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What is the advantage of not having cytokinesis after karyokinesis?

What is the advantage of not having cytokinesis after karyokinesis?



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In nature most of the cells are mononucleated, but in some cases like osteoclasts, umbrella cells of transitional epithelium, tapetal cells of another etc. are multinucleated. Wikipedia article about tapetum says that it is a strategy to increase the production of proteins. Why don't cells just up-regulate their genes for more production? What is the benefit of not having cytokinesis?


The absence of cytokinesis allows the two nuclei to remain in the same cell. If a cell has 2 nuclei instead of 1, it can transcribe DNA to mRNA at double the maximum speed, and so produce the desired proteins at double the rate. It's almost as if the cells become tetraploid, or hexaploid, and so on.

Learn more: http://www.nature.com/scitable/definition/cytokinesis-100

Why is it more desirable to have 1 cell with 2 nuclei than actually do cytokinesis and have 2 cells with 1 nucleus each (still 2 total)?

Because it is unfavorable to create two cells, which needs an extra investment in energy and nutrients to maintain, and is also harder to coordinate, and so it becomes more desirable to just have one single cell with the capacity of producing double the proteins.

Then why don't all cells adopt this strategy? What is the reason for such partiality?

First, because most cells don't need the extra transcription speed, as they don't produce a lot of proteins. Second, because of cell differentiation - to have cells doing different things they have to transcribe their genetic material differently, and so must be separate. Third, a lot of times it is desirable to have extra cells. For example, skin epithelial cells: their main purpose is to serve as a barrier or protection, and so there has to be a lot of them. That's why the basal cells are in constant mitosis.


Cytokinesis

Cytokinesis is a mechanical process during which a cell undergoes major mechanical deformation. Thus, an in-depth understanding of the effects of mechanical signals on cell mechanics and biochemistry is required. A diverse tool set is available to study cell mechanics during cytokinesis, allowing characterization of various mechanical parameters. While micropipette aspiration (MPA) studies are used to determine the elastic modulus and effective cortical tension, atomic force microscopy (AFM) measures the bending modulus. The elastic modulus quantifies the deformability of the cell surface, and the cortical tension is a complex parameter that measures the energy cost per unit increase in cell surface area. The bending modulus reflects the stress required for bending a material. During cytokinesis, the initial deformation of a roughly spherical cell requires deviation from its quasi-steady state. This is resisted by the cortical surface tension, which favors a spherical cell. However, as the furrow continues to ingress, the curvature in the cleavage furrow changes so that the Laplace pressure eventually favors bridge thinning and abscission. Laser tracking microrheology (LTM) can be used to measure cortical viscoelasticity noninvasively. Viscoelasticity represents the time-dependent cellular response to stresses and affects the kinetics of furrow ingression by dampening the mechanical deformation, thereby allowing sufficient time for activation and stabilization of biochemical factors.

Treatment with actin depolymerizing drugs such as Latrunculin-A has established that the actin cytoskeleton is the main contributor of cell mechanics, though the cell membrane and microtubules also make some contribution. The actin cytoskeleton also undergoes remodeling with the application of internal or external mechanical stresses. The impact of mechanical stresses has been uncovered using micropipette aspiration, which allows the application of an external stress on the cell, similar in magnitude to stresses generated internally during cytokinesis. Many mechanosensitive proteins such as myosin II, which localize to the cleavage furrow cortex, also accumulate at sites where mechanical stress has been applied.

In contrast to the mechanical activation of biochemical reactions, the mechanical properties of the cell can be controlled biochemically. Knockdown of some actin cross-linkers softens the cell cortex significantly, leading to altered furrow ingression kinetics and a reduced ability to perform cytokinesis in suspension culture (where cell–substrate adhesion is absent). Interestingly, the overall deformability of the furrow is lower than the polar cortex, even though furrow undergoes major deformation during cytokinesis, which is attributed to a differential cortical distribution of mechanosensitive proteins during cytokinesis. This further illustrates the intricate interplay between biochemical and mechanical pathways during cytokinesis.


The Forms of DNA

Except when a eukaryotic cell divides, its nuclear DNA exists as a grainy material called chromatin. Only when a cell is about to divide and its DNA has replicated does DNA condense and coil into the familiar X-shaped form of a chromosome, like the one shown in Figure (PageIndex<2>). Because DNA has already replicated, each chromosome actually consists of two identical copies. The two copies of a chromosome are called sister chromatids. Sister chromatids are joined together at a region called a centromere.

Figure (PageIndex<2>): Chromosome. After DNA replicates, it forms X-shaped chromosomes like the one shown here. 1. Chromatid, 2. Centromere, 3. short arm, 4. long arm. Centromere contains proteins called kinetochores (not shown) where spindles attach during mitosis.

The process in which the nucleus of a eukaryotic cell divides is called mitosis. During mitosis, the two sister chromatids that make up each chromosome separate from each other and move to opposite poles of the cell. Mitosis occurs in four phases. The phases are called prophase, metaphase, anaphase, and telophase. They are shown in Figure (PageIndex<3>) and described in detail below.

Prophase

Figure (PageIndex<4>): Prophase in later stage is called prometaphase. The spindle starts to form during the prophase of mitosis. The spindles start to attach to the Kinetochores of centromeres of sister chromatids during Prometaphase.

The first and longest phase of mitosis is prophase. During prophase, chromatin condenses into chromosomes, and the nuclear envelope (the membrane surrounding the nucleus) breaks down. In animal cells, the centrioles near the nucleus begin to separate and move to opposite poles of the cell. Centrioles are small organelles found only in eukaryotic cells that help ensure the new cells that form after cell division each contain a complete set of chromosomes. As the centrioles move apart, a spindle starts to form between them. The blue spindle, shown in Figure (PageIndex<4>), consists of fibers made of microtubules.

Metaphase

During metaphase, spindle fibers fully attach to the centromere of each pair of sister chromatids. As you can see in Figure (PageIndex<5>), the sister chromatids line up at the equator, or center, of the cell. The spindle fibers ensure that sister chromatids will separate and go to different daughter cells when the cell divides. Some spindles do not attach with the centromeres of chromosomes, rather, they attach with each other and grow longer. The elongation of spindles not attached to the centromeres. They elongate the whole cell. This is visible in the figure below:

Figure (PageIndex<5>): Chromosomes, consisting of sister chromatids, line up at the equator or middle of the cell during metaphase. The blue lines are spindles, and the orange rectangles at the cell poles are centrioles. Some spindles from the opposing centrioles attach with each other, and some spindles attach to the kinetochores of the sister chromosomes from their respective sides. Each chromosome is attached to two spindles.

Anaphase

During anaphase, sister chromatids separate and the centromeres divide. The sister chromatids are pulled apart by the shortening of the spindle fibers. This is a little like reeling in a fish by shortening the fishing line. One sister chromatid moves to one pole of the cell, and the other sister chromatid moves to the opposite pole (see Figure (PageIndex<6>)). At the end of anaphase, each pole of the cell has a complete set of chromosomes

Figure (PageIndex<6>): Anaphase: Sister chromatids break apart and move to the opposite pole with the help of spindles. The newly separated sister chromatids are called chromosomes now.

Telophase

The chromosomes reach the opposite poles and begin to decondense (unravel), relaxing once again into a stretched-out chromatin configuration. The mitotic spindles are depolymerized into tubulin monomers that will be used to assemble cytoskeletal components for each daughter cell. Nuclear envelopes form around the chromosomes, and nucleosomes appear within the nuclear area (see Figure (PageIndex<7>).

Figure (PageIndex<7>): Telophase: The chromosomes decondense, spindles start to disappear, two nuclei form in a cell.


