RBSE Class 11 Biology Important Questions Chapter 10 Cell Cycle and Cell Division

Rajasthan Board RBSE Class 11 Biology Important Questions Chapter 10 Cell Cycle and Cell Division Important Questions and Answers.

RBSE Class 11 Biology Chapter 10 Important Questions Cell Cycle and Cell Division

Multiple Choice Type Questions

Question 1.    
Crossing over occurs between:
(a) non sister chromatid of bivalent
(b) two bivalents
(c) sister chromatids of a bivalent
(d) two nuclei
Answer:
(a) non sister chromatid of bivalent

Question 2.    
Synaptonemal complex is formed during:
(a) zygotene
(b) leptotene
(c) diakinesis
(d) pachytene
Answer:
(b) leptotene

Question 3.    
Centromeres takes part in:
(a) transcription
(b) crossing over
(c) poleward movement of chromosome
(d) cytoplasmic cleavage
Answer:
(c) poleward movement of chromosome

Question 4.    
Crossing over requires an enzyme called:
(a) ligase    
(b) recombinase
(c) endonuclease    
(d) polymerase
Answer:
(b) recombinase

Question 5.    
Name the phase of prophase - I when synapto - nemal complex dissolves, chromatids become clear and bivalents are called tetrads
(a) pachytene    
(b) diakinesis
(c) diplotene    
(d) zygotene
Answer:
(c) diplotene

Question 6.    
Yeast completes the cell cycle in:
(a) 30 minutes
(b) 60 minutes
(c) 90 minutes
(d) 120 minutes
Answer:
(c) 90 minutes

Question 7.    
In which stage are the chromosomes arranged in equatorial plate?
(a) Metaphase    
(b) Telophase
(c) Anaphase    
(d) Prophase
Answer:
(a) Metaphase    

Question 8.    
Correct sequence of different phases of cell cycle is:
(a) S - G₁ - G₂ - M    
(b) G₁ - S - G₂ - M
(c) G - G₂ - S - M    
(d) G₂ - S - G₁ - M
Answer:
(b) G₁ - S - G₂ - M

Question 9.    
Main protein of mitotic spindle is:
(a) dynein    
(b) tubulin
(c) myosin    
(d) tropomyosin
Answer:
(b) tubulin

Question 10.    
Exchange of segment between paternal and maternal chromosomes during meiosis is:
(a) synapsis
(b) dyad formation
(c) bivalent formation
(d) crossing over
Answer:
(d) crossing over

Question 11.    
Synapsis is complete. The cell is said to have entered:
(a) zygotene    
(b) pachytene
(c) diplotene    
(d) diakinesis
Answer:
(b) pachytene

Question 12.    
Complex formed by a pair of synapsed homologous chromosomes is called:
(a) axoneme
(b) equatorial plate
(c) kinetochore
(d) bivalent
Answer:
(d) bivalent

Question 13.    
Synapsis occurs between:
(a) spindle fibres and centromeres
(b) mRNA and ribosomes
(c) a male and a female gamete
(d) two homologous chromosomes
Answer:
(d) two homologous chromosomes

Question 14.    
DNA replication occurs in:
(a) G₁ phase    
(b) G₂ phase
(c) S phase    
(d) G₀ phase
Answer:
(c) S phase    

Question 15.    
The enzyme recombinase is required in which stage of meiosis?
(a) Zygotene    
(b) Pachytene
(c) Diplotene    
(d) Diakinesis
Answer:
(b) Pachytene

Question 16.    
Some cells in the adult animal do not divide. They exit Gx phase and enter an inactive stage which is called as:
(a) G₂ phase    
(b) G₀ phase
(c) S - phase    
(d) M - phase
Answer:
(b) G₀ phase

Question 17.    
In a DNA molecule, distance between two basis is:
(a) 2 nm/20 Å    
(b) 0.2 nm/2 Å
(c) 3 nm/34 Å    
(d) 0.34 nm/3.4 Å
Answer:
(d) 0.34 nm/3.4 Å

Question 18.    
Which of the following condition is called monosomic?
(a) 2n + 1    
(b) 2n + 2
(c) n + 1    
(d) 2n - 1
Answer:
(d) 2n - 1

Question 19.    
At metaphase, chromosomes are attached to the spindle fibres by their:
(a) satellites
(b) secondary constrictions
(c) kinetochores
(d) centromeres
Answer:
(c) kinetochores

Question 20.    
Identify the meiotic stage in which the homologous chromosomes separate while the sister chromatids remain associated at their centromeres.
(a) Metaphase - I    
(b) Metaphase - II
(c) Anaphase - I    
(d) Anaphase - II
Answer:
(a) Metaphase - I    

