Chapter 10 : Cell Cycle and Cell Division

Class 11 biology Chapter 10 Cell Cycle and Cell Division Exercise Questions and Answers :

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

Answer : The average cell cycle duration for a mammalian cell is typically around 16 to 24 hours, but it can vary depending on the cell type and environmental conditions. The cell cycle consists of the , S,  and M phases, with the S phase (DNA replication) lasting about 6-8 hours.

2. Distinguish cytokinesis from karyokinesis.

Answer : The Distinguish between karyokinesis and cytokinesis :

Karyokinesis	Cytokinesis
The process of nuclear division during the M phase, which results in the separation of daughter chromosomes into two distinct nuclei.	The division of the cell's cytoplasm into two daughter cells, occurring after karyokinesis, completing the cell division process.
Starts at the beginning of the M phase and concludes before cytokinesis begins.	Begins after karyokinesis is complete and is the final step of the M phase.
The main outcome is two daughter nuclei, each with a full set of chromosomes identical to the parent cell.	The main outcome is the formation of two separate daughter cells, each with its own nucleus and organelles.

3. Describe the events taking place during interphase.

Answer :  Interphase is a crucial period in the cell cycle, encompassing the time between cell divisions.

The interphase is divided into three phases are :

(a) Phase (Gap 1) :

(i) The cell increases in size by synthesizing proteins, lipids, and organelles. This phase is essential for preparing the cell for DNA replication.

(ii) Cellular metabolism is active, with an emphasis on energy production and enzyme synthesis.

(iii) Cells undergo a checkpoint to ensure that conditions are favorable for cell division. If conditions are not suitable, the cell may enter a quiescent state ( phase) or cease division.

(b) S Phase (Synthesis) :

(i) The cell's DNA is replicated, resulting in the formation of two identical sister chromatids for each chromosome. This step is crucial for ensuring that each daughter cell receives a full and accurate set of genetic information.

(ii) The centrioles in animal cells also replicate during this phase, which is important for organizing microtubules during cell division (mitosis or meiosis).

(c) Phase (Gap 2) :

 (i) The cell continues to grow and synthesize the necessary proteins, organelles, and enzymes for the upcoming cell division.

(ii) Another checkpoint occurs to confirm that DNA replication is complete and accurate. The cell assesses whether conditions are favorable for entering the M phase (mitosis or meiosis). If everything is in order, the cell proceeds to cell division.

4. What is (quiescent phase) of cell cycle?

Answer : The phase, also known as the quiescent stage of the cell cycle, is a state in which cells exit the phase and become metabolically active but do not actively proliferate. Cells in do not divide unless required by the organism, such as in response to injury or specific signals. Many adult animal cells, like heart cells, remain in the phase and only divide when necessary for tissue maintenance or repair.

5. Why is mitosis called equational division ?

Answer : Mitosis is called equational division because the production of daughter cells with an identical genetic complement to the parent cell. This division is crucial for the growth of multicellular organisms, maintaining the nucleo-cytoplasmic ratio, and repairing and replacing cells, as seen in the upper epidermal cells, gut lining cells, and blood cells, contributing to overall organism development and maintenance.

6. 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.

Answer : The name of the stage of cell cycle are :

(i) Chromosomes are moved to the spindle equator during Metaphase of mitosis or meiosis.

(ii) Centromere splits and chromatids separate during Anaphase of mitosis or meiosis.

(iii) Pairing between homologous chromosomes takes place during Prophase I of meiosis.

(iv) Crossing over between homologous chromosomes takes place during Prophase I of meiosis .

7. Describe the following:

(a) synapsis (b) bivalent (c) chiasmata

Draw a diagram to illustrate your answer .

Answer :  (a) Synapsis : Synapsis is the process that occurs during the zygotene stage of meiotic prophase I. During this process, homologous chromosomes (chromosomes with the same genes but potentially different alleles) come together and pair up. This pairing is a crucial step for genetic recombination and the formation of chiasmata. The association of homologous chromosomes during synapsis is accompanied by the formation of a complex structure called the synaptonemal complex, which helps align the chromosomes and facilitates genetic exchange.

                 

(b) Bivalent : A bivalent, also known as a tetrad, is a complex structure formed by a pair of synapsed homologous chromosomes during meiotic prophase I. This structure consists of four chromatids, with each homologous chromosome contributing two chromatids. Bivalents are more clearly visible during the pachytene stage of prophase I when the four chromatids of each chromosome become distinct and clearly appear as tetrads. This is the stage at which genetic recombination (crossing over) between non-sister chromatids of homologous chromosomes occurs.

           

(c) Chiasmata : Chiasmata are X-shaped structures that form at the sites of crossovers between non-sister chromatids of homologous chromosomes during the diplotene stage of meiotic prophase I. These structures are a result of the physical exchange of genetic material (crossing over) between homologous chromosomes and represent points of genetic recombination. Chiasmata mark the locations where chromatids have broken and recombined, leading to the mixing of genetic material between homologous chromosomes .

            

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

Answer : The difference from Plant cells and animal cells : 

Aspect of Cytokinesis	Plant Cells	Animal Cells
Cytokinesis Structure	Formation of a cell plate made of vesicles containing cell wall materials.	Formation of a cleavage furrow created by a contractile ring of actin filaments.
Cell Wall Involvement	Essential due to the presence of a rigid cell wall in plant cells.	Not relevant, as animal cells lack a rigid cell wall.
Cytokinesis Location	Can occur anywhere within the cell, allowing multiple daughter cells to form from a single parent cell.	Typically occurs at the center, leading to the division of a single parent cell into two daughter cells.
Vesicle Formation	Involves the production and fusion of vesicles for the formation of the cell plate.	No vesicle formation is involved; instead, the contractile ring is responsible for cleavage furrow formation.

