Cell Division

Mitosis

When does DNA Replication takes place?

G0 Phase – Terminally differentiated cells withdraw from cell cycle indefinitely

M Phase – Mitosis (nuclear division) and cytokinesis (cell division) yield two daughter cells

G2 Phase – No DNA synthesis,

RNA and protein synthesis continue

S Phase – DNA REPLICATION

where DNA synthesis doubles the amount of

DNA in the cell. RNA and protein also

synthesised

Restriction point – A cell that passes this point is committed to pass into S phase

G1 Phase – RNA and protein synthesis. No DNA synthesis

Reentry point – A cell returning from G0 enters at early G1 phase

The cell cycle can be divided into four major stages:• M phase• G1 phase• S phase• G2 phase

Lehninger et al. (2000) Principles of Biochemistry, 3rd ed.

Chromatin is a mass of uncoiled DNA and associated proteins called histones.

When cell division begins, DNA coils around the proteins forming visible structures called chromosomes

Diploid cells (2N) have two complete sets of chromosomes. The body cells of animals are diploid.

Haploid cells have one complete set of chromosomes. In animals, gametes (sperm and eggs) are haploid.

Learning objectives Explain that there are two different types of cell

division in most organisms, both with different purposes

Using diagrams, describe the behaviour of chromosomes during mitotic cell cycle in eukaryotes. Include reference to : mitosis, growth (G1 and G2 ), and DNA replication.

Describe the difference between sister chromatids and homologous chromosomes.

Learning objectives Identify and describe the following events in

mitosis: prophase, metaphase, anaphase and telophase

With respect to both plant and animal cells, understand the term cytokinesis, and distinguish between nuclear division and division of cytoplasm

Explain the importance of mitosis in growth and repair, and asexual reproduction

Introduction to Cell and Nuclear Division- MITOSIS

• Mitosis: process which a nucleus divides to produce two genetically identical daughter nuclei, each with same number and same kind of chromosomes as original nucleus in parent cell.

Mitosis is conventionally divided into five phases:• Prophase• Metaphase• Anaphase• Telophase

Cytokinesis is well underway by late telophase

BioFlix: MitosisBioFlix: Mitosis

Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Fig. 12-5

S(DNA synthesis)

MITOTIC(M) PHASE

Mitosis

Cytokinesis

G1

G2

Mitosis is conventionally divided into five phases:• Prophase• Metaphase• Anaphase• Telophase

Cytokinesis is well underway by late telophase

DNA REPLICATION BEFORE MITOSIS TAKES PLACE ie DURING INTERPHASE

Fig. 12-4 0.5 µm Chromosomes

Chromosomeduplication(including DNAsynthesis)

Chromo-some arm

Centromere

Sisterchromatids

DNA molecules

Separation ofsister chromatids

Centromere

Sister chromatids

G2 of InterphaseCentrosomes(with centriolepairs)

Chromatin(duplicated)

Nucleolus Nuclearenvelope

Plasmamembrane

Early mitoticspindle

Aster Centromere

Chromosome, consisting of two sister chromatids

Prophase Prometaphase

Fragmentsof nuclearenvelope

Nonkinetochoremicrotubules

Kinetochore Kinetochoremicrotubule

Metaphase

Metaphaseplate

Spindle Centrosome atone spindle pole

Anaphase

Daughterchromosomes

Telophase and Cytokinesis

Cleavagefurrow

Nucleolusforming

Nuclearenvelopeforming

Prophase

Fig. 12-6a

G2 of Interphase

Fig. 12-6b

ProphaseG2 of InterphaseAster CentromereEarly mitotic

spindleChromatin

(duplicated)Centrosomes(with centriolepairs)

Nucleolus Nuclearenvelope

Plasmamembrane

Chromosome, consistingof two sister chromatids

Mitosis (Prophase)

Qn: What are the key events in prophase?

Condensation of chromatin to visible chromosomes

Formation of spindle fibres

Breakdown of nuclear membrane and nucleolus.

Fig. 12-6d

Metaphase Anaphase Telophase and Cytokinesis

Cleavagefurrow

Nucleolusforming

Metaphaseplate

Centrosome atone spindle pole

SpindleDaughterchromosomes

Nuclearenvelopeforming

Fig. 12-6c

Metaphase Anaphase Telophase and Cytokinesis

Fig. 12-7

Microtubules Chromosomes

Sisterchromatids

Aster

Metaphaseplate

Centrosome

Kineto-chores

Kinetochoremicrotubules

Overlappingnonkinetochoremicrotubules

Centrosome 1 µm

0.5 µm

Mitosis (Metaphase)

Metaphase: Chromosomes migrate to equator of spindle.

Sister chromatids align right angles to spindle fibres.

Chromosomes attached to spindle fibres at kinetochore of centromeres.

kinetochore

Mitosis (Anaphase)Anaphase: Centromeres duplicate, so each sister chromatid move to

opposite poles. Hence, sister chromatids separate

Each sister chromatid with own centromere called a chromosome.

Paired chromosomes pulled by shortening of centromere-to-pole spindle fibres.

Energy requiring process

Mitosis (Telophase)

Telophase: Chromosomes reach respective poles of

spindle.

They uncoil and lengthen into chromatin threads again.

Mitosis (Telophase)

Spindle fibres disintegrate.

Nuclear envelope reforms around chromosomes at each pole.

Nuceloli reappear.

Cytokinesis

Mitosis is usually followed by cytokinesis (cell splitting).

In animal cells: Cytokinesis begins in mid or late telophase. Constriction of cell surface membrane which cuts

across equator of spindle.

Cytokinesis in animal cell

A ring of actin microfilaments where cleavage furrow develops involved in constriction.

Cytokinesis (Plant cells)In plant cells: Plants have rigid cell walls and cannot divide by

pinching-in.

Fig. 12-9b

Daughter cells

(b) Cell plate formation in a plant cell (TEM)

Vesiclesformingcell plate

Wall ofparent cell

New cell wallCell plate

1 µm

Cytokinesis (Plant cells)

Golgi vesicles line up in middle of parent cell.

They fuse to form a cell plate extending across equator.

Golgi vesicles contents contribute to new cell wall.

Their membranes form plasma membranes of daughter cells.

Spreading cell plate fuses with parent cell wall, separating two daughter cells.

The Significance of Mitosis

1. Genetic stability Two nuclei same number and same kind of chromosomes as

parent cell

Chromosome constitution preserved because:• Chromosomes of parent nucleus replicate at interphase.

• Arrangement of sister chromatids at equator of spindle such that each chromatid faces opposite poles at metaphase.

• Separation of identical sister chromatids at anaphase.

Fig. 12-2

100 µm 200 µm 20 µm

(2) Reproduction (3) Growth and development

(4) Tissue renewal

The Significance of Mitosis

2. Asexual reproduction Binary fission in bacteria and vegetative

propagation in plants.

Offspring genetically similar as parent (clone).

Offspring have advantages of parents in mastering same habitat, and also parents’ disadvantages.

Fig. 12-11-4

Origin ofreplication

Two copiesof origin

E. coli cell Bacterialchromosome

Plasmamembrane

Cell wall

Origin Origin

The Significance of Mitosis

3. Growth and development Increase in cell number for growth and

development.

E.g. development of foetus into adult human being.

The Significance of Mitosis

4. Regeneration and cell replacement

Regeneration of missing parts in flat worm, wound healing.

Damaged tissues repaired and new cells genetically identical to cells being replaced.