Solving the Cell Cycle and Cell Division Multiple Choice Questions of Class 11 Biology Chapter 10 MCQ can be of extreme help as you will be aware of all the concepts. These MCQ Questions on Cell Cycle and Cell Division Class 11 with answers pave for a quick revision of the Chapter thereby helping you to enhance subject knowledge. Have a glance at the MCQ of Chapter 10 Biology Class 11 and cross-check your answers during preparation.

I. Select the correct answer from the following Questions:

Question 1.
Life starts from a single cell in plants and animals called
(a) Cell
(b) Zygote
(c) Tissue
(d) Growth

Question 2.
A typical eukaryotic cell cycle is illustrated by human cells in culture, which divide approximately every:
(a) 12 hours
(b) 10 hours
(c) 24 hours
(d) 6 hours

Question 3.
Yeast cell can progress through all the four stages of the cell cycle in only about:
(a) 60 minutes
(b) 90 minutes
(c) 30 minutes
(d) 45 minutes.

Question 4.
The interphase is divided into.
(a) G1 phase (Gap1)
(b) S phase (Synthesis)
(c) G2 phase (Gap2)
(d) ail of these stages.

Answer: (d) All of these stages.

Question 5.
The S phase marks the period during which replication of DNA takes place. It is during this time that the content of DNA doubles, from
(a) 2C to 4C
(b) 4C to 2C
(c) (1n or 2n)
(d) (2n or 1n)

Question 6.
The centrioles, in animal cells, initiate their replication in the cytoplasm during.
(a) G1 phase
(b) G2 phase
(c) S phase
(d) None of these phases.

Question 7.
In plants apical cells and the cambium tissue continue to divide all their life, they are called.
(a) Meristemic tissue
(b) cambium tissue
(c) equational division
(d) syneytium

Answer: (a) Meristemic tissue.

Question 8.
Mitosis is divided into
(a) Prophase
(b) Metaphase
(c) Anaphase
(d) Telophase
(e) All of these phases.

Answer: (e) All of these phases.

Question 9.
The small disc shaped structure at the surface of centromeres is called.
(a) Kinetochores
(b) sister chromatids
(c) microtubule
(d) Golgi complex

Question 10.
Mitosis accomplishes the segregation of duplicated chromosomes into daughter nuclei (karyokinesis), but the cell itself is divided into two daughter cells by a separate process called.
(a) Cytokinesis
(b) Karyokinesis
(c) Nucleolous
(d) Chromosome clusters.

Question 11.
In some organisms karyokinesis is not followed by cytokinesis as a result of which multinucleate condition arises which is called:
(a) Syncytium
(b) Meiosis I
(c) Cell-plate
(d) Meiosis II

Question 12.
The cells having more than two complete sets of chromosomes are called
(a) Diploid
(b) Haploid
(c) Polyhybrid
(d) Polyploid.

Question 13.
In Meiosis, the chromatids separate during
(a) Metaphase I
(b) Anaphase I
(c) Anaphase II
(d) Metaphase II

Question 14.
In the meiotic cell division four daughter ceils are produced by two successive division in which
(a) First division is reductional and second is equationai.
(b) First division is equationai, second is reductional.
(c) Both division are equationai.
(d) Both division are reductional.

Answer: (a) First division is reductional and second is equationai.

Question 15.
Meosis is
(a) Reductional division
(b) Equationai division
(c) Multiplicational division
(d) Disjunctional division.

Answer: (a) Reductional division.

Question 16.
The term meiosis was coined by
(a) Blackman
(b) Flemming
(c) Robertson
(d) Former and Moore.

Answer: (d) Former and Moore.

Question 17.
Chromosomes counting is best done during
(a) Metaphase
(b) Telophase
(c) Late prophase
(d) Late anaphase.

Question 18.
Meisosis II bring about
(a) Sepration of chromatids
(b) Separation of homologous chromosomes.
(c) Synthesis of DNA and centromere
(d) Separation of sex chromosomes.

Answer: (a) Sepration of chromatids

Question 19.
In which stage the chromosomes appear as thin long thread?
(a) Leptotene
(b) Zygotene
(c) Prophase
(d) Pachytene.

Question 20.
Anastral mitosis is found in
(a) All living organisms
(b) Lower animals.
(c) Higher plants
(d) Higher animals.

Question 1.
Meiosis ends with telophase II, in which the …………… are once again enclosed by a nuclear envelope, cytokinesis follows, resulting in the formation of tetrad of cells i.e., four haploid ……………

Answer: chromosomes, daughter cells

Question 2.
Anaphase begins with the simultaneous splitting of the ………….. which hold the sister chromatids together, allowing them to move toward …………….

Answer: centromeres, opposite poles of the cell

Question 3.
Metaphase II the chromosomes align on the equator with micro¬tubules from opposite poles of the spindle get attached to the …………. of sister chromatids.

Question 4.
Prophase II meiosis II initiates immediately after ………….. usually before the …………. have fully elongated.

Answer: cytokinesis, chromosomes

Question 5.
The stage between the two meiotic divisions is called ………….. and is generally short lived.

Question 6.
Diplotene X-shaped structures are called ……………

Question 7.
The complex formed by a pair of synapsed homologous chromosomes is called a …………. or a tetrad.

Question 8.
Zygotene is the second stage of prophase I during which certain chromosomes start pairing together and this process of association is called ……………

Question 9.
Meiosis involves two sequential cycles of nuclear and cell division, called ………… and ………….. but only a single cycle of DNA replication.

Answer: meiosis I, Meiosis II

Question 10.
M phase is the most dramatic period of the cell cycle, involing a major recoganization of virtually all cell components. Since the chromosome number (ploidy) of parent and progeny cell is the same it is also called as ………….

Answer: equational division

III. Mark the statement true (T) or false (F)

Question 1.
All organisms, even the largest, start their life from a single cell.

Question 2.
Growth and reproduction are characteristic of cells, indeed of all living organisms.

Question 3.
Cell division is a very important process in all organisms.

Question 4.
The requence of events by which a cell duplicates its genome, synthesies the other constituent of the cell and eventually divides into two daughter cells is termed cell cycle.

Question 5.
Yeast for example, can progress through the cell cycle in only about 24 hours.

Question 6.
The cell cycle is divided into two basic phases:
(1) M phase (mitosis phase)
(2) Interphase.

Question 7.
The 24 hour overage duration of cell cycle of a human cell, cell division proper lasts only about an hour. Hence, 95% of the progression of cell cycle is spent in interphase the period between two successive mitosis or cell division.

Question 8.
Interphase though called 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.

Question 9.
Prophase which is the second stage of mitosis follows the S and G2 phases of interphase.

Question 10.
In an animal cell this is achieved by the appearance of a furrow in the plasma membrane. The furrow gradually deepens and ultimately joins in the centre dividing the cell cytoplasm into two.

