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All eukaryotic, multicellular organisms each develop from

a single original cell, i.e. the fertilised egg (zygote)Repeated divisions of the zygote, by a process of cell division called

mitosis, give rise to all the cells that make up a multicellular organism

The fertilised egg

divides repeatedlyby mitosis

to initially produce a ball ofgenetically identical cells

- the early embryo.

The function of mitosis is to produce new cells, each of which receivesa set of chromosomes identical to those of the original cell that gave

rise to them; genetically identical cells are the result of mitosis

Cell DivisionCell Division

Mitosis is involved in:DEVELOPMENT – the formation of a multicellular organism from a single fertilised egg

GROWTH – involves an increase in size through the division of cells by mitosis

CELL REPLACEMENT – many cells within a multicellular organism areconstantly dying and being replaced by mitosis

ASEXUAL REPRODUCTION – asexual reproduction in certain plants and lower eukaryotic animals involves the process of mitosis

Budding in this species of Hydra is a formof asexual reproduction; new individualsare produced as outgrowths of the parent

by the process of mitosis

Significance of MitosisSignificance of Mitosis

The epithelial cells lining the intestine are continually being replaced by mitosissuch that the entire lining is replaced every five days

Epithelial cells

The dead layers of cells at the surface of the skin are constantly being lost andare replaced by the mitotic divisions of the cells in the layer beneath them

Dead cells

Constantly dividing cellsreplacing those lost at the

surface

These nerve cells have lost the ability to undergo mitosis – once formed theydo not divide

This diagram represents a parent cell containinga single pair of chromosomes – the pair of chromosomes arethe same size and shape and are therefore described as being

homologous chromosomes. One member of the pair wasdonated by the male at reproduction and the other member

by the female. Each pair of homologous chromosomes thus hasone member that is paternal in origin and another member thatis maternal in origin. Humans possess 23 pairs of chromosomes

in each body cell. Cells in which the chromosomes occur inhomologous pairs are termed DIPLOID (two sets of chromosomes)

maternal inorigin

paternal inorigin

Each chromosome replicates itself to form a pair of identicalchromatids called sister chromatids that remain attached to

one another at a region called the centromerecentromere

The cell divides into two and each daughter cell receives one of each of the sister chromatids

The daughter cells are genetically identicalto the parent cell

The function of mitosis is to construct an exactcopy of each chromosome and then to distribute,through division of the parent cell, an identical

set of chromosomes to each of thetwo daughter cells

The Process of MitosisThe Process of Mitosis

Mitosis is a continuous process but for convenience of description is dividedinto FOUR MAIN STAGES:

Prophase

Metaphase

Anaphase

Telophase

Please

Make

Another

Two

The following slides describe the process of mitosis in an animal cell containingtwo pairs of homologous chromosomes

The Process of MitosisThe Process of Mitosis

InterphasePrior to mitosisthe cell is in astage calledInterphaseDuring

interphase theDNA of the

chromosomesreplicates and

new cellorganelles aremanufactured

Although thechromosomes

haveduplicated, they

are looselycoiled withinthe nucleus

and visible onlyas granular

material at thisstage

This granularchromosomematerial is known aschromatin The centrioles replicate (animal cells only)

Two pairs ofhomologous

chromosomes –visible only as

granular chromatin at

this stage

Nucleolus

Nuclearmembrane

Transmission electronmicrograph showing

the granular chromatinof an interphase nucleus

Interphase prepares thecell for division and thecell now enters the firstphase of mitosis known

as prophase

During early prophase thechromosomes spiralise and

condense – they becomeshorter and thicker and visible as clear threads

Early ProphaseThe nucleolus shrinks and

the centriolesmove to

opposite polesof the cell

Spindle fibresbegin to

form in theregions of

the centrioles

By late prophase, the chromosomes

have condensedfurther and eachchromosome can

be seen toconsist of twochromatids

joined at the centromere

Late ProphaseThe nucleolus shrinks and

the centriolesmove to

opposite polesof the cell

Spindle fibresbegin to

form in theregions of

the centrioles

By late prophase, the chromosomes

have condensedfurther and eachchromosome can

be seen toconsist of twochromatids

joined at the centromere

Each pair ofsister chromatids

is a pair ofduplicated

chromosomesresulting fromthe replicationof DNA that

occurred duringinterphase

Sister chromatids

Centromere

At the end ofprophase the

nuclearmembrane

begins todisintegrateand the cellenters thenext phaseknown as

metaphase

During metaphase, the spindle fibres grow across the cell and the replicated chromosomes (pairs of chromatids) line up independently along

the equator of the spindle attaching to the spindle at their centromeres

MetaphaseAs the cell

enters anaphase,the centromeres

divide intotwo, separating

the sisterchromatids of

eachchromosome

This electron micrograph showsa metaphase chromosome about

to enter anaphase

Centromere

Sister chromatids

As the cell enters anaphase, thedivided centromeres repel one

another and the chromatids beginto move apart

Early AnaphaseSpindle activitypulls the

chromatids apart and the

separatedchromatids

(know calledchromosomes)

move toopposite poles

of the cell

AnaphaseSpindle activitypulls the

chromatids apart and the

separatedchromatids

(know calledchromosomes)

move toopposite poles

of the cell

Telophase

A single chromatid from each chromosome has reached the poles ofthe spindle - the chromatids are now described as chromosomes

