NCERT Solutions for Class 11 Science Biology Chapter 10 Cell Cycle And Cell Division are provided here with simple step-by-step explanations. These solutions for Cell Cycle And Cell Division are extremely popular among Class 11 Science students for Biology Cell Cycle And Cell Division Solutions come handy for quickly completing your homework and preparing for exams. All questions and answers from the NCERT Book of Class 11 Science Biology Chapter 10 are provided here for you for free. You will also love the ad-free experience on Meritnation’s NCERT Solutions. All NCERT Solutions for class Class 11 Science Biology are prepared by experts and are 100% accurate.
Page No 49:
Question 1:
Meiosis results in diploid organisms
a. production of gametes
b. reduction in the number of chromosomes
c. introduction of variation
d. all of the above
Answer:
Meiosis is a reductional division where chromosome number is reduced to half during gamete formation. In diploid (2n) organisms, meiosis results in the production of haploid (n) gametes. During meiosis, recombination that is exchange of genetic material takes place in the pachytene stage of prophase I which leads to variations. Thus, all the options are the characteristic features of meiosis.
Hence, the correct answer is option D.
Page No 49:
Question 2:
At which stage of meiosis does the genetic constitution of gametes is finally decided?
a. Metaphase I
b. Anaphase II
c. Metaphase II
d. Anaphase I
Answer:
During anaphase I of meiosis, each cell receives half the chromosome number, i.e. the actual reduction of chromosome number from 2n in parent cell to n in daughter cell occurs. During this phase, two homologoues of each chromosome pair separate and moves towards the opposite poles of the cell. This cannot be done during metaphase, as chromosomes are only arranged by themselves at the metaphase plate.
Hence, the correct answer is option D.
Page No 49:
Question 3:
Meiosis occurs in organisms during
a. sexual reproduction
b. vegetative reproduction
c. both sexual and vegetative reproduction
d. none of the above
Answer:
Meiosis occurs in organisms during sexual reproduction because this type of cell division results in the formation of gametes with half the number of chromosomes i.e. haploid content. Gametes fuse to form the diploid offspring so as to maintain constant chromosome number in the progeny.
Hence, the correct answer is option A.
Page No 49:
Question 4:
During anaphase-I of meiosis
a. homologous chromosomes separate
b. non-homologous chromosomes separate
c. sister chromatids separate
d. non-sister chromatids separate
Answer:
During Anaphase I of meiosis, the homologous chromosomes separate, while sister chromatids stay connected at their centromeres. As a result, each of the bivalent pairs separates while the sister chromatids remain together. This separation of homologous chromosomes at anaphase I is called disjunction.
Hence, the correct answer is option A.
Page No 49:
Question 5:
Mitosis is characterised by
a. reduction division
b. equal division
c. both reduction and equal division
d. pairing of homologous chromosomes
Answer:
Mitosis is called equational division as the number of chromosomes in the parent and offsprings is the same. Reductional division is the characteristic feature of meiosis.
Hence, the correct answer is option B.
Page No 50:
Question 6:
A bivalent of meiosis-I consists of
a. two chromatids and one centromere
b. two chromatids and two centromere
c. four chromatids and two centromere
d. four chromatids and four centromere
Answer:
In prophase I of meiosis I, pairing of chromosomes, that is the process of synapsis occurs and a complex is formed by the pair of synapsed homologous chromosomes called a bivalent or a tetrad. A bivalent consists of four chromatids and two centromeres.
Hence, the correct answer is option C.
Page No 50:
Question 7:
Cells which are not dividing are likely to be at
a. G1
b. G2
c. G0
d. S phase
Answer:
Some cells in the body like heart cells do not divide and enter an inactive stage called quiescent stage (G0) of the cell cycle. G signifies growth and 0 signifies no growth. Cells in G0 stage remain metabolically active but no longer proliferate unless called on to do so depending on the requirement of the organism.
Hence, the correct answer is option C.
Page No 50:
Question 8:
Which of the events listed below is not observed during mitosis?
a. chromatin condensation
b. movement of centrioles to opposite poles
c. appearance of chromosomes with two chromatids joined together at the centromere.
d. crossing over
Answer:
Recombination or exchange of genetic material between two homologous chromosomes takes place in meiotic cell division with the help of an enzyme recombinase. Crossing over occurs in the pachytene stage of prophase-I of meiosis-I. Other options such as chromatin condensation, movement of centrioles to opposite poles and appearance of chromosomes with two chromatids joined together at the centromere are observed in both meiosis and mitosis.
Hence, the correct answer is option D.
Page No 50:
Question 9:
Identify the wrong statement about meiosis.
a. Pairing of homologous chromosomes
b. Four haploid cells are formed
c. At the end of meiosis the number of chromosomes are reduced to half
d. Two cycle of DNA replication occurs
Answer:
Either it is mitosis or meiosis, cell division involves only a single cycle of DNA replication that is during S phase of the cell cycle.
Meiosis involves two sequential cycles of nuclear and cytoplasmic division called meiosis-I, meiosis-II, cytokinesis I and cytokinesis II respectively, but only a single cycle of DNA replication. Rest of the options are the events of meiosis.
Hence, the correct answer is option D.
Page No 50:
Question 10:
Select the correct statement about G1 phase.
a. Cell is metabolically inactive
b. DNA in the cell does not replicate
c. It is not a phase of synthesis of macromolecules
d. Cell stops growing
Answer:
G1 phase is the gap 1 phase which is 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. DNA replicates in the S phase. During the G2 phase, proteins are synthesized in preparation for mitosis while cell growth continues.
Hence, the correct answer is option B.
Page No 50:
Question 1:
Between a prokaryote and a eukaryote, which cell has a shorter cell division time?
Answer:
Prokaryotic cells have shorter cell division time than eukaryotic cells. Prokaryotic cell such as E.coli divides in 20 minutes and eukaryotic cells such as human cells divide once in approximately every 24 hours.
Page No 50:
Question 2:
Which of the phases of cell cycle is of longest duration?
Answer:
In about 24 hour cycle of cell division, Interphase which is the phase between two successive M phases, lasts more than 95% of the duration of cell cycle (about 20-22 hours). Interphase includes G1, S and G2 phases of the cell cycle. M phase takes about approximately 1-2 hours of the cell cycle.
Page No 51:
Question 3:
Name a stain commonly used to colour chromosomes.
Answer:
Stains used to colour the chromosomes are giemsa stain and acetocarmine. Chromosomes are stained for karyotyping or visualizig chromosomes in mitotic studies.
Page No 51:
Question 4:
Which tissue of animals and plants exhibits meiosis?
Answer:
Meiosis is a type of cell division that takes place in sexually reproducing organisms for the formation of gametes. Meiosis undergo reductional division thus occur in the germ cells of male and female reproductive organs of plants and animals. In animals germ cells are present in gonads (testis and ovary in males and females respectively).
Page No 51:
Question 5:
Given that the average duplication time of E.coli is 20 minutes, how much time will two E.coli cells take to become 32 cells?
Answer:
Each cell division in E.coli takes place in every 20 minutes.
Two cells of E.coli forms 32 cells in 4 cycles of cell division.
Thus, total time will be 4 x 20 = 80 minutes
= 1 hour 20 minutes
Page No 51:
Question 6:
Which part of the human body should one use to demonstrate stages in mitosis?
Answer:
Except the germinal cells (male and female germ cells), all the cells in the human body are somatic cells and are diploid in nature. The somatic cells undergo mitotic cell division for growth and regeneration which can be used to demonstrate mitosis.
Page No 51:
Question 7:
What attributes does a chromatid require to be classified as a chromosome?
Answer:
A chromatid is one of two identical halves of a replicated chromosome. Thus, two identical sister chromatids attached at the center by the centromere can be classified as a chromosome.
Page No 51:
Question 8:
The diagram shows a bivalent at prophase-I of meiosis. Which of the four chromatids can cross over?
Answer:
In a bivalent, non-sister chromatids of homologous chromosomes cross over in pachytene stage of prophase-I.
Page No 51:
Question 9:
If a tissue has at a given time 1024 cells, how many cycles of mitosis had the original parental single cell undergone?
Answer:
Each mitotic cycle produces 2 daughter cells.
If 'n' is the number of cycle
2n = 1024
2n = (32)2
2n = (25)2
n = 10
Therefore, 210 = 1024
Thus, a cell must undergo 10 rounds of mitosis to produce 1024 cells.
Page No 51:
Question 10:
An anther has 1200 pollen grains. How many pollen mother cells must have been there to produce them?
Answer:
A single pollen mother cell or PMC (2n) undergoes meiotic cell division and each cell produces four pollen grains with haploid (n) number of chromosomes.
4 pollen grains produced by = 1 PMC
1200 pollen grains produced by = 1200/4
= 300
Hence, 300 pollen mother cells will produce 1200 pollen grains.
Page No 51:
Question 11:
At what stage of cell cycle does DNA synthesis take place?
Answer:
DNA synthesis occurs at S phase or synthesis phase of the cell cycle.
Page No 51:
Question 12:
It is said that the one cycle of cell division in human cells (eukaryotic cells) takes 24 hours. Which phase of the cycle, do you think occupies the maximum part of cell cycle?
Answer:
An average human cell takes 24 hours to complete its one cycle of cell division where interphase stage would spend maximum of the time that is about 18-20 hours in its cycle to prepare the itself for cell division.
Page No 51:
Question 13:
Fill in the blanks.
It is observed that heart cells do not exhibit cell division. Such cells do not divide further and exit ___________ phase to enter an inactive stage called ___________ of cell cycle.
Answer:
It is observed that heart cells do not exhibit cell division. Such cells do not divide further and exit G1 phase to enter an inactive stage called quiescent stage (G0) of cell cycle.
Page No 51:
Question 14:
In which phase of meiosis are the following formed? Choose the answers from hint points given below.
| a. Synaptonemal complex | __________________________ |
| b. Recombination nodules | __________________________ |
| c. Appearance/activation of enzyme recombinase | __________________________ |
| d. Termination of chiasmata | __________________________ |
| e. Interkinesis | __________________________ |
| f. Formation of dyad of cells | __________________________ |
Answer:
| a. Synaptonemal complex | Zygotene |
| b. Recombination nodules | Pachytene |
| c. Appearance/activation of enzyme recombinase | Pachytene |
| d. Termination of chiasmata | Diakinesis |
| e. Interkinesis | After Telophase-I /before Meosis-II |
| f. Formation of dyad of cells | Telophase-I /After Meiosis-I |
Page No 52:
Question 1:
State the role of centrioles other than spindle formation.
Answer:
Two cylindrical structures called centrioles lie perpendicular to each other in a centrosome. Centrosome is found only in animal cells. Each centriole has an organisation like a cart wheel. Apart from forming the spindle fibres, centrioles form the basal body of cilia and flagella and also helps in the formation of microtubules and sperm tail.
Page No 52:
Question 2:
Mitochondria and plastids have their own DNA (genetic material). What is known about their fate during nuclear division like mitosis?
Answer:
Mitochondria and plastids have their own DNA. It is not related to nuclear DNA and are called extrachromosomal DNA. Due to this, during mitosis, no effect is seen on extrachromosomal DNA of mitochondria and plastids. During cytokinesis, these organelles get distributed between the two daughter cells.
Page No 52:
Question 3:
Label the diagram and also determine the stage at which this structure is visible.
Answer:
This stage shows the transition to metaphase stage of mitosis.
Page No 52:
Question 4:
A cell has 32 chromosomes. It undergoes mitotic division. What will be the chromosome number (N) during metaphase? What would be the DNA content (C) during anaphase?
Answer:
During mitosis, the chromosome number remains same as it is an equational division which occurs in somatic cells of an organism. So the number of chromosomes remain same i.e. 32 (N).
DNA replication takes place during the S phase, thus DNA content gets doubled after the S phase; and will remain so during mitosis. Thus, the DNA content (C) during anaphase = 2C.
Page No 52:
Question 5:
While examining the mitotic stage in a tissue, one finds some cells with 16 chromosomes and some with 32 chromosomes. What possible reasons could you assign to this difference in chromosome number. Do you think cells with 16 chromosomes could have arisen from cells with 32 chromosomes or vice versa?
Answer:
Condition where two different population of cells with different genotypes is observed in a single individual is called mosaicism. It can be the result of mechanisms of anaphase lag, non-disjuction, or endoreplication.
In this case cells with 16 chromosomes have arisen from cells with 16 chromosomes may be due to anaphase lag. As the tissue was in mitotic stage vice versa (formation of 16 chromosomes containing cells from 32 chromosomes containing cell) is not possible.
Page No 52:
Question 6:
The following events occur during the various phases of the cell cycle, Name the phase against each of the events.
| a. Disintegration of nuclear membrane | __________________________ |
| b. Appearance of nucleolus | __________________________ |
| c. Division of centromere | __________________________ |
| d. Replication of DNA | __________________________ |
Answer:
| a. Disintegration of nuclear membrane | Prophase |
| b. Appearance of nucleolus | Telophase |
| c. Division of centromere | Anaphase |
| d. Replication of DNA | S-phase |
Page No 52:
Question 7:
Mitosis results in producing two cells which are similar to each other. What would be the consequence if each of the following irregularities occur during mitosis?
a. Nuclear membrane fails to disintegrate.
b. Duplication of DNA does not occur.
c. Centromeres do not divide.
d. Cytokinesis does not occur.
Answer:
a) If nuclear membrane fails to disintegrate then the spindle fibres will not be able to reach the chromosome, resulting in failure of movement of chromosomes towards opposite poles.
b) If DNA duplication does not occur in the S phase, the cell cycle will stop as the cell will not be able to reach the M phase.
c) If centromeres do not divide one of the daughter cells will receive a complete pair of chromosomes and other cell will not get any of them.
d) If cytokinesis does not occur then a syncitium or multinucleate coenocyte may produce as it occurs in Rhizopus, Vaucheria etc.
Page No 53:
Question 8:
Both unicellular and multi-cellular organisms undergo mitosis. What are the differences, if any, observed in the process between the two?
Answer:
Both unicellular and multi-cellular organisms undergo mitosis. Growth of multicellular organisms is due to mitosis of somatic cells, on the other hand, unicellular organisms reproduce by mitotic cell division.
| Mitosis in Unicellular Organisms | Mitosis in Multicellular Organisms |
| 1. The nucleus and cytoplasm are divided directly without the formation of chromosomes. | 1. The nucleus and cytoplasm are not divided directly and involves formation of chromosomes. |
| 2. Phases of cell cycle are not observed. | 2. Phases of cell cycle are observed. |
| 3. Duplication of DNA takes place simulaneously with the division of nucleus. | 3. Separate synthetic phase called S phase in the cell cycle for DNA duplication. |
| 4. |
4. |
Page No 53:
Question 9:
Name the pathological condition when uncontrolled cell division occurs.
Answer:
The pathological condition when cells loose control over cell division is malignancy. A malignant tumor can grow and spread to other parts of the body very rapidly leading to disease called cancer.
Page No 53:
Question 10:
Two key events take place, during S phase in animal cells, DNA replication and duplication of centriole. In which parts of the cell do events occur?
Answer:
DNA replication occurs in the nucleus and duplication of centrioles takes place in the cytoplasm during S phase of the cell cycle. Centrioles are involved in the formation of spindle fibres which direct the movement of chromosomes towards the opposite poles. The position of the nucleus is determined by the position of centrioles.
Page No 53:
Question 11:
Comment on the statement – Meiosis enables the conservation of specific chromosome number of each species even though the process per se, results in reduction of chromosome number.
Answer:
Meiosis is a reductional division where chromosome number is reduced to half in daughter cells as compared to the mother cell. When the male and female gamete fuses in sexually reproducing organisms, meiosis enables the conservation of specific chromosome number of each species across generations. In case if meiosis does not occur, two diploid gametes will produce that will result in the formation of tetraploid zygote. Such organism is non-viable in nature. Thus, meiosis is important for conservation of chromosome number in species. Meiosis also results in genetic variability in the population of organisms from one generation to another.
Page No 53:
Question 12:
Name a cell that is found arrested in diplotene stage for months and years. Comment in 2-3 lines how it completes cell cycle?
Answer:
Mammalian oocytes generally get arrested at diplotene stage of meiosis.
In females, meiosis occurs at an early embryonic stage where an oocyte grows and forms secondary follicle and then gets arrested at diplotene stage. This arrest is under hormonal control. First meiosis gets completed in the egg when the women attains sexual maturity. Meiosis II again stops at the metaphase of meiosis II before ovulation. Meiosis II completes only if the secondary oocyte fuses with a sperm.
Page No 53:
Question 13:
How does cytokinesis in plant cells differ from that in animal cells?
Answer:
| Cytokinesis in Plant Cells | Cytokinesis in Animal Cells |
| 1. Cytokinesis takes place by cell plate formation. | 1. Cytokinesis takes place through cleavage. |
| 2. Cell plate starts from the centre of the cell and ends at the periphery. | 2. Cleavage starts from the periphery and ends at the centre of the cell. |
| 3. After anaphase, the spindles degenerate. | 3. Spindle persists during cytokinesis. |
| 4. Cytokinesis is achieved by the formation of the new cell wall called the cell-plate. | 4. Cytokinesis is achieved by the appearance of a furrow in the plasma membrane. |
| 5. |
5. |
Page No 53:
Question 1:
Comment on the statement – Telophase is reverse of prophase.
Answer:
| Prophase | Telophase |
| 1. First phase of karyokinesis in cell division. | 1. Last phase of karyokinesis in cell division. |
| 2. Chromatin condense and forms chromosomes. | 2. Chromosomes decondense and forms chromatin. |
| 3. Chromosomes become clearly visible. | 3. Chromosomes reach the opposite poles and become invisible. |
| 4. Nuclear envelope and nucleolus breaks down. | 4. Nuclear envelope reappear and daughter nuclei are formed. |
| 5. Spindle fibres appear around the poles. | 5. Spindle fibres disappear around the poles. |
| 6. Golgi apparatus, ER gets disorganised. | 6. Golgi apparatus, ER forms agaim. |
| 7. Cytoplasm and nucleoplasm disappears. | 7. Nucleoplasm appears and becomes distinct from cytoplasm. |
| |
|
Page No 53:
Question 2:
What are the various stages of meiotic prophase-I? Enumerate the chromosomal events during each stage?
Answer:
Prophase of meiosis I is longer and more complex as compared to prophase of mitosis. It is because genetic variation occurs due to this phase in sexually reproducing organisms.
Prophase is divided into 5 phases based on chromosomal behaviour: Leptotene, zygotene, pachytene, diplotene and diakinesis.
(a) Leptotene: During this stage, the chromosomes are slowly visible under a light microscope. The nuclear membrane starts to disappear and chromatin material starts condensing. The centriole move towards opposite poles. The compaction of chromosomes continues until the end of this stage. The sister chromatids are tightly bound so that they cannot be differentiated from each other. This stage is followed by the second stage of prophase I, i.e. zygotene.
(b) Zygotene: In this stage the chromosomes start forming pairs. and this process of pairing is called synapsis. Synaptonemal complex is formed along with chromosome synapsis. Each pair formed in this process is called a bivalent or tetrad.
(c) Pachytene: In this stage, recombination nodules starts to appear. The recombination nodule is the site at which the crossing over of the non-sister chromatids takes place. Crossing over occurs between non-sister chromatids of the homologous chromosomes. Crossing over or recombination helps in the exchange of genetic material between two homologous chromosomes.
(d) Diplotene: In this stage, the synaptonemal complex starts to break down. The reconnected chromosomes begin to segregate from each other. But they are connected at the site of crossing over. This makes an X-shaped structure called chiasmata. This stage in mammalian oocytes may lasts for months or years.