IV. Match the item of column I with the items of column II

Column I Column II
(a) Mitosis is divided 1. Quiescent stage (Gg) of the cell cycle.
(b) Resting phase 2. at metaphase is referred to as the metaphase plate.
(c) G1 phase to enter meristematically 3. Prophase, Metaphase, Anaphase Telophase.
(d) Prophase which is the first stage 4. (1) centromeres split and chromatids separate
(2) Chromatids move to opposite poles.
(e) The plane of alignment of the chromosomes 5. at the end of meiosis II.
(f) Anaphase stage is characterised by the key events. 6. Interphase
(g) Four haploid cells are formed 7. called meiosis I and meiosis II
(h) Meiosis involves two sequential cycles of nuclear and cell division, 8. of mitosis follows the S and S2 phases of interphase.
(i) Zygotene 9. second stage of prophase I.
(j) Crossing over is also an enzyme mediated process and the enzyme involved 10. is called recombinase.
(k) Diakinesis 11. This is the final stage of meiotic prophasel, marked by terminalisation of chaismata.
(l) Metaphase I 12. Meiosis ends with telophase II, in which the two groups of chromosomes once again get enclosed.
(m) Telophase I 13. It begins with the simultaneous splitting of the centromeres of each chromosome.
(n) Ananaphase II 14. The bivalent chromosomes align the equitorial plate.
(o) Telophase II 15. The nuclear membrane reap-pears, cytokinesis follows and this is called as diad of cells.
Answer

Column I Column II
(a) Mitosis is divided 3. Prophase, Metaphase, Anaphase Telophase.
(b) Resting phase 6. Interphase
(c) G1 phase to enter meristematically 1. Quiescent stage (Gg) of the cell cycle.
(d) Prophase which is the first stage 8. of mitosis follows the S and S2 phases of interphase.
(e) The plane of alignment of the chromosomes 2. at metaphase is referred to as the metaphase plate.
(f) Anaphase stage is characterised by the key events. 4. (1) centromeres split and chromatids separate
(2) Chromatids move to opposite poles.
(g) Four haploid cells are formed 5. at the end of meiosis II.
(h) Meiosis involves two sequential cycles of nuclear and cell division, 7. called meiosis I and meiosis II
(i) Zygotene 9. second stage of prophase I.
(j) Crossing over is also an enzyme mediated process and the enzyme involved 10. is called recombinase.
(k) Diakinesis 11. This is the final stage of meiotic prophasel, marked by terminalisation of chaismata.
(l) Metaphase I 14. The bivalent chromosomes align the equitorial plate.
(m) Telophase I 15. The nuclear membrane reap-pears, cytokinesis follows and this is called as diad of cells.
(n) Ananaphase II 12. Meiosis ends with telophase II, in which the two groups of chromosomes once again get enclosed.
(o) Telophase II 13. It begins with the simultaneous splitting of the centromeres of each chromosome.

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Section Summary

In both prokaryotic and eukaryotic cell division, the genomic DNA is replicated and then each copy is allocated into a daughter cell. In addition, the cytoplasmic contents are divided evenly and distributed to the new cells. However, there are many differences between prokaryotic and eukaryotic cell division. Bacteria have a single, circular DNA chromosome but no nucleus. Therefore, mitosis is not necessary in bacterial cell division. Bacterial cytokinesis is directed by a ring composed of a protein called FtsZ. Ingrowth of membrane and cell wall material from the periphery of the cells results in the formation of a septum that eventually constructs the separate cell walls of the daughter cells.


The Mitosis Type of Cell Division | Cell Biology

The division of the cell is initiated by the division of the nucleus. In the ordinary method of division a nucleus passes through many stages, and the whole complicated process is known as mitosis. The details of mitosis were worked out in the later part of the nineteenth century by W. Flaming and others. Commonly this type of cell division is found in the vegetative parts of the plant body. In the process of mitosis prior to the cell division, the number of chromosomes is always duplicated.

Image Courtesy : medical-institution.com/wp-content/uploads/2013/04/Mitosis_diagram.jpg

For example, if a corn plant possesses 20 chromosomes in the somatic cell, then prior to each cell division the 20 chromosomes are duplicated, then the division takes place resulting in the formation of two daughter cells each containing 20 chromosomes again.

The different stages of mitosis may easily be recognised in the root apices of onion. From the study point of view, the process of mitosis may be differentiated into two main phases — The karyokinesis and the cytokinesis. The actual division of the nucleus is known as karyokinesis whereas the division of the cytoplasm of the cell is called cytokinesis.

Thus, during the process of mitosis the nucleus undergoes several changes which may easily be studied in the onion root apex by special cytological techniques. The chief function of mitosis seems to be to divide all parts of the chromatin equally between the two daughter nuclei. The important phases of mitosis are — prophase, metaphase, anaphase and telophase.

Prophase:

In the resting nucleus the chromatin is spread out as a reticulum. It is actually composed of a number of separate units, the chromosomes. The number of chromosomes in the nuclei is definite in different species. Gradually the chromosomes become thick and condensed and each of them splits lengthwise forming two chromatids.

The chromatids remain coiled around each other throughout their length. Gradually, they become much more thick and smooth. The chromatids coil around each other spirally and each chromosome itself remains surrounded by a membrane.

In well fixed chromosomes some unstained gaps or constrictions are seen they are the attachment regions, called centromeres. The nucleoli lose their staining power and disappear completely. The nucleus then rapidly passes into the next stage, the metaphase, through a complicated series of changes.

Metaphase:

The nuclear membrane disappears and simultaneously a new structure, the spindle, appears in the cytoplasm, which chemically, consists of long chain protein molecules oriented longitudinally between two poles. The fibres of the spindle, however, are really fine tubules, not just protein threads.

Chemical analysis of the cells has indicated that approximately 15 per cent of the cytoplasmic proteins go into its make-up. Once the spindle is formed, the chromosomes move through the cytoplasm to it, and become fastened by their centromeres to a region midway between the poles called the equator of the spindle, a position of apparent equilibrium. The centromere of each chromosome always contacts the spindle at the equator the arms of the chromosomes, not being so restricted, are randomly oriented.

The centromere is the organ of movement. Without it, a chromosome cannot orient on the spindle, and the chromatids cannot separate from each other later. The position of centromere is visible in a chromosome during metaphase by a constriction, and since the position of the constriction is characteristic for each chromosome, the centromere divides the chromosome into two arms of varying lengths. Very few chromosomes have strictly terminal centromeres.

Anaphase:

Anaphase follows metaphase. At the end of the metaphase the centromeres of each pair of chromatids appear to repel each other. The centromeres now divide so that each chromatids has its own centromere they then move apart from each other to initiate a slow movement that will take sister chromatids to opposite poles. Termination of anaphase movement occurs when the chromosomes form a densely packed group at the two poles.

Telophase:

As soon as the chromosomes reach the poles, they collect into a more or less solid-appearing mass. This marks the beginning of telophase. The mass of chromosomes gradually converts into a nucleus. A new nuclear membrane forms. Spindle gradually disappears.

The formation and enlargement of the spaces containing nucleoplasm continue until the chromosomes again become scattered in the form of a network typical of the resting stage. As the mass of chromosomes becomes more and more spread on by the formation of nucleoplasm a new nucleolus makes its appearance. The newly formed nucleus contains the same number of chromosomes, as this was in parent nucleus.

Cytokinesis:

Just after the nuclear division, the division of cytoplasm takes place which is known as cytokinesis. The cytokinesis takes place in two ways. According to one method, much of the cellulose is being deposited in the centre of the cell, and the cell is resulted. This method is known as cell plate method. According to other method after the formation of young nuclei, a furrow develops in the cytoplasm and the cytoplasm is being divided into two equal parts, thus completing the cytokinesis.

Duplication of DNA and its transfer to daughter cells:

With the result of mitotic cell division one parental cell gives rise to two daughter cells, and this process continues indefinitely. The newly formed daughter cells behave in the similar way as their parent cells. This shows that the daughter cells bearing the DNA molecule of one type, and they are also similar in quantity. As we know, the DNA molecules consist of two spirally coiled threads. This model of DNA is known as double helix DNA.

During cell division, because of the presence of weak hydrogen bonds the threads of DNA helix separate from each other. In prophase stage of mitosis each chromosome splits into two chromatids. One of the DNA threads goes to one chromatids and the other to another chromatids.

All the chemical substances that give rise to the new thread of DNA are found in the protoplasm of the daughter cell. The newly formed thread coils around the old DNA thread and forms the double helix of DNA. The newly developed DNA is similar to that of original DNA of parental nucleus. By this process, the DNA molecules reach in the same quantity to each of the daughter cells.

Significance of mitosis:

With the result of mitosis, the chromosomes split lengthwise into two chromatids. Each chromatid bears all those characteristics which were present in mother chromosome. In other words, with the result of mitosis, two identical cells have the same genetic constitution, qualitatively and quantitatively, as the parental cell from which they arose.