The chromosomes begin touncoil and a nuclear membranebegins to form around each set

of chromosomes

Cytokinesis (division of thecytoplasm) now begins as the

cell membrane begins to constricttowards the centre of the cell

The spindlefibres begin to

disintegrate

CytokinesisAs the membrane continues to constrict, the cytoplasm becomes completely

divided forming two genetically identical daughter cellsEach cell now possesses an exact copy of each chromosome that was present in

the nucleus of the original cell

In plant cells, constriction of the membrane at cytokinesis cannot occur due tothe presence of the cell wall

In plant cells, a cell membrane forms in the middle of the dividing cell and thismembrane secretes cell wall material on each side to form

new cell walls for the daughter cells

Interphase – cell preparesfor division; DNA replicates,

new organelles are manufactured and cell grows;

chromosomes present asgranular material (chromatin)

Early Prophase – chromosomesspiralise and condense and become visible as threads; centrioles move to opposite poles of the cell and spindle

fibres begin to form

Late Prophase – chromosomes spiralise and

condense further andcan now be seen to consist

of two sister chromatidsjoined at the centromere

Metaphase –nuclear membrane

has disintegrated andspindle fibres have

grown across the cell;chromosomes line upindependently alongthe equator of the

spindle attaching tothe fibres via their

centromeres

Early Anaphase –as the cell

enters anaphase,the centromeres

divide intotwo separating

the sisterchromatids of

each chromosome

Summary of MitosisSummary of Mitosis

Anaphase – spindle activity pulls the chromatids apart and the separated

chromatids move to opposite poles of the cell

Telophase – the chromatids are nowdescribed as chromosomes and they beginto uncoil. The spindle fibres disintegrateand the cell begins to constrict along its

central axis. A nuclear membrane begins to form around each set of chromosomes

Cytokinesis – as the membranecontinues to constrict, the

cytoplasm becomes dividedforming two genetically

identical daughter cells. Eachcell now possesses an exact copy

of each chromosome that waspresent in the nucleus of

the original cell

Daughtercells

Summary of MitosisSummary of Mitosis

These photographs are taken from prepared slides of onion

root tip cells that wereundergoing mitosis:

Identify the photograph showing Interphase and the

photographs showing the fourstages of mitosis – Prophase,Metaphase, Anaphase and

Telophase

A B C

D E

BThis cell is inInterphase

The chromosomesare not visible as

threads but appearas chromatin

(granular material)in the nucleoplasm

During this stagethe cell prepares for mitosis – DNAreplicates and new

organelles aremanufactured

The nucleolus isclearly visible

AnswersAnswers

EThis cell is in Prophase

The chromosomeshave spiralised andcondensed – theyare shorter and

thicker and visible as clear threads

The nucleolus hasshrunk in size

Spindle fibres arebeginning to form

close to the nucleusand the nuclear

membrane disintegrates

At this stage thechromosomes can be

seen to havereplicated with each

chromosome nowconsisting of two

chromatids

AThis cell is in Metaphase

Spindle fibreshave grownacross the

cell

Each replicatedchromosome

lines up independently

along the equator

of the cellReplicatedchromosomesattach to thespindle fibres

by theircentromeres

DThis cell is in Anaphase

The centromeresof each chromosome

replicate and thechromatids repel

one another

The spindle fibrescontract and pull

the separated chromatids to

opposite poles ofthe cell

CThis cell is in Telophase

A single chromatidfrom each

chromosome hasreached the poles of

the spindle – thechromatids are now

described aschromosomes

The chromosomesbegin to uncoil and appear as chromatinonce again; a nuclear

membrane formsaround each set of

chromosomes

Cytokinesis (division

of the cytoplasm) follows telophaseIn animal cells this

involves constrictionof the cell

membrane along itscentral axis and

division of the cellinto daughter cells

In the plant cell shown in the photograph, this involves the formation of a cell membrane in the middle of the cell followed by secretion

of cell wall material on either side of this

membrane

The stages through which a cell passes from one cell division to the nextconstitute the cell cycle

The cell cycle is divided into two major phases: the M phase or mitotic phaseand the interphase

