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

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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.

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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

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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

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