Case Study Questions Class 11 Biology Chapter 10 Cell Cycle and Cell Division
CBSE Class 11 Case Study Questions Biology Cell Cycle and Cell Division. Important Case Study Questions for Class 11 Board Exam Students. Here we have arranged some Important Case Base Questions for students who are searching for Paragraph Based Questions Cell Cycle and Cell Division.
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CBSE Case Study Questions Class 11 Biology Cell Cycle and Cell Division
CASE 1
Cell division is a very important process in all living organisms. During the division of a cell, cell division, DNA replication, and cell growth, hence, have to take place in a coordinated way to ensure correct division and formation of progeny cells containing intact genomes. The sequence of events by which a cell duplicates its genome, synthesises the other constituents of the cell and eventually divides into two daughter cells is termed cell cycle. Although cell growth (in terms of cytoplasmic increase) is a continuous process, DNA synthesis occurs only during one specific stage in the cell cycle. The replicated chromosomes (DNA) are then distributed to daughter nuclei by a complex series of events during cell division. These events are themselves under genetic control.
The cell cycle is divided into two basic phases: Interphase and M Phase (Mitosis phase). The M Phase represents the phase when the actual cell division or mitosis occurs and the interphase represents the phase between two successive M phases.
The M Phase starts with the nuclear division, corresponding to the separation of daughter chromosomes (karyokinesis) and usually ends with division of cytoplasm (cytokinesis). The interphase, though called the resting phase, is the time during which the cell is preparing for division by undergoing both cell growth and DNA replication in an orderly manner. The interphase is divided into three further phases:
- G1 phase (Gap 1)
- S phase (Synthesis)
- G2 phase (Gap 2)
G1 phase corresponds to the interval between mitosis and initiation of DNA replication. During G1 phase the cell is metabolically active and continuously grows but does not replicate its DNA. S or synthesis phase marks the period during which DNA synthesis or replication takes place. During this time the amount of DNA per cell doubles. If the initial amount of DNA is denoted as 2C then it increases to 4C. However, there is no increase in the chromosome number; if the cell had diploid or 2n number of chromosomes at G1, even after S phase the number of chromosomes remains the same, i.e., 2n.
In animal cells, during the S phase, DNA replication begins in the nucleus, and the centriole duplicates in the cytoplasm. During the G2 phase, proteins are synthesised in preparation for mitosis while cell growth continues.
Cells in the adult animals do not appear to exhibit division (e.g., heart cells) and many other cells divide only occasionally, as needed to replace cells that have been lost because of injury or cell death. These cells that do not divide further exit G1 phase to enter an inactive stage called quiescent stage (G0) of the cell cycle. Cells in this stage remain metabolically active but no longer proliferate unless called on to do so depending on the requirement of the organism.
1.) ___________is the procedure in which cell nucleus division occurs through series of events and the daughter chromosome get separated into two daughter nuclei.
a) Cytolysis
b) Karyokinesis
c) Karyocytosis
d) Cytokinesis
2) _____________is the sequence of events through cell duplicates its genome, synthesises of cell constituent takes place and divides into daughter cells.
a) Karyokinesis
b) Cell-division cycle
c) Replication
d) Cytokinesis
3) Name the phase of cell cycle in which DNA replication occurs.
4) Define cytokinesis.
5) What is cell cycle?
Answer key
1) b
2) b
3) Synthesis phase (S phase) is the phase in cell cycle marks the period during which DNA replicates. During this time the amount of DNA per cell doubles.
4) Cytokinesis is the process of cell division, which divides the cytoplasm of a parental cell and leads to formation of two daughter cells.
5) Cell cycle is defined as, the series of events through cell duplicates its genome, synthesises of other cell constituent takes place and, divides into daughter cells.
The cell cycle is divided into two basic phases:
- Interphase
- M Phase (Mitosis phase).
The M Phase represents the phase when the actual cell division or mitosis occurs and the interphase represents the phase between two successive M phases.
