NCERT Solutions Class 11 Biology Chapter 10 Cell Cycle and Cell Division

NCERT Solutions for class 11 Chapter-10 Cell Cycle and Cell Division: The chapter on Cell Cycle and Cell Division is important for students approaching the board exams. This article introduces NCERT solutions designed to help students explain the concepts of further learning and how to write to get good grades on exams. The solutions are presented in very simple language for ease of understanding.

NCERT CBSE Chapter 10 Solutions Cell Cycle and Cell Division of Class 11 explains The field of biology recognizes growth and reproduction as the two fundamental characteristics of cells in all living organisms. These attributes are extensively explored in this chapter.  Revise the basic concepts of the Cell Cycle and Cell Division for quick revision and class notes.

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

Answer:

A mammalian cell typically takes approximately 24 hours to complete one cycle of cell division.

Q2: Distinguish cytokinesis from karyokinesis.

Answer: 

Cytokinesis is the term used to describe the division of the cytoplasm. It occurs after the M-phase has ended. Karyokinesis, on the other hand, describes the division of the nucleus. It takes place in the M-phase.

Q3: Describe the events taking place during the interphase.

​Answer:

​Interphase is the stage in the cell cycle that prepares the cell and its nucleus for division. It consists of three sub-phases known as the G1 phase, the S phase, and the G2 phase.

1. G1 phase: This phase, also known as the first growth phase or post-mitotic gap phase, is the longest stage of interphase. During the G1 phase, both the cell and its nucleus increase in size. RNA, proteins, nucleotides, amino acids for histones, and energy-rich compounds are synthesized. The cell undergoes a checkpoint called G1, where the decision is made whether the cell will enter the G0 stage (resting phase) or continue with the cell cycle.
2. S phase: In this phase, chromosomes replicate their DNA. The amount of DNA doubles and chromosomes stay the same. After replication, the daughter chromosomes remain connected at the centromere region, and if a centrosome is present, it begins to divide.
3. G2 phase: This phase is characterized by increased synthesis of RNA and proteins. Cell organelles multiply, and the cell continues to grow in size. The G2 phase is also referred to as the second growth phase or pre-mitotic gap phase 

Q4: What is the Go (quiescent phase) of the cell cycle?

Answer:

The G0 phase, also known as the quiescent phase, signifies the inactivation of the cell cycle caused by the absence of mitogens and energy-rich compounds. Cells in the G0 phase remain metabolically active and serve as reserve cells, undergoing growth and differentiation for specific functions. For example, cells located in the quiescent center of the root tip enter this stage.

Q5: Why is mitosis called equational division?

Answer: 

Mitosis is referred to as equational division because, upon completion of mitosis, the number of chromosomes remains unchanged between the parental cells and the resulting daughter cells. Additionally, the ploidy, or the number of complete sets of chromosomes, remains the same in both the parental and daughter cells during mitosis

Q6: Name the stage of the cell cycle at which one of the following events occurs:

• (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 prophase I of meiosis I
• (iv)​ pachytene of prophase I of meiosis I. 

Q7: Describe the following:

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

Draw a diagram to illustrate your answer.

Answer:

• (a) Synapsis is the process of pairing homologous chromosomes while they are in the zygotene stage of prophase I of “meiosis I.” A synaptonemal complex, also known as a bivalent or tetrad, is created by the homologous chromosome during synapses.
• (b) It refers to the structure that is created when a pair of homologous chromosomes undergo synapsis and become aligned together
• (c) Chiasmata are the points of attachment that occur between homologous chromosomes during their separation from the diplotene stage to the metaphase I stage of meiosis. Initially, chiasmata are formed in the areas where non-sister chromatids undergo crossing over. However, as the process progresses, these chiasmata tend to shift sideways.

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

Answer: 

Cytokinesis happens in plants through the cell plate method. Throughout cytokinesis, the spindle structure remains present. The cell plate grows outwardly in a centrifugal direction. The new cell membrane is formed from vesicles originating from the Golgi apparatus.

In contrast, animal cells undergo cytokinesis through the cleavage method. The spindle begins to disintegrate following the anaphase stage. Cleavage occurs inwardly, in a centripetal manner. The formation of the new cell wall in animal cells is derived from the endoplasmic reticulum

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

Answer: 

Meiosis is a type of cell division known as reductional division, which results in the production of four daughter cells. These daughter cells have half the number of chromosomes compared to the parent cell. Meiosis specifically occurs during gametogenesis, the process of producing gametes. In humans, both spermatogenesis (sperm production) and oogenesis (egg production) involves meiotic division.