Thus, the maintenance of the genetic integrity of the cell population and ultimately of the organism and its dependent depends upon the mechanism of mitosis. This process has been proved to be beneficial to vegetative reproduction. In the similar way, the characters of the plants grown by vegetative reproduction may be preserved for long time.


Mammalian Myocardial Regeneration

Bin Zhou , . William T. Pu , in Muscle , 2012

Normal Myocardial Growth and Cell Cycle Activity

The fetal myocardium grows by cardiomyocyte proliferation. Post-natally, cardiomyocyte cell division largely stopped by 3 days after birth in rats (7) . Subsequently, cardiomyocyte number was constant, but cardiomyocyte volume increased 2.5-fold between day 3 and 12, indicating that post-natal myocardial growth occurs primarily by increasing cardiomyocyte size.

One hallmark of cellular proliferation is DNA synthesis, and therefore cardiomyocyte DNA synthesis has been exhaustively studied (reviewed in ( 8 )). In rodents, intense DNA synthesis peaked at post-natal day 10 (P10) and declined to adult levels by P20 (7,9) . Between P3 and P12, cardiomyocytes no longer underwent cell division (cytokinesis) but continued to synthesize DNA and to undergo nuclear division ( karyokinesis ), a form of endoreduplication known as acytokinetic mitosis. As a result, by day 12 cardiomyocytes reached their adult level of binucleation of 90% (7,10) . Acytokinetic mitosis was associated with formation of stable, highly ordered and functional sarcomeres, suggesting that the organized contractile apparatus impairs cytoplasmic division (11,12) .

In addition to increased ploidity from multinucleation, cardiomyocytes can become polyploid through DNA synthesis in the absence of karyokinesis, a form of endoreduplication known as endocycling or endomitosis. The extent of endocycling was measured in mouse by FACS sorting of cardiomyocyte nuclei (13) . Murine fetal cardiomyocyte nuclei were mononuclear and diploid. At birth, only 65% of cardiomyocyte nuclei remained diploid, with most of the remaining nuclei being tetraploid. The fraction of diploid nuclei reached a stable level of

55% by P21 (13) , corresponding to the time by which intense DNA synthesis had halted (7) . Importantly, endoreduplication is stimulated by cardiomyocyte stress and activation of specific signaling pathways (14,15) , making it essential for experiments that address upregulation of cell cycle markers in myocardial injury models to demonstrate productive formation of new cardiomyocytes rather than more limited forms of cell cycle activity.

In the human heart, cardiomyocyte growth similarly changes from hyperplasia to hypertrophy in infancy, although the timing of this transition is less clearly defined (16) . From infancy to adulthood, the number of cardiomyocytes in the normal human heart was constant (17,18) . However, unlike myocytes from mice, rats, dogs, and pigs, mononucleated cells predominated and binucleated cells were in the minority (77% and 22%, respectively) (18) . This proportion did not change with age or ischemic or hypertrophic heart disease (18) . Polyploidization through endocycling continued in humans up to 10 years of age, considerably longer than observed in rodents (19) . As with rodents, myocardial injury was observed to stimulate endomitosis and to increase cardiomyocyte ploidity (20) .

The newborn heart also grows through expansion of the non-myocyte compartment. In mice, fetal and neonatal myocardium contains few non-myocytes. Post-natally, the non-myocyte fraction expands rapidly from 13% on post-natal day 1 (P1) to 80% at P20 (10) . In adult mice, the non-myocyte cell number fraction is

85%. This expansion involves fibroblast expansion as well as rapid growth of the vascular bed, which increases by more than four-fold during post-natal cardiac growth (17) .

A series of proteins promote or inhibit cell cycle progression ( Figure 39.1 , reviewed in ( 21 )). To study the mechanisms governing post-natal cardiomyocyte cell cycle exit, the expression of cell cycle regulators was investigated in human and rodent heart (reviewed in ( 11,22 )). Cell cycle regulators that promote cell cycle activity, including Cyclins A, B, D1/D2/D3, and cyclin-dependent kinases (CDKs) CDK1 (also known as CDC2) and CDK2, were highly expressed in fetal heart and markedly downregulated in adult heart (10,13,23–25) . The E2F family of transcription factors, pivotal regulators of the G1/S phase transition, were also markedly downregulated between neonatal and adult cardiomyocytes (26) . Activity of E2F factors is normally held in check by the pocket protein family, containing the retinoblastoma susceptibility gene (Rb) and its relatives p107 and p130. During hyperplastic heart growth, CDK2/CyclinE/CyclinA and CDK4/CyclinD complexes phosphorylate pocket proteins, releasing inhibition of E2F and driving cell cycle activity. During hypertrophic heart growth, downregulation of these kinases leads to pocket protein hypophosphorylation and inhibition of the cell cycle promoting activity of E2F (27,28) . The CDK inhibitors p21 and p27 act as important brakes on cell cycle activity by inhibiting Cdk2/CyclinE/CyclinA and Cdk4/CyclinD activity and thereby repressing E2F. p21 and p27 were markedly upregulated during the transition from hyperplastic to hypertrophic growth (25,29) . In summary, cardiomyocyte expression of cell cycle regulators is carefully regulated, so that during hyperplastic growth the profile of factors favors cell cycling. During hypertrophic growth, the profile is reversed, leading to cell cycle exit. Important future directions will uncover the molecular pathways that coordinately regulate the profile of cell cycle regulator expression.

Figure 39.1 . Cell cycle regulation.

Cell cycle genes that promote cycling are shown in green, and those that inhibit cycling are shown in red. RTK, receptor tyrosine kinase P13K, phosphoinositol-3-kinase. Symbols with solid blue outline have been overexpressed in mouse heart, while those with dashed blue outlines have been knocked out in mouse heart.

Based on changes in expression of cell cycle regulators during the transition from hyperplastic to hypertrophic cardiomyocyte growth, concerted efforts were made to promote adult cardiomyocyte cell cycle reentry by direct manipulation of cell cycle regulators ( Figure 39.1 ). Knockout of the cell cycle inhibitor p27 and the redundant pocket protein genes Rb and p107 increased heart size, cardiomyocyte number, and adult cardiomyocyte DNA synthesis (28,30) . Transgenic overexpression of SV40 T antigen robustly stimulated cardiomyocyte cell cycle reentry, but these mice showed extensive cardiac pathology and died before weaning (31) . Ectopic cardiomyocyte expression of the E2F family member E2F1 resulted in increased DNA synthesis, but unfortunately caused cardiomyocyte apoptosis and death (32) . Forced cardiomyocyte expression of Cyclin B and Cdk1 drove adult cardiomyocyte cell cycle reentry in cell culture (33) , although extension of this observation in vivo has not been reported. D-cyclins are regarded as sensors of the extracellular environment that link mitogenic pathways to the cell cycle machinery, and cyclins D1-3 are required for fetal cardiomyocyte proliferation (34) . Transgenic overexpression of cyclin D1, D2, or D3 promoted cardiomyocyte DNA synthesis and multinucleation without affecting the cardiomyocyte differentiation (24,35) . Cardiomyocyte-specific cyclin D2 overexpression increased the fraction of cardiomyocytes labeled by 3 H-thymidine by over

500-fold in adult heart. Immediately following experimental left anterior descending coronary artery (LAD) ligation, infarct size in cyclin D2 transgenic mice was not distinguishable from littermate controls. However, 2 and 6 months after infarction, infarct size was markedly smaller in transgenic mice, indicating substantial myocardial repair (35,36) . Likewise, transgenic cyclin A2 overexpression enhanced early post-natal cardiomyocyte cell cycle activity. Although this effect was not sustained in normal adult heart, myocardial infarction elicited new cardiomyocyte formation that improved ventricular function compared to controls (37) . The promising results from transgenic cyclin D2 and A2 mice provide proof of concept that driving cardiomyocyte cell cycle reentry may be a viable strategy for stimulating cardiac regeneration.