The Cell CycleThe Cell Cycle

Interphase is dividedinto G1, S and G2 phases

G1

G2

S

INTERPHASE

MITOSIS (M PHASE)

The Cell CycleThe Cell Cycle

G1

G2

S

MPROPHASE

CYTOKIN

ESIS

METAPHASE

ANAPHASE

TELOPHASE

The M phase consists of the process ofmitosis, when the chromosomes separate, together with cytokinesis, when the entire

cell is physically divided into two daughter cells

The M phaseoccupies only

a small portionof the cycle,lasting from

about 30minutes toone hour

Interphase occupiesthe largest portion

of the cycle

The Cell CycleThe Cell Cycle

MPROPHASE

CYTOKIN

ESIS

METAPHASE

ANAPHASE

TELOPHASE

During interphase, preparations formitosis take place

G2 - Second

Growth PhaseThe cell grows and prepares

for mitosis

G1

- First Growth PhaseThe cell grows andnew organelles and

proteins aremanufactured

SDNA replication

takes place

The Cell CycleThe Cell Cycle

G1

G2

S

M

G1 is the most variableof all the phases

The rapidly dividing epithelialcells lining the human intestine

remain in the G1 phase foronly about 2 hours

Slowly dividing liver cellsmay take many months to

move through G1 to theS phase

Some cells, such as nerveand muscle cells never divide;they may be considered to bepermanently in the G1 phaseand never enter the S phase

Variations in the Cell CycleVariations in the Cell Cycle

The multiplication of cells is a closely regulated process

Cell division is under genetic control, and it is known that there arespecific genes which code for proteins that ‘switch on’ and

‘switch off’ the process

Cancer is a disease that results from uncontrolled cell divisions

Normal cells become transformed into cancer cells when the genes thatcontrol cell division mutate and become ONCOGENES

Environmental cancer-causing agents, known as carcinogens, play a partin causing the alteration of DNA structure that leads to oncogene formation

Known carcinogens include ultraviolet radiation, cigarette smoke and X-rays

Cancer: The Cell Cycle Out of ControlCancer: The Cell Cycle Out of Control

Scanning Electron Micrograph of Dividing Cancer Cells

When a normal body cell mutates it may divide to produce a clone of cells that form a tumour

normalbody cell

mutatedbody cell

tumour

mitosismutation

Many such tumours are found to be BENIGN and do not spread fromtheir site of origin – they may nevertheless compress and damage adjacent tissues

Malignant, cancerous tumours may spread from their site of origin

These tumours develop their own blood and lymph supply which can transportmalignant cells from the tumour to other sites in the body

malignant tumour

malignant cancercells carried toother body sites

secondarytumour

This is calledmetastasis

these cells invadeother body regionsto form secondary

cancers

Cancer: The Cell Cycle Out of ControlCancer: The Cell Cycle Out of Control

Plant roots grow by mitotic division of the cells at the root tip

Onion root tips are an ideal source of material for observing the stagesof mitosis

• A scalpel is used to cut about 4 mm from the tip of the growing onion root• Acetic acid is added to the tip on a watch glass and warmed gently for about 5 minutes; the acid helps to macerate the cells• The root tip is transferred to a slide where two or three drops of aceto-orcein stain are added; this stain is taken up by the chromosomes and makes them more visible as they stain red• The tip is gently broken up with a mounted needle and the cells are spread across the slide• A coverslip is placed over the root preparation and gently squashed• The slide is examined for stages of mitosis using an optical microscope

Observing Mitosis in Plant TissuesObserving Mitosis in Plant Tissues

Photomicrograph showing cells from an onion root tipNote that many of the cells are in interphase

Meiosis is another form of cell division that is associatedwith reproduction in many organisms

In humans, meiosis is responsible for the formationof the reproductive cells or gametes

SPERMCELL

EGGCELL

In humans, these are the egg and sperm cellsWhereas most body cells have a complement of

23 pairs of chromosomes, human gametes possessonly 23 single chromosomes. A gamete’s complement of

23 single chromosomes is constituted by one chromosometaken from each of the 23 pairs of chromosomes

Within the human ovaries and testes, gametes areproduced by meiosis and this process halves the

chromosome number

Human body cells are DIPLOID as they possess two sets ofchromosomes (23 pairs)

Human gametes are described as being HAPLOID as theypossess only one set of chromosomes (23 chromosomes)

If the gametes were diploid then the number ofchromosomes would double at every generation after

fertilisation

An Introduction to MeiosisAn Introduction to Meiosis

MEIOSISMITOSIS

Diploid bodycell

Two diploid daughter cells

The nucleusdivides twice

Four haploid, genetically different gametes are produced

Meiosis is important as it ensuresthat, when the gametes fuse at

fertilisation, the normal diploid numberof chromosomes is maintained; meiosis

is also an important source of genetic variation