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108

Learning ObjectivesCOMPANION WEBSITE

5 Cell DivisionTHEME: Investigating the Cell as a Basic Unit of Living Things

CHAPTER FORM 4

SPM Topical AnalysisYear

Paper

Section

Number of questions

2007 2008 2009 2010 2011

1 2 3 1 2 3 1 2 3 1 2 3 1 2 3

A B A B A B A B A B

2 1 3 1 2 2 1 3 1

ONCEPT MAP

Cell cycle

G1, S, G2 phases (interphase)

CELL DIVISION

The effects of controlled and

uncontrolled celldivision

Meiosis I Meiosis II

Prophase I Prophase II

Metaphase I Metaphase II

Anaphase I Anaphase II

Telophase I

Differences

Importance

Definition

Application: Tissue culture Cloning

Prophase

Metaphase

Anaphase

Telophase

Cytokinesis

Stages of mitosis

Mitosis Meiosis

Stages of meiosis

Importance

Definition

M stage

Telophase II

Learning ObjectivesCOMPANION WEBSITE

Cell cycle

G1, S, G2 phases (interphase)

CELL DIVISION

The effects of controlled and

uncontrolled celldivision

Meiosis I Meiosis II

Prophase I Prophase II

Metaphase I Metaphase II

Anaphase I Anaphase II

Telophase I

Differences

Importance

Definition

Application: Tissue culture Cloning

Prophase

Metaphase

Anaphase

Telophase

Cytokinesis

Stages of mitosis

Mitosis Meiosis

Stages of meiosis

Importance

Definition

M stage

Telophase II

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1 Cells in the body are continuously dividing, growing, and dying. Dead cells need to be replaced with new cells. All organisms grow and change through cell division.

2 (a) New cells are produced from existing cells, through a process known as mitotic cell division. (b) Mitotic cell division involves the

process of nuclear division called mitosis, followed by a cytoplasmic division called cytokinesis.

The types of cells that undergo mitosis

1 (a) In plants, mitotic cell division occurs actively in the meristematic tissues of the root tips and bud tips.

(b) Meristematic tissues are also found in terminal buds, the vascular cambium and cork cambium.

(c) Active cell division in meristematic tissues allows growth and elongation of a plant to take place at a faster rate.

2 (a) In animals, growth takes place in every part of the body and is not just confined to certain parts as in plants.

(b) For example, the human skin has Malpighian layers that undergo mitotic cell division to produce new skin cells to replace dead skin cells. During the growth process, the Malpighian layers also add to the skin surface area.

Mitosis is the process of nuclear division which results in the formation of two genetically identical daughter nuclei.

The meaning and significance of mitosis

(a) Mitosis replaces dead cells. For example, skin cells can live for only two weeks, after which new cells are formed through mitosis.

(b) It allows damaged cells to be repaired, replaced, or even regenerated, for example, liver cells can regenerate themselves following an injury through the process of mitosis to replace the damaged or lost part.

(c) It is the basis of asexual reproduction in unicellular organisms such as Amoeba sp. The daughter cells produced are genetically identical to the parent cell. This type of cell division, which produces two new organisms, is also known as binary fission.

(d) It increases the number of cells in all living organisms, thus, allowing growth and development in multicellular organisms. In multicellular organisms, the zygote divides and grows into two cells, then four, eight and

eventually into millions of cells that make up a multicellular organism. All the cells that are formed are genetically identical. This means that all the cells in our body

have the same genes; be it a cell in the liver, a cell in the skin or a cell in the brain.

(e) It results in the formation of two daughter nuclei which are genetically identical to each other and to the parent nucleus. Each nucleus contains the same number of chromosomes and the same genetic material as the parent cell.

Refer Form 4, Chapter 2, Unit 2.2

5.1 Mitosis

The necessity for the production of new cells in living organisms

The significance of mitosis

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1 The cells in a sexually reproducing organism can be divided into(a) somatic cells(b) reproductive cells or gametes

2 (a) Somatic cells comprise all the cells in an organism, except for the reproductive cells. (b) Somatic cells are formed through mitosis.

3 Reproductive cells are formed through meiosis.

4 Every cell has thread-like structures in its nucleus called chromosomes.

5 The number of chromosomes present in the cells of each species of an individual organism is constant. This number is referred to as the chromosomal number of the species.

6 (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. For example, onions have 16 chromosomes while the fruit fly, Drosophila melanogaster, has eight chromosomes.

(b) Since chromosomes in the nucleus exist in pairs, the chromosomal number is said to be diploid and is designated as 2n. Therefore, for the onions, 2n = 16 and for Drosophila melanogaster, 2n = 8.

7 The gametes contain only half the number of chromosomes or only one of each pair of chromosomes, that is, a single set. The chromosomal number is said to be haploid, and is designated as n. Therefore, in an onion, n = 8 and in a Drosophila melanogaster, n = 4.

8 (a) All somatic cells in the human body have 46 chromosomes.

(b) Each gamete only has 23 chromosomes. (c) Red blood cells do not have nuclei, and

consequently no chromosomes. 9 All somatic cells have two sets of chromosomes:

one set inherited from each parent. Therefore, one set of the chromosomes is of paternal origin, whereas the other is of maternal origin.

10 The presence of two sets of chromosomes in the nucleus of a cell is known as the diploid number of chromosomes (2n).

11 In humans, one set of chromosomes consists of 23 chromosomes. Hence, our somatic cells have 46 chromosomes arranged in 23 pairs or 2n = 46 while each gamete only has 23 chromosomes.

12 The two chromosomes in each pair have the same structural features and are referred to as homologous chromosomes. Each member of the pair is called a homologue.

13 Both chromosomes of each pair carry genes for the same trait (for example, eye colour) at the same location.

14 Cells with two sets of homologous chromosomes are called diploid cells (for example, somatic cells) while cells which contain only one set of chromosomes are called haploid cells (for example, sperm and egg cells).

15 Of the 23 pairs of homologous chromosomes in humans, one pair is the sex chromosomes. Females have two X chromosomes (XX) while males have an X chromosome and a Y chromosome (XY).

16 Each of the gametes or reproductive cells contains only one set of chromosomes or one of each kind of chromosome found in a somatic cell. Therefore, each human gamete only contains one set of 23 chromosomes or haploid number of chromosomes (n).

Mitosis maintains the chromosomal number of species and ensures genetic material is passed on to the offspring

1 (a) Each daughter cell that is formed through mitosis receives genetic material inherited from the parent cell.

