NCERT Solutions Class 11 Biology Chapter 10

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

Ans-

The average cell cycle span for a typical mammalian cell is about 24 hours. These cells divide once every 24 hours to form two daughter cells.

Q.2 Distinguish cytokinesis from karyokinesis.

Ans-

Q.3 Describe the events taking place during Interphase.

Ans-

During the course of the cell cycle, the time span a living cell spent in between two consecutive M- phase (mitotic- phase) is called Interphase. This is the metabolically active phase during which the cell grows in size, duplicating its genome (DNA) and accumulating nutrients required for mitosis. It is also called the resting phase as no change in chromatin structure is observed under the microscope during this phase. On the basis of molecular and biochemical events during Interphase, it has been divided into three sub-phases.

Gap 1 phase (G1): This is the duration between mitosis and initiation of DNA replication where the cell is metabolically active, and grow without DNA replication. This phase is marked by the increase in cell size where the cell prepares for DNA replication

Synthesis phase (S): This phase begins with DNA synthesis or DNA replication. By the end of this phase the DNA content of the cell doubles but without increase in the chromosomal number. Thus if the initial chromosomal number of a cell was 2n and DNA content was 2C, at the end of S-phase the DNA content will become 4C while chromosomal number remains the same (2n).In S-phase initiation of centriole duplication also takes place in the cytoplasm.

Gap 2 phases (G 2): In G2 phase the cell continues to grow and synthesize the regulatory proteins and enzymes necessary for mitotic phase.

Q.3 What is G₀ (quiescent phase) of cell cycle?

Ans-

G₀ or quiescent phase of the cell cycle is the phase when the cell has come out of G₁ phase to enter an inactive stage. The cells are metabolically active however do not undergo division unless required depending upon the environmental condition. It is also known as the resting phase. The adult myocardiocytes which never undergo division of cells which divide rarely to replace cells lost due to some injury or cell death remain in G₀ or quiescent phase of the cell cycle.

Q.4 Why is mitosis called equational division?

Ans-

Mitosis is called equational division because the two daughter cells formed at the end of mitosis acquire the same number and kinds of chromosomes as the parent cell nucleus. There is a duplication of each chromosome before the onset of mitosis and is composed of two chromatids which get separated equally during the process of mitosis. The orderly distribution of chromosomes on the mitotic spindle helps in equal separation of chromatids of each chromosome. Also, since there is no crossing over during the process of mitosis, it results in the same kinds of the chromosome as the parent cell.

Q.5 Name the stage of cell cycle at which one of the following events occur:

1. Chromosomes are moved to spindle equator.
2. Centromere splits and chromatids separate.
3. Pairing between homologous chromosomes takes place.
4. Crossing over between homologous chromosomes takes place.

Ans-

1. Metaphase
2. Anaphase
3. Zygotene of Meiosis I
4. Pachytene of Meiosis I

Q.6 Describe the following:

1. Synapsis
2. Bivalent
3. Chiasmata.

 Draw a diagram to illustrate your answer.

1. Synapsis: It is the pairing up of duplicated homologous chromosomes (one from each parent) during the process of meiosis in such a way that the DNA of the non-sister chromatids is aligned. It takes place during the zygotene stage of prophase I of meiosis. Synapsis of homologous chromosomes is accompanied by the formation of a complex structure called synaptonemal complex.
2. Bivalent: It is the complex formed by a pair of synapsed homologous chromosomes. It is a bivalent because there are two chromosomes in close association. It is also known as tetrad because bivalent contains four chromatids. Bivalents condense and become visibly distinct during the pachytene stage of prophase I of meiosis. Crossing over occurs between the non-sister chromatids of the bivalent. This results in non-identical sister chromatids of a chromosome.
3. Chiasmata: It is an X-shaped structure of chromosomes formed during the diplotene stage of meiosis I. The synaptonemal complex dissolves resulting in separation of recombined homologous chromosomes of the bivalents except at the site of crossing over.

Q.7 How does cytokinesis in plant cells differ from that in animal cells?

