Rajasthan Board RBSE Class 11 Biology Chapter 11 Cell Division
RBSE Class 11 Biology Chapter 11 Multiple Choice Objective Questions
Question 1.
In which phase of cell cycle, DNA synthesis occurs-
(a) G1
(b) S
(c) G2
(d) M
Question 2.
Bacterium cell divides by-
(a) Amitosis
(b) Mitosis
(c) Meiosis
(d) None
Question 3.
In which phase of cell division, synapsis can be seen-
(a) Pachytene
(b) Zygotene
(c) Diplotene
(d) Diakinesis
Question 4.
Cell division in which chromosome number is reduced to half-
(a) Meiosis
(b) Mitosis
(c) Amitosis
(d) All
RBSE Class 11 Biology Chapter 11 Very Short Answer Questions
Question 1.
Who described cell cycle and when ?
Answer:
Howard & Pelc (1953).
Question 2.
Give main symptom of amitosis.
Answer:
Dumbble – shaped nucleus.
Question 3.
What is synapsis ?
Answer:
The pairing of homologous chromosomes during zygotene subphase of meiosis.
Question 4.
What is chiasma ?
Answer:
The point of exhcnage of genetic material between homologous chromosomes.
RBSE Class 11 Biology Chapter 11 Short Answer Questions
Question 1.
Describe in brief the amitosis.
Answer:
This is a simplest cell division. It takes place in prokaryotes eg Bacteria and some lower plants eg algae, fungi etc. In this division, the nucleus elongates to become dumble shaped and finally divides into two. Simultaneously, the cytoplasm also divides into 2 and there is formation of 2 daughter cell. The genetic material is approximately divided in this division.
Question 2.
Give the importance of mitosis.
Answer:
Significance of Mitosis
- The mitosis helps in body growth by forming new cells.
- In mitosis, the number of the chromosomes remain unchanged. Hence, the duaghter cells become similar to the parent cell.
- The mitosis help to maintain proper karyoplasmic index in a cell.
- The mitosis helps in the repairing of worn out tissues & healing of wounds.
Question 3.
Why meiosis is essential in living organisms.
Answer:
- It keeps the number of chromosomes constant in a species.
- The exchange of genetic material causes new gentic variations which help in speciation.
- It is essential to complete sexual cycle.
- Lack of meiosis will cause polyploidy. It may be useful in plants but it will be deleterious for animals.
Question 4.
Write the names of subphases of prophase – I.
Answer:
Lepotene, Zygotene, Pachytene. Diplotene & Diakinesis.
RBSE Class 11 Biology Chapter 11 Essay Type Questions
Question 1.
What do you mean by cell cycle ? Describe cell cycle with help of diagram.
Answer:
The cell cycle is an orderly sequence of events by which a growing cell duplicates its contents and divides into two. In other word, the life cycle of any specific cell is called as cell cycle It was first described by Howard and Pelc (1953). The cell cycle is a complex and cyclic process of eukaryotic cells. It includes cell growth, duplication of genome, nuclear division & cytoplasmic division. The cell cycle of a somatic cell consists of two phases viz. –
(A) Interphase and
(B) Mitotic phase (M-phase)
Interphase:
It is the metabolically active phase that is further divisible into 3 phases viz. -G1 phase, S-phase & G2 phase.
- G1 phase [Gap-1 phase]
It is the longest phase of interphase. The chromosomes are found in the form of invisible chromatin network. Synthesis of the proteins and RNA which are required for DNA replication. The DNA value is 2C. - S-phase [Synthetic phase]
There is Synthesis of DNA by replication. 1 he DNA value will increase gradually. There is duplication of centrosome.
- G2 Phase [Gap 2-phase]
There is synthesis of RNA and proteins which are required during mitosis. No DNA replication and the DNA value becomes 4C. There is duplication of cytoplasmic orgenelles.
Note – In G1 – phase, the cell decide to divide or not to divide.
During this phase, the cell enters into prophase, metaphase, anaphase and telophase and finally forms two daughter cells.
Question 2.
Explain various phases of mitosis
Answer:
[Gr. Mitos = Wrap thread]
This is the most common type of cell division that occurs both in somatic and germ cells. It results in the formation of two equal daughter cells in which the chromosomes number remains unchanged. The term “mitosis” was coined by Walther Flemming (1882).
The mitosis can be studied as follows-
(A) Karyokinesis:
It involves division of the nucleus. It is further divided into four phases –
- Prophase – The chromosomes become visible like thin and long threads due to dehydration. The centromere is distinct. The nuclear membrane and the nucleolous finally get disintegrated. Both the centrioles reach ultimately to the opposite poles. The centrosphere part forms the arsters and the spindle fibres (Fig. 11.3).
- Metaphase – Further shortning and thickning of the chromosomes due to spiralization and chromatids become distinct. Attachment of the spindle fibres to the centromere parts of the chromosomes. The chromosomes get arranged at the equitorial plate (Meta phasic plate) with help of the spindle fibres.
