Divided into

5.1 MITOSIS

Cell division

Process by which a cell, called the parent cell, divides into two cells, called daughter

cells.

Mitosis

A process of nuclear division which produces two genetically identical daughter

nuclei.

The significance of mitosis

1. Replaces dead cells. E.g skin cells

2. Repaired and replaced damaged cells.

3. Basis of asexual reproduction in unicellular organism. – binary fission

4. Increases the number of cells in all living organisms, thus, allowing growth and

development in multicellular organism.

5. Results in the formation of two daughter nuclei which are genetically identical to

each other and to parent nucleus. The nuclei contain the same number of

chromosomes, same genetic material as parent nucleus. This is to ensure that

the daughter cells perform the same function as the parent cell.

Chromosomes and chromosomal number

1. Two type of cells in sexually reproducing organism

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Nuclear division cytoplasmic division (cytokinesis)

Cell division (2 stages)

Mitosis

Meiosis

Chromosome:

A thread-like structure which composed of a long and linear DNA molecule that carries

genes in a linear sequence which determines the individual characteristics of an organism.

a. Somatic cell

- Comprise all the cells in an organism, except for the reproductive cell

- Formed through mitosis

b. Reproductive cells – formed through meiosis

2. Chromosomes are found in nucleus.

3. The number of chromosomes present in the cells of each species of an individual

organism is constant.

a. All individuals of the same species have the same chromosomal number but

the cells of individuals of a different species have a different chromosomal

number.

b. For example,

- Onions have 16 chromosomes.

- Fruit fly, Drosophila melanogaster, has 8 chromosomes.

- Human has 46 chromosomes.

4. The gametes contain only half the number of chromosomes (single set). The

chromosomal number is said to be haploid and is designated as n.

5. In human, one set of chromosomes consists of 23 chromosomes.

6. All somatic cell have two sets of chromosomes. Our somatic cells have 46

chromosomes arranged in 23 pairs while each gamete only has 23

chromosomes.

7. The presence of two sets of chromosomes in the nucleus of a cell is known as

the diploid number of chromosomes (2n).

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Male chromosome set Female chromosome set

8. Each pair of the chromosomes is referred as homologous chromosomes. They

have same structural features.

Mitosis

Definition:

• Process of nuclear division

• Formation of two daughter cells

• Two daughter cells are genetically identical to the parent cell

• Maintains the chromosomal number of species

• Ensures genetic material is passed on to the offspring

Cell cycle:

The period that extends from the time a new cell is produced until the time the cell

completes a cell division.

2 major phases:

a. Interphase

• G1 (Growth phase 1)

• S phase (DNA synthesis)

• G2 (Growth phase 2)

•

b. Mitotic cell division (M Phase)

• Mitosis

i. Prophase

ii. Metaphase

iii. Anaphase

iv. Telophase

• Cytokinesis

Interphase:

- Interphase accounts for about 90% of the cell cycle.

- Interphase is also the stage at which the cell grow larger and prepare for cell

division

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Interphase Description

G1 (gap or growth phase) • Begins to acquire and synthesis the materials

required for cell division.

• Synthesis new organelles and proteins, cells

grow larger

• Metabolic rate of the cell is high.

• Crucial phase: the cell will decide whether o not

to divide and complete the cycle to form new

cells

• Chromosomes at this stage are known as

chromatin.

S phase • DNA undergoes replication.

• A duplicated chromosome consists of two

identical sister chromatids = 2 identical DNA

double helices

G2 • Cell continues grow and remains metabolically

active.

• Enzymes and proteins are synthesized for cell

division

• Cell accumulates energy and completes its final

preparations for division.

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Mitotic Cell Division (M Phase):

Prophase

Chromosomes condense, become shorter,

thicker and visible under light microscope

Each chromosome consists of two sister

chromatids joined together at the

centromere

In the cytoplasm, spindle fibres begin to

form between the centrioles

Each pair of centrioles migrates to the

opposite poles of cell

The spindle fibres are attached to the

centromeres of each sister chromatid

In plant cells, spindle forms without the

presence of centrioles

Nucleolus disappears and nuclear

membrane disintegrates at the end of

prophase.

Metaphase

All the chromosomes line up at the

metaphase plate with the centromeres

attached to the spindle fibres.