Question 21.    
Meiosis takes place in:
(a) conidia    
(b) gemmules
(c) megaspore    
(d) meiocyte
Answer:
(d) meiocyte

Question 22. 
Meiosis results in:
(a) production of gametes
(b) reduction in number of chromosomes
(c) introduction of variation
(d) All of the above
Answer:
(d) All of the above

Question 23.    
At which stage of meiosis does the genetic constitution of gametes is finally decided?
(a) Metaphase - I
(b) Anaphase - II
(c) Metaphase - II
(d) Anaphase - I
Answer:
(d) Anaphase - I
 
Question 24.    
Meiosis occurs in organisms during:
(a) sexual reproduction
(b) vegetative reproduction
(c) both sexual and vegetative reproduction
(d) None of the above
Answer:
(a) sexual reproduction

Question 25.    
During anaphase - I of meiosis:
(a) homologous chromosomes separate
(b) non homologous autosomes separate
(c) sister chromatids separate
(d) non - sister chromatid separate
Answer:
(a) homologous chromosomes separate

Question 26.    
Mitosis is characterised by:
(a) reduction division
(b) equal division
(c) both reduction and equal division
(d) None of the above
Answer:
(a) reduction division

Question 27.    
A bivalent of meiosis - I consists of:
(a) two chromatids and one centromere
(b) two chromatids and two centromeres
(c) four chromatids and two centromeres
(d) four chromatids and four centromeres
Answer:
(c) four chromatids and two centromeres

Question 28.    
Cells which are not dividing are likely to be at:
(a) G₁    
(b) G₂
(c) G₀    
(d) S - phase
Answer:
(c) G₀

Question 29.    
Which of the events listed below is not observed during mitosis?
(a) Chromation condensation
(b) Movement of centrioles to opposite poles
(c) Appearance of chromosomes with two chromatids joined together at the centromere
(d) Crossing over
Answer:
(d) Crossing over

Question 30.    
Identify the wrong statement about meiosis:
(a) Pairing of homologous chromosomes
(b) Four haploid cells are formed
(c) At the end of meiosis the number of chromosomes are reduced to half
(d) Two cycles of DNA replication occur
Answer:
(d) Two cycles of DNA replication occur

Question 31.    
Select the correct statement about G1 phase:
(a) Cell is metabolically inactive
(b) DNA in the cell does not replicate
(c) It is not a phase of synthesis of macromolecules
(d) Cell stops growing
Answer:
(b) DNA in the cell does not replicate

Question 32.    
During mitosis ER and nucleolus begin to disappear at:
(a) early prophase
(b) late prophase
(c) early metaphase
(d) late metaphase
Answer:
(b) late prophase

Question 33.    
When, during meiosis, chromosomes first appear duplicated, each bivalent is better called:
(a) tetrad    
(b) tetraxon
(c) tetraploid    
(d)tetramerous
Answer:
(a) tetrad    

Question 34.    
Exchange of paternal and maternal chromosome material during cell division is:
(a) synapsis
(b) crossing over
(c) dyad formation
(d) bivalent formation
Answer:
(b) crossing over

Question 35.    
The term synaptonemal complex refers to side of:
(a) chromosome alignment and recombination
(b) chromatid separation
(c) spindle attachment
(d) replication
Answer:
(a) chromosome alignment and recombination

Question 36.    
Zygotic meiosis occurs in:
(a) Pinus
(b) Marchantia
(c) Dryopteris
(d) Chlamydomonas
Answer:
(d) Chlamydomonas

Question 37.    
Which of the following algae exhibits gametic meiosis?
(a) Chara
(b) Sargassum
(c) Polysiphonia
(d) Oldogonium
Answer:
(b) Sargassum

Question 38.    
Meiosis takes place in:
(a) conidia    
(b) gemmules
(c) meiocyte    
(d) megaspore
Answer:
(c) meiocyte    

Question 39.    
In mitosis, where does the chromosome duplication occur?
(a) Interphase
(b) Prophase
(c) Late prophase
(d) Late telophase
Answer:
(a) Interphase

Question 40.    
The centrosome duplicate during the:
(a) S phase of cell cycle
(b) G₂ phase of cell cycle
(c) Prophase of cell cycle
(d) G₁ phase of cell cycle
Answer:
(a) S phase of cell cycle

Question 41.    
During which phase(s) of cell cycle, amount of DNA in a cell remains at 4C level, if the initial amount is denoted as 2C?    
(a) G₂ and M
(b) G₀ and G₁
(c) G₁ and S
(d) Only G₂
Answer:
(d) Only G₂

Question 42.    
What are the spindle fibres that connect the centromere of chromosome to the respective poles called?
(a) Astral rays
(b) Interpolar fibres
(c) Chromosomal fibres
(d) Inter chromosomal fibres
Answer:
(c) Chromosomal fibres

