7/28/2019 Biology (Form 4) [2]

1/25

-CHAPTER 5 (SUBTOPIC 5.2)-

7/28/2019 Biology (Form 4) [2]

2/25

1 ) T H E S I G N I F I C A N C E O F M E I O S I S2 ) T H E P R O C E S S E S A N D S TA G E S O F M E I O S I S 13 ) T H E P R O C E S S E S A N D S TA G E S O F M E I O S I S 24 ) T H E D I F F E R E N C E S A N D S I M I L A R I T I E S

B E T W E E N M I T O S I S A N D M E I O S I S

5 ) T H E I M P O RTA N C E O F M E I O S I S6 ) A P P R E C I AT I N G T H E M O V E M E N T O F

C H R O M O S O M E S D U R I N G M I T O S I S A N D M E I O S I S

SUBTOPIC 5.2 :MEIOSIS

7/28/2019 Biology (Form 4) [2]

3/25

THE SIGNIFICANCE OF MEIOSIS

1. Mitosis produces daughter cells that have exactly thesame number of chromosomes as the original parentcells.

2. If mitosis is the only means of nuclear division, theneach gamete produced by the reproductive organswould contain a complete set of chromosomes, thatis, each gamete would have a diploid number of chromosomes (2n).

7/28/2019 Biology (Form 4) [2]

4/25

3. This means that each offspring formed through thefertilisation of the male and female gametes would havetwice the chromosomal number of the parent cell.

4. Hence, in order for the offspring to possess the samechromosomal number as their parents, their reproductiveorgans that produce the gametes must undergo meiosis.

5. The number of chromosomes in the nucleus of someorganisms is given in the Table 5.2.

7/28/2019 Biology (Form 4) [2]

5/25

Table 5.2 : Diploid chromosomal number of someorganisms.

ORGANISM CHROMOSOMAL NUMBER

Yeast 32

Corn 20Cat 38

Chicken 78

Tomato 24

Housefly 12

Sheep 54

Horse 64

Human 46

7/28/2019 Biology (Form 4) [2]

6/25

THE NECESSITY FOR THE PRODUCTION OFHAPLOID GAMETES

1. Meiosis is a process of nuclear division that reduces thenumber of chromosomes in daughter cells to half that of parentcell.

2. Meiosis produces haploid gametes. Gametes are called haploidcells (n) because the contain half of the genetic material or half the number of chromosomes of the parent cells (diploid cells,2n)

3. As each gamete receives onlyone chromosome from everypair of homologous chromosomes , this means in humans,the gametes contain only23 chromosomes or haploidnumber of chromosomes (n).

7/28/2019 Biology (Form 4) [2]

7/25

4. During sexual reproduction, the fusion of two gametes(the sperm and the ovum) restores the completenumber of chromosomes and genetic material,forming a diploid zygote with 46 chromosomes.

5. This means the offspring inherits traits from bothparents to ensure a continuation of life.

7/28/2019 Biology (Form 4) [2]

8/25

If human reproductive organs divide by mitosis, theresulting daughter cells (gametes) would be likesomatic cells, having 46 chromosomes (2n=46).

Fertilisation of two gametes would then bring thenumber of chromosomes to 92.

If this happens the offspring would not be humananymore.

7/28/2019 Biology (Form 4) [2]

9/25

The type of cells thatundergo meiosis

IN ANIMALS

The testes(in males)

The ovaries(in females)

INPLANTS

In anthersof flowers

In ovariesin flowers

7/28/2019 Biology (Form 4) [2]

10/25

THE PROCESS OF MEIOSIS

MEIOSIS (consist of twoseparate nuclear division)

Meiosis 1

Prophase 1Metaphase

1

Anaphase 1

Telophase 1

Meiosis 2

Prophase 2

Metaphase2

Anaphase 2

Telophase 2

7/28/2019 Biology (Form 4) [2]

11/25

1. Meiosis 1 begins with a single diploid parent cell. Atthe end of meiosis 2, four haploid daughter cells areproduced, each genetically different from the othersand from the parent cell.

2. In meiosis, even though the cell undergoes twonuclear divisions, the DNA of each chromosome onlyreplicates once.

7/28/2019 Biology (Form 4) [2]

12/25

THE STAGES OF MEIOSIS

Interphase1. The cell replicates its DNA and duplicates its

chromosomes.2. After replication, each chromosome consists of two

identical sister chromatids, held together by a centromere.3. The cell now has twice the amount of genetic material, but

the same number of chromosomes as before.4. Chromosomes are not condensed and therefore are not

visible under the microscope.

7/28/2019 Biology (Form 4) [2]

13/25

5. The nuclear membrane and the nucleolus are still present.6. In animal cells, a pair of centrosomes is also formed in the

cytoplasm. Each centrosome consists of a pair of

centrioles.