In animal cells, a furrow develops in the plasma membrane. This cleavage furrow deepens gradually and ultimately joins in the centre resulting in the division of the cell cytoplasm into two halves. However, the plant cells are surrounded by a rigid and inextensible cell wall due to which no cell furrow formation takes place. The process of cytokinesis is very different in plant cells as compared to that of animal cells. The major differences are:

1. Cell plate formation: The cell wall formation begins with the precursor called cell plate at the centre of the cell.
2. Extension of cell plate: The cell plate extends outwardly in all direction and ultimately unites with the exiting lateral wall and thus function as middle lamella between the walls of two adjacent cells
3. Deposition of cell wall components: Once the cell plate is fused to the cell wall of the cell, deposition of cell wall components such as cellulose takes place to make a proper cell wall.

Q.8 Find examples where the four daughter cells differ from meiosis are equal in size and where they are found unequal in size?

An example where equal-sized daughter cells are formed at the end of meiosis: Spermatogenesis results in the production of equal-sized haploid sperms.

An example where unequal sized daughter cells are formed at the end of meiosis: During the process of oogenesis, unequal sized daughter cells are formed at the end of meiosis.

Q.9 Distinguish anaphase of mitosis from anaphase I of meiosis.

Q.11 List the main differences between mitosis and meiosis.

Q.12 What is the significance of meiosis?

Significance of meiosis:

• Since gametes in sexually reproducing organisms are produced by the process of meiosis (resulting in the reduction of chromosome number to half), it helps in conserving the specific chromosome number of each species across generations.
• It also increases genetic variability due to the process of crossing over happening during the process. This results in gaining new characteristics by the population over generations. Such variations help in the process of evolution.

Q.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.

(i) Cell division occurs in insects and lower plants, like algae (Spirogyra and Chlamydomonas), bryophytes and pteridophytes. Haploid gametes are produced through mitosis and the zygote formed after fertilisation undergoes meiosis to produce haploid organisms.

(ii) In higher plants, antipodals and synergids of the embryo sac are haploid and do not undergo cell division.

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

No, mitosis cannot take place without DNA replication in ‘S’phase. Mitosis is an equational division resulting in the production of two genetically identical daughter cells. This is achieved because the DNA of each chromosome duplicates by the process of replication during the S or synthetic phase of the cell cycle. The amount of DNA per cell doubles. This is followed by segregation of the two chromatids of each chromosome during the anaphase stage of mitosis bringing the DNA content identical to that of the parent cell.

Q.15 Can there be DNA replication without cell division?

Yes, there can be DNA replication without cell division. Endoreplication is the replication of DNA of the cell without the cell division. This results in a large number of copies of the same DNA molecule(s) within the cell nucleus resulting in polyploidy. One example is polytene chromosomes of the salivary glands of fruit flies. The repeated division of chromosomes without any cell division results in a large number of sister chromatids tightly bound to each other. DNA replication without cell division is a common phenomenon in plants.

Q.16 Analyze the events during every stage of cell cycle and notice how the following two parameters change

1. Number of chromosomes (N) per cell
2. Amount of DNA content (C) per cell

Mitotic Cell Cycle:

• The number of chromosomes (N) per cell: Number of chromosomes (N) per cell never changes in the mitosis. If the cell has diploid (2n) number of chromosomes, even after the synthetic phase where the DNA replication occurs, the chromosome number remains 2n.

• Amount of DNA content (C) per cell: During the S phase (synthetic) of interphase in mitosis, DNA synthesis or replication takes place. It results in duplication of the amount of DNA per cell i.e. from the initial amount of 2C, it becomes 4C. It is during the anaphase that the sister chromatids separate due to splitting of the centromere, thereby bringing the DNA content per cell back to 2C amount.

Meiotic Cell Cycle:

• The number of chromosomes (N) per cell: Meiosis results in the reduction of the number of chromosome per cell to half. The cell starts with a 2N number. Before meiosis, the DNA replication occurs and each chromosome gets 2 sister chromatids. It is during the anaphase I of meiosis I that homologous chromosomes segregate reducing the chromosome number to N. There is no further reduction in the chromosome number in meiosis II.
• Amount of DNA content (C) per cell: Amount of DNA content per cell is also reduced to half during the process of meiosis. Let us assume the initial DNA content is to be 1C per cell before the onset of meiosis. During the replication process before the onset of meiosis I, each chromosome undergoes replication thereby doubling the DNA content (2C). During anaphase I, homologous chromosomes segregate to opposite poles however there is no splitting of the centromere. The DNA content of the 2 daughter cells produced becomes 1C again. It is during the anaphase II in meiosis that the centromeres split resulting in separation of sister chromatids further into 4 daughter cells. This results in a reduction of DNA content to half as compared to the parent cell.