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FORM 4 THEME: Investigating the Cell as a Basic Unit of Living Things
CHAPTER
5
Cell Division
SPM Topical Analysis Year
2007
Paper
1
Section Number of questions
2
2
2008 3
A
B
1
–
–
1 3
2009
2
3
A
B
1
–
–
1 2
2010
2
3
A
B
–
–
–
1 2
2
2011 3
A
B
1
–
–
1
2
3
3
A
B
1
–
–
ONCEPT MAP CELL DIVISION Cell cycle
G1, S, G2 phases (interphase)
M stage Mitosis
Meiosis Differences
Definition
Definition Importance
Importance
Stages of meiosis
Stages of mitosis Prophase
Application: • Tissue culture • Cloning
Metaphase
Anaphase
The effects of controlled and uncontrolled cell division
Telophase
Meiosis I
Meiosis II
Prophase I
Prophase II
Metaphase I
Metaphase II
Anaphase I
Anaphase II
Telophase I
Telophase II
Cytokinesis
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COMPANION WEBSITE
Learning Objectives
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.
The necessity for the production of new cells in living organisms 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 meaning and significance of mitosis Mitosis is the process of nuclear division which results in the formation of two genetically identical daughter nuclei. The 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
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The types of cells that undergo mitosis
Mitosis
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5.1
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5
Chromosomes and chromosomal number 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. Cell Division
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 SPM are called diploid cells (for example, somatic ’09/P2 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).
Photograph 5.1 The human karyotype consists of a total of 46 chromosomes arranged in matching pairs
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. 110
Refer Form 5, Chapter 5, Unit 5.3
What is a chromosome?
DNA replication
one chromatid
the chromosome condenses
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.
duplicated chromosome in a condensed state centromere
DNA double helix
sister chromatids
Each duplicated chromosome contains two identical DNA double helices. Each sister chromatid contains a DNA double helix.
Sister chromatids separate and become independent daughter chromosomes during anaphase. Each chromatid carries an identical DNA double helix.
Figure 5.1 Chromosome duplication and condensation
1 A chromosome consists of DNA molecule and protein.
7 Each DNA double helix is contained within a sister chromatid. Hence, the two sister chromatids contain identical copies of DNA molecules.
2 DNA carries the genetic material that organisms inherit from their parents.
8 During mitosis, the two sister chromatids separate and each becomes an independent daughter chromosome.
3 A DNA molecule consists of hundreds or thousands of genes.
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.
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.
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A chromosome which consists of a DNA double helix.
5
chromosome duplication
F O R M 4
4
CHAPTER
F O R M
5
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.
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.
THE CELL C
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.
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.
Photograph 5.2 A cell at interphase Two identical DNA double helices
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• • • •
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. SPM ’11/P1
CHAPTER
5
G1 (growth phase 1)
SPM ’05/P1
CYCLE
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 identical copies of the chromosome’s DNA molecule.
G2 (growth phase 2) • 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. Figure 5.2 The cell cycle consists of G1, S, G2, mitosis and cytokinesis
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CHAPTER
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5
The processes of mitosis and cytokinesis
SPM ’09/P2
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: PROPHASE
SPM ’10/P1
METAPHASE
SPM ’11/P1
spindle fibres nucleolus • The chromosomes condense and become tightly coiled. The chromosomes become shorter, thicker chromosome and visible under a centrioles light microscope. centromere • Each chromosome Figure 5.3(a) Prophase 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.
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centromere
metaphase plate
SPM ’09/P1
spindle fibres
sister chromatids
Figure 5.3(b) Metaphase
• 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.
5 CHAPTER
ANAPHASE
TELOPHASE
nuclear membrane
cleavage furrow
pole
daughter chromosomes
Figure 5.3(c) Anaphase
• 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
nucleolus
Figure 5.3(d) Telophase
• Telophase begins when both sets of chromo somes 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 re-forms in each nucleus. • The process of mitosis is now complete.
Cell Division
F O R M 4
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.
Cytokinesis 1 Following mitosis, the cytoplasm of the cell divides through a process called cytokinesis to form two daughter cells, each having one nucleus.
4
CHAPTER
F O R M
5
Cytokinesis in animal cells
cleavage furrow
1 Actin filaments in the cytoplasm contract to pull a ring of the plasma membrane inwards, forming a groove called the cleavage furrow.
2 The cleavage furrow pinches at the equator of the cell.
3 The cleavage furrow deepens progressively until the cell separates into two daughter cells.
Figure 5.4 Cytokinesis in an animal cell
Photograph 5.3 The formation of a cleavage furrow in an animal cell
Cytokinesis in plant cells 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. cell wall
cell plate
newly formed cell wall
vesicles
1 • Membrane-enclosed vesicles collect at the equator between the two nuclei. • The vesicles join to form a cell plate.