(b) The genetic material, the DNA, is carried in the chromosomes.

2 The DNA consists of a double helix which contains hundreds or thousands of genes.

3 Each gene in the chromosomes of a parent cell is a unit of inheritance that must be passed down to its offspring.

4 This genetic information is passed down to the offspring when the nucleus divides to produce two identical nuclei by mitosis.

5 Each daughter cell contains the same chromosomal number and genetic material as the parent cell.

6 Hence, mitosis doubles the number of cells without changing the genetic content of the cell.

Refer Form 5, Chapter 5, Unit 5.3

Chromosomes and chromosomal number

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Photograph 5.1 The human karyotype consists of a total of 46 chromosomes arranged in matching pairs

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5 1 A chromosome consists of DNA molecule and

protein.

2 DNA carries the genetic material that organisms inherit from their parents.

3 A DNA molecule consists of hundreds or thousands of genes.

4 When the chromosomes are not condensed and visible as thread-like structures, they are called chromatin.

5 During the S phase, the DNA molecule replicates, forming two identical DNA double helices.

6 The replication of DNA produces a duplicated chromosome with two sister chromatids.

7 Each DNA double helix is contained within a sister chromatid. Hence, the two sister chromatids contain identical copies of DNA molecules.

8 During mitosis, the two sister chromatids separate and each becomes an independent daughter chromosome.

9 When cell division begins, the chromatin becomes condensed, coiled and folded. At this stage, the chromosome becomes compact and thick and can be easily seen under the light microscope. It has a narrow region in the centre called the centromere.

onechromatid

centromere

sister chromatids

DNAdouble helix

A chromosome which consists of a DNA double helix.

the chromosomecondenses

DNA replication

chromosome duplication

When a DNA double helix replicates, it becomes two DNA double helices. The chromosome is said to have duplicated. A duplicated chromosome consists of two identical sister chromatids.

Sister chromatids separate and become independent daughter chromosomes during anaphase. Each chromatid carries an identical DNA double helix.

duplicated chromosome in a condensed state

Each duplicated chromosome contains two identical DNA double helices. Each sister chromatid contains a DNA double helix.

Figure 5.1 Chromosome duplication and condensation

What is a chromosome?

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THE CELL CYCLE

1 The cells of a multicellular organism progress through a well-defined sequence of stages leading to the division and formation of new cells.

2 A cell cycle extends from the time a new cell is produced until the time the cell completes a division.

3 The cell cycle is divided into two major phases:(a) Interphase (G1, S and G2 sub-phases)(b) Mitotic cell division or the M phase

4 The different phases of the cell cycle are outlined in Figure 5.2.

Interphase

1 In humans, the cell cycle occurs gradually and continuously for 8 to 24 hours.

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

3 Interphase is also the stage at which cells grow larger and prepare for cell division.

4 During interphase, the nucleus is big and well defined (Photograph 5.2).

5 The chromosomes are not condensed and are visible as thread-like structures called chromatin.

The cell cycle

Photograph 5.2 A cell at interphase

6 A pair of centrosomes (found only in animal cells) is also formed in the cytoplasm. Each centrosome consists of a pair of centrioles.

7 Each pair of centrioles will later migrate towards the opposite poles of the cell and help in the formation of the spindle fibres.

8 After a period of time, depending on the type of cell and the nutrients available, the cell will start to divide.

9 Interphase is divided into three shorter stages or sub-phases:(a) G1 phase (gap or growth phase 1)

(b) S phase (DNA synthesis) (c) G2 phase (gap or growth phase 2) 10 The events that take place at each sub-phase

are detailed in Figure 5.2.

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What is DNA replication?When one DNA double helix replicates, two identical DNA double helices are formed. Each DNA double helix has the original strand and a new strand.

Two identical DNA double helices

G1(growth phase 1)

G2(growth phase 2)

During this stage, the cell begins to acquire and synthesise the materials required for cell division. Proteins and new organelles are being synthesised. The metabolic rate of the cell is high. G1 is a crucial phase because during this phase, cells will decide whether or not to divide and

complete the cycle to form new cells. If the external conditions are conducive for growth, then the cell enters the S phase.

During G1, chromosomes are extremely fine and cannot be seen under the light microscope. At this stage, the chromosomes are known as chromatin.

The cell continues to grow and remains metabolically active.

Enzymes and proteins are synthesised for cell division.

The cell accumulates energy and completes its final preparations for division.

S phase(DNA synthesis)

Synthesis of DNA (genetic material) occurs.

The DNA undergoes replication. A duplicated chromosome consists

of two identical sister chromatids. Both sister chromatids contain identi-

cal copies of the chromosomes DNA molecule.

THE CELL CYCLE

Figure 5.2 The cell cycle consists of G1, S, G2, mitosis and cytokinesis

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PROPHASE ANAPHASEMETAPHASE TELOPHASE

spindlefibres

nucleolus

chromosome

centromerecentrioles

daughter chromosomes

pole

nuclearmembrane

cleavagefurrow

nucleolus

centromere spindlefibres

sisterchromatids

metaphaseplate

The processes of mitosis and cytokinesis

1 After the interphase stage, the dividing cells enter the M phase.

2 The M phase or mitotic cell division phase can be divided into two major parts:(a) mitosis(b) cytokinesis

3 Mitosis can be further subdivided into four phases, namely,(a) prophase(b) metaphase(c) anaphase(d) telophase

4 The phases are continuous, with each merging into the next one.

The phases of mitosis in animal cells:

The chromosomes condense and become tightly coiled. The chromosomes become shorter, thicker and visible under a 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 then migrates to lie at the opposite poles

of the cell. Each pair of centrioles acts as a central point from which the

spindle fibres radiate. The central point is known as the spindle pole.

The spindle fibres from the opposite spindle poles are attached to the centromeres of each sister chromatid.

In plant cells the spindle forms without the presence of centrioles. At the end of prophase, the nucleolus disappears and the

nuclear membrane disintegrates.

Figure 5.3(b) MetaphaseFigure 5.3(a) Prophase

The centromeres of all the chromosomes are lined up on the equator of the cell called the metaphase plate.

The spindle fibres are now fully formed.

The chromosomes are arranged randomly at the metaphase plate.

The two sister chromatids of each chromosome are still attached to each other at the centromere.

Metaphase ends when the centromeres divide.

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PROPHASE ANAPHASEMETAPHASE TELOPHASE

spindlefibres

nucleolus

chromosome

centromerecentrioles

daughter chromosomes

pole

nuclearmembrane

cleavagefurrow

nucleolus

centromere spindlefibres

sisterchromatids

metaphaseplate

Figure 5.3(c) AnaphaseFigure 5.3(d) Telophase

The two sister chromatids of each chromosome separate at the centromere.