During spermatogenesis, the four daughter cells or sperm cells that are formed are equal in size. In contrast, during oogenesis, the daughter cells produced are unequal in size. Out of the four daughter cells, one large mature ovum (egg) is formed, while the remaining three are smaller structures known as polar bodies

​​Q10: Distinguish the anaphase of mitosis from anaphase I of meiosis.

Answer:

During anaphase of mitosis, each chromosome divides at the centromere, resulting in two sister chromatids. These sister chromatids then move towards opposite poles of the cell.

In contrast, during anaphase I of meiosis I, the separation occurs between homologous chromosomes, while the chromatids themselves remain attached to their respective centromeres.

Q11: List the main differences between mitosis and meiosis.

Answer:

Mitosis occurs in somatic cells, and these cells can be either diploid or haploid. It involves a single division that results in the production of two daughter cells. The daughter cells formed by mitosis are identical to their parent cell, both in terms of genetic content and the number of chromosomes. Mitosis plays a role in cell multiplication, as well as in the processes of healing and repair.

On the other hand, meiosis takes place in germ cells. These cells cannot be haploid; they are always diploid. Meiosis involves two divisions, resulting in the formation of four daughter cells. The daughter cells produced through meiosis are distinct from both the parent cell and from one another. The number of chromosomes is reduced to half in these daughter cells after meiosis. Meiosis is primarily involved in the production of gametes (sperm and egg cells).

​​Q12: What is the significance of meiosis?

Answer:

Meiosis is an important process in reproducing organisms. It specifically takes place in reproductive cells, leading to the formation of gametes that possess half the number of chromosomes compared to the reproductive cells. The fusion of two gametes, one from each parent, results in the formation of a zygote with a doubled number of chromosomes.

The primary role of meiosis is to maintain the chromosome number in sexually reproducing organisms. Additionally, it serves other significant purposes, such as:

1. Gamete formation for reproduction.
2. Maintenance of a fixed number of chromosomes during gametogenesis.
3. Independent assortment of paternal and maternal chromosomes during meiosis, leads to the reshuffling of chromosomes and the traits they control. This variation is beneficial for breeders in improving useful plants and animals.
4. Introduction of new combinations of traits or variations through meiosis.
5. Occurrence of chromosomal and genomic mutations due to irregularities in the meiotic division. Some mutations can be advantageous for the organism.

Q13: Discuss with your teacher about

1. haploid insects and lower plants where cell division occurs, and
2. some haploid cells in higher plants where cell division does not occur.

Answer:

1. In certain lower plants and haploid insects like drones of a honey bee, meiosis takes place within the zygote. This particular form of meiosis is referred to as zygotic meiosis, and it is associated with a life cycle known as the haplontic life cycle.
2. In higher plants, there are haploid cells called synergids and antipodals that do not undergo cell division.

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

Answer: 

DNA replication in the “S” phase is an essential prerequisite for mitosis to take place. This step involves the duplication of DNA, which is crucial for accurate chromosome segregation. Without DNA replication, the daughter cells would have a reduced number of chromosomes, preventing proper cell division. Therefore, mitosis is dependent on DNA replication, as it ensures the integrity and fidelity of chromosome distribution

Q15: Can there be DNA replication without cell division?

Answer: 

Indeed, DNA replication can occur independently of cell division. Polyteny is an example of such a condition where chromosomes undergo repeated replication without subsequent cell division. This leads to the accumulation of DNA within the cell without the formation of new cells

Q16: Analyze the events during every stage of the 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:

• (i) The number of chromosomes per cell undergoes a change during the anaphase I stage of meiosis I. At this stage, homologous chromosomes separate and migrate toward opposite poles of the cell. Consequently, the bivalent structures formed by the pairing of homologous chromosomes are divided into individual sister chromatids. As a result, the number of chromosomes is reduced by half in the resulting daughter cells.
• (ii) During anaphase II of the meiotic cycle, the separation of chromatids occurs due to the splitting of the centromere, which is responsible for holding the sister chromatids together. This separation causes the chromatids to move towards their respective poles. As a result, each pole contains a haploid number of chromosomes and a haploid amount of DNA. In contrast, during mitosis, the number of chromosomes remains unchanged. In mitotic anaphase, the duplicated DNA from the S phase is separated into two daughter cells. Consequently, the DNA content (C) in the newly-formed daughter cells remains the same.