The spindle fibres are now fully formed

The two sister chromatids of each

chromosome are still attached to each other

at the metaphase plate

Metaphase ends when the centromeres

divide

Anaphase

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The two sister chromatids of each

chromosome separate at the centromere

The daughter chromosomes are pulled

apart to the opposite poles by shortening of

the spindle fibre.

Once separate, the chromatids are referred

to as daughter chromosomes

Anaphase ends when the chromosomes

reach the poles of the cell

Telophase

two sets of chromosomes reach opposite

poles.

Chromosomes start to uncoiled and revert

to their thin and extended state (chromatin).

Spindle fibres disappear.

New nuclear membrane forms around each

set of chromosoms.

Nucleolus re-forms in each nucleus.

Cytokinesis- Cytokinesis is the process of cytoplasmic division

- It usually begins before nuclear division is complete

Cytokinesis in animal cell Cytokinesis in plant cell

Actin filaments in cytoplasm

contract to pull a ring of the plasma

membrane inwards, forming a

groove called the cleavage furrow

The cleavage furrow pinches at the

equator of the cell

The cleavage furrow deepens

progressively until the cell

separates into two daughter cells

Vesicles collect at the equator

between the two nuclei

The vesicles join to form a cell

plate

The cell plate grows outwards

until its edges fuse with the

plasma membrane

New cell walls and plasma

membranes are formed from the

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contents of the cell plate

Cell plate divides the cell into two

daughter cells

Cellulose fibres are produced by

the cells to strengthen the new

cell walls

Controlled mitosis

1. The cell cycle is controlled by genes of the chromosomes. Each type of cell

has its own timing and rate of cell division. The ability of the cell to divide at

its own rate and timing is called controlled mitosis.

2. Mitosis ensures that the genetic content and the number of chromosomes in

the parent cells are maintained in the daughter cells from one generation to

the next

3. The rate and timing of cell division are important for normal growth,

development and maintenance of the organism.

4. For example:

- Regenerate a lizard’s lost tail

- Heals skin in a wound.

5. Cell with different division frequencies:

- Skin = throughout lifespan

- Liver = replace damaged & injured tissues

- nerve & muscle = do not divide once mature

Uncontrolled mitosis

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1. Uncontrolled mitosis is the situation where cells undergo uncontrolled

division when the gene that regulate the cell cycle are mutated or damaged.

2. When a cell divides by mitosis repeatedly, without control and regulation, it

can produce cancer cell.

3. Cancer (disease) caused by uncontrolled mitosis

4. Cancer cells compete with other normal cells to obtain sufficient nutrients

and energy for their own growth.

5. A cancer cell will divide uncontrollably to form a tumour (abnormal mass of

cells)

6. a) Benign tumour- remain at the original site, do not cause serious problem,

can be removed.

b) Malignant tumour – invasive, spread to other to neighbouring tissue,

impairing the functions of one or more organs.

7. Cancer can caused by many factors such as:

- damage of DNA

- Change in genes (mutation) that control cell division.

- radioactive rays (X-rays, ultraviolet rays and gamma rays)

- Carcinogenic compound, e.g. formaldehyde

Normal cells Cancerous cells

Controlled growth Uncontrolled growth

A single-organised layer Multi-layered and disorganised

Nuclei and number of

chromosomes are normal

Nuclei and number of chromosomes

are abnormal

Cells are differentiated and carry

out specialized functions

Cells are undifferentiated and do not

carry out specialised functions.

The application of knowledge of mitosis in cloning

The knowledge of mitosis is applied in cloning and the tissues culture technique.

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Cloning

The process of producing clones or genetically identical organisms through

asexual reproduction.

1. Many unicellular organisms and plants can reproduce asexually through mitosis

to produce clones.

2. The clones share the same genetic content and chromosomal number with one

another as well as with the parent organism.

Cloning of animal

1. Somatic cells (from the mammary gland) are removed from a donor and grown in

a low culture medium.

2. The starved cells stop diving and enter a non-dividing phase.

3. An unfertilized egg cell is obtained. The nucleus is sucked out, leaving the

cytoplasm and organelles without the chromosomes.

4. An electric pulse stimulates the fusion between the somatic cell and the egg cell

without nucleus.

