10.7 Exercise Problems

Q1. What is the average cell cycle span for a mammalian cell?

Answer: The average cell cycle span for a mammalian cell is approximately 24 hours.

Q2. Distinguish cytokinesis from karyokinesis.

Answer: 

\(
\begin{array}{|l|l|l|l|}
\hline & {\text { Cytokinesis }} & & {\text { Karyokinesis }} \\
\hline \text { (i) } & \begin{array}{l}
\text { Cytokinesis is the biological } \\
\text { process involving the division of a } \\
\text { cell’s cytoplasm during mitosis or } \\
\text { meiosis. }
\end{array} & \text { (i) } & \begin{array}{l}
\text { Karyokinesis is the biological } \\
\text { process involving the division of a } \\
\text { cell’s nucleus during mitosis or } \\
\text { meiosis. }
\end{array} \\
\hline \text { (ii) } & \begin{array}{l}
\text { It is divided into four stages – } \\
\text { prophase, metaphase, anaphase, } \\
\text { and telophase. }
\end{array} & \text { (ii) } & \begin{array}{l}
\text { Stages such as prophase, } \\
\text { metaphase, anaphase, and } \\
\text { telophase are not present in } \\
\text { karyokinesis. }
\end{array} \\
\hline
\end{array}
\)

Q3. Describe the events taking place during the interphase.

Answer: Interphase involves a series of changes that prepare a cell for division. It is the period during which the cell experiences growth and DNA replication in an orderly manner. Interphase is divided into three phases.
– \(G_1\) phase
– S phase
– \(\mathrm{G}_2\) phase
\(\mathrm{G}_1\) phase – It is the stage during which the cell grows and prepares its DNA for replication. In this phase, the cell is metabolically active.
\(\mathrm{S}\) phase – It is the stage during which DNA synthesis occurs. In this phase, the amount of DNA (per cell) doubles, but the chromosome number remains the same.
\(\mathrm{G}_2\) phase – In this phase, the cell continues to grow and prepares itself for the division. The proteins and RNA required for mitosis are synthesised during this stage.

Q4. What is \(\mathrm{G}_0\) (quiescent phase) of the cell cycle?

Answer: \(\mathrm{G}_0\) or quiescent phase is the stage wherein cells remain metabolically active, but do not proliferate unless called to do so. Such cells are used for replacing the cells lost during injury.

Q5. Why is mitosis called equational division?

Answer: Mitosis is the process of cell division wherein the chromosomes replicate and get equally distributed into two daughter cells. The chromosome number in each daughter cell is equal to that in the parent cell, i.e., diploid. Hence, mitosis is known as equational division.

Q6. Name the stage of cell cycle at which one of the following events occur:
(i) Chromosomes are moved to the spindle equator.
(ii) Centromere splits and chromatids separate.
(iii) Pairing between homologous chromosomes takes place.
(iv) Crossing over between homologous chromosomes takes place.

Answer: (i) Metaphase
(ii) Anaphase
(iii) Zygotene of meiosis I
(iv) Pachytene of meiosis I

Q7. Describe the following:
(a) synapsis
(b) bivalent
(c) chiasmata
Draw a diagram to illustrate your answer.

Answer:

(a) Synapsis: The pairing of homologous chromosomes is called synapsis. This occurs during the second stage of prophase I or zygotene.

(b) Bivalent: Bivalent or tetrad is a pair of synapsed homologous chromosomes. They are formed during the zygotene stage of prophase I of meiosis.

(c) Chiasmata: Chiasmata is the site where two sister chromatids have crossed over. It represents the site of cross-over. It is formed during the diplotene stage of prophase I of meiosis.

Q8. How does cytokinesis in plant cells differ from that in animal cells?

Answer: 

\(
\begin{array}{|l|l|l|l|}
\hline & {\text { Cytokinesis in plant cells }} & &{\text { Cytokinesis is animal cells }} \\
\hline \text { (i) } & \begin{array}{l}
\text { The division of the cytoplasm takes } \\
\text { place by cell plate formation. }
\end{array} & \text { (i) } & \begin{array}{l}
\text { The division of the cytoplasm takes } \\
\text { place by cleavage. }
\end{array} \\
\hline \text { (ii) } & \begin{array}{l}
\text { Cell plate formation starts at the } \\
\text { centre of the cell and grows outward, } \\
\text { toward the lateral walls. }
\end{array} & \text { (ii) } & \begin{array}{l}
\text { Cleavage starts at the periphery } \\
\text { and then moves inward, dividing } \\
\text { the cell into two parts. }
\end{array} \\
\hline
\end{array}
\)

Q9. Find examples where the four daughter cells from metosis are equal in size and where they are found unequal in size.