Biology - Different Stages of Mitosis

I'm studying for a biology test at the moment and I'm reading about the M Phase of the cell cycle but I can't really grasp where the boundaries between each phase lies.

I'm reading my notes aswell as wikipedia but neither make it clear which stage each process occurs in. For example my notes under the Metaphase heading say "The microtubules have now formed mature spindle fibres that attach to chromosomes via the kinetichore.

Did the spindle fibres occur in the prophase or do they occur in the metaphase. Also what I'm not sure about is whether cytokinesis is part of mitosis or is a separate process. My notes make the distinction between karyokinesis (nuclear division) and cytokinesis (cell division) but they list cytokinesis as one of the stages of mitosis. Wikipedia on the other hand defines cytokinesis as a process that occurs directly after mitosis. Which is it? Is cytokinesis a part of mitosis? If not then would karyokinesis basically encompass all of mitosis?


Triggering p53 after cytokinesis failure

Cells that fail to divide during cytokinesis often arrest in the next G1 phase by a mysterious mechanism that depends upon p53. What triggers this arrest is unclear. New studies, including a report in this issue (Uetake and Sluder, 2004) suggest that this arrest does not occur because cells are polyploid, are binucleate, have multiple centrosome, or have failed cytokinesis, making this phenomenon even more puzzling.

A hallmark of most cancer cells is that they are highly aneuploid, whereas most somatic cells have stable ploidy. Polyploidy has even been postulated to generate genetic instability (Lengauer et al., 1998). It is unclear if normal somatic cells maintain their ploidy simply by faithful mitotic segregation of their chromosomes or if they have mechanisms to detect aneuploidy and either correct this problem or block aneuploid cells from further division cycles. A growing body of work suggests that cells that fail to undergo cytokinesis activate a “tetraploid checkpoint” that arrests them in the following G1 in a p53-dependent manner. However, recent papers suggest that polyploidy per se cannot trigger the p53 network, and the in vivo relevance of this arrest is still unclear.

It is well established that p53 blocks cell cycle progression in cells that fail cytokinesis, as many researchers have independently generated polyploid cells that arrest in the following G1 (Fig. 1). The original observation of this phenomenon preceded the discovery of p53. Hirano and Kurimura (1974) found SV40-infected cells did not arrest in G1 when treated with cytochalasin, a drug that poisons actin and, hence, prevents contraction of the cytokinetic furrow (Fig. 1 B). It is now known that SV40 infection inactivates p53. Reid and colleagues (Cross et al., 1995) incubated mouse embryo fibroblasts (MEFs) in nocodazole or colcemid, two different microtubule-depolymerizing drugs, for 22 h, and found that wild-type MEFs arrested with 4N ploidy, but P53 −/− MEFs had rereplicated their chromosomes and become 8N (Cross et al., 1995). Further studies demonstrated that even though the cells were in nocodazole, the 4N cells did not arrest in mitosis but escaped the spindle checkpoint and arrested in the subsequent G1 phase in a state that had many hallmarks of a p53 checkpoint arrest induced by DNA damage (Fig. 1 C) (Lanni and Jacks, 1998 Minn et al., 1996). It is worth pointing out that these experiments were first seen in mouse cells that have a functional spindle checkpoint but cannot maintain the mitotic arrest in nocodazole for nearly as long as human cells. Margolis's group generated binucleate cells with dihydrocytochalasin B (Fig. 1 B) (Andreassen et al., 2001), and once again p53-positive cells arrested in the subsequent G1 phase whereas p53-minus cells rereplicated their DNA to become 8N. While exploring how overexpression of the oncogene Aurora A generated multiple centrosomes, Erich Nigg's group found that excess Aurora A expression blocked cytokinesis (Fig. 1 B) (Meraldi et al., 2002). They went on to show that these cells also arrested in the following G1 in a p53-dependent manner. Although it still has to be formally established, it is likely that a common mechanism is activating p53 after each of these treatments.

Since cancer cells often have extra chromosomes, it has been postulated that there is an initial event causing cancer cells to become polyploid and then reduced fidelity of chromosome segregation results in subsequent aneuploidy that drives the loss of heterozygosity of tumor suppressors. Thus, the notion that p53 blocks the progression to S-phase in the cells that are polyploid is satisfying, as it further explains the almost universal loss of the p53 pathway during cancer progression. However, deeper thinking suggests that “normal” somatic cells are often polyploid, and the initial models may be naïve. Polyploidy, both autopolyploidy and allopolyploidy, is common among higher (angiosperm) plants but relatively rare among animals and not restricted to any particular genus. Muller (1925) was the first to suggest that polyploidy is rare in animals because of the evolution of sex chromosomes and a chromosomal basis for sex determination. Importantly, there are polyploid animals. A variety of frogs and toads are tetraploid, most famous among them is Xenopus laevis. The brine shrimp (Artemia franciscana) is tetraploid, whereas the pine sawfly (Diprion similie) has diploid males but tetraploid females. Increased ploidy has also been reported in humans. Triploid and tetraploid fetuses often die and are aborted in the first trimester, but there are many cases of fetuses that survive to the third trimester and a small number of cases of tetraploid live births (Edwards et al., 1994 Nakamura et al., 2003). There are certain cell types in humans that are polyploid for example, megakaryocytes increase in ploidy as part of their differentiation (Queisser et al., 1971). Although it is possible that polyploid organisms and cells undergo adaptive events, these observations suggest that polyploidy per se is not lethal at the cellular level.

A report in this issue provides new insight into the cause of p53-dependent arrest. Uetake and Sluder found that transient treatment with very low concentrations of cytochalasin D can block cytokinesis to generate binucleate cells but cells treated this way did not arrest at G1 (Fig. 1 D) (Uetake and Sluder, 2004). Using video microscopy, they followed binucleate cells formed in these low cytochalasin D concentrations and showed that they underwent mitosis and another round of cytokinesis. The lack of the arrest was not caused by the loss of the p53 pathway, since the same cells arrested at the higher concentrations of cytochalasin D. Similarly, Wong and Stearns fused human diploid foreskin fibroblasts (which can also arrest as binucleates with high concentrations of cytochalasin) and showed that the resulting binucleate hybridomas entered S-phase without a prolonged arrest (Wong, C., and T. Stearns, personal communication). These simple experiments argue strongly that p53-dependent arrest is not triggered by binucleation, polyploidy, multiple centrosomes, or failure of cytokinesis.

What is triggering the p53 network in tetraploid cells has become the central enigma in this field. One clue comes from the observation that there may be some cell type specificity. Margolis's group originally used rat embryonic fibroblasts (Ref52 cells) (Andreassen et al., 2001) and Uetake and Sluder found that these cells arrested even at the lower concentrations of cytochalasin D that did not block S-phase progression in hTert-RPE1 cells or human primary foreskin fibroblasts. Interestingly, the arrest in Ref52 cells could be relieved by plating the cells on fibronectin rather than directly on glass (Uetake and Sluder, 2004). It is unclear why fibronectin suppresses the arrest, but it is interesting that the binding of integrins to fibronectin can regulate the actin and microtubule cytoskeleton. Perhaps the disruption of the cytoskeleton during a failed cytokinesis generates a “dead end” cytoskeletal complex that is activating p53 and the pathways downstream of integrins can resolve these cytoskeleton network problems.

To understand if this p53-dependent arrest actually prevents cancer progression, not only does the signal need to be determined but the conditions by which the arrest is normally triggered must be described. Most studies have used drugs to trigger the arrest, with one exception from Brian Reid's group who found an increase in ploidy specifically in p53 −/− mice. 25 d after birth, the pancreases of 53 −/− mice have ∼23% of 4N cells as compared with 7% in wild type. Moreover, in transgenic mice that blocked p53 and other proteins by expressing SV40 T-antigen under the elastase promoter the number of polyploid cells in the pancreas was >45% (Cross et al., 1995). This report of p53 preventing polyploidy in vivo suggests that this mysterious pathway may still have an important role in preventing cancer progression.