The sister chromatids are pulled apart to the opposite poles by the shortening of the spindle fibres that connect the chromosomes to the poles.

Once separated, the chromatids are referred to as daughter chromosomes.

Anaphase ends when the chromosomes reach the poles of the cell.

Since the sister chromatids are identical copies of the original chromosomes, each pole of the cell will have a set of complete and identical chromosomes as in the parent cell.

115 Cell Division

Telophase begins when both sets of chromosomes reach the opposite poles of the cell.

The chromosomes start to uncoil and revert to their extended state (chromatin) again.

The spindle fibres disappear and a new nuclear membrane forms around each set of chromosomes.

The nucleolus reforms in each nucleus. The process of mitosis is now complete.

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5Cytokinesis

1 Following mitosis, the cytoplasm of the cell divides through a process called cytokinesis to form two daughter cells, each having one nucleus.

Figure 5.4 Cytokinesis in an animal cell

Actin filaments in the cytoplasm contract to pull a ring of the plasma membrane inwards, forming a groove called the cleavage furrow.

2 Through cytokinesis, the daughter cells formed have all the organelles, nutrients and other components needed to survive and maintain themselves.

3 Cytokinesis is the process of cytoplasmic division.

4 It usually begins before nuclear division is complete, that is, towards the end of telophase.

Photograph 5.3 The formation of a cleavage furrow in an animal cell

1The cleavage furrow pinches at the equator of the cell.

The cleavage furrow deepens progressively until the cell separates into two daughter cells.

Cytokinesis in animal cells

cleavagefurrow

2 3

1 Although plant cells undergo the same stages of mitosis as in animal cells, cytokinesis in plant cells occurs by a process which is different from that of animal cells.

2 After cytokinesis, the cell enters G1 of interphase, thus completing the cell cycle.

Figure 5.5 Cytokinesis in a plant cell

Membraneenclosed 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 contents of the cell plate.

Eventually, the cell plate divides the cell into two daughter cells.

Cellulose fibres are produced by the cells to strengthen the new cell walls.

cell wall cell plate newly formedcell wall

vesicles

Cytokinesis in plant cells

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1 Cells must divide in a controlled and orderly manner and be precise in distributing an exact copy of each of their chromosomes to the new cells.

2 This is important because the genetic infor-mation carried by the chromosomes is necessary for the proper functioning of an organism.

3 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.

4 The rate and timing of cell division is im -portant for normal cell growth, develop ment and maintenance.

5 Different cells divide at different frequencies. For example, human skin cells divide throughout their lifespan while liver cells only divide when necessary to replace damaged and injured tissues. Nerve and muscle cells do not divide at all once they mature.

6 The entire cell cycle and cell division is closely regulated.(a) Each cell has a system consisting of

specific proteins which control and direct the sequence and progression of phases in the cell cycle.

(b) The control system within the cells ensures that cell division is complete and the cell divides in a controlled manner.

(c) Certain genes are also involved in the synthesis of certain proteins that can stimulate the replication of chromatin during the S phase.

1 When a cell divides by mitosis repeatedly, without control and regulation, it can produce cancer cells.

2 Cancer is a disease caused by uncontrolled mitosis due to severe disruption to the mechanism that controls the cell cycle.

3 Cancer cells divide freely and uncontrollably without heeding the cell cycle control system.

4 Cancer cells compete with the surrounding normal cells to obtain sufficient nutrients and energy for their own growth.

5 A cancer cell that is not destroyed will divide uncontrollably to form a tumour, an abnormal mass of cells (Figure 5.6).

6 Cancer cells can intrude on and spread to other tissues which then lead to the malfunction of the tissues and ultimately death.

7 Cancer can be caused by many factors such as(a) damage to the DNA(b) changes in genes (mutation) that control

cell division(c) ionising radiation, for example, X-rays,

ultraviolet rays and gamma rays(d) certain chemical compounds like tar in

tobacco smoke(e) carcinogenic com pounds (cancer-

causing com pounds) such as formal-dehyde

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

Figure 5.6 A layer of normal cells that divide uncontrollably to become a tumour

Table 5.1 The differences between normal cells and cancer cells

Controlled growth

A single organised layer

Cells are differentiated and carry out specialised functions.

The nuclei and number of chromosomes are normal.

Normal cells Cancer cells

Uncontrolled growth

Multilayered and disorganised

Cells are undifferentiated and do not have specialised functions.

The nuclei and number of chromosomes are abnormal.

(a) Normal cells

(b) A tumour

The importance of controlled mitosis

The effects of uncontrolled mitosis

The application of knowledge of mitosis in cloning

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5Cloning

1 Cloning is the process of producing clones or genetically identical copies of a cell, tissue or an organism through asexual reproduction.

2 Animal cloning involves the transfer of the nucleus from a somatic cell to an ovum or embryonic cell with the nucleus removed.

3 Many animals have been successfully cloned ever since the first mammal, a sheep named Dolly, was cloned in 1996 (Figure 5.7).

4 Cloning is a form of asexual reproduction because the organisms produced have the same

genetic content and chromosomal number as the parent organism. This is a common charac-teristic of asexual reproduction.

5 The nucleus that directs the development of the offspring comes from a diploid cell produced through mitotic cell division and not through the fusion of gametes produced by meiotic cell division.

6 The successful cloning of Dolly has demonstrated that under the right conditions, inactive genes of specialised adult cells can be expressed and made functional once again.

How is animal cloning carried out?An animal is cloned using a nucleus obtained from an adult tissue.Dolly, the sheep, is genetically identical to the somatic cell donor.

Figure 5.7 The cloning of Dolly

3

4

2

5

Somatic cells (from themammary gland cells)are removed and grownin a low culture medium. The starved cells stop dividing and enter a non-dividing phase.

An electric pulsestimulates the fusionbetween the somaticcell and the egg cellwithout nucleus.

The cell dividesrepeatedly, formingan embryo.

Dolly, which is a clone of a somatic cell donor parent,was born in July 1996.

1

An unfertilised eggcell is obtained. Thenucleus is sucked out,leaving the cytoplasmand organelles withoutany chromosomes.

The embryo is thenimplanted into asurrogate mother(the same breed ofsheep as the ovumdonor sheep).

Dolly, the clonedsheep of the somaticcell donor, is born.