Answer: (a) Spermatogenesis or the formation of sperms in human beings occurs by the process of meiosis. It results in the formation of four equal-sized daughter cells.
(b) Oogenesis or the formation of ovum in human beings occurs by the process of meiosis. It results in the formation of four daughter cells which are unequal in size.

Q10. Distinguish anaphase of mitosis from anaphase I of meiosis.

Answer: 

\(
\begin{array}{|l|l|l|}
\hline \text { Anaphase of mitosis } & \text { Anaphase I of meiosis } \\
\hline \begin{array}{l}
\text { Anaphase is the stage during which } \\
\text { the centromere splits and the } \\
\text { chromatids separate. The } \\
\text { chromosomes move apart, toward } \\
\text { the opposite poles. These } \\
\begin{array}{l}
\text { chromosomes are genetically } \\
\text { identical. }
\end{array}
\end{array} & \begin{array}{l}
\text { chromosomes separate, while the } \\
\text { chromatids remain attached at their } \\
\text { centromeres. } \\
\text { Hence, in anaphase I, the chromosomes of } \\
\text { each bivalent pair separate, while the sister } \\
\text { chromatids remain together. }
\end{array} \\
\hline
\end{array}
\)

Q11. List the main differences between mitosis and meiosis.

Answer: 

Mitosis	Meiosis
1. In mitotic division, a single division results in two daughter cells.	1. Meiotic division involves two successive divisions – meiosis I and meiosis II. These divisions result in four daughter cells.
2. Mitosis is known as equational division. This is because the daughter cells have the same diploid number of chromosomes as the parent.	2. Meiosis I is known as the reductional division. This is because the chromosome number is reduced to half. Meiosis II is known as equational division. This is because the sister chromatids separate and the chromosome number remains the same.
3. Prophase is short and does not comprise any phase.	3. Prophase I is very long and comprises 5 phases -leptotene, zygotene, pachytene, diplotene, and diakinesis.
4. There is no pairing of chromosomes, crossing-over, or chiasmata-formation during prophase.	4. In the zygotene stage of prophase, the pairing of chromosomes occurs. During pachytene, the crossing-over occurs. The chiasmata are formed in the diplotene stage.
5. Synaptonemal complex is not formed.	5. Synaptonemal complex is formed during the zygotene stage of prophase I.
6. Anaphase involves the separation of the chromatids of each chromosome.	6. During anaphase I, the homologous chromosomes separate, while the chromatids remain attached at their centromeres. During anaphase II, the chromatids separate as a result of the splitting of the centromere.
7. Mitosis plays a significant role in the healing, repair, and growth of a cell.	7. Meiosis brings about variation and maintains the chromosome number from generation to generation.

Q12. What is the significance of meiosis?

Answer: Meiosis is a type of cell division that reduces the number of chromosomes in the parent cell by half and produces four gamete cells.
The significance of meiosis is in
Formation of gametes – Meiosis forms gametes that are essential for sexual reproduction.
Genetic information – Meiosis switches on the genetic information for the development of gametes.
Crossing over – It introduces a new combination of traits or variations.
Mutation – Mutations take place due to irregularities of meiotic division.

Q13. 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.

Answer: (i) In some insects and lower plants, fertilization is immediately followed by zygotic meiosis, which leads to the production of haploid organisms. This type of life cycle is known as haplontic life cycle.
(ii) The phenomenon of polyploidy can be observed in some haploid cells in higher plants in which cell division does not occur. Polyploidy is a state in which cells contain multiple pairs of chromosomes than the basic set. Polyploidy can be artificially induced in plants by applying colchicine to cell culture.

Q14. Can there be mitosis without DNA replication in ‘S’ phase?

Answer: Mitotic cell division cannot take place without DNA replication in the S phase. Two important events take place during S phase – one is the synthesis or duplication of DNA and the other is the duplication of the centriole. DNA duplication is important as it maintains the chromosome number in the daughter cells. Mitosis is an equational division. Therefore, the duplication of DNA is an important step.

Q15. Can there be DNA replication without cell division?

Answer: There can be DNA replication without cell division. During cell division, the parent cell gets divided into two daughter cells. However, if there is a repeated replication of DNA without any cell division, then this DNA will keep accumulating inside the cell. This would increase the volume of the cell nucleus, thereby causing cell expansion. An example of DNA duplication without cell division is commonly observed in the salivary glands of Drosophila. The chromosome undergoing repeated DNA duplication is known as a polytene chromosome.

