Cell Cycle and Cell DivisionClass 11 Biology NCERT Solutions
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Q1Exercises
What is the average cell cycle span for a mammalian cell?
Solution
The average cell cycle span for a mammalian cell, such as a human cell in culture, is approximately 24 hours. Of this time, the M phase (actual cell division) lasts for only about an hour, while the interphase lasts for more than 95% of the duration.
Q2Exercises
Distinguish cytokinesis from karyokinesis.
Solution
Karyokinesis and cytokinesis are two distinct events that occur during the M phase of the cell cycle.
-
Karyokinesis: This is the division of the nucleus. It involves the precise separation of the duplicated chromosomes into two daughter nuclei. It consists of four stages: prophase, metaphase, anaphase, and telophase.
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Cytokinesis: This is the division of the cytoplasm, which typically follows karyokinesis. It results in the separation of the parent cell into two distinct daughter cells, each with its own nucleus and cytoplasm.
Q3Exercises
Describe the events taking place during interphase.
Solution
Interphase is the preparatory phase between two successive M phases and is the longest phase of the cell cycle. It is divided into three further phases:
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phase (Gap 1): This is the post-mitotic phase where the cell is metabolically active and grows continuously. It synthesizes proteins and RNA, and most of the organelle duplication occurs during this stage. However, DNA replication does not occur.
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S phase (Synthesis): This is the phase where DNA synthesis or replication takes place. The amount of DNA per cell doubles (from 2C to 4C), but the number of chromosomes remains the same (e.g., 2n). In animal cells, the centriole also duplicates in the cytoplasm during this phase.
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phase (Gap 2): In this phase, cell growth continues, and proteins necessary for mitosis are synthesized. The cell is now fully prepared to enter the M phase.
Q4Exercises
What is (quiescent phase) of cell cycle?
Solution
The phase, or quiescent stage, is an inactive stage that some cells in adult animals enter when they exit the phase of the cell cycle. Cells in this stage do not divide further and are considered to be in a state of metabolic activity without proliferation. Examples include heart cells. These cells can remain in the phase for long periods, sometimes indefinitely, but they can be called upon to re-enter the phase and resume division if required by the organism, for instance, to repair an injury.
Q5Exercises
Why is mitosis called equational division?
Solution
Mitosis is called equational division because the number of chromosomes in the progeny cells (daughter cells) is the same as that in the parent cell. During mitosis, the duplicated chromosomes are separated equally into two daughter nuclei, ensuring that each daughter cell receives an identical set of chromosomes to the parent cell. This maintains the genetic complement from one generation of cells to the next.
Q6Exercises
Name the stage of cell cycle at which one of the following events occur:
(i)
Chromosomes are moved to spindle equator.
(ii)
Centromere splits and chromatids separate.
(iii)
Pairing between homologous chromosomes takes place.
(iv)
Crossing over between homologous chromosomes takes place.
Solution
(i)
Metaphase: Chromosomes are moved to the spindle equator and align along the metaphase plate.
(ii)
Anaphase: The centromere of each chromosome splits, and the sister chromatids separate.
(iii)
Prophase I (Zygotene stage) of Meiosis: The pairing of homologous chromosomes, a process called synapsis, takes place.
(iv)
Prophase I (Pachytene stage) of Meiosis: Crossing over, the exchange of genetic material between non-sister chromatids of homologous chromosomes, occurs.
Q7Exercises
Describe the following:
(a)
synapsis
(b)
bivalent
(c)
chiasmata
Draw a diagram to illustrate your answer.
Solution
(a) Synapsis: It is the process of pairing of homologous chromosomes that occurs during the zygotene stage of prophase I of meiosis. This precise alignment is facilitated by the formation of a proteinaceous structure called the synaptonemal complex between the homologous chromosomes.
(b) Bivalent: A bivalent, or a tetrad, is the structure formed by a pair of synapsed homologous chromosomes during prophase I of meiosis. Since each homologous chromosome has already replicated, a bivalent consists of four chromatids (two sister chromatids per chromosome).