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Tissue culture technique

1 Many types of plant and animal cells can be extracted from organisms and cultured in a nutrient medium outside the organisms.

2 Tissue culture technique involves the growth of cells or tissues outside the organisms in a suitable culture medium, which contains nutrients and growth hormones (in vitro methods).

3 In vitro literally means in glasses. The term refers to experiments conducted outside the body of an organism, namely in test tubes or conical flasks.

4 The main purpose of tissue culture is to produce plant and animal cells through asexual reproduction.

5 Each cell has the full genetic potential (just like a zygote) to form all parts of a mature organism. This means a single plant cell can develop to become a complete plant.

6 Different parts of plants that can be cultured include young shoots, meristematic tissues, leaves, roots, seeds, embryos, cells and proto plasm.

7 In Malaysia, the tissue culture technique is used to propagate plants such as oil palm, rubber trees, orchids and tomatoes.

How is the tissue culture technique carried out?

Small pieces of a plants leaf, shoot, bud, stem or root tissues are cut out.

These cut out plant tissues are called explants.

1

Alternatively, enzymes are used to digest the cell walls of tissues, for example, the mesophyll tissue from a leaf.

This results in naked cells without cell walls called protoplasts.

2

The explants or protoplasts are sterilised and then placed in a glass container which contains a nutrient solution with a fixed chemical composition. A culture medium or growth medium normally consists of a complex mixture of glucose, amino acids, minerals and other substances required for the growth of the tissues.

The culture medium and the apparatus used must be in sterile conditions and free from microorganisms which can contaminate the tissue culture.

The pH and temperature of the culture medium also need to be maintained at optimum levels.

3

The explants or protoplasts begin to divide by mitosis. Cell division produces aggregates of cells. The aggregate of cells develop into a callus; an

un differentiated mass of tissue.

4

The callus develops into a somatic embryo. The embryo develops into a plantlet which can later be

transferred to the soil for growth into an adult plant. All the plantlets produced this way are genetically

identical. Therefore, all the adult plants that develop from them share the same traits.

5

1

3

isolatedcells

Explant 2 Protoplasts

Explant/protoplastsin a culture medium

callusaggregatesof cells

somaticembryo

plantlet

4

5

Figure 5.8

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5 8 Through the tissue culture technique:

(a) thousands of new young plants or cloned plants with desirable characteristics and traits such as strong resistance towards diseases can be produced from somatic cells taken from the parent plant.

(b) thousands of identical young plants, all having the same characteristics and genetic content as the parent plant can be produced.

(c) a large number of identical plants can be grown or propagated for commercial purposes.

9 With the latest developments in genetic engin eering, the genes of a plant can be altered and engineered to produce higher yields.

10 These transgenic plants carry a foreign gene that has been introduced into their genetic constitution so that they possess new and different traits.

11 Transgenic plants have improved food quality. These plants can be propagated through the tissue culture technique.

12 Transgenic crops like wheat, soya bean and cotton which are resistant to herbicides, pests and diseases have been successfully created by biotechnologists.

Cloning allows biotechnologists to multiply copies of useful genes or clones. (a) For example, the bacterium Escherichia

coli has been genetically manipulated to produce bovine growth hormones.

(b) The clones of these bacteria can synthesise a large amount of the hormone.

(c) The hormone can then be injected into cows to increase the quality of their milk.

1

(a) Plants that reproduce from seeds take a long time to grow and produce fruits. Cloned plants, however, can produce flowers and fruits within a shorter period.

(b) Furthermore, as clones reach maturity in a shorter period of time, less time and effort are needed to properly supervise them in the earlier stages.

3

(f) The insulin is then purified and used in the treatment of diabetes mellitus.

(g) The problem with this method is that it is costly and the amount produced cannot meet the demand for insulin.

(h) Today, through genetic engineering, the gene that codes the synthesis of human insulin is inserted into the bacterias genome.

(i) The genetically modified bacteria are then grown on a large scale.

(j) The bacteria multiply rapidly by binary fission, and the human gene replicates together with the bacterias own genes.

(k) The bacterial clones or transgenic bacteria that are being produced are identical because each clone contains the gene to synthesise insulin.

(l) The bacterial cells are then lysed so that insulin can be extracted. Because bacteria multiply rapidly and can be grown in large numbers, insulin can be produced on a large scale for commercial purposes.

(m) Insulin produced in this way can be made in large quantities, is less expensive and more readily available.

Clones can be produced in a shorter time and in larger numbers. (a) In medicine, for example, the Escherichia

coli strain can be cloned to produce insulin. (b) Insulin is a hormone that lowers the level

of blood sugar by converting excess glucose into glycogen in the liver.

(c) Insulin is produced by the pancreas. A lack of insulin can cause diabetes mellitus.

(d) People with diabetes mellitus require a constant supply of insulin.

(e) In the past, insulin was obtained by extracting it from the pancreas of animals such as cows after they had been slaughtered.

2

Many transgenic crops like wheat, soya bean and cotton which are resistant to herbicides, pests and diseases have been created. (a) Plants are also engineered to produce better

quality yields. For example, a gene from the bacterium Bacillus thuringiensis (Bt) is transferred to the cotton plant to create a new transgenic cotton plant which is resistant to the Bt larvae. This gene codes the synthesis of the Bt protein which kills the larvae that feed on cotton plants.

(b) Delayed ripening in tomatoes is another example of the beneficial traits possessed by transgenic plants. This type of tomato appears fresh and firm and has a longer shelf life (Photograph 5.4).

(c) Transgenic plants can be cloned using the tissue culture technique to produce thousands of plantlets (clones) with similar resistance to pests and diseases. Farmers are now planting many of these genetically

modified (GM) crops.

4

Advantages of cloning

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Certain transgenic bacteria can be used to control environmental pollution. (a) For example, the gene for the synthesis of

lipase is isolated from animals and inserted into the bacterial genome to create a new strain of bacteria that can clean up oil spills in the ocean.

(b) There are also some bacterial clones which are able to break down toxic waste materials and help clean up toxic waste dumps.

(c) For example, one such bacterium is able to remove sulphur from coal before it is burnt.

(d) Therefore, transgenic bacteria are able to help humans overcome pollution by cutting down the time and cost of cleaning required for the removal of oil spills and toxic wastes.

6

(a) Cloning and tissue culture techniques involve vegetative reproduction which does not need pollinating agents.

(b) Thus, propagation can take place at any time without the need for pollination.

5

Photograph 5.4 A delayed-ripening tomato (left) does not rot when compared to a normal tomato (right) which rots after being kept for two weeks.

