LECTURE 8: MITOSIS AND CYTOKINESIS

INTRODUCTION1. Cell division is accomplished by

• mitosis = division of chromosomes and • cytokinesis = division of the cytoplasm

2. Mitosis is the division of somatic cells in which the appropriate number of chromosomes is maintained

3. A somatic cell is a body cell of an organism or a cell that is not involved in reproduction of a new organism. Somatic cells divide to recreate themselves

4. Cytokinesis is division of the cytoplasm to produce two daughter cells, usually begun during telophase

5. Difference between animals and plants: plants are divided by the formation of a cell plate, where animal cells are divided through the formation of a cell cleavage. Cell plate materials originate in the golgi complex.

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INTRODUCTION

4.  When the somatic cell has its complete complement of chromosomes it is said to be in the diploid condition

5. At the end of mitosis daughter cells are diploid

Terminology• Genome : cell’s genetic information• Somatic : body cells• Gametes : reproductive cells (sperm and

egg cells)• Chromosomes : DNA molecules• Diploid (2n) : 2 sets of chromosomes • Haploid (1n) : 1 set of chromosomes

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BASIC PRINCIPLECell Division: Key Roles

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Coordination of Mitosis and CytokinesisCell Division demands coordination of DNA replication (Mitosis) and division of the cytoplasm (Cytokinesis).

Cell Division at cellular level

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What’s so important about cell division?

The process

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The Knit of Identity - Mitosis Precisely and Evenly Divides Duplicated Chromosomes

Precisely dividing the duplicated chromosomes has the consequence of providing each new cell with an identical and complete set of genetic instructions.

interphase prophase metaphase

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Mitosis Precisely and Evenly Divides Duplicated Chromosomes

Cytokinesis is the process of cell division and it is distinct and separable from mitosis.

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In Animal Cells, a Cleavage Furrow Forms and Separates Daughter Cells

Cleave furrow in a dividing frog cell.

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The Plant Cell Wall Forces Cytokinesis to Play by Different Rules

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Cancer• Transformation• Tumor: benign or malignant• Metastasis

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Cancer Is One Outcome of A Runaway Cell Cycle

Licentious division - prostate cancer cells during division.

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Chromosome and DNA

What is the difference between DNA and Chromosome?What is Karyotype ?

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Cell Devision at chromosome level

CHROMOSOME DUPLICATION

AND

SEPERATION

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DNA Replication – Simple in Principle, Complicated in Practice

Cell Devision at DNA level

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DNA is Packaged into Chromosomes

The packaging is impressive – 2 meters of human DNA fit into a sphere about 0.000005 meters in diameter.

chromatin

duplicatedchromosome

DNA in the cell is virtually always associated with proteins.

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The Link Between DNA Replication and Chromosome Duplication

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DNA is Condensed into Visible Chromosomes Only For Brief Periods in

the Life of a Cell

95% of the time, chromosomes are like this.

Easily visible chromosomes are apparent perhaps 5% of the time in an actively growing cell and less in a non-growing cell.

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A normal human karyotype

Boy or girl?

Note that almost all chromosomes come in homologous pairs.

A Karyotype is an arranged picture of chromosomes at their most condensed State

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The Cell Cycle

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From Birth to Rebirth, a Cell Progresses Through Characteristic Stages That Constitute the Cell Cycle

In multicellular organisms like us, progress through the cell cycle is carefully regulated.

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The Cell Cycle1. Interphase (90% of

cycle)– G1 phase~ growth – S phase~ synthesis of

DNA– G2 phase~ preparation

for cell division2. Mitotic phase3. Mitosis~ nuclear

division4. Cytokinesis~

cytoplasm division

INTERPHASE - It is the time between divisions

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The Cell Cycle

• G1• S• G2Mitosis &cytokinesi

s

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Cell Cycle regulation

• Growth factors

• Density-dependent inhibition

• Anchorage dependence

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Stages of Mitosis

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Mitosis1. Prophase2. Prometapha

se3. Metaphase4. Anaphase5. Telophase

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Mitosis in Action

Blue shows DNA, green shows spindle fibers.

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Stages of mitosis

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Stages of mitosis

PROPHASE, METAPHASE, ANAPHASE,TELOPHASE

1. Prophase:• Chromosomes condense and become

visible, each Chromosome contains several cm of DNA condensed into 5 – 10 micrometers.

• During the S Phase each chromosome has been duplicated into a sister chromatid. (A chromotid is one of the two halves of duplpicated chromosomes)

S Phase = synthesis of DNA

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Prometaphase

• Nuclear membrane fragments

• Spindle interaction with chromosomes

• Kinetochore develops

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• Centrosomes at opposite poles• Centromeres are aligned• Kinetochores of sister chromatids attached to

microtubules (spindle)

2. Metaphase: 1. Chromosomes line up along the cell

“equator”.2. The cell itself condenses and the

chromosomes are more clearly visible than during any other phase of mitosis

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• Paired centromeres separate; sister chromatids liberated

• Chromosomes move to opposite poles• Each pole now has a complete set of

chromosomes

3. Anaphase1. is defined as the sister chromatids begin

to separate2. By now each chromatid is an independent

and functional chromosome3. The chromosomes are pulled toward

opposite poles. Anaphase is complete when a complete set of chromosomes reaches each pole.

4. Telophase: 1. The final stage of mitosis,

chromosomes uncoil into chomatin threads

2. A nuclear envelope forms around each set of chromosomes, nucleoli appear

• Daughter nuclei form• Nuclear envelopes arise• Chromatin becomes less coiled• Two new nuclei complete mitosis

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Structure of a replicatedchromosome

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Counting chromosomes and chromatids

• n = haploid number of chromosomes– Example: the humans have 23

different chromosomes (n=23).– Diploid cells have 2n chromosome

#. Human diploid cells, have 46

chromosomes (2n=46).• c = number of chromatids in

unreplicated (G1) haploid state.

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Replication of achromosome during

mitosis

1. Frequency of mitosis varies between species or even with location within an organism…skin cell versus nerve cells for example, but under similar conditions the length of the cell cycle is constant for a particular type of cell.

2.  The rate and frequency of mitosis in a multicelluar organism must be controlled…

3.  The cell requires a protein called maturation promoting factor (MPF). Scientists are still learning how MPF works and controls the cell mitosis.

CONTROLS ON DIVISION

•  Read about the effects of Colchicine and the condition of polypoidy

•  When mitosis occurs each daughter cell receives exactly the same number and kind of chromosomes as the parent

•  When a haploid cell undergoes mitosis, two haploid cells are produced.

The End