Q16. Analyse the events during every stage of cell cycle and notice how the following two parameters change
(1) number of chromosomes (N) per cell
(ii) amount of DNA content (C) per cell

Answer: During meiosis, the number of chromosomes and the amount of DNA in a cell change.
(i) Number of chromosomes (N) per cell
During anaphase I of the meiotic cycle, the homologous chromosomes separate and start moving toward their respective poles. As a result, the bivalents get divided into two sister chromatids and receive half the chromosomes present in the parent cell.
Therefore, the number of chromosomes reduces in anaphase I.
(ii) Amount of DNA content (C) per cell
During anaphase II of the meiotic cycle, the chromatids separate as a result of the splitting of the centromere. It is the centromere that holds together the sister chromatids of each chromosome. As a result, the chromatids move toward their respective poles. Therefore, at each pole, a haploid number of chromosomes and a haploid amount of DNA are present.
During mitosis, the number of chromosomes remains the same. The DNA duplicated in the S phase gets separated in the two daughter cells during anaphase. As a result, the DNA content (C) of the two newly-formed daughter cells remains the same.

Exemplar Section

VERY SHORT ANSWER TYPE QUESTIONS

Q1. Between a prokaryote and a eukaryote, which cell has a shorter cell division time?

Answer: Prokaryotic cells has shorter cell division time than eukaryotic cells. A typical eukaryotic cell cycle is illustrated by human cells in culture. These cells divide once in approximately every 24 hours. In bacteria (E.coli) cell cycle is of 20 minutes.

Q2. Which of the phases of cell cycle is of longest duration?

Answer: Interphase

Q3. Name a stain commonly used to colour chromosomes.

Answer:  Basic fuchsin, acetocarmine etc.

Q4. Which tissue of animals and plants exhibits meiosis?

Answer: Gohads (testes and ovary) in animals and sporangium in plants.

Q5. 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: \(2 \xrightarrow{I} 4 \xrightarrow{II} 8 \xrightarrow{III} 16 \xrightarrow{IV} 32\)
For formation of 32 cells, two E.coli cells takes 4 cycles. So total time will be 4 x 20 = 80 minutes

Q6. Which part of the human body should one use to demonstrate stages in mitosis?

Answer: Nail base or any somatic cell (diploid cell).

Q7. What attributes does a chromatid require to be classified as a chromosome?

Answer: Centromere

Q8. The diagram shows a bivalent at prophase-I of meiosis. Which of the four chromatids can cross over?

Answer: Sister chromatids of homologous chromosome.

Q9. If a tissue has at a given time 1024 cells, how many cycles of mitosis had the original parental single cell undergone?

Answer: \(1024=(2)^n\)
\(n=\) No. of cycles
\(
1024=2^{10}=2^n \Rightarrow n=10
\)
Thus, 10 cycles of mitosis are there in the original parental single cell.

Q10. An anther has 1200 pollen grains. How many pollen mother cells must have been there to produce them?

Answer: 4 pollen grains are produced by 1 pmc
1200 pollen grains are produced by = 1200/4 = 300 pmc

Q11. At what stage of cell cycle does DNA synthesis take place?

Answer: S-phase (interphase)

Q12. 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: It is significant to note that in the 24 hour average duration of cell cycle of a human cell, cell division proper lasts for only about an hour. The interphase lasts more than 95% of the duration of cell cycle.

Q13. 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. Fill in the blanks.

Answer: It is observed that heart cells do not exhibit cell division. Such cells do not divide further and exit \(G _1\) phase to enter an inactive stage called quiescent stage ( \(G _0\) ) of the cell cycle. Muscle cells when reaching a level of maturity, no longer divide and just perform their function all through its life. Similar is the case with nerve cells too which are highly specific and lack the ability to divide once they get specialised. They remain in that state until they die.

Q14. 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 ____________________________

Hints : 1) Zygotene, 2) Pachytene, 3) Pachytene, 4) Diakinesis, 5) After Telophase-I /before Meosis-II, 6) Telophase-I /After Meiosis-I.

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.

SHORT ANSWER TYPE QUESTIONS

Q1. State the role of centrioles other than spindle formation.

Answer: The centrioles form the basal body of cilia or flagella.

Q2. Mitochondria and plastids have their own DNA (genetic material). What is known about their fate during nuclear division like mitosis?