7/28/2019 Biology (Form 4) [2]

14/25

The similarity betweenmitosis and meiosis

Mitosis Meiosis

The process of cell division in which DNA replicates only once.

7/28/2019 Biology (Form 4) [2]

15/25

The differences betweenmitosis and meiosis

MITOSIS ASPECTS/EVENTS MEIOSIS

All somatic cells. Type of cell Cells in the reproductiveorgans.

Produces new cells forgrowth and repair.

Role Produces gametes forsexual reproduction.

Pairing of homologouschromosomes (synapsis)

doe not occur.

Synapsis Homologous chromosomespair up (synapsis) to form

bivalents.

Crossing over betweennon-sister chromatids doesnot occur during prophase.

Crossing over Crossing over betweennon-sister chromatids

occurs during prophase 1

The individualchromosomes are arranged

randomly at themetaphase plate.

Metaphase of mitosis andMetaphase 1 of meiosis

Homologous chromosomesline up side by side at the

metaphase plate.

7/28/2019 Biology (Form 4) [2]

16/25

Sister chromatids separateto move to the oppositepoles.

Anaphase of mitosis andAnaphase 1 of mitosis

Homologouschromosomes separate tomove to opposite poles. The sister chromatids stillremain attached to eachother.

One Number of cell divisions Two

Two daughter cellsparent cell

daughter cells

Number of daughter cellsthat produced at the endof the division

Four daughter cells(gametes) parent cell

daughter cells

Diploid (2n) or the samenumber of chromosomesas the parent cell.

(2n) Parent cell daughter cells (2n)

Chromosomal number of the daughter cells

Haploid (n) or half of thenumber of chromosomesof parent cell.

2n Parent cell daughter cells (n)

Genetically identical to theparent cell and to oneanother.

Genetic content Different from the parentcell and from one another.

7/28/2019 Biology (Form 4) [2]

17/25

There is no geneticvariation in any generation.

Genetic variation There is genetic variationfrom one generation to the

next.

7/28/2019 Biology (Form 4) [2]

18/25

THE IMPORTANCE OF MEIOSIS

1. In species that reproduce sexually, meiosis ensuresthat the diploid number of chromosomes ismaintained from one generation to the next.

2. Meiosis provides for genetic variation which occursfrom one generation to the next. Meiosis leads togenetic recombination in two key events which occur during meiosis 1.

7/28/2019 Biology (Form 4) [2]

19/25

During prophase 1, the process of crossing over results in theexchange of genetic material

between non-sister chromatids of abivalent. This results in formation of

new combinations of genes on achromosome.

During metaphase 1, each pair of

homologous chromosomes isarranged independently andrandomly (independent assortment)at the metaphase plate of the cell.

The paternal or maternalchromosomes or homologous maybe oriented to face either one of the

poles.

7/28/2019 Biology (Form 4) [2]

20/25

3. Both these events produce gametes with differentcombinations of chromosomes. The events that occur during Meiosis 1 and the random fertilisation of anovum by a sperm results in genetic variation in apopulation of organisms that reproduce sexually.

7/28/2019 Biology (Form 4) [2]

21/25

THE HUMAN LIFE CYCLE

7/28/2019 Biology (Form 4) [2]

22/25

APPRECIATING THE MOVEMENT OFCHROMOSOMES DURING MITOSIS ANDMEIOSIS

1. The ability of organisms to reproduce ensures thecontinuity of life on Earth.

2. Whether the organisms reproduce through mitotic celldivision or meiotic cell division, the ultimate aim is toensure the survival of each species from one generation tothe next.

3. Asexual reproduction through mitosis produces offspringthat are identical to the parent ;sexual reproductionthrough meiosis produces genetic variability in theoffspring.

7/28/2019 Biology (Form 4) [2]

23/25

4. Both processes are regulated in a precise manner.5. If meiosis does not occur properly, the gametes

formed will have an abnormal number of chromosomes. As a result, The zygote that is formedwould later become abnormal.

6. For example, Downs syndrome is the result of anextra chromosome 21, so that each body cell has atotal of 47 chromosomes instead of 46.

7/28/2019 Biology (Form 4) [2]

24/25

7. The affected individuals have certain characteristicswhich include small build and mental retardation.

Certain environmental agents such as radiations and certainchemicals are known to be carcinogenic and can disrupt the

processes of mitosis and meiosis.Food that contains preservatives such as sodium nitrite,benzene and formaldehyde are also known to change thestructure of DNA molecules.

8. Ways of preventing cancer would be to avoid contact withthese substances as well as adopting a healthy lifestyleand a diet that rich in fruits and vegetables.

7/28/2019 Biology (Form 4) [2]

25/25