of Cell Cycle
Checkpoints monitor cell for its progression
in cell cycle
Three main checkpoints that control cell cycle in eukaryotic
cells
1. G1 checkpoint
2. G2 checkpoint
3. Metaphase checkpoint
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 G2 boundary
CDKs Cyclins
Regulation of cell Cycle
RB/E2F
Growth Factors Can Stimulate Cells to
Divide
Platelets
Blood clotting
Platelet derived growth factor
Erythropoietin Bone Marrow cells Red Blood cells
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
29
Meiosis: Two Part Cell Division
Meiosis 1 Meiosis II
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
1. Prophase IPhases of Meiosis I
Chromatin material
condenses and become
visible
Pairing of homologous
chromosomes,
Coiling and shortening
chromosomes continue
Crossing over occurs
2. Prometaphase 1Phases of Meiosis I
• Nuclear envelop disappear
•
• Nucleolus disappear
•
• One kinetochore forms per
chromosome
•
• Chromosomes attach to
spindle fibers
3. Metaphase 1Phases of Meiosis I
Homologous chromosomes
line up on equatorial plate
4. Anaphase 1Phases of Meiosis I
Homologous chromosomes
move to opposite poles of
cell
5. Telophase 1Phases of Meiosis I
Chromosomes reach to
opposite poles
Chromosome stay
condensed
Interkinesis
• Period between meiosis I and meiosis II
• No S phase (no DNA replication)
• Typically brief
Prophase II Metaphase II Anaphase II Telophase II
Meiosis IIMeiosis II is similar to Mitosis
Video Meiosis I and II
Eukaryotic Chromosome
Chromosome: 2 meters
Nucleus: 5um or 0.000005 meter
5 families of Histones
Nucleosomes• Eight histones
• 146 bp or 1.65 turns of DNA
• H1 clamps DNA to Histone core
Nucleosomes
Condensation of Chromosomes
43
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