Disadvantages of cloning

Many ethical and moral issues regarding cloning have been raised. Many religious groups and organisations have questioned and strongly opposed cloning. Among the issues raised are as follows:

Disadvantages of cloning

The long-term side effects of using genetically modified viruses and bacterial clones in various fields such as medicine and industries are not yet known.

For example, many vaccines, antibodies and hormones are produced by genetically modified bacteria. The period of use and their side effects on humans have not been established.

1

The longterm effects and safety aspects of releasing bacterial clones to the environment to solve problems related to the environment such as pollution are not yet known. These organisms may mutate and become dangerous to the environment and other living organisms.

2

Clones do not show any genetic variations. For example, certain plant clones have adapted to the current environment. However, if a drastic change to the environment should occur in the future, the clones may be wiped out entirely, as they would be unable to adapt to the changes.

3

All clones have the same level of resistance towards certain diseases. If a new disease or pest emerges, then all the clones may be eliminated, as they are not resistant to the new diseases or pests.

4

Certain transgenic crops contain genes that are resistant to herbicides. These genes may be transferred to weeds through viruses. These weeds could then become resistant to herbicides.

6

For reasons still unknown, cloned animals have a shorter lifespan. Research is currently underway to find a solution to prolong the lives of cloned animals.

7

New clones may undergo natural mutations which can endanger mankind, as well as the environment. They may also disrupt the natural equilibrium of an ecosystem.

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1 Mitosis produces daughter cells that have exactly the same number of chromosomes as the original parent cells.

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

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

4 Hence, in order for the offspring to possess the same chromosomal number as their parents, the reproductive organs that produce the gametes must undergo meiosis.

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

The necessity for the production of haploid gametes

1 Meiosis is a process of nuclear division that reduces the number of chromosomes in daughter cells to half that of the parent cell.

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

3 As each gamete receives only one chromosome from every pair of homologous chromosomes, this means, in humans, the gametes contain only 23 chromosomes or haploid number of chromosomes (n).

4 During sexual reproduction, the fusion of two gametes (the sperm and the ovum)

Organism

Chromosomal number

Saccharomyces cerevisiae (yeast) 32

Zea mays (corn) 20

Felis domesticus (cat) 38

Gallus gallus (chicken) 78

Lycopersicon esculantum (tomato) 24

Musca domestica (housefly) 12

Orvis aries (sheep) 54

Equus caballus (horse) 64

Homo sapiens (human) 46

Table 5.2 Diploid chromosomal number of some organisms

The diagram shows a cell at one particular stage of mitosis.

Which cell is produced by the cell division?A B C D

CommentsThe stage shown in the diagram is prophase. The number of chromosomes in the cell is 4. At the end of mitosis, the number of chromosomes is also 4, consisting of 2 pairs of homologous chromosomes.

Answer B

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The significance of meiosis

1 Give the definition of mitosis.

2 State two reasons why mitosis is important in living organisms.

5.1

3 Describe the process that takes place during the S phase.

4 State two advantages of applying the tissue culture technique.

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restores the complete number of chromo-somes and genetic material, forming a diploid zygote with 46 chromosomes. This means the offspring inherits traits from both parents to ensure a continuation of life.

5 If human reproductive organs divide by mitosis, then the resulting daugther cells (gametes) would be like somatic cells, having 46 chromosomes (2n = 46). Fertilisation of two gametes would then bring the number of chromosomes to 92. If this happens the offspring would not be human anymore!

The types of cells that undergo meiosis

1 In animals, meiosis occurs in reproductive organs, that is the testes (in males) and ovaries (in females).

2 In plants, meiosis occurs in the anthers and ovaries of flowers.

1 Meiosis consists of two separate nuclear divisions: (a) meiosis I, which consists of prophase I,

metaphase I, anaphase I and telophase I.(b) meiosis II, which consists of prophase II,

metaphase II, anaphase II and telophase II. 2 Meiosis I begins with a single diploid parent

cell. At the end of meiosis II, four haploid daughter cells are produced, each genetically different from the others and from the parent cell.

3 In meiosis, even though the cell undergoes two nuclear divisions, the DNA of each chromo-some only replicates once.

Interphase

1 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.

centrosomes(with centriole pairs)

nucleolus

chromatin

nuclearenvelope

Figure 5.9 Interphase

The process of meiosis

The stages of meiosis

The diagram shows a pair of homologous chromosomes during prophase I of meiosis.

P

What is P?A SynapsisB ChiasmaC BivalentD Crossing over

Comments Synapsis is the process when homologous

chromosomes pair up. A bivalent consists of a pair of chromosomes,

one is of paternal origin, the other is of maternal origin.

Crossing over is the process in which nonsister chromatids exchange segments of DNA.

The point at which segments of chromatids cross over is called a chiasma.

Answer B

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5 The nuclear membrane and 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 (Figure 5.9).

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The chromosomes begin to condense. They become shorter, thicker and clearly visible.

Homologous chromosomes come together to form pairs of bivalents through a process called synapsis. One of the chromosomes is of paternal origin, whereas the other is of maternal origin.

Each bivalent consists of a fourpart structure called a tetrad. A tetrad consists of two homologous chromosomes, each of which is made up of two sister chromatids.

Nonsister chromatids ex change segments of DNA in a process known as crossing over.

Crossing over can occur at any locations or several locations on the chromosome at the same time.

Crossing over results in new combinations of genes on a chromosome.

The points at which segments of chromatids cross over are called chiasmata (singular, chiasma).

At the end of prophase I, the nucleolus and nuclear membrane disappear.

The two pairs of centrioles migrate to the opposite poles of the cell. Each pair of centrioles acts as a central point from which the spindle fibres radiate.

Metaphase I

The spindle fibres pull the tetrads to the middle of the cell. Pairs of homologous chromosomes align themselves at the metaphase

plate (equator of the cell). The homologous chromosomes are lined up side by side as tetrads. One chromosome of each homologous pair is attached to fibres from one

pole while its homologue is attached to fibres from the opposite pole. The centromere does not divide.

Meiosis i:Separates homologous

chromosomes ProPhase i MetaPhase i anaPhase iteloPhase i

and Cytokinesis

Prophase I

Anaphase I

The spindle fibres pull the homologous chromosomes apart from one another and move them to the opposite poles of the cell.

Each chromosome still consists of two sister chromatids which move as a single unit.

This means that each member of the homologous chromosomes is attached to spindle fibres that pull them towards the opposite poles.