Question 43.    
A cell at telophase state is observed by a student in a plant brought from the field. He tells his teacher that this cell is not like other cells at telophase stage. There is no formation of cell plate and thus the cell is containing more number of chromosomes as compared to other dividing cells. This would result in:
(a) polyteny
(b) aneuploidy
(c) polyploidy
(d) somaclonal variations
Answer:
(c) polyploidy

Question 44.    
Spindle fibres attach on to:
(a) telomere of the chromosome
(b) centromere of the chromosome
(c) kinetosome of the chromosome
(d) kinetochore of the chromosome
Answer:
(d) kinetochore of the chromosome

Question 45.    
Which of the following is not a characteristic feature during mitosis in somatic cells?
(a) Synapsis
(b) Spindle fibres
(c) Chromosome movement
(d) Disappearance of nucleolus
Answer:
(a) Synapsis

Question 46.    
In meiosis, crossing over is initiated at:
(a) zygotene
(b) diplotene
(c) pachytene
(d) leptotene 
Answer:
(c) pachytene

Question 47.    
During cell growth, DNA synthesis takes place in:
(a) G₁ phase
(b) M phase
(c) S phase
(d) G₁ phase 
Answer:
(c) S phase

Question 48.    
When cell has stalked DNA replication fork, which checkpoint should be predominantly activated?
(a) M    
(b) G₁ / S
(c) G₂ / M    
(d) Both G2M and M
Answer:
(c) G₂ / M

Very Short Answer Type Questions

Question 1. 
Name the stage of cell division in which paired homologous chromosomes get shortened and thickened?
Answer:
Pachytene.

Question 2. 
Which structure of animal cell forms the asters of spindle?
Answer:
Centrosome.

Question 3. 
Name the cells in which meiosis occur?
Answer:
Reproductive or germ cells.

Question 4. 
At which stage of meiosis crossing over of genetic material takes place?
Answer:
Pachytene.

Question 5. 
What is G₀ phase?
Answer:
A stage when cell cycle is arrested during interphase is called G₀ phase.

Question 6. 
Name the cell division concerned with cancer. 
Answer:
Mitosis.

Question 7. 
What are kinetochores?
Answer:
Small disc shaped structure at the surface of the centromeres.

Question 8. 
What is interkinesis?
Answer:
The stage between two meiotic divisions.

Question 9. 
Why is mitosis called equational division? 
Answer:
The chromosomes number in daughter cells is equal to that of the parent cell.

Question 10. 
Name the stage of meiosis during which synaptonemal complex is formed?
Answer:
Zygotene.

Question 11. 
Name the parts of plants and animals where mitosis takes place?
Answer:
Meristematic tissues in plants. Somatic cells in animals.

Question 12. 
What is the role of asters in cell division? 
Answer:
To push the centriole pairs to the opposite ends. 

Question 13. 
What is meant by term ‘congression’ in cell divison?
Answer:
The directed movement of the chromosomes to the equator of the spindle to form metaphase plate.

Question 14. 
Mention the relationship between cell plate and middle lamella in plant cell.
Answer:
A fully formed cell plate is called middle lamella. 

Question 15. 
In what features, the chromosome differ among themselves?
Answer:
Prom, size, position of centromere, secondary constriction and length of arms.

Question 16. 
What are tractile fibres?
Answer:
The chromosomal fibres of the mitotic spindle are also called tractile fibres. They link the chromosomes to the spindle poles.

Question 17. 
What are mitogenes? Give examples.
Answer:
Agents which induce mitosis e.g., lymphokines, growth factors.

Question 18. 
Which are haploid; gametes or spores?
Answer:
Both are haploid.

Short Answer Type Questions - I

Question 1. 
Define the terms: 
(i) Spireme stage 
(ii) Astral rays 
(iii) Aster.
Answer:
(i) Spireme stage: It is the stage of early prophase in which the chromosome overlap one another with their ends not discernible as yet.
(ii) Astral rays: These are radiating microtubular fibrils which appear around the centriole pair in dividing animal cell.
(iii) Aster: It is a star shaped complex consisting of a centriole pair and radiating astral rays found at the polar end of the spindle in dividing animal cells.

Question 2. 
What are metaphase and metaphasic plate?
Answer:
(a) Metaphase: It is second stage of karyokinesis in which the replicated chromosomes are brought over equatorial region of spindle for their equilable distribution.
(b) Metaphasic plate: Metaphasic or equatorial plate is a plate like configuration produced by coming together of all the chromosomes at the equator of the spindle.