Answer: At the time of cytoplasmic division, organelles like mitochondria and plastids get distributed between the two daughter cells.

Q3. Label the diagram and also determine the stage at which this structure is visible.

Answer: This is a transition to metaphase

Q4. 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: Chromosome number (N) during metaphase = 32 (N)
DNA content (C) during anaphase = 2C

Q5. 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:  Cells with 16 chromosomes are produced by meiosis while that with 32 chromosomes are produced by mitosis.
Cells with 16 chromosomes could have arisen from cells with 32 chromosomes through meiosis.
Cells with 32 chromosomes could have arisen from cells with 16 chromosomes through fertilisation or syngamy.

Q6. 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: Late prophase
b. Appearance of nucleolus: Telophase
c. Division of centromere: Anaphase
d. Replication of DNA: S-phase

Q7. 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. Nuclear membrane fails to disintegrate: In this condition, mitosis
takes place within nucleus. This is called endoduplication.
b. Duplication of DNA does not occur: There will be no mitosis
c. Centromeres do not divide: Polyploidy appears
d. Cytokinesis does not occur: In some organisms karyokinesis is not followed by cytokinesis as a result of which multinucleate condition arises leading to the formation of syncytium (e.g., liquid endosperm in coconut)

Q8. Both unicellular and multicellular organisms undergo mitosis. What are the differences, if any, observed in the process between the two?

Answer: The growth of multicellular organisms is due to mitosis.
The reproduction of unicellular organisms is due to mitosis.

Q9. Name the pathological condition when uncontrolled cell division occurs

Answer: Cancer

Q10. 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:  In animal cells, during the S phase, DNA replication begins in the nucleus, and the centriole duplicates in the cytoplasm.

Q11. Comment on the statement-Meiosis enables the conservation of specific chromosome number of each species even though the process per se, results in a reduction of chromosome number.

Answer: Meiosis is the mechanism by which conservation of specific chromosome number of each species is achieved across generations in sexually reproducing organisms, even though the process, per se, paradoxically, results in reduction of chromosome number by half. But fertilisation restores the chromosome number.

Q12. 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: In oocytes of some vertebrates, diplotene can last for months or years.
Lampbrush chromosomes or diplotene chromosome are found in diplotene stage of most animal oocytes of frog or amphibians.
Lampbrush chromosomes are observed in meiotic prophase. These chromosomes become normal after growth and thus completing the cell cycle.

Q13. How does cytokinesis in plant cells differ from that in animal cells?

Answer: In an animal cell, cytokinesis 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. Plant cells however, are enclosed by a relatively inexte’nsible cell wall, therefore they undergo cytokinesis by a different mechanism. In plant cells, wall formation starts in the centre of the cell and grows outward to meet the existing lateral walls. The formation of the new cell wall begins with the formation of a simple precursor, called the cell-plate that represents the middle lamella between the walls of two adjacent cells.

LONG ANSWER TYPE QUESTIONS

Q1. Comment on the statement – Telophase is reverse of prophase.

Answer: Prophase is marked by the initiation of condensation of chromosomal material. The chromosomal material becomes untangled during the process of chromatin condensation. At the beginning of the final stage of mitosis, i.e. telophase, the chromosomes that have reached their respective poles decondense and lose their individuality.
Cells at the end of prophase, when viewed under the microscope, do not show golgi complexes, endoplasmic reticulum, nucleolus and the nuclear envelope. In the telophase stage nuclear envelope assembles around the chromosome clusters. Nucleolus, golgi complex and ER reform.

Q2. What are the various stages of meiotic prophase-I? Enumerate the chromosomal events during each stage?

Answer:

Meiosis-I:
Prophase-I: Prophase of the first meiotic division is typically longer and more complex when compared to the prophase of mitosis. It has been further subdivided into the following five phases based on chromosomal behaviour, i.e. Leptotene, Zygotene, Pachytene, Diplotene and Diakinesis. During leptotene stage, the chromosomes become gradually visible under the light microscope.