5. The cell divides to form embryo.

6. The embryo is then implanted into a surrogate mother. (the same breed of sheep

as the ovum donor sheep)

7. The sheep that is born is identical to somatic cell donor sheep.

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Tissues culture

The growth of tissues of living organisms in a suitable and sterile culture

medium, containing nutrients and growth hormones.

1. The main purpose of tissues culture is to produce plant and animal cells

through asexual reproduction

2. Each cell has the full genetic potential to form all parts of a mature organism.

3. Different parts of plant that can be cultured include young shoots,

meristematic tissues, leaves, roots, seeds, embryo, cells and protoplasm.

Technique of tissue culture:

1. Small piece of tissues called explants are taken from the parent plant (leaf, shoot,

bud, stem or root)

2. The explants are sterilised to prevent the growth of pathogens,

3. Each sterile explants is placed onto a growth medium/culture medium

( containing nutrients and hormones)

4. The explants are incubated in a suitable temperature and at optimum pH level for

several weeks.

5. The explants divide by mitosis to produce callus ( an undifferentiated mass of

tissue)

6. The callus develop into an embryo

7. The embryo develops into a plantlet which can later be transferred to the soil for

growth into an adult plant.

Cloning

Advantages Disadvantages

Multiply copies of useful genes & clones The resistance of clones towards

diseases and pests is the same. If a

clone is affected with a disease /

attacked by pests, then all the clones

will also be affected ad die.

Clones can be produced in a shorter

time and in larger numbers

If the external environment changes, the

clones will be destroyed as cloning is

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carried out under controlled

environment.

Cloned plants can produce flowers and

fruits within a shorter period.

Clones may disrupt the natural

equilibrium of an ecosystem.

Cloning prevent endangered species

from extinction

The clones do not show genetic

variation

Cloning involves vegetative

reproduction which does not need

pollinating agents

For reasons still unknown, cloned

animals have a shorter lifespan

5.2 MEIOSIS

The Significance of meiosis

1. Meiosis (meio=reduce) is the process of nuclear division that reduces the number

of chromosomes in daughter cells to half that of the parent cell.

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2. Meiosis produces haploid gametes. The gametes contain half the number of

chromosomes of the parent cell.

Each gamete receives only one chromosome from every pair of homologous

chromosome

3. Meiosis I begins with a single diploid (2n) parent cell and the end of meiosis II, four

haploid (n) daughter cells are produced, each genetically distinct from the other and

from the parent cell.

4. During sexual reproduction, the fusion of two gametes restores the complete

numbers of chromosomes and genetic material, forming a diploid zygote. The

offspring inherit trait from both parents to ensure continuation of life.

Sexual reproduction

sperm (n) ovum (n)

diploid zygote (2n)

THE STAGES OF MEIOSIS

Meiosis consists of two separate nuclear divisions: meiosis I and meiosis II

a) Meiosis I

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2n

n

n

n

n

n

n

Parent cell 2 haploid cells Gamete cells

MEIOSIS

I

MEIOSIS

II

Organism Chromosomal number

Saccharomyces cerevisiae (yeast) 32Zea mays (corn) 20

Felis domesticus (cat) 38

Gallus gallus (chicken) 78Orvis aries (sheep) 54

Musca domestica(housefly) 12

Meiosis I is preceded by interphase.

PROPHASE I

1. The chromosomes begin to condense. They

become shorter, thicker and clearly visible.

2. Homologous chromosomes come together to form

pairs of bivalents through a process called

synapsis.

Each bivalent consists of a four-part structure

called a tetrad. A tetrad consists of two

homologous chromosomes

3. Crossing over

Non-sister chromatids exchange segment of

DNA

crossing over result in new combinations of

genes on a chromosome.

Chiasma – the point at which segments of

chromatids cross over.

4. Spindle fibres radiate from the centrioles.

5. The two pairs of centrioles move to the opposite

poles of the cell.

6. The nucleolus and nuclear membrane disappear.

METAPHASE I

1. The homologous chromosomes are lined up side

by side as tetrad on the metaphase plate.

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2. One chromosome of each homologous pair is

attached to the spindle fibres from one pole while

its homologue is attached to the spindle fibres

from opposite pole

ANAPHASE I

1. The spindle fibres pull the homologous

chromosomes away from one another and move

them to the opposite poles.