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

Answer :  Equal-sized daughter cells are common in meiosis II, where sister chromatids separate, resulting in four roughly equal haploid cells. Unequal-sized daughter cells occur during meiosis I, as seen in oogenesis, where one large secondary oocyte and one smaller polar body are produced, due to an unequal division of cytoplasm.

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

Answer : The difference between anaphase of mitosis and anaphase I of meiosis :

Aspect	Anaphase of Mitosis	Anaphase I of Meiosis
Chromosome Number	Daughter cells have the same chromosome number as the parent cell (diploid to diploid).	Daughter cells have half the chromosome number as the parent cell (diploid to haploid).
Homologous Chromosomes	No separation of homologous chromosomes occurs.	Homologous chromosomes are separated and move to opposite poles.
Sister Chromatids	Sister chromatids are separated and move to opposite poles.	Sister chromatids remain attached; they are separated in Anaphase II.
Genetic Diversity	Maintains the genetic makeup of the parent cell.	Introduces genetic diversity by reshuffling genetic material between homologous chromosomes.
Number of Divisions	Part of a single division in mitosis.	Part of the first division in meiosis.

11. List the main differences between mitosis and meiosis.

Answer : The main differences between mitosis and meiosis:

Aspect	Mitosis	Meiosis
Purpose	Growth, repair, asexual reproduction	Gamete formation, genetic diversity
Number of Divisions	One division	Two divisions
Chromosome Number	Daughter cells have the same chromosome number (diploid) as the parent cell	Daughter cells have half the chromosome number (haploid) as the parent cell
Genetic Diversity	Maintains genetic identity; daughter cells are genetically identical	Introduces genetic diversity through crossing over and random assortment
Number of Daughter Cells	Produces two daughter cells	Produces four daughter cells
Role in the Organism	Tissue growth, repair, somatic cell maintenance	Gamete production for sexual reproduction
Homologous Chromosomes	Do not pair or separate	Pair and separate during meiosis I
Sister Chromatids	Separate during anaphase	Remain attached in meiosis I and separate in meiosis II
Crossover (Crossing Over)	Crossover does not occur	Occurs during prophase I, leading to genetic recombination
End Result	Two diploid daughter cells, genetically identical to each other and the parent cell	Four haploid daughter cells, genetically unique from each other and the parent cell

12. What is the significance of meiosis ?

Answer : Meiosis is significant because it ensures the conservation of a specific chromosome number in sexually reproducing organisms across generations. While it paradoxically reduces the chromosome number by half, this reduction is essential to maintain species' genetic stability. Additionally, meiosis increases genetic variability through processes like crossing over and random assortment. This genetic diversity is vital for the evolutionary process, enabling organisms to adapt to changing environments and contributing to the survival and adaptation of species over time.

13. 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 haploid insects and lower plants, cell division plays a crucial role in the life cycle. These organisms exhibit alternation of generations, where they alternate between a haploid (n) and a diploid (2n) phase. In the haploid phase, cell division is essential for growth, development, and reproduction. Haploid cells divide to produce gametes (sperm and egg cells), which are necessary for sexual reproduction.

(ii) In higher plants, there are specific haploid cells where cell division does not occur. One such example is the male gametophyte (pollen grain) in angiosperms. During pollen development, the microspore undergoes a single round of cell division to form two haploid cells: a generative cell and a tube cell. After this division, cell division ceases in the generative cell. It is the generative cell that eventually divides to produce two sperm cells, which are non-dividing haploid cells. These sperm cells are transported to the female reproductive organs for fertilization.

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

Answer : No, DNA replication in the 'S' (synthesis) phase is a prerequisite for mitosis. DNA replication ensures that each daughter cell receives a complete and identical set of genetic material, which is crucial for accurate cell division.

15. Can there be DNA replication without cell division?

Answer :  Yes, DNA replication can occur without immediate cell division. During the cell cycle, DNA replication (S phase) precedes cell division (mitosis or meiosis). Cells can replicate their DNA in preparation for division, and the actual division may occur later in the cell cycle or under different conditions.

16. 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

Answer : The number of chromosomes (N) and the amount of DNA content (C) change during each stage of the cell cycle :

(a) Interphase ( Phase):

N: The number of chromosomes (N) remains constant. For humans, it's typically 46 (diploid).

C: DNA content (C) increases as DNA replication occurs, resulting in two identical sister chromatids per chromosome. The amount of DNA effectively doubles.

(b) Interphase (S Phase):

N: The number of chromosomes (N) remains constant.

C: DNA content (C) increases significantly as DNA replication proceeds, with each chromosome consisting of two sister chromatids.

(c) Interphase ( Phase):

N: The number of chromosomes (N) remains constant.

C: DNA content (C) remains unchanged since DNA replication is already completed.

(d) Mitosis (Prophase):

N: The number of chromosomes (N) remains constant.

C: DNA content (C) remains constant as well.

(e) Mitosis (Metaphase):

N: The number of chromosomes (N) remains constant.

C: DNA content (C) remains constant.

(f) Mitosis (Anaphase and Telophase):

N: The number of chromosomes (N) remains constant, but they are distributed to two daughter cells.

C: DNA content (C) remains constant in each daughter cell, which still has the same amount of DNA as the parent cell.

(g) Cytokinesis:

N: The number of chromosomes (N) remains constant, but they are now distributed into two separate daughter cells, so each daughter cell has half the original number of chromosomes.

C: DNA content (C) remains constant, with each daughter cell having the same DNA content as the parent cell but half the amount as the original cell.