At the end of anaphase I, each pole has only two chromosomes (each with two sister chromatids).

Telophase I

The chromosomes arrive at the poles. Each pole now has a haploid daughter nucleus because it contains only

one set of chromosomes. The spindle fibres disappear. The nuclear membrane reappears to surround each group of

chromosomes. The nucleolus then reappears in each nucleus.

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chiasmata spindlefibre

sisterchromatids

sisterchromatidsremainattached

metaphaseplate

cleavagefurrow

centrioleshomologouschromosomesseparate andpulled to the opposite poles

homologouschromosomesaligned at themetaphaseplate

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1. Cytokinesis usually occurs simultaneously with telophase I, resulting in two haploid daughter cells. Each daughter cell receives one chromosome from a homologous pair.

2. In some organisms, the newly formed daughter cells undergo a short interphase. However, for most organisms, there is no interphase between meiosis I and meiosis II.

3. In both situations, DNA repli-cation does not take place and the chromosomes remain in a condensed state.

4. The events which take place during meiosis II are identical to those of mitosis.

Meiosis ii: Separates sister chromatids

ProPhase ii MetaPhase ii anaPhase iiteloPhase ii

and Cytokinesis

Prophase II

The nuclear membrane disintegrates.

The spindle fibres reform in each daughter cell.

Metaphase II

The chromosomes, each still made up of two sister chromatids, are positioned randomly at the metaphase plate.

Each sister chromatid is attached to the spindle fibres at the centromere.

Anaphase II

The centromeres of the sister chromatids separate.

The sister chromatids of each chromosome are now individual chromosomes.

Each individual chromosome moves towards the opposite poles of the cell.

Telophase II

Finally, the nucleoli and nuclear membranes reform.

The spindle fibres break down. Cytokinesis follows and four haploid

daughter cells are formed. Each haploid cell contains half the number of chromosomes and is genetically different from the parent diploid cell. These haploid cells become gametes.

chiasmata spindlefibre

sisterchromatids

sisterchromatidsremainattached

metaphaseplate

cleavagefurrow

centrioleshomologouschromosomesseparate andpulled to the opposite poles

homologouschromosomesaligned at themetaphaseplate

sister chromatidsseparate

nuclearmembrane

haploid daughtercells forming

two haploiddaughter cells

four haploid daughter cells

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MeiosisMitosis Aspects/events

Cells in the reproductive organs

Produces gametes for sexual reproduction

Homologous chromosomes pair up (synapsis) to form bivalents.

Crossing over between nonsister chromatids occurs during prophase I.

Homologous chromosomes line up side by side at the metaphase plate.

Homologous chromosomes separate to move to the opposite poles.

The sister chromatids still remain attached to each other.

Two

Four daughter cells (gametes).

Haploid (n) or half the number of chromosomes of the parent cell.

Different from the parent cell and from one another.

There is genetic variation from one generation to the next.

All somatic cells

Produces new cells for growth and repair

Pairing of homologous chromosomes (synapsis) does not occur.

Crossing over between nonsister chromatids does not occur during prophase.

The individual chromosomes are arranged randomly at the metaphase plate.

Sister chromatids separate to move to the opposite poles.

One

Two daughter cells.

Diploid (2n) or the same number of chromosomes as the parent cell.

Genetically identical to the parent cell and to one another.

There is no genetic variation in any generation.

Type of cell

Role

Synapsis

Crossing over

Metaphase of mitosis Metaphase I of meiosis

Anaphase of mitosis Anaphase I of meiosis

Number of cell divisions

Number of daughter cells produced at the end of the division

Chromosomal number of the daughter cells

Genetic content

Genetic variation

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

Similarity

Differences

MeiosisMitosis

The differences and similarity between mitosis and meiosis

1 State two differences between meiosis I and meiosis II.

2 Identify the event that occurs during prophase I which brings about genetic variation in the daughter cells being formed.

3 Explain what will happen if the cells in the reproductive organs do not divide by meiotic cell division.

5.2

Cell Division

(b) During metaphase I, each pair of homolo gous chromosomes is arranged inde-pendently and randomly (independent assortment) at the metaphase plate of the cell. The paternal or maternal chromo-somes or homo logues may be oriented to face either one of the poles.

3 Both these events produce gametes with different combinations of chromosomes. The events that occur during meiosis I and the random fertilisation of an ovum by a sperm results in genetic variation in a population of organisms that reproduce sexually.

Figure 5.10 The human life cycle

sperm (n)

Fertilisation

Meiosis

diploid zygote(2n = 46)

testisovary

haploid gametes (n = 23)

multicellular diploidadults (2n = 46)

ovum (n)

Mitosis anddevelopment

1 In species that reproduce sexually, meiosis ensures that the diploid number of chromosomes is maintained from one generation to the next (Figure 5.10).

2 Meiosis provides for genetic variation which occurs from one generation to the next. Meiosis leads to genetic recombination in two key events which occur during meiosis I.(a) During prophase I, the process of cross

ing over results in the exchange of genetic material between non-sister chromatids of a bivalent. This results in the formation of new combinations of genes on a chromosome.

Refer Form 5, Chapter 6, Unit 6.2

The importance of meiosis

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Meiosis increases the genetic variation of the population. The diploid cell of an organism which undergoes meiosis can produce 2n different chromosomal combinations, where n is the haploid number. In humans, the number is 223, which is more than eight million different combinations.

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

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

3 Asexual reproduction through mitosis pro-duces offspring that are identical to the parent; sexual reproduction through meiosis produces genetic variability in the offspring.

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

formed would later become abnormal. For example, Downs syndrome is the result of an extra chromosome 21, so that each body cell has a total of 47 chromosomes instead of 46. The affected individuals have certain charac-teristics which include small build and mental retardation.

6 (a) Certain environmental agents such as radiation and certain chemicals are known to be carcinogenic and can disrupt the processes of mitosis and meiosis.

(b) Food that contains preservatives such as sodium nitrite, benzene and formaldehyde are also known to change the structure of DNA molecules.

7 Ways of preventing cancer would be to avoid contact with these substances as well as adopting a healthy lifestyle and a diet rich in fruits and vegetables.

Refer Form 5, Chapter 5, Unit 5.2

5.3 Appreciating the Movement of Chromosomes during Mitosis and Meiosis

1 Mitosis is the process of nuclear division which results in the formation of two genetically identical daughter nuclei.