Question 3. 
Where do we find 
(a) Mid body
(b) Phragmoplast?
Answer:
(a) Mid body: It is equatorial region of dividing animal cells in the begining of cytokinesis.
(b) Phragmoplast: Equatorial part of spindle in dividing plant cells in the begining of cytokinesis.

Question 4. 
What is the difference between sister and non sister chromatids?
Answer:
Sister chromatids are the two chromatids of the same chromosome, while the non sister chromatids are chromatids of two different chromosomes of a homologous pair.

Question 5. 
What is terminalisation? Where will you find this event?
Answer:
Terminalisation: It is the shifting of chiasmata outwards towards the ends of a bivalent. It can be observed in diplotene stage of prophase - I.

Short Answer Type Questions - II

Question 1. 
Distinguish between metaphase of mitosis and metaphase - I of meiosis.
Answer:

Metaphase of Mitosis	Metaphase - I of Meiosis
1. Each chromosome consists of two chromatids which are held together by centromere.	1. Homologous chromosomes form bivalent. Each bivalent consists of four chromatids and two centromeres.
2. The chromosomes line up in one plane to make the equatorial plate.	2. Bivalents become arranged in the plane of the equator forming equatorial plate.

Question 2. 
Write three processes which take place in interphase.
Answer:
There are following three main phases of interphase:
(a) G₁ Phase: Cells increase in size, cells remain metabolically active and perform their normal activities.
(b) S - Phase: DNA replication occurs during this phase.
(c) G₂ Phase: The cells will continue to grow and many organelles divided during this phase.

Question 3. 
Enumerate the significance of mitosis.
Answer:

• The number of chromosomes in mitosis remains constant in daughter cells.
• Asexual reproduction occurs with the help of mitosis.
• Size of cell is controlled by mitosis.
• Growth and development of the zygote is maintained through mitosis.

Question 4. 
Give the terms for the following:
(a) The phase between two successive mitotic divisions.
(b) Cell division in which chromosome number is halved.
(c) Phase in cell cycle where DNA is synthesized.
(d) Division of nuclear material.
(e) Method of cytokinesis in animal cell.
(f) Method of cytokinesis in plant cell.
Answer:
(a) Interphase 
(b) Meiosis 
(c) S - phase 
(d) Karyokinesis 
(e) Furrowing method 
(f) Cell plate formation method.

Question 5. 
What is significance of meiosis?
Answer:

• It reduces number of chromosomes to half in daughter cells.
• Gametes are formed as a result of meiosis. Each gamete possesses half the number of chromosome present in somatic cells.
• It is very essential phenomenon in life cycle of sexually reproducing organisms as it restores the fixed number of chromosomes.
• It avoids the multiplication of chromosomes and thus maintain the stability and constant number of chromosomes of the species.
• During the crossing over, exchange of genetic material between non sister chromatids results variations that produce genetic recombinations (recombinants).

Long Answer Type Questions

Question 1. 
Discuss the various stages of cell cycle.
Answer:
In prokaiyotes, the cell cycle is quite simple : The cell grows, its DNA replicates, and the cell divides. This form of division in prokaryotes is called asexual reproduction. The duration of cell cycle (also called generation time) in prokaryotes is less than eukaryotes e.g., in bacteria it is of 20 minutes.
In Eukaryotes, the cell cycle is complicated. The generation time of eukaryotic cell like a mammalian cell is about 24 hours. The cell cycle of eukaryotes can be divided into two main phases : A long inter phase (I phase) and a shorter phase called mitotic phase (M. phase) or the phase of nuclear division. These main phases are followed by a still shorter phase of cytoplasmic division called cytokinesis or C phase.


(A) Interphase:
It prepares the cell to enter the mitotic phase so also called preparatory phase. Interphase of the eukaryotic cell cycle can be subdivided into following three phases :

1. Growth phase - 1 (G₁): During this phase, a cell undergoes rapid growth and the cell performs its routine functions. During this phase, the biosynthetic and metabolic activities of the cell occur at a high rate. It involves the synthesis of amino acids for protein synthesis, synthesis of nucleotides and various enzymes required for DNA replication. The time taken in G₁ phase is 6 hours in human cell cycle.

2. Synthesis Phase (S): In this phase, DNA synthesis or replication takes place. Since two daughter cells will be formed after mitotic division therefore, the genetic material (DNA) must be copied. This is why, the amount of DNA per cell becomes double during this phase. If the initial amount of DNA is denoted as 2C then it increases to 4C. However, there is no increase in the chromosome number. If the cell had diploid (2 n) number of chromosomes at G₁, even after ‘S’ phase the number of chromosomes remains same (i.e., 2 n). In animal cells, during the S phase, DNA replication begins in the nucleus and the centriole duplicates in the cytoplasm.