(c) Chiasmata: These are the X-shaped structures that become visible during the diplotene stage of prophase I. Chiasmata are the points of contact between non-sister chromatids of homologous chromosomes and represent the sites where crossing over has occurred. They hold the homologous chromosomes together after the synaptonemal complex dissolves.
(A diagram would show two homologous chromosomes, each with two chromatids. The pairing of these chromosomes illustrates synapsis. The entire four-chromatid structure is the bivalent. The points where the non-sister chromatids cross over each other are labeled as chiasmata.)
Q8Exercises
How does cytokinesis in plant cells differ from that in animal cells?
Solution
Cytokinesis differs between plant and animal cells primarily due to the presence of a rigid cell wall in plant cells.
| Animal Cell Cytokinesis | Plant Cell Cytokinesis |
|---|---|
| It occurs by the formation of a cleavage furrow in the plasma membrane. | It occurs by the formation of a cell plate. |
| The process is centripetal; the furrow starts at the periphery and deepens towards the center. | The process is centrifugal; the cell plate forms in the center and grows outwards towards the lateral walls. |
| It pinches the cell into two daughter cells. | The cell plate matures into the middle lamella, forming a new cell wall between the two daughter cells. |
Q9Exercises
Find examples where the four daughter cells from meiosis are equal in size and where they are found unequal in size.
Solution
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Equal Size: The four daughter cells from meiosis are equal in size in the formation of male gametes (spermatogenesis) in animals, such as humans. The process results in four equal-sized spermatids. Similarly, in the formation of microspores (pollen grains) in plants, the meiotic division of the microspore mother cell produces a tetrad of four equal-sized microspores.
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Unequal Size: The four daughter cells are unequal in size during the formation of the female gamete (oogenesis) in most animals, including humans. Meiosis results in one large haploid ovum (egg cell) and two or three small polar bodies. This unequal cytokinesis ensures that the ovum retains most of the cytoplasm and nutrients, which are essential for the development of the embryo after fertilization.
Q10Exercises
Distinguish anaphase of mitosis from anaphase I of meiosis.
Solution
The key differences between anaphase of mitosis and anaphase I of meiosis are as follows:
| Feature | Anaphase of Mitosis | Anaphase I of Meiosis |
|---|---|---|
| Centromere Splitting | The centromere of each chromosome splits. | Centromeres do not split. |
| Separation | Sister chromatids separate and move to opposite poles. | Homologous chromosomes separate and move to opposite poles. |
| Structures Moving to Poles | Individual chromatids (now called daughter chromosomes) move to the poles. | Each chromosome moving to a pole still consists of two sister chromatids joined at the centromere. |
| Chromosome Number | The chromosome number at each pole is the same as the parent cell (e.g., 2n). | The chromosome number is halved at each pole (e.g., n). |
Q11Exercises
List the main differences between mitosis and meiosis.
Solution
| Feature | Mitosis | Meiosis |
|---|---|---|
| Type of Division | Equational division | Reductional division |
| Occurs in | Somatic (body) cells | Germline (reproductive) cells |
| Number of Divisions | One | Two (Meiosis I and Meiosis II) |
| Synapsis | Does not occur | Occurs during Prophase I |
| Crossing Over | Absent | Occurs between homologous chromosomes in Prophase I |
| Daughter Cells | Two diploid (2n) cells, genetically identical to the parent | Four haploid (n) cells, genetically different from the parent |
| Chromosome Number | Remains the same in daughter cells | Is halved in daughter cells |
| Purpose | Growth, repair, and asexual reproduction | Production of gametes for sexual reproduction and genetic variation |
Q12Exercises
What is the significance of meiosis?
Solution
Meiosis is a crucial process in sexually reproducing organisms and has two major points of significance:
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Conservation of Chromosome Number: Meiosis reduces the chromosome number from diploid (2n) to haploid (n) in the gametes. This is essential because during fertilization, the fusion of two haploid gametes restores the diploid chromosome number in the zygote. This mechanism ensures the conservation of the specific chromosome number of a species across generations.
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Introduction of Genetic Variation: Meiosis increases genetic variability in a population from one generation to the next. This is achieved through:
- Crossing Over: The exchange of genetic material between homologous chromosomes during Prophase I creates new combinations of genes.
- Independent Assortment: The random orientation and separation of homologous chromosome pairs during Metaphase I and Anaphase I lead to various combinations of chromosomes in the gametes. This genetic variation is the raw material for natural selection and is fundamental to the process of evolution.
Q13Exercises
Discuss with your teacher about
(i)
haploid insects and lower plants where cell-division occurs, and
(ii)
some haploid cells in higher plants where cell-division does not occur.
Solution
(i)
Haploid insects and lower plants where cell-division occurs:
- Haploid Insects: An example mentioned in the text is the male honey bee (drone). Drones develop by parthenogenesis from unfertilized haploid eggs. Their bodies are composed of haploid cells, and they grow by mitosis. To produce gametes (sperm), their haploid germ cells undergo mitosis, not meiosis.
- Lower Plants: In many lower plants like algae, fungi, and bryophytes (mosses), the dominant life cycle stage is the haploid gametophyte. The gametophyte grows from a haploid spore through mitotic divisions. It then produces gametes (which are also haploid) by mitosis.
(ii)
Some haploid cells in higher plants where cell-division does not occur:
- In higher plants such as angiosperms, the male gametes are found within the pollen grain, and the female gamete is the egg cell within the ovule. These gametes are haploid cells that are terminally differentiated. They do not undergo further cell division. Their function is to fuse during fertilization to form a diploid zygote, which then divides and develops into an embryo.
Q14Exercises
Can there be mitosis without DNA replication in 'S' phase?
Solution
No, mitosis cannot occur without DNA replication in the S phase. The cell cycle is a sequence of events where the S phase (DNA synthesis) must precede the M phase (mitosis). The purpose of mitosis is to distribute the duplicated chromosomes equally into two daughter cells. If DNA replication does not occur, there would be no sister chromatids to separate. An attempt to divide would result in daughter cells with half the genetic material, which is not mitosis. The cell cycle has checkpoints that prevent a cell from entering mitosis if DNA replication is not completed successfully.
Q15Exercises
Can there be DNA replication without cell division?
Solution
Yes, DNA replication can occur without cell division. This phenomenon is known as endoreduplication, where a cell undergoes multiple rounds of S phase without entering mitosis or cytokinesis, leading to an increase in the number of chromosome sets (polyploidy). This occurs in certain specialized cells. A related example mentioned in the text is the formation of a syncytium, like the liquid endosperm in coconut. In this case, repeated nuclear divisions (karyokinesis) occur after DNA replication, but they are not followed by cytoplasmic division (cytokinesis), resulting in a large, multinucleated cell.
Q16Exercises
Analyse the events during every stage of cell cycle and notice how the following two parameters change
(i)
number of chromosomes (N) per cell
(ii)
amount of DNA content (C) per cell
Solution
Let's analyze the changes for a diploid cell, where the initial chromosome number is 2n and the initial DNA content is 2C.
Mitotic Cell Cycle:
| Stage | Chromosome Number per cell | DNA Content per cell |
|---|---|---|
| S | ||
| Prophase | ||
| Metaphase | ||
| Anaphase | (temporarily, as sister chromatids separate) | |
| Telophase | (in each forming nucleus) | (in each forming nucleus) |
| After Cytokinesis | (in each daughter cell) | (in each daughter cell) |
Meiotic Cell Cycle:
| Stage | Chromosome Number per cell | DNA Content per cell |
|---|---|---|
| Meiosis I | ||
| Prophase I | ||
| Metaphase I | ||
| Anaphase I | (homologous chromosomes separating) | |
| Telophase I | (in each forming nucleus) | (in each forming nucleus) |
| After Cytokinesis I | (in each of the 2 cells) | (in each of the 2 cells) |
| Meiosis II | ||
| Prophase II | ||
| Metaphase II | ||
| Anaphase II | (temporarily, as sister chromatids separate) | |
| Telophase II | (in each forming nucleus) | (in each forming nucleus) |
| After Cytokinesis II | (in each of the 4 daughter cells) | (in each of the 4 daughter cells) |