- Anaphase – The centromeres divide to form daughter chromosomes. The daughter chromosomes separate and begin to move towards the opposite poles. Gradually, the daughter chromosomes reach to the respective poles
- Telophase – Disappearance of the asters and the spindle fibres. Reformation of the nuclear membrane and the nucleolous. The chromosomes become long and thread like due to despiralization. Finally, the chromosomes become invisible due to hydration. The parent cell has two distinct nuclei in this stage.
(B) Cytokinesis:
The karyokinesis is followed by cvtokinesis which divides the cytoplasm into two almost eqal parts. the cytokiness is different in the animal cells and plant cells. In the plant cells the cytkinesis beings wth the formation of cell plate. In the animal cells, it beings with the formation of a peripheral groove. In plant cells, the granules of calcium & magnesium pectate get accumulated during formation of cell plate.
Significance of Mitosis:
- The mitosis helps in body growth by forming new cells.
- In mitosis, the number of the chromosomes remain unchanged. Hence, the duaghter cells become similar to the parent cell.
- The mitosis help to maintain proper karvoplasmic index in a cell.
- The mitosis helps in the repairing of worn out tissues & healing of wounds.
Question 3.
Describe the various phases of prophase – I of meiosis. Give diagrams.
Answer:
The meiosis is a special cell division that takes place in deploid primary spermatocytes, primary oocytes and zygotes of thallophytes. It results in the formation of haploid gametes. The number of chromosomes in the gametes is half of the parent cell. The meiosis involves double division.
I. Meiosis – I
It is also called as reductional division. It results in half number of chromosomes. It completes in two phases viz-
(A) Karyokinesis
(B) Cytokinesis.
(A) Karyokinesis:
It divides the nuclear substances and involves following four phases viz –
(a) Prophase – I:
It is the longest phase of the meiosis. It is a complex phase and it is further divided into four subphases :
- Leptotene : The chromosomes appear like long and thin thread due to dehydration. These chromosomes show bead- loke chromomers. It is also called as bouquet-phase. Formation of asters and centrioles begin to move to the opposite poles.
- Zygotene : Further shortning and thicking of the chromosomes due to spiralization. There is pairing of homologous chromosomes which is called as synapsis. The homologous pair becomes bivalent.
- Pachytene: There is further shortning and thicking of the chromosomes, each chromosome develops two chromatids, hence, the homologous pair forms tetravalent or tetrad. There is exchange of genetic material between chromatids of homologous chromosomes (non-sister chromatids) which is called as crossing over. This process involves recombinant enzymes. Little DNA replication also takes place during this phase.
- Diplotene : There is incomplete separation of homologous chromosomes. There is formation of charsmata. The chiasma is the point of where both the homologous chromosomes remain linked, the number of chiasmata depends upo nthe length of the chromosomal arms.
- Diakinesis : There is further separation of the homologous chromosomes as a result of the chiasmata more towards the ends. It is called as terminalization of chiasmata. Formation of spindle fibres and disappearance of nuclear membrane and nucleolous.
(b) Metaphase – I: The spindle figres get attached to the homologous chromosomes. These homologous pairs get arranged at the equitorial plate or metaphasic plate. The centromeres remain towards the poles and the arms remain at the metaphasic plate.
(c) Anaphase – I : The homologous chromosomes move apart and form daughter chromosomes. The daughter chromosomes begin to move and finally reach to the respective poles. Hence, every pole receives half number of chromosomes.
(d) Telophase – I : Disappearance of the asters and the spindle fibres. Reappearance of the nuclear membrane and nucleolous. Elongation of the chromosomes due to despiralization.
(B) Cytokinesis – I
It results in the formation of two haploid daughter cells. It’s process is similar to the mitosis cytokinesis.
II. Meiosis – II
It is also called as equational division. It is almost similar to the mitosis. The interphase – II is normally absent as all the required preparation takes place in interphase – I.
(A) Karyokinesin – II
- Prophase – II : Similar to mitosis prophase.
- Metaphase – II : Similar to mitosis prophase.
- Telophase – II : Similar to mitosis prophase.
Cytokinesis – II
Similar to mitosis cyto-kinesis. It finally results in the formation of four haploid daughter cells.
Significance of Meiosis:
- It keeps the number of chromosomes constant in a species.
- The exchange of genetic material causes new gentic variations which help in speciation.
- It is essential to complete sexual cycle.
- Lack of meiosis will cause polyploidy. It may be useful in plants but it will be deleterious for animals
Question 4.
Differentiate mitosis & meiosis
Answer:
| S.NO | Mitosis | Meiosis |
| 1 | It occurs in somatic cells & germ cells. | It occurs only in primary oocytes & primary spermatocytes. |
| 2 | It results in body growth | It forms gametes. |
| 3 | It results in the formation of 2 daughter cells. | It forms 4 daughter cells. |
| 4 | The chromosome number. remains unchanged. |
The chromosome number becomes half. |
| 5 | Prophase is comparatively shorter | Prophase-I is longest phase. |
| 6 | Prophase is simple. | Prophase-I is complex & consists of 5 subphases. |
| 7 | No exhcnage of genes | Exchange of gene takes place during pachytene. |
| 8 | Formation of bivalent. | Formation of tetravalent. |
| 9 | Centromeres remain in one line during metaphase. | Centromeres form two line during metaphase-I. |
| 10 | No synapsis formation. | Formation of synapsis. |