ICSE Solutions for Class 10 Biology – Cell Division

APlusTopper.com provides ICSE Solutions for Class 10 Biology Chapter 1 Cell Division for ICSE Board Examinations. We provide step by step Solutions for ICSE Biology Class 10 Solutions Pdf. You can download the Class 10 Biology ICSE Textbook Solutions with Free PDF download option.

Short Questions

Question 1: What is direct cell division ? Explain with an example.
Answer: Amitosis is the direct cell division. It is the simplest type of cell division in which there is no spindle formation or condensation of fibres. Nucleus is directly divided into two, e.g., bacteria.

Question 2: Name the two kinds of cell division found in living organisms.
Answer: Meiosis and Mitosis.

Question 3: What type of cell division does occur in somatic cells of the body ?
Answer: The mitotic cell division occurs in somatic cells of the body.

Question 4: Where does the meiosis occur in our body ?
Answer: In our body meiosis occurs in germ cells i.e. in gonads.

Question 5: What do you mean by cell-cycle ?
Answer: Every cell capable of cell division passes through different stages or phases in a cyclic maimer. It is called the cell cycle.

Question 6: Write the name of various steps of cell cycle.
Answer: Cell Cycle

Question 7: Name the structure which initiates cell division ?
Answer: Centriole (Centrosome).

Question 8: Why gametes have a haploid number of chromosomes ?
Answer: The gametes are produced as a result of meiosis hence they have haploid number of chromosomes.

Question 9: Mention three significant changes that occur in a cell during interphase.
Answer: The three significant changes that occur in a cell during interphase are:
(i) The cell grows in size.
(ii) New DNA is synthesized as per the old DNA templet.
(iii) Synthesis of RNA and protein takes place.

Question 10: What is cytokinesis ?
Answer: During cell division karyokinesis (division of nucleus) is followed by the division of cytoplasm. It is called cytokinesis. Or in other words cytokinesis is the division of cytoplasm.

Question 11: How does colchicine act as mitotic poison ? Is there any advantage of it ?
Answer: Colchicine is an alkaloid obtained from Autumn crocus (Colchicum autumnale). It inhibits the formation of mitotic spindle. As a result, chromosomes duplicate but they remain within the same cell, increasing in number (endoduplication). Such cells are called polyploid cells.
Its advantage is that, plant breeders have used colchicine-induced polyploidy as a means of producing variants of agricultural and horticultural crops.

Question 12: Explain the significance of mitosis.
Answer: (i) It helps to maintain linear heredity of an organism by keeping the chromosome number constant in daughter cells.
(ii) It helps in development of organism from zygotic stage to adult stage.
(iii) It is the means of repair and regeneration of cells.
(iv) Asexual reproduction is accomplished only through mitosis.
(v) Details of mitosis are similar in all organisms which emphasizes the unity of life.

Question 13: Why is meiosis referred to as reduction division ?
Answer: The meiosis is referred to as reduction division because the number of chromosomes in the daughter cells is half than that of the mother cell.

Question 14: What is the importance of meiosis in creating variations ?
Answer: During meiosis, the exchange of chromosomal material takes place between the non-sister chromatids forming new combinations. These new combinations give rise to variations which result in the evolution of species and even in the origin of new species.

Question 15: State how does meiosis maintain chromosome number in a species.
Answer: The gametes are formed by meiosis. During meiosis the number of chromosomes is reduced to half i.e. the gametes contain haploid number of chromosomes. The male and female gametes fuse to form a diploid zygote. In this way meiosis maintains chromosome number in a species.

Question 16: How prophase-I of meiosis differs from prophase of mitosis in an essential way ? Describe how it affects the daughter cells ?
Answer: Prophase-I of meiosis has five sub-stages namely Leptotene, Zygotene, Pachytene, Diplotene and Diakinesis. In pachytene exchange of genetic material between non-sister chromatids takes place through crossing over and chiasma formation which does not occur in prophase of mitosis. As a result, the daughter cells have a variation in their genetic composition contrary to identical daughter cells of mitosis.

Question 17: What is the importance of chiasma formation ?
Answer: Chiasma is the region where crossing-over takes place. By the formation of chiasma, exchange of genetic material between non-sister chromatids of the homologous chromosomes is accomplished. So, chiasma is the means of bringing about recombination of characters and thus variations in multicellular organisms.

Question 18: What is the importance of meiosis?
Answer: The meiosis is important to maintain the constant number of chromosomes in a species. It also brings about variations which result in the evolution or origin of new species.

Give Reasons

Question 1: The mitosis is called equational division.
Answer: Mitosis is called equational division because during mitosis the cell divides equally into two identical daughter cells.

Question 2: The meiosis is called reductional division.
Answer: The meiosis is called reductional cell division since the four daughter cells formed have half the number of chromosomes than the mother cell.

Question 3: Gametes must be produced by meiosis for sexual reproduction.
Answer: The number of chromosomes in sex cell is halved.

Question 4: Chromosomes are the carriers of heredity.
Answer: The chromosomes contain gene which carry specific features to the offsprings.

Differentiate

Question 1: Mitosis and Meiosis.
Answer:

Mitosis Meiosis
(i) It occurs in somatic cells. It occurs in generative cells.
(ii) It involves a single division resulting into two daughter cells. It involves two successive divisions resulting in the formation of four daughter nuclei.
(iii) Prophase is short and simple. Prophase is of longer duration and complex.
(iv) Number of chromosomes in daughter cells is equal to that of parent cell. Number of chromosomes in daughter cells is half to that of the mother cells.
(v) Equational division. Reductional division.
(vi) Mitosis brings about growth, repair and healing. Meiosis forms gametes and spores and maintains the chromosome number constant from generation to generation.

Question 2: Chromatin and Chromosome.
Answer:

Chromatin Chromosome
(i) Uncondensed form of nucleoprotein. Condensed form of nucleoprotein.
(ii) Seen in interphase stage of cell division. Seen in M-phase.
(iii) Control of metabolic activities. Vehicles of heredity.

Question 3: Centrifugal cytokinesis and Centripetal cytokinesis.
Answer:

Centrifugal cytokinesis Centripetal cytokinesis
During the partition of the cytoplasm following karyokinesis, when the cell plate formation begins in the centre and proceeds towards outwards, the division is said to be centrifugal. When the cell membrane starts constricting from the sides and proceeds inwards, till the mother cell is divided into two daughter cells, the division is known as centripetal cytokinesis.
All plant cells follow centrifugal cytokinesis by cell plate formation. All animal cells follow centripetal cytokinesis through cell furrow formation.

Question 4: Anaphase of Mitosis and Anaphase of Meiosis-I.
Answer:

Anaphase of mitosis Anaphase of meiosis-I
During this phase of mitosis the centromeres divide, the spindle fibres contract and move towards opposite poles, pulling the daughter chromosomes apart. With the contraction of microtubules of the spindle apparatus each homologous chromosome with its two chromatids and unbroken centromeres (unlike anaphase of mitosis) start moving towards the opposite poles of the cell.

Question 5: Gametic meiosis and Zygotic meiosis.
Answer:

Gametic meiosis Zygotic meiosis
When the reproductive cells of a diploid organism undergoes meiosis to produce haploid gametes, it is called gametic meiosis. Some algae and fungi are haploid adults. They produce haploid gametes which upon fertilization form a diploid zygote. This‘zygote undergoes meiosis to form haploid spores, which on repeated mitotic division form the adult body.

Question 6: Cytokinesis and Karyokinesis.
Answer:

Cytokinesis Karyokinesis
It is the division of cytoplasm. It is the division of nucleus.
It is followed by Karyokinesis. It is the first division.

Question 7: Chiasmata and crossing over.
Answer:

Chiasmata Crossing Over
It is the part of attachment of non-sister chromatids of homologous chromosomes where crossing over takes place. It is exchange of genetic material between non-sister chromatids of homologous chromosomes.

Question 8: Centrosome and centromere.
Answer:

Centrosome Centromere
It is an organelle of the animal cell. It is a non-stainable part of chromo-some at which two chromatids join.
It contains two centrioles which move towards the opposite poles and forms spindle fibres during cell division. It provides attachment of spindle fibres during cell division.

Question 9: Cytokinesis in plant and animal cell.
Answer:

Cytokinesis in plant cell Cytokinesis in animal cell
It starts with a plate formation. Plate formation is absent. A constriction forms in the middle of cell membrane.
It is centrifugal. It is centripetal.

Diagram Based Questions

Question 1: Identify the stages of mitosis given below and label the figures.

Answer:
(a) Anaphase, (b) Metaphase, (c) Telophase.
1. Centriole, 2. Spindle fibres, 3. Chromosomes, 4. Centromere. 5. Daughter nuclei.

Question 2: Identify the stages of meiosven below and label them.

Answer:
(a) Anaphase I, (b) Telophase I.

Question 3: The diagram below represents a certain stage of a cell.

(i) Is it an animal cell or a plant cell ? Give one reason in support of your answer.
(ii) Label the parts numbered 1 – 3.
(iii) Which stage (phase) of mitosis is represented in this diagram.
Answer: (i) It is a plant cell because it has cell wall.
(ii) 1. Chromatids 2. Spindle fibres 3. Centromere.
(iii) Anaphase.

Question 4: (i) Draw a neat labeled diagram to show the metaphase stage of mitosis in an animal cell having ‘6’ chromosome.
(ii) How many daughter cells are formed at the end of mitosis and at the end of meiosis?
(iii) With reference to cell division explain the following terms:
(Chromatid, Centromere, Haploid).
(iv) Name the type of cell division that occurs during:
1. Growth of shoot 2. Formation of pollen grains.
3. Repair of worn out tissues.
Answer: (i) See diagram.

(ii) Mitosis: two daughter cells.
Meiosis: four daughter oeils.
(iii) Chromatid: Duplicated chromosomes consist of two identical strands, each of these is called a chromatid.
Centromere: It is the point at which the two chromatids remain attached. It is also the point of attachment for spindles.
Haploid: A cell having only one set of chromosomes is called haploid.
(iv) 1. Mitosis
2. Meiosis
3. Mitosis

Question 5: The diagram below represents a stage during cell division. Study the same and then answer, the questions, that follow :

(i) Name the parts labelled 1, 2 and 3.
(ii) Identify the above stage and give a reason to support your answer.
(iii) Mentldh where in the body this type of cell division occurs.
(iv) Name the stage prior to this stage and draw a diagram to represent the same.
Answer: (i) 1. Centriole
2. Spindle fibres
3. Chromatid
(ii) Anaphase—The daughter chromosomes are reaching to the opposite poles of the cell.
(iii) In the somatic cells of the body.
(iv) Metaphase

Question 6: In the given diagram name the parts labeled 1, 2, 3,4 and 5 and describe about them in short.

Answer:
1. Pellicie: The matrix of chromosome is enclosed in a sheath called as pellicle.
2. Matrix: The chromatin of chromosome is embedded in the achromatic substance known as matrix.
3. Chromatin: Chromatin is the heredity material made-up of long fibres of DNA combined with proteins.
4. Centromere: A narrow constriction is seen in the chromosome at metaphase or anaphase is called primary constriction. The distinct area of light colour inside the primary constriction is called centromere.
5. Chromatids: Each metaphase chromosome consisi lied chromatids.

Question 7: Given below is a diagram representing a stage during mitotic cell division. Study it carefully and answer the questions that follow:

(i) Is it a plant cell or an animal cell? Give a reason to support your answer.
(ii) Identify the stage shown.
(iii) Name the stage that follows the one shown here. How is that stage identified?
(iv) How will you differentiate between mitosis and meiosis on the basis of the chromosome number in the daughter cells?
Answer: (i) It is plant cell, because centrosome is absent and spindle apparatus not connected to it
(ii) Prophase.
(iii) Metaphase: In this stage the chromosome lie in one plane at equator and gets attached to a spindle fibre by its centromere.
(iv) Mitosis: Same diploid number of chromosomes are present in the daughter cell.
Meiosis: Haploid number of chromosomes are present in the daughter cells.

Question 8: The figure below shows a certain stage of mitosis:

(i) Name the stage,
(ii) Label tie parti-4
(iii) How many chromosomes are shown here?
Answer: (i) Metaphase
(ii) 1. Chromatid
2. Centromere
3. Centriol
4. Spindle fibre

Sketch and Label the Diagram

Question 1: Give a labeled diagram to illustrate amitosis.
Answer:

Question 2: Draw a labeled schematic representation of mitosis cell division.
Answer:

Question 3: Draw a duplicate chromosome and label its part.
Answer:

Question 4: Draw a well labelled diagram to show the anaphase stage of mitosis in arplant cell having four chromosomes.
Answer:

Explain the Terms

Question:
1. Leptotene
2. Zygotene
3. Pachytene
4. Diplotene
5. Diakinesis
6. Cell division
7. Chromatids
8. Centromeres
9. Centrioles
10. Spindle
11. Cell Plate
12. Cleavage furrow
13. Chromosomes
14. Chromatin
Answer: 1. Leptotene: Th this step the chromosomes become visible as single threads.
2. Zygotene: Pairing of homologous chromosomes (synapsis) occur in this stage. Each pair is a bivalent.
3. Pachytene: The crossing-over begins at the end of this stage.
4. Diplotene: Crossing-over continues and two homologous chromosomes in each pair begin to separate. They are held together at chiasmata.
5. Diakinesis: In this stage nuclear membrane and nucleolus disappear. Spindle begins to be formed at the end of this stage.
6. Cell division: Process by which a cell divides into two new daughter cells.
7. Chromatids: Two identical parts of a chromosome called “sister” chromatids.
8. Centromeres: Part of a chromosome. Located near the middle of the chromatids. (Some lie at the ends)
9. Centrioles: Two tiny structures located in the cytoplasm near the nuclear envelope (membrane that surrounds the nucleus).
10. Spindle: A fanlike micròthbule structure that helps separate the chromosomes.
11. Cell plate: Structure that forms in plant cells when the cytoplasm divides during cytokinesis.
12. Cleavage furrow: Structure that forms in animal cells when the cytoplasm divides during cytokinesis.
13. Chromosomes: Made up of DNA. Carry genetic information.
14. Chromatin: Material in the nucleus that condenses during cell division to form chromosomes.

Name the Following

Question:
1. The process by which cell divides into two equal daughter cells.
2. The type of cell division present in unicellular organisms.
3. The two kinds of cell division found in living organisms.
4. Mitosis takes place in which cells.
5. Replacement of dead cells is accomplished by which process.
6. The kind of division normally seen at the tip of root and shoot system.
7. Microtubules forifTarbipolar spindle in which stage.
8. The structure responsible for initiating cell division in animal cells.
9. The part of the cell associated with heredity.
10. Process by which gametes are produced by. .
11. The process responsible for variation.
12. The kind of division takes place in the reproductive tissues.
13. The largest phase of a normal cell cycle.
14. The stage when chromosomes arrange at the equator.
15. Separation of sister chromatids takes place in which stage.
16. Stage in which the crossing-over takes place.
17. The point at which the explicated chromosomes are joined.
18. Name the stage during which nuclear membrane and nucleoide reappear.
19. ‘V’ shaped chromosome having the centromere at the centre.
20. Nuclear envelope and nucleoli reappear in which stage.
21. Result of uncontrolled cell division.
Answer:
1. Cell division
2. Amitosis
3. Mitosis, Meiosis
4. Somatic cells
5. Mitosis
6. Mitosis
7. Metaphase
8. Centrioles
9. Chromosome
10. Meiosis
11. Crossing-over
12. Meiosis
13. Prophase
14. Metaphase
15. Anaphase
16. pachytene
17. Centromere
18. Telophase
19. Metacentric
20. Telophase
21. Cancer

Give Technical Terms

Question:
1. The stage in mitosis when the nucleolus start disappearing.
2. The stage at which spindle fibres begin to be formed.
3. The shortest phase of mitosis.
4. The stage when sister chromosomes separate from their paired condition.
5. The period between two successive mitotic division.
6. Point at which two sister chromatids are held together.
7. The stage at which chromosomes occurs reach the opposite poles.
8. The process of cytoplasmic division.
9. Division of nucleus.
10. During cytokinesis when the cell plate begins in the centre and moves towards the wall.
11. The phase of the cell cycle during which the cell grows.
12. The phase of the cell cycle in which DNA replication takes place.
13. Division which brings about vegetative growth.
14. The largest phase of a normal cell cycle.
15. The stage at which progressive condensation and coiling of chromatin fibres.
16. The stage at which sydapsis in chromosomes to form bivalents.
17. The stage at which formation of chiasmata occurs. .
18. Crossing over occurs during thisjsubstage of meiosis.
19. The stage at meiosis at which there are two cells, each with sister chromatids aligned at the equator.
20. The phase usually skipped in meiosis.
21. The phase of meiosis at which homologous chromosomes are separated.
22. The process during which the meiosis occurs in human beings.
23. Period between Meiosis-I and Meiosis-II.
Answer:
1. Prophase
2. Late prophase or early Metaphase
3. Anaphase
4. Anaphase
5. Interphase
6. Centromere
7. Anaphase
8. Cytokinesis
9. Karyokinesis
10. Centrifugal
11. G1 phase
12. S phase
13. Mitosis
14. Interphase
15. Leptotene
16. Zygotene
17. Pachytene
18. Prophase I
19. Anaphase I
20. Telophase 1
21. Metaphase II
22. Gamete formation
23. Interkinesis

Fill in the Blanks

Complete the following sentences with appropriate words :
1. The type of cell division that occurs in apical meristem of plants is Mitosis .
2. Karyokinesis means splitting of nucleus.
3. The stage between Meiosis-I and Meiosis-II is called Interkinesis .
4. Colchicine arrests cell division at Metaphase .
5. Centromere is the point at which sister chromatids are held together.
6. The spindle fibres are made of Microtubules .
7. The pairing of homologous chromosomes is called Synapsis .
8. Chromosomes are Hereditary material.
9. Polytene chromosomes are found in Salivary glands of fly larvae.

True & False

Mention, if the following statements are True or False. If false rewrite the wrong statement in its correct form:
1. Somatic cells of a multicellular organisms arise from a single cell by mitosis. (True)
2. Mitosis results in four daughter cells. (False, meiosis results in four daughter cells)
3. Mitosis keeps the chromosome number constant through the generations. (False, meiosis keeps the chromosome number constant through the generations.)
4. Germ cells divide meiotically to produce gametes. (True)
5. The alkaloid coichicine inhibits formation of mitotic spindle. (True)
6. Asexual reproduction is accomplished through mitosis. (True)
7. Chromosomes other than sex-chromosomes are autonomous. (True)
8. Cytokinesis takes place through cleavage furrow in animal cells. (True)
9. Chromosomes are arranged in the form of chromatids at the equator in prophase. (False, chromosomes are arranged in the form of chromatids at the equator in metaphase.)
10. Chromosomes are the thickest and shortest in telophase. (False, chromosomes are thickest and shortest in anaphase.)
11. Meiosis is also called heterotypic division. (True)
12. Prophase of meiosis-I has five sub-stages. (True)
13. Meiosis leads to recombination of characters. (True)

State the Location

Name Location
Asters Around the centriole at each pole
Cell plate In the centre of the cell.
Chromosomes In nucleus, mitochondria and chloroplast.
Polytene chromosome In salivary glands of fly Larur.

State the Function

Write the functional activity of the following structures:

Name Function
Chromosome Heredity, i.e., transmission of characters from parents to offsprings.
Spindle fibres Support chromosomes at the time of cell division.
Chiasmata Crossing-over, in which genes are transferred from one part to another.
Colchicine. It inhibits the formation of mitotic spindle.

Choose the Odd One Out

1. Amitosis, Mitosis, Meiosis, Cell cycle. (Cell cycle)
2. Prophase, Metaphase, Anaphase, Telophase, Meiosis. (Meiosis)
3. Leptotene, Zygotene, Pachytene, Diplotene, Telophase. (Telophase)

Multiple Choice Questions

1. Cytokinesis is the division of:
(a) Cell (b) Cytoplasm
(c) Cell wall (d) Nucleus

2. Karyokinesis is the division of:
(a) Cytoplasm (b) Nucleus
(c) Celiwall (d) Pollen grains

3. Cell division occurring in somatic cells is:
(a) Mitosis (b) Meiosis
(c) Diplotene (d) Diakinesis

4. In meiotic cell division four daughter cells are produced by two successive divisions in which:
(a) First division is equational and second is reductional
(b) First division is reductional and second is equational
(c) Both divisions are reductional
(d) Both divisions are equational.

5. Duplication of DNA occurs in:
(a) G1-phase (b) G2-phase
(c) S-phase (d) M-phase

6. The nuclear membrane disappears in:
(a) Prophase (b) Anaphase
(c) Zygotene (d) Pachytene

7. How many chromosomes are found in a cell of human?
(a) 20 Pairs (b) 46
(c) 23 (d) 46 Pairs

8. The nuclear membrane and nucleolus become indistinguishable during:
(a) Telophase (b) Metaphase
(c) Prophase (d) Interphase

9. The disappearance of spindle and uncoiling of chromosomes takes place in:
(a) Anaphase (b) Telophase
(c) Pachytene (d) Meiosis

10. The regions where crossing-over takes place are called:
(a) Chiasmata (b) Cell plate
(c) Spindle fibres (d) Chromosomes

11. Duplicated chromosomes are joined at a point termed:
(a) Centrosome (b) Centromere
(c) Centriole (d) Chromatid

12. The œntromeredivides into two in:
(a) Prophase (b) Metaphase
(c) Anaphase (d) Telophase

13. After mitotic cell division, a female human cell will have:
(a) yy + xx chromosome (b) yy + xy chromosome
(c) 22 + x chromosome (d) 22 + y chromosome

14. The period between two successive mitotic divisions is:
(a) Diakinesis (b) Interphase
(c) Anaphase (d) Mitosis

15. The term meiosis was coined by:
(a) Farmer and Moore (b) Winiwarter
(c) Flemming (d) Strasburger

16. Meiosis is also known as:
(a) Equational division (b) Reductional division
(c) Direct cell division (d) All of the above

17. Meiosis occurs in:
(a) Vegetative cells (b) Reproductive cells
(c) Meristematic cells (d) None of the above

18. The process of meiosis takes place to produce:
(a) Cells of the body (b) Cells of the brain
(c) Sperms and ova (d) Testis and ovary

19. Leptotene, Zygotene and Diplotene phases are found in:
(a) Mitosis (b) Prophase of Meiosis-I
(c) Interphase (d) Prophase of Meiosis-U

Match the Column

Column ‘II’ is a list of items related to ideas in Column ‘I’. Match the term in Column ‘II’ with the suitable idea given in Column ‘I’.


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