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

                (ii) amount of DNA content (C) per cell

The amount of DNA content per cell changes during S phase and anaphase. In S phase, the number of chromosomes remains same but DNA replicates. Due to DNA replication, the content of DNA doubles up from 1C to 2C or from 2C to 4C depending on whether the cell is diploid or haploid. In anaphase II, the DNA Content decreases to one half from 2C to 1C. 

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

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 starts 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. 

15. Can there be DNA replication without cell division?

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

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

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

Q13.    Discuss with your teacher about

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

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

13.  Discuss with your teacher about

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

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 haplontic life cycle.   

12. What is the significance of meiosis?

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...

11. List the main differences between mitosis and meiosis.

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...

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

Anaphase of mitosis Anaphase I of meiosis                                                                                                                  Anaphase of mitosis involves splitting of each chromosome at centromere into two sister chromatids which start moving towards the opposite poles. Anaphase I of meiosis I involve the separation of homologous chromosomes, while the...

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

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 spermatogenesis and oogenesis in humans. During spermatogenesis, the daughter cells or sperms formed are equal in size while during oogenesis the daughter cells are unequal in size. 

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

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 Spindle starts to degenerate after anaphase Cell plate grows centrifugally  Cleavage takes place centripetally The new cell membrane is derived from vesicles of Golgi...

7. Describe the following:

            (c) chiasmata

Draw a diagram to illustrate your answer.

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

7. Describe the following:

            (b) bivalent

Draw a diagram to illustrate your answer.

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

Q7.    Describe the following:

            (a) synapsis

Draw a diagram to illustrate your 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.  

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

            (iv) Crossing over between homologous chromosomes takes place.

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

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

            (iii) Pairing between homologous chromosomes takes place.

     (iii) Pairing between homologous chromosomes takes place        The pairing between homologous chromosomes takes place during zygotene of prophase I of meiosis I. As a result of this pairing bivalents are formed

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

            (ii) Centromere splits and chromatids separate.

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

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

            (i) Chromosomes are moved to spindle equator.

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

5. Why is mitosis called equational division?

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

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

Go (quiescent phase)- It refers to the stage of inactivation of 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 centre of the root tip
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