2 • 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. Figure 5.5 Cytokinesis in a plant cell
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3 • Eventually, the cell plate divides the cell into two daughter cells. • Cellulose fibres are produced by the cells to strengthen the new cell walls.
The importance of controlled mitosis 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 information 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, development 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.
The effects of uncontrolled mitosis
SPM ’05/P2
(a) Normal cells
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 compounds (cancercausing com pounds) such as formaldehyde Table 5.1 The differences between normal cells and cancer cells
Normal cells
SPM ’08/P2
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).
Cancer cells
Controlled growth
Uncontrolled growth
A single organised layer
Multi-layered and disorganised
Cells are differentiated and carry out specialised functions.
Cells are undifferentiated and do not have specialised functions.
The nuclei and number of chromosomes are normal.
The nuclei and number of chromosomes are abnormal.
The application of knowledge of mitosis in cloning The knowledge of mitosis is applied in cloning and the tissue culture technique. 117
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Figure 5.6 A layer of normal cells that divide uncontrollably to become a tumour
5
(b) A tumour
F O R M 4
Cloning
4
5 CHAPTER
F O R M
genetic content and chromosomal number as the parent organism. This is a common characteristic 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.
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
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.
SPM ’07/P1
Dolly, which is a clone of a somatic cell donor parent, was born in July 1996.
1 Somatic cells (from the mammary gland cells) are removed and grown in a low culture medium. The starved cells stop dividing and enter a non-dividing phase.
2 An unfertilised egg cell is obtained. The nucleus is sucked out, leaving the cytoplasm and organelles without any chromosomes. 5 The embryo is then implanted into a surrogate mother (the same breed of sheep as the ovum donor sheep).
3 An electric pulse stimulates the fusion between the somatic cell and the egg cell without nucleus. 4 The cell divides repeatedly, forming an embryo.
Dolly, the cloned sheep of the somatic cell donor, is born.
Figure 5.7 The cloning of Dolly Cell Division
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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 protoplasm. 7 In Malaysia, the tissue culture technique is used to propagate plants such as oil palm, rubber trees, orchids and tomatoes.
CHAPTER
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.
5
Tissue culture technique
How is the tissue culture technique carried out? SPM
’06/P2 1 • Small pieces of a plant’s leaf, shoot, bud, stem or root tissues are cut out. • These cut out plant tissues are called explants.
2 • 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. 3 • 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. 4 • The explants or protoplasts begin to divide by mitosis. • Cell division produces aggregates of cells. • The aggregate of cells develop into a callus; an undifferentiated mass of tissue. 5 • 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. 119
SPM ’11/P1
isolated cells
1
2
Explant
Protoplasts
3 Explant/protoplasts
in a culture medium
4 aggregates of cells
callus
5 plantlet
somatic embryo
Figure 5.8
Cell Division
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F O R M
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 engineering, 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.
(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 bacteria’s 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 bacteria’s 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. 3 (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.
Advantages of cloning 1 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.
4 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.
2 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. Cell Division
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5 (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.
Disadvantages of cloning 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:
1 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.
4 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.
5 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.
2 The long-term 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.
6 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.
3 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.
7 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.
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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.
CHAPTER
6 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.
F O R M 4
5.1 1 Give the definition of mitosis.
3 Describe the process that takes place during the S phase.
2 State two reasons why mitosis is important in living organisms.
4
5 CHAPTER
F O R M
5.2
4 State two advantages of applying the tissue culture technique.
1
Meiosis
The diagram shows a cell at one particular stage of mitosis.
The significance of meiosis 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.
Which cell is produced by the cell division? A B C D
Comments The 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
Table 5.2 Diploid chromosomal number of some organisms
The necessity for the production of haploid gametes
Chromosomal Organism 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
Cell Division
SPM Clone
’09
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 SPM are called haploid cells (n) because they ’04/P1 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) 122
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).
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!
chromatin
The types of cells that undergo meiosis
nucleolus
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.
Figure 5.9 Interphase
2
The process of meiosis
SPM Clone
’08
The diagram shows a pair of homologous chromosomes during prophase I of meiosis.
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.
P
What is P? A Synapsis B Chiasma C Bivalent D 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 non-sister chromatids exchange segments of DNA. • The point at which segments of chromatids cross over is called a chiasma. Answer B
The stages of meiosis 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 chromo somes as before. 4 Chromosomes are not condensed and therefore are not visible under the microscope. 123
Cell Division
CHAPTER
centrosomes (with centriole pairs)
5
nuclear envelope
F O R M 4
Meiosis I: Separates homologous chromosomes
visible. 4 • 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 four-part structure called a tetrad. A tetrad consists of two homologous chromosomes, each of which is made up of two sister chromatids. • Non-sister chromatids exchange 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.