3. Growth Phase - 2 (G₂): It is also called post synthetic or premitotic phase. During this phase, synthesis of spindle proteins, RNA molecules (on DNA template) and ATP molecules occur for the ‘M’ phase. The cell organelles like mitochondria, plastids and centrioles becomes double. The time taken by this phase is 6 hours in human cell cycle.

(B) Mitotic Phase or M Phase
During this phase, the nucleus divides as the chromosomes are equally separated. One nucleus becomes two nuclei, each with an identical set of chromosomes. The phases of mitosis (prophase, metaphase, anaphase and telophase) are followed by cytokinesis. When the cytoplasm divides, resulting in two cells. After cytokinesis, the cell division is complete. Thus a parent cell (dividing cell) forms two genetically identical daughter cells. After M phase, formed daughter cells may enter either the G₁ phase of next cell cycle or a G₀ phase.

G₀ Phase
It is an inactive stage, also called quiescent stage. In this phase, the cell has left the cycle and has stopped dividing. These cells may remain in G₀ phase for long period e.g., heart cells, neurons. However, some cells of G₀ phase like fibroblasts, which help in healing of wounds, grow and divide again according to need of the body cells in this stage remain metabolically active but no longer proliferate unless called on to do so depending on the requirement of the organism.

Question 2. 
Discuss the behaviour of chromosomes during the cell division in a somatic cells. Draw the diagram also.
Answer:
It is also called somatic cell division or equational division. In this division a somatic cell divides into two daughter cells in which number of chromosome remains same as parent cell. The formed daughter cells are quantitatively as well as qualitatively similar to the parent cell so it is called equational division.

Discovery
It was discovered in plant cells by a German biologist Strasburger (1875 A.D.) and in animal cells by another German biologist W. Flemming (1879 A.D.). The term mitosis was coined by W. Flemming in 1882. Its detail here was given by Schneider (1887 A.D.).

Duration
It ranges from 30 minutes to 3 hours. Duration is specific for different species and it also depends upon types of tissues and temperature.

Occurence
Mitosis occurs in both somatic cells (during growth) as well as germ cells of the gonads (during multiplication phase of gametogenesis). In plants, mitosis occurs in the meristematic cells (e.g., root apex and shoot apex). In man, it occurs is skin, bone marrow etc. It also occurs during embryonic development.

Phases of Mitosis
Mitosis can be divided into two phases:    
(A) Karyokinesis 
(B) Cytokinesis.

(A) Karyokinesis: It involves following four stages: prophase, metaphase, anaphase and telophase.
Prophase
It is the stage of longest duration, it can be further divided into three sub phases:

(i) Early Prophase: It is characterised by formation of asters and condensation of chromatin. Centrospheres are disappeared and proteinous microtubules are developed between two centriole pairs. Centriole pairs (diplosome) start moving towards opposite poles. Each centriole pair develops fine, radiating protein microtubules called astral rays forming star shaped body called aster. Each centriole pair is separated from the astral rays by a clear cytoplasmic area called pericenfriolar cloud. Nuclear chromatin condenses into long, thin, thread like chromosomes by dehydration, spiralization.

Each chromosome is formed of two sister chromatids joined at primary constriction or centromere or kinetochore. Kinetochore is a discoidal area on each chromatid and is the site of attachment of chromosomal spindle fibre. Two chromatids are coiled on their own to form minor coils as well as around each other to form major or somatic coils. Chromosomes overlap one another forming a mass, like a ball of wool called spireme stage (Gk spirema =a coil).

(ii) Mid Prophase: It is characterised by more condensation of chromosomes so that these become shorter and thicker. The chromosomes become distinct from one another. So somatic coils increase in size but decrease in number. Two asters move away further from each other due to more elongation of the microtubules. Nuclear membrane (fragments merge into E.R.) and nucleolus (sheds its RNA over the chromosomes) start disappearing.

(iii) Late Prophase: It is also called prometaphase. It is characterised by complete disappearance of nuclear membrane and nucleolus. So chromosomes come freely in cytoplasm. Two asters migrates and reach on their respective (opposite) poles. The proteinous microtubules between them take the form of spindle. The central broader part of spindle is called equator, while narrow ends of spindle having asters are called poles. Such spindle with two asters, one at each pole of the spindle is called bipolar or amphiastral spindle. It is characteristic of animal cell. While in plant cells the spindle is anastral as these have no centrosome. The spindle fibres are of two types:

Discontinuous or Chromosomal fibres: These extend from pole to kinetochores of the chromosome.

Continuous or Interpolar fibres: These extend from pole to pole without coming in contact with centromere. Each spindle fibre is formed of 4 - 20 microtubules which are chemically formed of mainly tubulin protein. Both the spindle and aster are collectively called mitotic apparatus. But the astral rays and centrioles do not contribute to spindle formation.