2. Each chromosome still consists of two sister

chromatids

3. Although the cell started with four chromosomes,

only two chromosomes move towards each pole.

(based on diagram / question)

TELOPHASE I

1. The chromosomes arrive at the poles.

2. Each pole has a haploid daughter nucleus (contain

only one set of chromosomes)

3. The spindle fibres disappear.

4. The nuclear membrane reappears to surround

each set of chromosomes. The nucleolus then

reappears in each nucleus.

b) Meiosis II

The stages of meiosis II are identical to mitosis which results in the

separation of sister chromatids.

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Cytokinesis Occurs simultaneously with telophase I, resulting in two haploid daughter cells. For most organisms, there is no interphase between meiosis I and meiosis II Another cell division is required because the chromosomes are still duplicated Chromosomes remain in a condensed state.

PROPHASE II

1. The nuclear membrane disintegrates

2. The spindle fibres reform in each daughter cell

.

METAPHASE II

1. The chromosomes (still made up of two sister

chromatids) line up randomly at the metaphase

plate.

2. Each sister chromatid is attached to the spindle

fibres at the centromere.

ANAPHASE II

1. The centromeres of the sister chromatids separate

2. The sister chromatids of each chromosome are

now individual chromosomes.

3. Each individual chromosomes moves to the

opposite poles of the cell

TELOPHASE II

1. The spindle fibres disappear

2. The nucleoli and nuclear membrane re-form.

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3. Cytokinesis follows and four haploid daughter cells

are formed. (contain half the no. of chromosomes)

4. Each haploid cell is genetically different from

parent diploid cell. These haploid cells become

gametes.

THE IMPORTANCE OF MEIOSIS

1. Meiosis ensures that the diploid number of chromosomes is maintained

from one generation to the next.

2. Meiosis provides for genetic variation which occurs from one generation to

the next.

Meiosis leads to genetic recombination

crossing over (prophase I)

Independent assortment (metaphase I) – The homologous

chromosomes is arranged independently and randomly at the

metaphase plate of the cell. The paternal or maternal

chromosomes may be oriented to face either one of the poles.

COMPARISON BETWEEN MITOSIS AND MEIOSIS

Mitosis Similarities Meiosis

1. Nuclear division happen

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2. DNA replicates only once.

MitosisDifferences

Aspects/eventsMeiosis

All somatic cells Type of cell Cells in the reproductive

organs

Produces new cells for growth

and repair

Role Produces gametes for

sexual reproduction

Synapsis does not occur Synapsis Homologous chromosomes

pair up to form bivalents.

Crossing over between non-

sister chromatids does not

occur during prophase

Crossing over Crossing over between non-

sister chromatids occur

during prophase I

The individual chromosomes

are arranged randomly at the

metaphase plate

Metaphase of

mitosis/ Metaphase

I of meiosis

Homologous chromosomes

line up side by side at the

metaphase plate

Sister chromatids separate to

move to the opposite poles

Anaphase of

mitosis/ Anaphase I

of meiosis

- Homologous

chromosomes separate

to move to the opposite

poles.

- The sister chromatids still

remain attached to each

other

One Number of cell

divisions

Two

Two daughter cells Number of

daughter cells

produced at the

end of the division

Four daughter cells

(gametes)

Diploid (2n)/ the same number

of chromosomes as the parent

cell

Chromosomal

number of the

daughter cells

Haploid (n)/ half the number

of chromosomes of the

parent cell

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Genetically identical to the

parent cell and to one another

Genetic content Different from the parent cell

and form one another

No genetic variation in any

generation

Genetic variation There is genetic variation

from one generation to the

next.

5.3 APPRECIATING THE MOVEMENT OF CHROMOSOMES DURING MITOSIS

AND MEIOSIS

1. The ability of organisms to reproduce ensures the continuity of life.

2. The organisms reproduce through mitotic or meiotic cell division is to ensure

the survival of each species from one generation to the next.

3. Asexual reproduction through mitotic division produces offspring that are

identical to the single parent; sexual reproduction through meiotic division

produces genetic variation in offspring.

4. If meiosis does not occur properly, the gametes formed will have an abnormal

number of chromosomes. As a result, the zygote that is formed later would

become abnormal.

Exp: down syndrome (2n = 47 chromosomes)

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