CASE 2
Mitosis has been divided into four stages of nuclear division (karyokinesis), it is very essential to understand that cell division is a progressive process and very clear-cut lines cannot be drawn between various stages. Karyokinesis involves following four stages: Prophase, Metaphase, Anaphase and Telophase.
Prophase – which is the first stage of karyokinesis of mitosis follows the S and G2 phases of interphase. In the S and G2 phases the new DNA molecules formed are not distinct but intertwined. Prophase is marked by the initiation of condensation of chromosomal material. The chromosomal material becomes untangled during the process of chromatin condensation. The centrosome, which had undergone duplication during S phase of interphase, now begins to move towards opposite poles of the cell. The completion of prophase can thus be marked by the following characteristic events:
- Chromosomal material condenses to form compact mitotic chromosomes. Chromosomes are seen to be composed of two chromatids attached together at the centromere.
- Centrosome which had undergone duplication during interphase, begins to move towards opposite poles of the cell. Each centrosome radiates out microtubules called asters. The two asters together with spindle fibres forms mitotic apparatus.
Cells at the end of prophase, when viewed under the microscope, do not show golgi complexes, endoplasmic reticulum, nucleolus and the nuclear envelope.
Metaphase – The complete disintegration of the nuclear envelope marks the start of the second phase of mitosis, hence the chromosomes are spread through the cytoplasm of the cell. By this stage, condensation of chromosomes is completed and they can be observed clearly under the microscope. This then, is the stage at which morphology of chromosomes is most easily studied. At this stage, metaphase chromosome is made up of two sister chromatids, which are held together by the centromere. Small disc-shaped structures at the surface of the centromeres are called kinetochores. These structures serve as the sites of attachment of spindle fibres (formed by the spindle fibres) to the chromosomes that are moved into position at the centre of the cell. Hence, the metaphase is characterised by all the chromosomes coming to lie at the equator with one chromatid of each chromosome connected by its kinetochore to spindle fibres from one pole and its sister chromatid connected by its kinetochore to spindle fibres from the opposite pole. The plane of alignment of the chromosomes at metaphase is referred to as the metaphase plate. The key features of metaphase are:
- Spindle fibres attach to kinetochores of chromosomes.
Chromosomes are moved to spindle equator and get aligned along metaphase plate through spindle fibres to both poles
1.) ____________ is the initial stage of karyokinesis
a) Anaphase
b) Metaphase
c) Prophase
d) Telophase
2.) Prophase is characterised by ______________________________
a) Condensation of chromosomes
b) Condensation of chromatin into chromosomes
c) Replication of DNA
d) Both a and b
3.) What is metaphase plate?
4.) Define kinetochores
5.) Give the characteristics events which indicates the completion of prophase.
6.) What are the stages involved in karyokinesis?
Answer key
1.) c
2.) d
3.) In metaphase, the microtubules start pulling the chromosomes with equal force and the chromosome ends up in the middle of the cell. This region is known as the metaphase plate. Thus, each cell gets an entire functioning genome.
4.) Kinetochores are Small disc-shaped structures at the surface of the centromeres, which serve as the sites of attachment of spindle fibres to the chromosomes that are moved into position at the centre of the cell.
5.) The completion of prophase is marked by the following characteristic events:
-
- Chromosomal material condenses to form compact mitotic chromosomes. Chromosomes are seen to be composed of two chromatids attached together at the centromere.
- Centrosome which had undergone duplication during interphase, begins to move towards opposite poles of the cell. Each centrosome radiates out microtubules called asters. The two asters together with spindle fibres forms mitotic apparatus.
6.) Karyokinesis involves following four stages:
- Prophase
- Metaphase
- Anaphase
- Telophase
CASE 3
Anaphase – At the onset of anaphase, each chromosome arranged at the metaphase plate is split simultaneously and the two daughter chromatids, now referred to as daughter chromosomes of the future daughter nuclei, begin their migration towards the two opposite poles. As each chromosome moves away from the equatorial plate, the centromere of each chromosome remains directed towards the pole and hence at the leading edge, with the arms of the chromosome trailing behind. Thus, anaphase stage is characterised by the following key events:
- Centromeres split and chromatids separate.
