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1800-102-2727In 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.
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:
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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.
(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.
Q8. How does cytokinesis in plant cells differ from that in animal cells?
Answer:
The process of cytokinesis differs in plant and animal cells
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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:
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Q11. List the main differences between mitosis and meiosis.
Answer:
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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
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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