Metaphase
In this phase the chromosomes now become still more shorter and thicker and are clearly visible. So metaphase is considered to be the best stage to study chromosomes in a cell by scientists. Now the chromosomes start arranging themselves on the median line or equatorial plane of the cell. In metaphase, the centromeres of all chromosomes exactly lie on the median line of the cell with the two chromatids of the chromosomes facing opposite poles of the cell.

Anaphase
It is easily recognisable stage. In this phase, the centromere of each chromosome split so that two sister chromatids separate having their own centromeres. They are now called daughter chromosomes. Thus, each daughter chromosome is with only one chromatid. Daughter chromosomes move towards opposite poles. They may be V shaped called isobrachial or homobrachial or rod shaped called telocentric. Anaphase ends when all the daughter chromosomes reach the opposite poles. So anaphase divides the nuclear matter into two equal and similar halves so called equational division.


The forces which are responsible for anaphase movement of chromosomes is created by:

• Contraction of chromosomal spindle fibres in the presence of energy provided by ATP.
• Stretching of continuous fibres resulting elongation of spindle.
• Repulsion between the centromeres of daughter chromosomes.
• Elongation of interzonal fibres.

About 30 ATP molecules are used to move one chromosome from equator to pole.

Telophase
Mitosis ends with telophase or the stage at which the chromosomes reach the poles. The nuclear membrane then reforms, and the chromosomes begin to decondense into their interphase conformations. Nucleolus is reformed from nucleolar organising region of nuclear chromosome. Astral and spindle fibres are absorbed in cytoplasm. These microtubules undergo depolymerisation of tubulin subunits. Centrioles, at each pole, are surrounded by centrosphere to complete the centrosome. 

(B) Cytokinesis: It involves division of cytoplasm.

(i) In animal Cells: In animal cells, during cytokinesis, a contractile ring and actin filaments assemble in the equator of the cell. The contractile ring is made up of non muscle myosin II. Myosin II moves along these actin filaments using the free energy released during the ATP hydrolysis. Then the cell membrane constricts and forms a cleavage furrow. Due to continuous hydrolysis, this cleavage moves inwards. The cell furrow deepens and finally divides the cell into two daughter cells which are about half the volume of original mother cell. This type of cytokinesis is called furrowing or cleavage.

(ii) In Plant Cells: In plant cell, cytokinesis occurs by cell plate formation. A number of elements called phragmoplasts (each about 20 pm) are derived from E.R. and Golgi body. These elements line up at equator and later fuse to form a cell plate which finally grows on both sides and divide a parent cell into two daughter cell.

Question 3. 
Explain the various phases of meiosis - II division.
Answer:
Both the daughter cells obtained from meiosis - II are haploid (n). But their chromosomes have 2 sister chromatids. These sister chromatids require separation by further division. Therefore, each cell act as parent cell and undergoes meiosis-II to form two haploid daughter cells. Since the number of chromosomes remain same in parent as well as daughter cells during the meiosis-II therefore, it is called homotypic or equational division. It can also be divided into two steps, karyokinesis - II and cytokinesis - II.

(A) Karyokinesis - II: It involves the separation of two chromatids of each chromosome and their movement into separate cells. It is divisible into four phases.

1. Prophase - II: It is very short stage, it occurs simultaneously in both the nuclei formed by meiosis - I centrioles, already duplicated, move apart in pairs. Each pair develops astral rays round it to form an aster. Spindle is laid down between the 2 pairs of centrioles. The chromosomes, each comprising two chromatids, become visible in the nucleus. They are set free in the cytoplasm by breakdown of the nuclear envelope. Nucleoli disappear.

2. Metaphase - II: The chromosomes take up positions at the equator of the spindle, forming a single metaphase plate. The chromatid of each chromosome are joined at their kinetochores by spindle fibres extending from the opposite poles of the spindle as in mitosis.

3. Anaphase - II: The two chromatids of each chromosome start moving away from each other. Finally they reach the poles of the spindle. Here they are called chromosomes. Each pole has haploid number of chromosomes and haploid amount of DNA. This amount is one fourth of the DNA present in the original cell that started meiosis.


4. Telophase - II: The group of chromosomes at each pole of the spindle gets enclosed by a nuclear envelope. Nucleoli are laid down. Astral rays and spindle are lost. Cytokinesis - II: It is always present and occurs by cell furrow formation in animal cell and cell plate formation in plant cell. The cytokinesis is called successive type (when it occurs after each karyokinesis) or simultaneous type (when cytokinesis - I is absent and both cytokinetic divisions after karyokinesis - II. So, by the meiosis, a diploid parent cell divides twice forming four haploid gametes or sex cells, each having half the DNA amount than that of the parental cell and one - fourth of DNA present in the cell at the time of beginning of meiosis. These four haploid cells may be arranged in a linear of isobilateral tetrad.

