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

iacst-2022

In Biology, it is said that in all living organisms, the two most important attributes of cells are growth and reproduction. These characteristics of cells have been discussed in this whole NCERT Solutions for class 11 Biology Chapter 10.

Cells can reproduce by dividing themselves into two where each parental cell (i.e. the one reproducing) gives birth to two new daughter cells every time they divide. Now, these fresh sets of daughter cells grow on their own and divide themselves to form a new cell population, created because of the growth and division of one parental cell and its progeny. This continuous process of cell growth and division is known as Cycles of Cell Growth and Division that let a single cell to create a structure comprising millions of cells. Other than this, a few other subtopics are also a part of this chapter, like Cell Cycle, Importance of Mitosis, Meiosis, Importance of Meiosis, and M Phase. So, let's have an overlook of the topics of this chapter.

  • Cycle of Cells
  • M-phase
  • Importance of Mitosis
  • Meaning of Meiosis
  • Importance of Meiosis

Chapter 10 'Cell Cycle and Cell Division' is a part of Unit IV - 'Plant Physiology' of class 11 Biology syllabus, where a description of the phases of cell development has been given. The significance of Meiosis is a pivotal part of this chapter.

Meiosis helps in preserving the outgrowing chromosome numbers of every living species occurring across ages. Such variables are essential for the advanced stages of the cycle. Meiosis, chromosomal changes, etc., are immensely important in successfully developing an organism. The two most important aspects of this chapter are Mitosis and Meiosis. The difference between the two is that the former one takes place in somatic cells, giving rise to two daughter cells and germ cells. Whereas, the latter also happens in somatic cells, but gives birth to four daughter cells.
 

Q1. What is the average cell cycle span for a mammalian cell?
Answer: The average cell cycle span for a mammalian cell is 24 hours.

Q2. Distinguish cytokinesis from karyokinesis.
Answer: The differences between cytokinesis and karyokinesis are as follows:
 

 

Cytokinesis

 

Karyokinesis

 

The process of division of cytoplasm is called cytokinesis

 

The process of division of the nucleus is called karyokinesis

 

Cytokinesis takes place after the end of the M-phase

 

Karyokinesis takes place during the M-phase.

 

Q3. Describe the events taking place during the interphase.

Answer: Interphase refers to the phase in the cell cycle which prepares the cell and its nucleus for division. Interphase is divided into three sub-phases like G1 phase, S Phase and G2 phase.

1. G1 phase- It is the longest stage of interphase, also called the first growth phase or post-mitotic gap phase. Both the cell and its nucleus grow in size. There is the synthesis of RNA, proteins, nucleotides, amino acids for histones, and energy-rich compounds. There is a checkpoint called G1 , where the decision about the entry in the G0 stage is taken. If the decision is made the cell cycle goes on uninterrupted or the cell undergoes the G0 stage. It refers to the resting phase in which the cell is arrested and not allowed to divide.

2. S Phase- In this phase, chromosomes along with their DNA replicates. The content of DNA doubles up, however, the number of chromosomes remains the same. After replication, the daughter chromosomes remain attached in the region of the centromere. The centrosome, if present begins to divide.

3. G2 phase- This phase is characterized by increased synthesis of RNA and proteins. The cell organelles undergo multiplication in this phase while the cell grows in size. The G2 phase is also called as second growth phase or pre-mitotic gap phase.

 

Q4. What is the Go (quiescent phase) of the cell cycle?
Answer: G o (quiescent phase)- It refers to the stage of inactivation of the cell cycle due to the absence of mitogens and energy-rich compounds. The cells of this stage are metabolically active and they act as reserve cells i.e. they undergo growth and differentiation for a specific function. E.g Cells in the quiescent center of the root tip.

 

Q5. Why is mitosis called equational division?
Answer: Mitosis is called equational division because, at the end of mitosis, the numbers of chromosomes remain the same in parental cells and daughter cells. Similarly, the ploidy of the parental and daughter cells also remains the same in mitosis.