Cell Division
Prophase I
Metaphase I
Anaphase I
Telophase I And Cytokinesis
SPM ’08/P2
F • The chromosomes begin O to condense. They become R shorter, thicker M and clearly
CHAPTER
CHAPTER
4
5
Prophase I F O R M
SPM ’10/P2
chiasmata
sister chromatids remain attached
spindle fibre
cleavage furrow
sister chromatids
metaphase plate
Metaphase I
homologous chromosomes aligned at the metaphase plate
centrioles
homologous chromosomes separate and pulled to the opposite poles
SPM ’04/P1
• 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.
Anaphase I
SPM ’11/P1
• 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.
124
Anaphase II sister chromatids separate
T elophase II A nd C ytokinesis F O R M 4
nuclear membrane
5
Metaphase II
CHAPTER
Prophase II
SPM ’08/P2
CHAPTER
Meiosis II: Separates sister chromatids
haploid daughter cells forming
two haploid daughter cells
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 replication 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.
four haploid daughter cells
Prophase II • The nuclear membrane disintegrates. • The spindle fibres re-form 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.
125
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 re-form. • 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.
Cell Division
F O R M 4
The differences and similarity between mitosis and meiosis Mitosis
Meiosis Similarity
The process of cell division in which DNA replicates only once. Differences
CHAPTER
4
5
Mitosis F O R M
Aspects/events
Meiosis
All somatic cells
Type of cell
Cells in the reproductive organs
Produces new cells for growth and repair
Role
Produces gametes for sexual reproduction
Pairing of homologous chromosomes (synapsis) does not occur.
Synapsis
Homologous chromosomes pair up (synapsis) to form bivalents.
Crossing over between non-sister chromatids does not occur during prophase.
Crossing over
Crossing over between non-sister chromatids occurs 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) 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.
Cell Division
Chromosomal number of the daughter cells
Haploid (n) or half the number of chromosomes of the parent cell.
Genetic content
Different from the parent cell and from one another.
Genetic variation
There is genetic variation from one generation to the next.
126
haploid gametes (n = 23) ovum (n) sperm (n)
Meiosis Fertilisation ovary
testis
Mitosis and development diploid zygote (2n = 46) multicellular diploid adults (2n = 46)
Figure 5.10 The human life cycle
5.2 1 State two differences between meiosis I and meiosis II.
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.
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. Refer Form 5, Chapter 6, Unit 6.2
127
Cell Division
CHAPTER
(b) During metaphase I, each pair of homologous chromosomes is arranged inde pendently and randomly (independent assortment) at the metaphase plate of the cell. The paternal or maternal chromo somes or homologues 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.
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.
5
The importance of meiosis
F O R M 4
formed would later become abnormal. For example, Down’s 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.
Appreciating the Movement of Chromosomes during Mitosis and Meiosis
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 F duces offspring that are identical to the O R reproduction through meiosis parent; sexual M produces genetic variability in the offspring. 4 Both processes are regulated in a precise 4 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 CHAPTER
4
CHAPTER
F O R M
5
5.3
(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.
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.
Cell Division
Refer Form 5, Chapter 5, Unit 5.2
128
(b) Meiosis II: (i) Prophase II – Spindle fibres re-form. (ii) Metaphase II – Individual chromosomes are positioned 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.
5.1
4
Mitosis
1 Stages K, L, M and N in Diagram 1 occur during mitosis in a cell. SPM Clone
’08
CHAPTER
Multiple-choice Questions
CHAPTER
F O R M
5
5 4 Diagram 4 shows the process of SPM cloning a sheep. Clone ’07
K
L
M
N
Diagram 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
ovum
diplod cell
2 Diagram 2 shows a type of cell division. embryo
surrogate mother
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
offspring Z
Diagram 4
3 Diagram 3 shows the phases in the cell cycle. SPM Clone
Prophase
’06
Anaphase
P
Q
Which of the following is the offspring Z? A C
Diagram 3
Which statements about the chromosomes at stages P and Q are correct?
Stage P
Stage Q
A Each chromosome consists of two sister chromatids.
The homologous chromosomes form pairs of bivalents.
B The chromosomes condense and become tightly coiled.
The sister chromatids separate and move to the opposite poles.
C The chromosomes duplicate to form sister chromatids.
The chromosomes are long and not visible.
D The chromosomes line up at the metaphase plate.
The chromosomes reach the opposite poles of the cell.
129
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? Cell Division
F O R M 4
Gamete Somatic Embryonic cells cells cells 12 12 12 A 6 12 6 B 6 12 12 C 12 6 12 D
10 At which stages of the cell cycle do these events occur?
6 G1, M, G2 and S are the phases of SPM Clone a cell cycle in an organism. S
5 CHAPTER
4
G2
Prophase
Prophase
Anaphase
Interphase
Anaphase
Metaphase
C
Interphase
Prophase
Anaphase
D
Interphase
Interphase
Anaphase
M
Human cells
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
A
G1
Diagram 5
Chromosomal
A Red blood cells
number 0
B Ova
23
C Intestinal cells
46
D Skin cells
8 The diploid chromosomal number SPM Clone (2n) of an animal is 42. If one of ’08 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
12 Diagram 7 shows a cell at metaphase during mitosis.
Cell Division
A
B
D 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.