2 Somatic cells (formed through mitosis) comprise all the cells in an organism except reproductive cells.

3 Reproductive cells are formed through meiosis. 4 The cell cycle is divided into two major phases: (a) Interphase (G1, S and G2) (b) Mitotic cell division or the M phase 5 Interphase is the stage at which cells grow bigger

and prepare for cell division. The three sub-phases are:

(a) G1 phase Proteins and new organelles are synthesised.

(b) S phase Synthesis of DNA occurs. DNA undergoes replication where duplication of chromosomes occurs.

(c) G2 phase Enzymes and proteins are synthesised.

6 The M phase can be divided into mitosis and cytokinesis.

7 Mitosis is sub-divided into four phases: (a) Prophase The chromosomes condense and

become tightly coiled. Spindle fibres begin to form.

(b) Metaphase The chromosomes are arranged at the metaphase plate.

(c) Anaphase The two sister chromatids separate and are pulled apart to the opposite poles.

(d) Telophase The chromosomes reach the opposite poles of the cell.

8 Cytokinesis is the process where the cytoplasm is divided into two daughter cells, each with a nucleus.

9 Cloning is the process of producing clones or genetically identical copies of a cell, tissue or an organism through asexual reproduction.

10 Tissue culture involves the growth of cells or tissues outside the organisms in a suitable culture medium, which contains nutrients and growth hormones.

11 Meiosis is a process of nuclear division that reduces the number of chromosomes in daughter cells to half that of the parent cell.

12 Meiosis consists of two separate nuclear divisions: (a) Meiosis I: (i) Prophase I Crossing over which results

in new combinations of genes on a chromosome.

(ii) Metaphase I Pairs of homologous chromosomes align themselves at the metaphase plate.

(iii) Anaphase I Spindle fibres pull the homologous chromosomes apart from one another and move them to the opposite poles of the cell.

(iv) Telophase I and cytokinesis Each pole now has a haploid daughter nucleus. Two haploid daughter cells are produced.

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(b) Meiosis II: (i) Prophase II Spindle fibres re-form. (ii) Metaphase II Individual chromosomes

are posi tioned randomly at the metaphase plate.

(iii) Anaphase II The centromeres of the

sister chromatids separate. The sister chromatids move to the opposite poles.

(iv) Telophase II and cytokinesis Spindle fibres break down and four haploid daughter cells are formed.

1 Stages K, L, M and N in Diagram 1 occur during mitosis in a cell.

K L M NDiagram 1

Which of the following shows the correct sequence of mitosis? A K, L, M, N C M, K, L, N B N, K, M, L D N, M, K, L

2 Diagram 2 shows a type of cell division.

Diagram 2

Which cell undergoes this type of cell division? A Skin cell C Secondary oocyte B Red blood cell D Embryo sac mother cell

3 Diagram 3 shows the phases in the cell cycle.

Prophase AnaphaseP Q

Diagram 3

Which statements about the chromosomes at stages P and Q are correct?

A

B

C

D

4 Diagram 4 shows the process of cloning a sheep.

diplod cell

embryo

surrogatemother

offspring Z

ovum

Diagram 4

Which of the following is the offspring Z?

A C

B D

5 If the chromosomal number of an organism is 12, what is the chromosomal number of gamete cells, somatic cells and embryonic cells of the organism?

Stage P Stage Q

Each chromosome consists of The homologous chromosomes two sister chromatids. form pairs of bivalents.

The chromosomes condense The sister chromatids separate and become tightly coiled. and move to the opposite poles.

The chromosomes duplicate to The chromosomes are long form sister chromatids. and not visible.

The chromosomes line up at The chromosomes reach the the metaphase plate. opposite poles of the cell.

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5Multiple-choice Questions

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5 10 At which stages of the cell cycle do these events occur?

DNA replicationBreakdown of nuclear

membraneDivision of centromere

A Prophase Prophase Anaphase

B Interphase Anaphase Metaphase

C Interphase Prophase Anaphase

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Gamete cells

Somatic cells

Embryonic cells

A 12 12 12B 6 12 6C 6 12 12D 12 6 12

6 G1, M, G2 and S are the phases of a cell cycle in an organism.

G1

G2

S

M

Diagram 5

Which sequence of the phases during the interphase is correct?

A G1 S G2 B G1 G2 S C M G1 G2 D M S G2

7

Nuclear membrane disintegrates.

Spindle fibres are formed.

During which phase in mitosis do the events take place?

A Interphase C Metaphase B Prophase D Anaphase

8 The diploid chromosomal number (2n) of an animal is 42. If one of the homologous chromosome pairs does not separate during meiosis I, how many chromosomes can be found in the gametes?

A 19 C 21 B 20 D 42

9 Diagram 6 shows an animal cell undergoing mitosis.

Diagram 6

What is the stage of the mitosis? A Prophase B Metaphase C Anaphase D Telophase

14 Which of these illustrates the condition of a somatic cell and a reproductive cell of an insect after undergoing mitosis and meiosis respectively, if the number of chromosomes in a diploid cell is 4?

After mitosis After meiosis

A

B

C

D

15 If mitosis continues to occur without cytokinesis, the daughter cells will

A lack nuclei B grow unusually big C have more than one nucleus D not undergo interphase

16 Which phase in the interphase is responsible for the synthesis of DNA?

A G1 C S B G2 D M

5.2 Meiosis

17 Which sequence of meiosis I is correct?

A Prophase I Anaphase I Metaphase I Telophase I

B Metaphase I Telophase I Prophase I Anaphase I

C Anafase I Metaphase I Telophase I Prophase I

D Prophase I Metaphase I Anaphase I Telophase I

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11 Which of these human cells do not have the correct chromosomal number?

Human cellsChromosomal

numberA Red blood cells 0

B Ova 23

C Intestinal cells 46

D Skin cells 23

12 Diagram 7 shows a cell at metaphase during mitosis.

Diagram 7

What is the chromosomal number in the daughter cells after cell division is completed?

A 2 C 8 B 4 D 16

13 Diagram 8 shows the different stages of mitosis.

Diagram 8

What are the correct sequence of stages?

A P, Q, R, S C Q, R, P, S B S, R, P, Q D S, R, Q, P

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18 Crossing over occurs between A two different kinds of

chromosomes B two different kinds of bivalents C sister chromatids of the same

chromosomes D non-sister chromatids of a

bivalent

19 Which diagram represents metaphase I?

A

B

C

D

20 Diagram 9 shows the different stages of meiosis in a diploid cell, 2n = 4.

I II III IV

I II III IVDiagram 9

Which is the correct sequence of the stages?