 

(i) Chromosomes are moved to the spindle equator.
Answer:(i)Chromosomes are moved to the spindle equator
During metaphase, the chromosomes move to the spindle equator and align themselves there.

 

(ii)Centromere splits and chromatids separate.
Answer: (ii)Centromere splits and chromatids separate
During anaphase, the centromere splits, and chromatids are separated. These chromatids after separation move to the opposite poles.

 

(iii)The pairing between homologous chromosomes takes place.
Answer: (iii)The pairing between homologous chromosomes takes place
The pairing between homologous chromosomes takes place during zygotene of prophase I of meiosis I. 

 

(iv)Crossing over between homologous chromosomes takes place.
Answer: (iv)Crossing over between homologous chromosomes takes place
Crossing over between homologous chromosomes takes place during pachytene of prophase I of meiosis I.

 

Q7. Describe the following:
Draw a diagram to illustrate your answer.

(a)synapsis
Answer: Synapsis- The process of the pairing of homologous chromosomes during the zygotene stage of prophase I of “meiosis I” is called synapsis. During synapse, a synaptonemal complex is formed by the homologous chromosome which is referred to as a bivalent or a tetrad.

 

(b) bivalent

Draw a diagram to illustrate your answer.

Answer:

Bivalent- it refers to the structure formed by a pair of the synapsed homologous chromosome.


Chomosome



 

(c)chiasmata


Answer:

Chiasmata- These are points of attachment between homologous chromosomes during their separation in diplotene to the metaphase I stage of meiosis. Chiasmata are initially formed in the regions of crossing over between non-sister chromatids but later on, tend to shift sideways.

Chiasmata


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


Answer:

The process of cytokinesis differs in plant and animal cells

 

 

Cytokinesis in plants

 

Cytokinesis in animals

 

It occurs by cell plate method

 

It takes place by cleavage method

 

The spindle persists during cytokinesis

 

The spindle starts to degenerate after anaphase

 

Cell plate grows centrifugally

 

Cleavage takes place centripetally

 

The new cell membrane is derived from vesicles of the Golgi apparatus

 

The new cell wall 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 the reductional division that leads to the formation of four daughter cells which have half the number of chromosomes than the parent cell. Meiosis takes place during gametogenesis. Spermatogenesis and oogenesis in humans need meiotic division. During spermatogenesis, the four daughter cells or sperms formed are equal in size while during oogenesis the daughter cells are unequal in size. Out of the 4 daughter cells 1 big mature ovum and 3 small polar bodies are formed

 

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

Answer:

 

 

Anaphase of mitosis

 

Anaphase I of meiosis

Anaphase of mitosis involves the splitting of each chromosome at centromere into two sister chromatids which start moving towards

Anaphase I of meiosis I involves the separation of homologous chromosomes, while the chromatids remain attached to their centromere.

 

the opposite poles.

 

Anaphase of mitosis







 
 

 

Q11. List the main differences between mitosis and meiosis.


Answer:

 

Mitosis

 

Meiosis 

 

It takes place in somatic cells

 

It takes place in germ cells

 

The cells undergoing mitosis can be diploid or haploid

 

The cells that undergo meiosis cannot be haploid

 

It involves a single division producing two daughter cells at the end

 

It involves two divisions that give rise to four daughter cells

 

The daughter cells formed after mitosis are exactly similar to their parent cell

 

The daughter cells formed after meiosis are neither similar to the parent one nor to one another

 

The number of chromosomes remains the same after meiosis

 

The number of chromosomes is reduced to half after meiosis.

 

Mitosis helps in the multiplication of cells, healing, and repair.

 

Meiosis is involved in the formation of gametes.


 

Q12. What is the significance of meiosis?


Answer:

The process of meiosis is essential for all sexually reproducing organisms. It occurs in reproductive cells to form gametes that have half the number of chromosomes of the reproductive cells. The two gametes from reproductive cells fuse with each other to form a zygote. As a result, the zygote comes to have a double number of chromosomes.