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 SPM responsible for the synthesis of Clone ’11 DNA? A G1 C S B G2 D M
5.2
’09
Diagram 6
After mitosis After meiosis
C
9 Diagram 6 shows an animal cell SPM Clone undergoing mitosis.
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?
23
• 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
Division of centromere
B
11 Which of these human cells do not have the correct chromosomal number?
’05
F O R M
Breakdown of nuclear membrane
DNA replication
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
130
Meiosis
17 Which sequence of meiosis I is SPM correct? Clone ’11 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
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
21 Diagram 10 shows a sequence of SPM stages during meiosis. Clone ’04
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.
19 Which diagram represents metaphase I? A
D 20 Diagram 9 shows the different SPM stages of meiosis in a diploid cell, Clone ’07 2n = 4.
I
II
III
IV Diagram 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
22 If an insect species has a diploid SPM number of chromosomes, Clone ’04 2n = 12, in each of its nuclei, which is true?
III
Number Number of of nuclear chromosomes division in gametes during IV after meiosis 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
131
CHAPTER
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
C
II
5
Diagram 10
B
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 III B 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 II B III and IV C I, II and IV D II, III and IV
Cell Division
F O R M 4
Structured Questions 1 Diagram 1.1 shows part of the stages of meiosis in an animal cell. SPM Clone
CHAPTER
4
CHAPTER
F O R M
5
’08
Stage K
F O R M
Stage L
Stage M
Meiosis I
Stage N
Stage O
Meiosis II Diagram 1.1
4
The chromosomal behavior during stage N is not shown. (a) Name the structure labelled P.
2 Diagram 2.1 shows the nucleus of an animal cell.
[1 mark]
nuclear membrane
(b) Diagram 1.2 shows process X which takes place during stage K.
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]
Diagram 1.2
(i) Draw the chromosomes at the end of process X.
(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,
[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]
draw the correct number of chromosomes of the nucleus shown in Diagram 2.1, as they would appear (i) after mitosis (ii) after meiosis
(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]
Cell Division
Nucleus of a cell produced after division by mitosis
132
Diagram 2.2
Nucleus of a cell produced after division by meiosis [4 marks]
(d) Diagram
2.3 shows two homologous chromosomes and the loci of two genes. Q
Q
q
q
r
r
R
R
4 Diagram 4.1 shows two cells, X and Y, undergoing cell 4 division. SPM Clone ’05
Q P
Diagram 2.4
cell X
[4 marks]
cell Y
Diagram 4.1
(a) (i) Name the structures labelled P and Q. [2 marks]
3 Diagram 3.1 shows three stages of meiosis, K, L and SPM M, in an animal cell. Clone
(ii) State the stages of division of cells X and Y.
[2 marks]
’07
(b) If cell X undergoes three consecutive cell divisions, how many daughter cells are produced? [1 mark]
K
L
(c) (i) Cell Y undergoes the first nuclear division. Complete Diagram 4.2 to show the chromosomes in the daughter cells produced. [2 marks]
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]
Diagram 4.2 F4/28
(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.
(ii) State the number of chromosomes in each daughter cell. [1 mark] (iii) State one organ where cell Y can be found.
133
[1 mark] Cell Division
CHAPTER
F O Diagram 3.2 R [2 marks] M
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.
CHAPTER
5
Diagram 2.3 F O R M 4
(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]
(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]
(d) Explain how radiation can stop the growth of cancer cells. [2 marks]
Essay Questions
4
5
5 (a) Diagram 5.1 shows the process of mitosis.
CHAPTER
F O R M
Diagram 5.1
Explain the significance of mitosis. [4 marks]
6 (a) Explain the principles used in the cloning technique. [3 marks]
(b) Explain the similarities and differences between mitosis and meiosis. [6 marks]
(b) Diagram 6 shows how animal cloning is carried out.
(c) Diagram 5.2 shows a tissue culture technique used to clone or propagate carrot plants.
Step 1
egg
Step 2
black-faced sheep
explant
Step 4
explant in culture medium
egg fused with cell
Step 3 white-faced sheep callus
somatic cell
embryo
Step 5
surrogate mother somatic embryo
plantlet
offspring P
somatic embryo
Step 6
Step 7
Diagram 5.2
Diagram 6
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]
(c) Discuss the advantages and disadvantages of the cloning technique to mankind. [10 marks]
Cell Division
134
Based on Diagram 6, explain how the cloning of offspring P is carried out. [7 marks]
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