Question 4. 
Illustrate the various sub stage of prophase - I of a reductional division by giving suitable figures.
Answer:
Mitosis can be divided into two phases:
(A) Karyokinesis 
(B) Cytokinesis.
(A) Karyokinesis: It involves following four stages: prophase, metaphase, anaphase and telophase.

Prophase
It is the stage of longest duration, it can be further divided into three sub phases:
(i) Early Prophase: It is characterised by formation of asters and condensation of chromatin. Centrospheres are disappeared and proteinous microtubules are developed between two centriole pairs. Centriole pairs (diplosome) start moving towards opposite poles. Each centriole pair develops fine, radiating protein microtubules called astral rays forming star shaped body called aster. Each centriole pair is separated from the astral rays by a clear cytoplasmic area called pericenfriolar cloud.

Nuclear chromatin condenses into long, thin, thread like chromosomes by dehydration, spiralization. Each chromosome is formed of two sister chromatids joined at primary constriction or centromere or kinetochore. Kinetochore is a discoidal area on each chromatid and is the site of attachment of chromosomal spindle fibre. Two chromatids are coiled on their own to form minor coils as well as around each other to form major or somatic coils. Chromosomes overlap one another forming a mass, like a ball of wool called spireme stage (Gk spirema =a coil).

(ii) Mid Prophase: It is characterised by more condensation of chromosomes so that these become shorter and thicker. The chromosomes become distinct from one another. So somatic coils increase in size but decrease in number. Two asters move away further from each other due to more elongation of the microtubules. Nuclear membrane (fragments merge into E.R.) and nucleolus (sheds its RNA over the chromosomes) start disappearing.

(iii) Late Prophase: It is also called prometaphase. It is characterised by complete disappearance of nuclear membrane and nucleolus. So chromosomes come freely in cytoplasm. Two asters migrates and reach on their respective (opposite) poles. The proteinous microtubules between them take the form of spindle. The central broader part of spindle is called equator, while narrow ends of spindle having asters are called poles. Such spindle with two asters, one at each pole of the spindle is called bipolar or amphiastral spindle. It is characteristic of animal cell. While in plant cells the spindle is anastral as these have no centrosome. 

Question 5. 
Write the difference between mitosis and meiosis.
Answer:
There are three types of cell division occurs in plants and animals. I. Amitosis, II. Mitosis, III. Meiosis.
I. Amitosis (Direct Nuclear Division): It is also called incipient or direct cell division. It is the most uncommon, primitive and simplest type of cell division. It is very rare and of little genetic importance. It is visible only in some unicellular organisms like bacteria, yeast, Amoeba, diatoms etc. It may also be seen in the higher plants but in some very old cells which are degenerating. This may be readegraded as a primitive form of mitosis.

Mechanism of Amitosis: It comprises two steps: 
(A) Karyokinesis 
(B) Cytokinesis.

(A) Karyokinesis: (Gk. karyon = nucleus, kinesis = movement), it involves division of nucleus. The nucleus starts elongating, thus a constriction appears approximately in the middle. The construction gradually deepens and eventually gives rise to two daughter nuclei. The two daughter nuclei thus formed are not equal in size.

(B) Cytokinesis: It involves division of cytoplasm. In this stage, a constriction appears in the cytoplasm which divides it into two parts, each with a daughter nucleus. But the division of cytoplasm is not essential. Some times it does not occur after nuclear division. The result is multinucleate mass of protoplasm known as coenocyte and syncytium in case of plants and animals respectively.


Meiosis is a process where a single cell divides twice to produce four cells containing half the original amount of genetic information. These cells are sex cells as sperms in males, as eggs in females.

Occurence
Meiosis occurs in diploid sex cells or gamete forming cells present in sex organs that are testis and ovaries. In male these cells are primary spermatocytes while in female these are primary oocytes. Each diploid spermatocyte divides by meiosis to produce four haploid sperm cells (male gamete). Similarly, a diploid oocyte produces haploid ovum by meiosis. In plants, meiosis occurs in pollen mother cells (microsporocytes) of another and ovules of ovary to produce haploid spores. The cells undergoing meiosis are called meiocyte.

Discovery
Meiosis was discovered and described for the first time in sea urchin eggs in 1876 by the German biologist Oscar Hertwig. It was described again in 1883, at the level of chromosomes by Van Beneden in ascaris eggs. The term ‘Meiosis’ was given by Farmer and Moore (1905 A.D.) from a Greek word meiosum which means to reduce.