Thus, meiosis maintains the chromosome number of organisms. apart from this, there are other significances of meiosis also. Some of these are as follows

 

  1. Meiosis form gametes that are required for sexual reproduction
  2. Meiosis maintains the fixed number of chromosomes in sexually reproducing organisms by having the same during gametogenesis
  3. In meiosis, paternal and maternal chromosomes assort independently. It causes a reshuffling of chromosomes and the traits controlled by them. The variations help the breeders in improving the races of useful plants and animals.
  4. Meiosis introduces a new combination of traits or variations.
  5. Chromosomal and genomic mutations occur by irregularities of meiotic division. Some of these mutations are useful to the organism.

 

Q13. Discuss with your teacher about


(i)haploid insects and lower plants where cell division occurs


Answer:

In some lower plants and haploid insects such as drones of a honey bee, meiosis occurs in the zygote. This type of meiosis is called zygotic meiosis whereas this type of life cycle is called the haplontic life cycle.

 

(ii)some haploid cells in higher plants where cell division does not occur.


Answer:

Synergids and antipodals are haploid cells in higher plants where cell division does not occur.

 

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

Answer:

Without DNA replication in the ‘S' phase, mitosis cannot occur. During DNA replication, DNA duplicates and this is a very important step. If DNA duplication does not occur in mitosis, there will be a reduction in the number of chromosomes of daughter cells. Thus, mitosis cannot occur without DNA replication.

 

Q15. Can there be DNA replication without cell division?


Answer:

Yes, DNA replication can occur without being followed by cell division. For example, polyteny is a condition in which chromosomes repeatedly replicate but the cells do not divide and as a result of this, DNA accumulates in the cell.

 

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


Answer:

The number of chromosomes (N) per cell changes during the anaphase I stage of meiosis I. During this stage, the homologous chromosomes get separated and start moving towards the opposite poles. Thus, the bivalents get divided into sister chromatids. This leads to a reduction of the number of chromosomes to half in daughter cells.
 

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Also See    
NCERT Solutions for Class 11 Biology Chapter 1 - The Living World NCERT Solutions for Class 11 Biology Chapter 2 - Biological Classification NCERT Solutions for Class 11 Biology Chapter 3 - Plant Kingdom
NCERT Solutions for Class 11 Biology Chapter 4 - Animal Kingdom NCERT Solutions for Class 11 Biology Chapter 5 - Morphology of Flowering Plants NCERT Solutions for Class 11 Biology Chapter 6 - Anatomy of Flowering Plants
NCERT Solutions for Class 11 Biology Chapter 7 - Structural Organization in Animals NCERT Solutions for Class 11 Biology Chapter 8 - Cells: The Unit of Life NCERT Solutions for Class 11 Biology Chapter 9 - Biomolecules
NCERT Solutions for Class 11 Biology Chapter 11 - Transport in Plants NCERT Solutions for Class 11 Biology Chapter 12 - Mineral Nutrition NCERT Solutions for Class 11 Biology Chapter 13 - Photosynthesis in Higher Plants
NCERT Solutions for Class 11 Biology Chapter 14 - Respiration in Plants NCERT Solutions for Class 11 Biology Chapter 15 - Plant Growth and Development NCERT Solutions for Class 11 Biology Chapter 16 - Digestion and Absorption
NCERT Solutions for Class 11 Biology Chapter 17 - Breathing and Exchange of Gases NCERT Solutions for Class 11 Biology Chapter 18 - Body Fluids and Circulation NCERT Solutions for Class 11 Biology Chapter 19 - Excretory Products and their Elimination
NCERT Solutions for Class 11 Biology Chapter 20 - Locomotion and Movement NCERT Solutions for Class 11 Biology Chapter 21 - Neural Control and Coordination NCERT Solutions for Class 11 Biology Chapter 22 - Chemical Coordination and Integration

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