(e) Diakinesis: In this stage, the chiasmata is terminated. The nucleolus and the nuclear membrane disappears completely in this process. During this phase the chromosomes are fully condensed and the meiotic spindle is assembled to prepare the homologous chromosomes for separation. This stage marks the transition to metaphase.
Page No 53:
Question 3:
Differentiate between the events of mitosis and meiosis
Answer:
| Events | Mitosis | Meiosis |
| Prophase | Prophase is of shorter duration. Chromosomes are not easily noticeable and no crossing over takes place during this stage. This stage is not divided into sub-stages. No bouquet stage, chiasmata, synaptonemal complex, recombination nodules are recorded. | Prophase is of longer duration. Chromosomes are easily noticeable and crossing over takes place during this stage between non sister chromatids of homologous chromosomes. This stage is divided into 5 sub-stages. Bouquet stage, chiasmata, synaptonemal complex, recombination nodules etc, are recorded. |
| Metaphase | A single metaphasic plate is formed during metaphase. Centromeres are connected by spindle fibres coming from both the poles. Chromosomes are independent in this phase and do not show connections. | Two metaphasic plates are formed during metaphase–I but a single metaphasic plate is formed during metaphase–II. The Homologous chromosomes are interconnected. |
| Anaphase | During this stage, centromere splits along its length to form two centromeres. Chromosomes are single stranded and move towards the opposite poles. | Centromeres do not require segregation during anaphase–I. However, segregation of centromere takes place during anaphase–II. Double-stranded chromosomes are visible in anaphase–I, but during anaphase–II, single-stranded chromosomes are visible. |
| Telophase | Telophase is of longer duration which also produces the interphase nuclei. | Telophase is of shorter duration in meiosis. |
| Cytokinesis | Cytokinesis is a process that happens after every mitosis in which two daughter cells are formed. | Cytokinesis is a process that does not happens after meiosis I but occurs after meiosis II in which four haploid daughter cells are formed. |
Page No 53:
Question 4:
Write brief note on the following
a. Synaptonemal complex
b. Metaphase plate
Answer:
a) Synaptonemal complex: During the zygotene stage of prophase I homologous chromosomes start pairing together. This process of pairing is called synapsis and the complex formed by a pair of synapsed homologous chromosomes is called a bivalent or a tetrad. Following the formation of tetrad, a complex structure of protein called synaptonemal complex is also formed. Synaptonemal complex is made up of protein formed between two homologous chromosomes. This helps to keep the homologous chromosomes together and facilitates crossing over.
b) Metaphase plate: In metaphase, the centromeres of the chromosomes arrange themselves on the metaphase plate. It is an imaginary line which is at equal distances from the two opposite poles. The chromosomes are aligned because of the opposing position of the kinetochore microtubules. At this plate, sister chromatids are attached to the spindle fibres.
Page No 53:
Question 5:
Write briefly the significance of mitosis and meiosis in multi-cellular organism.
Answer:
Significance of Mitosis:
- Life starts from a single cell, zygote to a full grown organism with the help of mitosis. It helps in cellular division.
- Mitosis helps in increasing the size and growth of an organism.
- It aids in repairing worn out or damaged tissues.
- It is involved in asexual reproduction of lower organisms such as binary fission, vegetative propagation in flowering plants and regeneration.
- Meiosis ensures that the gametes have haploid set of chromosomes. This ensures the conservation of particular chromosomal numbers in species.
- Meiosis is important in introducing genetic variability in populations through recombination. This occurs due to crossing over during prophase–I.
- Variations occured in sexually reproducing organisms are very important for the process of evolution.
Page No 53:
Question 6:
An organism has two pair of chromosomes (i.e., chromosome number = 4). Diagrammatically represent the chromosomal arrangement during different phases of meiosis-II.
Answer:
The following diagram shows different phases of meiosis—II of an organism which has two pairs of chromosomes.
View NCERT Solutions for all chapters of Class 11