Question 6. 
Write short notes on the following:
(i) Amitosis.
(ii) Importance of mitosis for living organisms.
(iii) Difference between cytokinesis in animal cell and plant cell.
(iv) Types of meiosis.
Answer:
(i) There are three types of cell division occurs in plants and animals. I. Amitosis, II. Mitosis, III. Meiosis.
I. Amitosis (Direct Nuclear Division): It is also called incipient or direct cell division. It is the most uncommon, primitive and simplest type of cell division. It is very rare and of little genetic importance. It is visible only in some unicellular organisms like bacteria, yeast, Amoeba, diatoms etc. It may also be seen in the higher plants but in some very old cells which are degenerating. This may be readegraded as a primitive form of mitosis.
Mechanism of Amitosis: It comprises two steps: 
(A) Karyokinesis 
(B) Cytokinesis.

(A) Karyokinesis: (Gk. karyon = nucleus, kinesis = movement), it involves division of nucleus. The nucleus starts elongating, thus a constriction appears approximately in the middle. The construction gradually deepens and eventually gives rise to two daughter nuclei. The two daughter nuclei thus formed are not equal in size.

(B) Cytokinesis: It involves division of cytoplasm. In this stage, a constriction appears in the cytoplasm which divides it into two parts, each with a daughter nucleus. But the division of cytoplasm is not essential. Some times it does not occur after nuclear division. The result is multinucleate mass of protoplasm known as coenocyte and syncytium in case of plants and animals respectively.

(ii) Genetic Stability: Mitosis helps in the splitting of chromosomes during cell division and generates two new daughter cells. Therefore, the chromosomes form from the parent chromosome by copying the exact DNA. Therefore, the daughter cells formed as genetically uniform and identical to the parent as well as to each other. Thus, mitosis helps in preserving and maintaining the genetic stability of a particular population.

Growth: Mitosis help in increasing the number of cells in a living organism thereby playing a significant role in the growth of a living organism.

Replacement and Regeneration of New Cell: Regeneration and replacement of worn out and damaged tissues is a very important function of mitosis in living organisms. Mitosis helps in the production of identical copies of cells and thus helps in repairing the damaged tissue or replacing the worn out cells. But the degree of regeneration and replacement in multicellular organisms vary from one another. For example, mitosis process is used in order to regrowth the legs of newts and crustaceans.
Asexual Reproduction: Mitosis is used in the production of genetically similar offspring. For example budding of Hydra and yeast, binary fission in Amoeba etc.

(iii)

Animal Cell Mitosis	Plant Cell mitosis
1. An animal cell becomes rounded before cell division.	1. Plant cell do not change shape before the division.
2. A number of hormones are known to induce cell division but a specific cell division hormone is not known.	2. It is induced by a specific plant hormone called cytokinin.
3. Centrosome is essential for it.	3. Centrosome does not occur.
4. Mitotic apparatus contains aster, spindle is amphiastral.	4. Mitotic apparatus is without asters. Spindle is anastral.
5. Spindle degenerates at the time of cytokinesis.	5. A major part of spindle persists as phragmoplast at the time of cytokinesis.
6. A mid body may be formed during cytokinesis.	6. Mid body is absent.
7. Cytokinesis occurs through cleavage.	7. Cytokinesis occurs commonly by cell plate method.
8. Microfilaments are involved in cytokinesis.	8. Microfilaments do not seem to have any major function of cytokinesis.
9. Cleavage proceeds centripetally. (from out to in).	9. Cell plate grows centrifugally. (from in to out).
10. A furrow is formed between two daughter cells.	10. A solid middle lamella develops in between the two daughter cells for permanent adhesion.
11. It occurs in bone marrow and many epithelial cells.	11. It is found in meristematic cells.

(iv) Depending upon the stage, when meiosis occur, the meiosis is of three types: Gametic, zygotic and sporic.
1. Gametic or Terminal Meiosis: In most animals and some lower plants meiosis takes place during the formation of gametes (gametogenesis). Such a meiosis is described as gametic or terminal meiosis. When two gametes fuse to fertilisation, the diploid zygote is formed. Genetic meiosis results in diplontic life cycle.

2. Zygotic or Initial Meiosis: In some lower plants meiosis takes place in the zygote and the resulting organisms are haploid. It is called zygotic or initial meiosis. Organisms having zygote meiosis have haplontic life cycle.

3. Sporic or Intermediate Meiosis: In most of the plants, meiosis occur at the time of sporogenesis. It is called sporic or intermediate meiosis. Spore gives rise to a new 
gametophytic phase in the life cycle. The gametophyte produces gametes. The life of plant is said to be diplohaplontic because of the presence of diploid and haploid multicellular phases.