A III, II, IV, I B I, III, IV, II C III, IV, II, I D II, IV, III, I

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21 Diagram 10 shows a sequence of stages during meiosis.

Diagram 10

During stage P, the homologous chromosomes

A become condensed and thickened

B pair up and crossing over occurs

C separate and move towards the opposite poles

D arrange themselves randomly at the metaphase plate

22 If an insect species has a diploid number of chromosomes, 2n = 12, in each of its nuclei, which is true?

Number of nuclear division during meiosis

Number of chromosomes

in gametes after meiosis

A 1 6

B 2 3

C 2 6

D 2 12

23 During which phase of meiosis are chiasmata formed?

A Prophase I B Metaphase I C Anaphase I D Telophase II

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24 During which stage of meiosis do hormologous chromosomes separate?

A Prophase I C Prophase II B Anaphase I D Anaphase II

25 Diagram 11 shows a stage during cell division.

Diagram 11

Which of these statements are true about the cells?

I Four chromosomes are present in each daughter cell.

II Homologous chromosomes separate and move towards the opposite poles of the cells.

III The number of daughter cells produced at the end of the cell division for each cell is 8.

IV Sister chromatids are attached together at the centromere and move as a unit.

A I and III C I, II and IIIB II and IV D II, III and IV

26 Which statements explain the importance of meiosis?

I Haploid cells are produced during meiosis.

II The chromosomal number is reduced to half in the daughter cell.

III The chromosomal number is maintained after each cell division.

IV Causes genetic variation from one generation to the next.

A I and IIB III and IVC I, II and IVD II, III and IV

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Cell Division

2 Diagram 2.1 shows the nucleus of an animal cell.

nuclearmembrane

Diagram 2.1

(a) (i) Name the structures seen inside the nucleus in Diagram 2.1.

(ii) What is the chromosomal number of the nucleus in Diagram 2.1? [2 marks]

(b) When one nucleus divides, what is the normal number of daughter nuclei formed from this nucleus as a result of division by

(i) mitosis? (ii) meiosis? [2 marks]

(c) Within the outlines of the nuclei in Diagram 2.2, draw the correct number of chromosomes of the nucleus shown in Diagram 2.1, as they would appear

(i) after mitosis (ii) after meiosis

Nucleus of a cell produced after division by mitosis

Nucleus of a cell produced after division by meiosis

Diagram 2.2 [4 marks]

1 Diagram 1.1 shows part of the stages of meiosis in an animal cell.

Diagram 1.1

Stage K

Meiosis I Meiosis II

Stage L Stage M Stage N Stage O

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The chromosomal behavior during stage N is not shown.

(a) Name the structure labelled P. [1 mark]

(b) Diagram 1.2 shows process X which takes place during stage K.

Diagram 1.2

(i) Draw the chromosomes at the end of process X.

[1 mark]

(ii) Name process X. State one importance of process X to an organism. [2 marks]

(c) (i) In Diagram 1.1, complete the diagram to show the chromosomal behaviour during stage N. [1 mark]

(ii) Explain the behaviour of chromosomes during stage N. [1 mark]

(d) Cancer cells are formed after normal cells are exposed to several factors.

(i) Explain the formation of cancer cells. [2 marks]

(ii) State two factors that cause the formation of cancer cells. [2 marks]

(iii) State two ways to prevent the development of cancer cells. [2 marks]

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Diagram 3.2 [2 marks]

4 Diagram 4.1 shows two cells, X and Y, undergoing cell division.

cell X cell Y

P

Q

Diagram 4.1

(a) (i) Name the structures labelled P and Q. [2 marks] (ii) State the stages of division of cells X and Y. [2 marks]

(b) If cell X undergoes three consecutive cell divisions, how many daughter cells are produced? [1 mark]

(c) (i) Cell Y undergoes the first nuclear division. Complete Diagram 4.2 to show the chromo-somes in the daughter cells produced.

[2 marks]

F4/28Diagram 4.2

(ii) State the number of chromosomes in each daughter cell. [1 mark]

(iii) State one organ where cell Y can be found. [1 mark]

(d) Diagram 2.3 shows two homologous chromosomes and the loci of two genes.

Q Q q q

R Rr r

Diagram 2.3

If crossing over occurs between the allele Q and allele q, and between the alleles R and r, complete Diagram 2.4 to show four possible gametes formed at the end of meiosis.

Diagram 2.4

[4 marks]

3 Diagram 3.1 shows three stages of meiosis, K, L and M, in an animal cell.

K L M

Diagram 3.1

(a) Name the stages K, L and M in Diagram 3.1. [3 marks]

(b) Explain what happens at stage M. [2 marks]

(c) State the chromosomal behaviour at the following stages:

(i) stage K (ii) stage L [2 marks]

(d) Explain the role of mitosis in the cloning technique. [3 marks]

(e) Diagram 3.2 shows a cell at a certain phase. If chromosome P is not separated, draw the diagrams of the two daughter cells which will be formed in the next phase in the space provided.

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5 (a) Diagram 5.1 shows the process of mitosis.

Diagram 5.1

Explain the significance of mitosis. [4 marks]

(b) Explain the similarities and differences between mitosis and meiosis. [6 marks]

(c) Diagram 5.2 shows a tissue culture technique used to clone or propagate carrot plants.

plantlet

explantexplant inculture medium

callus

somaticembryo

somaticembryo

Diagram 5.2

Based on Diagram 5.2, explain how the process is carried out.

Explain the advantages of using this method of reproduction compared to growing plants from seeds to a fruit grower. [10 marks]

6 (a) Explain the principles used in the cloning technique. [3 marks]

(b) Diagram 6 shows how animal cloning is carried out.

Step 4

Step 5

Step 6Step 7

Step 1

Step 3

Step 2egg

egg fusedwith cell

embryo

surrogate mother

somatic cell

offspring P

white-faced sheep

black-faced sheep

Diagram 6

Based on Diagram 6, explain how the cloning of offspring P is carried out. [7 marks]

(c) Discuss the advantages and disadvantages of the cloning technique to mankind. [10 marks]

(iv) If the number of chromosomes in a somatic cell of an insect is 14, what is the number of chromosomes in the daughter cells produced at the end of the type of cell division shown by cell Y ? [1 mark]

(d) Explain how radiation can stop the growth of cancer cells. [2 marks]

(e) A farmer plans to produce a large number of bananas within a short period of time.

(i) Suggest the best technique that can be employed by the farmer.

(ii) State one aspect that must be considered before he chooses to use this technique.

[2 marks]

Essay Questions

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