- Chromatids move to opposite poles.
Telophase –At the beginning of the final stage of karyokinesis, i.e., telophase, the chromosomes that have reached their respective poles de-condense and lose their individuality. The individual chromosomes can no longer be seen and each set of chromatin material tends to collect at each of the two poles. This is the stage which shows the following key events:
- Chromosomes cluster at opposite spindle poles and their identity is lost as discrete elements.
- Nuclear envelope develops around the chromosome clusters at each pole forming two daughter nuclei.
- Nucleolus, Golgi complex and ER reform.
1.) Anaphase is characterised by ______________________________
a) Separation of chromatids
b) Condensation of chromatin
c) Splitting of Centromeres
d) Both a and c
2.) In anaphase, chromatids migrates to _________________
a) Centromeres
b) Opposite poles
c) Similar poles
d) Both b and c
3.) Name the phase in which de-condensation of chromosomes takes place.
4.) Enlist the characteristic events that takes place during the telophase.
5.) Write the characteristic events that takes place in anaphase?
Answer key
1) d
2) b
3) In telophase, the chromosomes reached their respective poles and de-condense. In this phase chromosomes lose their individuality.
4) Characteristic events that takes place during the telophase.
-
- Chromosomes cluster at opposite spindle poles and their identity is lost as discrete elements.
- Nuclear envelope develops around the chromosome clusters at each pole forming two daughter nuclei.
- Nucleolus, Golgi complex and ER reform.
5) Anaphase stage is characterised by the following key events:
-
- Centromeres split and chromatids separate.
- Chromatids move to opposite poles.
CLASS 4
The production of offspring by sexual reproduction includes the fusion of two gametes, each with a complete haploid set of chromosomes. Gametes are formed from specialised diploid cells. This specialised kind of cell division that reduces the chromosome number by half results in the production of haploid daughter cells. This kind of division is called meiosis. Meiosis ensures the production of haploid phase in the life cycle of sexually reproducing organisms whereas fertilisation restores the diploid phase. We come across meiosis during gametogenesis in plants and animals. This leads to the formation of haploid gametes. The key features of meiosis are as follows:
- Meiosis involves two sequential cycles of nuclear and cell division called meiosis I and meiosis II but only a single cycle of DNA replication.
- Meiosis I is initiated after the parental chromosomes have replicated to produce identical sister chromatids at the S phase.
- Meiosis involves pairing of homologous chromosomes and recombination between non-sister chromatids of homologous chromosomes.
- Four haploid cells are formed at the end of meiosis II. Meiotic events can be grouped under the following phases:
Meiosis 1 –Prophase 1 – Metaphase 1 – Anaphase 1 – Telophase 1
Meiosis 1 – Prophase 2 – Metaphase 2 – Anaphase 2 – Telophase 2
1.) Progeny or offspring produced by fusion of two gametes, each gametes contains _________ set of chromosomes.
a) Diploid
b) Haploid
c) Triploid
d) Both a and b
2.) Identify incorrect statement
Statement 1 – Gametes are formed from diploid cells.
Statement 2 – Sexual reproduction includes the fusion of two gametes.
Statement 3 – Meiosis across during gametogenesis in plants and animals.
Statement 4 – Meiosis involves three sequential cycles i.e. prophase, anaphase, telophase.
1.) Only 1
2.) Both 3 & 4
3.) Only 4
4.) None of the above
3.) Define meiosis.
4.) Enlist the cycles involved meiosis.
5.) Explain the key features of meiosis? Name the Meiotic events.
Answer key
1.) b
2.) c
3.) Cell division that reduces the chromosome number by half results in the production of haploid daughter cells. This kind of division is called as meiosis.
4) Meiosis involves two sequential cycles of
Nuclear division calledmeiosis 1
Cell division called & meiosis 2.
5) The key features of meiosis are as follows:
- Meiosis involves two sequential cycles of nuclear and cell division called meiosis I and meiosis II but only a single cycle of DNA replication.
- Meiosis I is initiated after the parental chromosomes have replicated to produce identical sister chromatids at the S phase.
- Meiosis involves pairing of homologous chromosomes and recombination between non-sister chromatids of homologous chromosomes.
- Four haploid cells are formed at the end of meiosis II. Meiotic events can be grouped under the following phases:
Meiosis 1 – Prophase 1 – Metaphase 1 – Anaphase 1 – Telophase 1
Meiosis 1 – Prophase 2 – Metaphase 2 – Anaphase 2 – Telophase 2
CASE 5
Prophase of the first meiotic division is typically longer and more complex when compared to prophase of mitosis. It has been further subdivided into the five phases based on chromosomal behaviour, i.e., Leptotene, Zygotene, Pachytene, Diplotene and Diakinesis.
During leptotene stage the chromosomes become gradually visible under the light microscope. The compaction of chromosomes continues throughout leptotene. This is followed by the second stage of prophase I called zygotene.
During zygotene this stage chromosomes start pairing together and this process of association is called synapsis. Such paired chromosomes are called homologous chromosomes. Electron micrographs of this stage indicate that chromosome synapsis is accompanied by the formation of complex structure called synaptonemal complex.
. During pachytene this stage, the four chromatids of each bivalent chromosomes becomes distinct and clearly appears as tetrads. This stage is characterised by the appearance of recombination nodules, the sites at which crossing over occurs between non-sister chromatids of the homologous chromosomes. Crossing over is the exchange of genetic material between two homologous chromosomes. Crossing over is also an enzyme-mediated process and the enzyme involved is called recombinase. Crossing over leads to recombination of genetic material on the two chromosomes. Recombination between homologous chromosomes is completed by the end of pachytene, leaving the chromosomes linked at the sites of crossing over.
The beginning of diplotene is recognised by the dissolution of the synaptonemal complex and the tendency of the recombined homologous chromosomes of the bivalents to separate from each other except at the sites of crossovers. These X-shaped structures, are called chiasmata. In oocytes of some vertebrates, diplotene can last for months or years.
The final stage of meiotic prophase I is diakinesis. This is marked by terminalisation of chiasmata. During this phase the chromosomes are fully condensed and the meiotic spindle is assembled to prepare the homologous chromosomes for separation. By the end of diakinesis, the nucleolus disappears and the nuclear envelope also breaks down. Diakinesis represents transition to metaphase.
1.) In______________ phase chromatids of bivalent chromosomes becomes distinct and clearly appears as tetrads.
a) Leptotene
b) Zygotene
c) Pachytene
d) Diplotene
2.) Pachytene stage is characterised by ______________________________
a) Condensation of chromosomes
b) Recombination nodules
c) Separation of chromatin
d) Replication of DNA
3.) What is Chiasmata?
4.) Name the sites at which crossing over occurs between non-sister chromatids of the homologous chromosomes.
5.) What is synaptonemal complex? Name the phase in which it get dissolved.
Answer key
1.) c
2.) b
3.) Chiasmata is the X-shaped structure formed at diplotene stage, due to the point of contact between paired chromatids during meiosis.
4.) Recombination nodules is the sites at which crossing over occurs between non-sister chromatids of the homologous chromosomes. Crossing over is the exchange of genetic material between two homologous chromosomes. Crossing over is also an enzyme-mediated process and the enzyme involved is called recombinase.
5) In zygotene phase chromosomes start pairing together and this process of association is called synapsis. Such paired chromosomes are called homologous chromosomes. Electron micrographs of this stage indicate that chromosome synapsis is accompanied by the formation of complex structure called synaptonemal complex. At the beginning of diplotene dissolution of the synaptonemal complex take place.