• The compaction of chromosomes continues throughout leptotene. This is followed by the second stage of prophase-I called zygotene. During this stage chromosomes start pairing together and this process of association is called synapsis. Such paired chromosomes are called homologous chromosomes. Electron micrographs of this stage indicate that chromosome synapsis is accompanied by the formation of complex structure called synaptonemal complex.
• The complex formed by a pair of synapsed homologous chromosomes is called a bivalent or a tetrad. However, these are more clearly visible at the next stage. The first two stages of prophase-I are relatively short-lived compared to the next stage that is pachytene. During this stage bivalent chromosomes now clearly appears as tetrads. This stage is characterised by the appearance of recombination nodules, the sites at which crossing over occurs between non-sister chromatids of the homologous chromosomes. Crossing over is the exchange of genetic material between two homologous chromosomes.
• Crossing over is also an enzyme-mediated process and the enzyme involved is called recombinase. Crossing over leads to the recombination of genetic material on the two chromosomes. Recombination between homologous chromosomes is completed by the end of pachytene, leaving the chromosomes linked at the sites of crossing over.
• The beginning of diploteneis recognised by the dissolution of the synaptonemal complex and the tendency of the recombined homologous chromosomes of the bivalents to separate from each other except at the sites of crossovers. These X-shaped structures, are called chiasmata. In oocytes of some vertebrates, diplotene can last for months or years.
• The final stage of meiotic prophase-I is diakinesis. This is marked by terminalisation of chiasmata. During this phase the chromosomes are fully condensed and the meiotic spindle is assembled to prepare the homologous chromosomes for separation. By the end of diakinesis, the nucleolus disappears and the nuclear envelope also breaks down.

Q3. Differentiate between the events of mitosis and meiosis 

Answer: 

Mitosis	Meiosis
1. Take place in the somatic cells of the body.	1. Take place in the germ cells.
2. Occurs in both sexually as well as asexually reproducing organisms.	2. Occurs only in sexually reproducing organisms.
3. Mitosis involves only one cycle of nuclear and cell division.	3. Meiosis involves two sequential cycles of nuclear and cell division called meiosis-l and meiosis-II.
4. The DNA replicates once for one cell division.	4. The DNA replicates once for two cell divisions.
5. The prophase is shorter.	5. Prophase is typically longer.
6. Prophase is comparatively simple.	6. Prophase of the first meiotic division is more complex when compared to prophase of mitosis.
7. The cell divides only once and the chromosomes also divide only once.	7. There are two cell divisions but the chromosomes divide only once.
8. Mitosis does not involves pairing of homologous chromosomes and recombination between them.	8. Meiosis involves pairing of homologous chromosomes and recombination between them.
9. Two cells are formed at the end of mitosis.	9. Four haploid cells are formed at the end of meiosis.

Q4. Write brief note on the following
a. Synaptonemal complex
b. Metaphase plate 

Answer: a. Synaptonemal complex: During zygotene stage chromosomes start pairing together and this process of association is called synapsis. Such paired chromosomes are called homologous chromosomes. Electron micrographs of this stage indicate that chromosome synapsis is accompanied by the formation of complex structure called synaptonemal complex. The complex formed by a pair of synapsed homologous chromosomes is called a bivalent or a tetrad. However, these are more clearly visible at the next stage.
b. Metaphase plate: At this stage, metaphase chromosome is made up of two sister chromatids, which are held together by the centromere. Small disc-shaped structures at the surface of the centromeres are called kinetochores. These structures serve as the sites of attachment of spindle fibres (formed by the spindle fibres) to the chromosomes that are moved into position at the centre of the cell. Hence, the metaphase is characterised by all the chromosomes coming to lie at the equator with one chromatid of each chromosome connected by its kinetochore to spindle fibres from one pole and its sister chromatid connected by its kinetochore to spindle fibres from the opposite pole. The plane of alignment of the chromosomes at metaphase is referred to as the metaphase plate.

Q5. Write briefly the significance of mitosis and meiosis in multicellular organism.

Answer: Significance of Mitosis:
Mitosis or the equational division is usually restricted to the diploid cells only. However, in some lower plants mitosis usually results in the production of diploid daughter cells with identical genetic complement. The growth of multicellular organisms is due to mitosis. A very significant contribution of mitosis is cell repair. The cells of the upper layer of the epidermis, cells of the lining of the gut, and blood cells are being constantly replaced. Mitotic divisions in the meristematic tissues —the apical and the lateral cambium, result in a continuous growth of plants throughout their life.

Significance of Meiosis:
Meiosis is the mechanism by which conservation of specific chromosome number of each species is achieved across generations in sexually reproducing organisms, even though the process, per se, paradoxically, results in reduction of chromosome number by half. It also increases the genetic variability in the population of organisms from one generation to the next. Variations are very important for the process of evolution.

Q6. An organism has two pair of chromosomes (i.e., chromosome number = 4). Diagrammatically represent the chromosomal arrangement during different phases of meiosis-II. 

Answer: