CBSE Class 11 Biology Cell Cycle And Cell Division Notes

Cell Cycle and Cell Division Class 11 Notes

POINTS TO REMEMBER

Cell cycle : The sequence of events by which a cell duplicates its genome, synthesis the other constituents of the cell and eventually divides into two daughter cells.

Phases of cell cycle :

Interphase :

• G1 Phase : Cell metabolically active and grows continuously.

• S Phase : DNA synthesis occurs, DNA content increases from 2C to 4C. but the number of chromosomes remains some (2N).

• G2 Phase : Proteins are synthesised in preparation for mitosis while cell growth continues.

M Phase (Mitosis Phase) : Starts with nuclear division, corresponding to the separation of daughter chromosomes (karyokinesis) and usually ends with division of cytoplasm (cytokinesis).

Quiescent stage (G0) : Cells that do not divide and exit G1 phase to enter an

inactive stage called G0. Cells at this stage remain metabolically active but do not proliferate.

MITOSIS

Prophase : (i) Replicated chromosomes, each consisting of 2 chromatids, condense and become visible.

(ii) Microtubules are assembled into mitotic spindle.

(iii) Nucleolus and nuclear envelope disappear.

(iv) Centriole moves to opposite poles.

Metaphase : (i) Spindle fibres attached to kinetochores (small disc-shaped structures at the surface of centromers) of chromosomes.

(ii) Chromosomes line up at the equator of the spindle to form a metaphase plate.

Anaphase : (i) Centromeres split and chromatids separate.

(ii) Chromatids move to opposite poles.

Telophase : (i) Chromosomes cluster at opposite poles.

(ii) Nuclear envelope assembles around chromosome cluster.

(iii) Nucleolus, golgi complex, ER reform.

Cytokinesis : Is the divison of protoplast of a cell into two daughter cells after Karyokinesis (nuclear division).

Animal cytokinesis : Appearance of furrow in plasma membrane which deepens and joins in the centre dividing cell cytoplasm into two.

Plant cytokinesis : Formation of new cell wall begins with the formation of a simple precursor  cell plate which represents the middle lamella between the walls of two adjacent cells.

Significance of Mitosis :

1. Growth addition of cells.

2. Maintenance of surface/volume ratio.

3. Maintenance of chromosome number.

4. Regeneration.

5. Reproduction in unicellular organism.

6. Repair and wound healing.

Meiosis :

• Specialised kind of cell division that reduces the chromosome number by half, resulting in formation of 4 haploid daughter cells.

• Occurs during gametogenesis in plants and animals.

• Involves two sequential cycles of nuclear and cell division called Meiosis I and Meiosis II.

• Interphase occurs prior to meiosis which is similar to interphase of mitosis except the S phase is prolonged.

• 4 haploid daughter cells are formed.

Meiosis I

Prophase I : Subdivided into 5 phases.

Leptotene :

• Chromosomes make their as single stranded structures.

• Compaction of chromosomes continues.

Zygotene :

• Homologous chromosomes start pairing and this process of association is called synapsis.

• Chromosomal synapsis is accompanied by formation of synaptonemal complex.

• Complex formed by a pair of synapsed homologous chromosomes is called bivalent or tetrad.

Pachytene : Crossing over occurs between non-sister chromatids of homologous chromosomes.

Diplotene : Dissolution of synaptonemal complex occurs and the recombined chromosomes separate from each other except at the sites of crossing over. These X-shaped structures are called chaismata.

Diakinesis : • Terminalisation of chaismata.

• Chromosomes are fully condensed and meiotic spindles assembled.

• Nucleolus disappear and nuclear envelope breaks down.

Anaphase I : Homologous chromosomes separate while chromatids remain associated at their centromeres.

Telophase I :
• Nuclear membrane and nucleolus reappear.
• Cytokinesis follows (diad of cells).
Interkinesis : Stage between two meiotic divisions. (meiosis I and meiosis II)

Meiosis II

Prophase II

• Nuclear membrane disappears.
• Chromosomes become compact.

Metaphase II
• Chromosomes align at the equator.
• Microtubules from opposite poles of spindle get attached to kinetochores of sister chromatids.

Anaphase II
• Simultaneous splitting of the centromere of each chromosome, allowing them to move towards opposite poles of the cell.

Telophase II
• Two groups of chromosomes get enclosed by a nuclear envelope.
• Cytokinesis follows resulting in the formation of tetrad of cells i.e., 4 haploid cells.
Significance of Meiosis

1. Formation of gametes : In sexually reproducing organisms.

2. Genetic variability

3. Maintenance of chromosomal number : By reducing the chromosome number in gametes. Chromosomal number is restored by fertilisation of gametes.

MCQ Questions for NCERT Class 11 Biology Cell Cycle and Cell Division

Question. Which of the following statements is correct about cell cycle?
(a) In G1-phase, the amount of cytoplasm and DNA gets duplicated.
(b) In S-phase, the DNA replication occurs and chromosome number is doubled.
(c) DNA replication and duplication of centrioles occurs in S-phase along with histone synthesis.
(d) DNA amount is doubled and chromosome number is doubled in G2-phase.

Answer : C

Question. Which of the following statements is not true?
(a) With end of cytokinesis, mitosis is considered to be completed.
(b) In animal cell, the division starts as a furrow in the plasma membrane which deepens to progress and meet in the centre to cut out two free cells.
(c) Cytokinesis procedure is similar in both plant and animal cells.
(d) The two daughter cells are exactly similar to the parent cell in mitosis.

Answer : C

Question. Which of the following statements is/are correct for meiosis?
I. Meiosis is a double division. It gives rise to four cells.
II. The cells undergoing meiosis may be haploid or diploid.
III. No bouquet stage is recorded.
IV. Pairing or synapsis of homologous chromosomes takes place during zygotene of prophase-I and continues upto meta phase-I.
(a) I only
(b) I and IV
(c) II and III
(d) I, II, III and IV

Answer : B

Question. Mitotic anaphase differs from metaphase in possessing
(a) same number of chromosomes and same number of chromatids
(b) half number of chromosomes and half number of chromatids
(c) half number of chromosomes and same number of chromatids
(d) same number of chromosomes and half number of chromatids.

Answer : D

Question. Diplotene stage of prophase-I is characterised by
(a) dissolution of synaptonemal complex
(b) separation of synapsed homologous chromosomes except at the site of crossovers
(c) formation of X-shaped structures called chiasmata
(d) all of these.

Answer : D

Ques. Which of the following options gives the correct sequence of events during mitosis ?
(a) Condensation → Nuclear membrane disassembly → Arrangement at equator → Centromere division → Segregation → Telophase
(b) Condensation → Crossing over → Nuclear membrane disassembly → Segregation → Telophase
(c) Condensation → Arrangement at equator → Centromere division → Segregation → Telophase
(d) Condensation → Nuclear membrane disassembly → Crossing over → Segregation → Telophase 

Answer: A

Ques. Anaphase Promoting Complex (APC) is a protein degradation machinery necessary for proper mitosis of animal cell. If APC is defective in a human cell, which of the following is expected to occur?
(a) Chromosomes will be fragmented.
(b) Chromosomes will not segregate.
(c) Recombination of chromosome arms will occur.
(d) Chromosomes will not condense. 

Answer: B

Ques. Spindle fibres attach on to
(a) centromere of the chromosome
(b) kinetosome of the chromosome
(c) telomere of the chromosome
(d) kinetochore of the chromosome. 

Answer: D

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

Answer: B

Ques. During the metaphase stage of mitosis, spindle fibres attach to chromosomes at
(a) kinetochore
(b) both centromere and kinetochore
(c) centromere, kinetochore and areas adjoining centromere
(d) centromere.

Answer: C

Ques. Select the correct option with respect to mitosis.
(a) Chromatids separate but remain in the centre of the cell in anaphase.
(b) Chromatids start moving towards opposite poles in telophase.
(c) Golgi complex and endoplasmic reticulum are still visible at the end of prophase.
(d) Chromosomes move to the spindle equator and get aligned along equatorial plate in metaphase.

Answer: D

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

Answer: C

Ques. During mitosis, ER and nucleolus begin to disappear at
(a) late prophase
(b) early metaphase
(c) late metaphase
(d) early prophase.

Answer: D

Ques. If you are provided with root-tips of onion in your class and are asked to count the chromosomes, which of the following stages can you most conveniently look into?
(a) Metaphase
(b) Telophase
(c) Anaphase
(d) Prophase 

Answer: A

Ques. Which one of the following precedes reformation of the nuclear envelope during M phase of the cell cycle?
(a) Decondensation from chromosomes and reassembly of the nuclear lamina
(b) Transcription from chromosomes and reassembly of the nuclear lamina
(c) Formation of the contractile ring and formation of the phragmoplast
(d) Formation of the contractile ring and transcription from chromosomes 

Answer: C

Ques. Mitotic spindle is mainly composed of which protein?
(a) Actin
(b) Myosin
(c) Actomyosin
(d) Myoglobin 

Answer: C

Ques. Best material for the study of mitosis in laboratory is
(a) anther
(b) root tip
(c) leaf tip
(d) ovary. 

Answer: B

 Important Questions for NCERT Class 11 Biology Cell Cycle and Cell Division

Question. G1 phase of the cell cycle is usually absent in

(a) Hela cells in culture
(b) senescent fibroblast cells
(c) early cleavage stage cells
(d) intestinal epithelial cells.

Answer : C

Question. Swollen bead like areas considered to be genes can be observed on chromosomes in ______ subphase of meiosis.
(a) leptotene
(b) zygotene
(c) pachytene
(d) none of these

Answer : A

Question. In which stage of cell cycle nucleus becomes bigger?
(a) G1
(b) G2
(c) S
(d) M

Answer : C

Question. There are 14 chromosomes in each cell and 2C DNA after M-phase in a plant. Find out the correct match in the following table.
Stage Chromosome No. DNA content
A. G1 14 2C
B. S 28 4C
C. G2 14 4C
D M 14 2C
(a) A and B are correct.
(b) A, B and D are correct.
(c) A, C and D are correct.
(d) B, C and D are correct.

Answer : C

Question. Which of the following are essential features of meiosis?
(i) Two successive divisions but no DNA replication prior to second division.
(ii) Crossing over
(iii) Formation of chiasmata
(a) (i) and (ii) (b) (ii) and (iii)
(c) (i) and (iii) (d) (i), (ii) and (iii)

Answer : D

Ques. Spindle fibre unite with which structure of chromosomes?
(a) Chromocentre
(b) Chromomere
(c) Kinetochore
(d) Centriole 

Answer: C

Ques. Microtubule is involved in the
(a) muscle contraction
(b) membrane architecture
(c) cell division
(d) DNA recognition. 

Answer: C

Ques. How many mitotic divisions are needed for a single cell to make 128 cells?
(a) 28
(b) 32
(c) 7
(d) 14 

Answer: C

Ques. During cell division in apical meristem, the nuclear membrane appears in
(a) telophase
(b) cytokinesis
(c) metaphase
(d) anaphase. 

Answer: A

Ques. Which of the following structures will not be common to mitotic cell of a higher plant?
(a) Centriole
(b) Spindle fibre
(c) Cell plate
(d) Centromere 

Answer: A

Ques. Colchicine is an inhibitory chemical, which
(a) stops the functioning of centriole
(b) prevents attaching of centromeres with rays
(c) prevents the spindle formation in mitosis
(d) prevents the formation of equatorial plane.

Answer: C

Ques. Which of the following represents the best stage to view the shape, size and number of chromosomes?
(a) Prophase
(b) Metaphase
(c) Interphase
(d) Telophase 

Answer: B

Ques. Mitotic anaphase differs from metaphase in possessing
(a) same number of chromosomes and same number of chromatids
(b) half number of chromosomes and half number of chromatids
(c) half number of chromosomes and same number of chromatids
(d) same number of chromosomes and half number of chromatids. 

Answer: D

Ques. Dissolution of the synaptonemal complex occurs during
(a) pachytene
(b) zygotene
(c) diplotene
(d) leptotene.

Answer: C

Ques. Match the following with respect to meiosis.
(A) Zygotene     (i) Terminalization
(B) Pachytene   (ii) Chiasmata
(C) Diplotene    (iii) Crossing over
(D) Diakinesis   (iv) Synapsis
Select the correct option from the following
     (A) (B) (C) (D)
(a) (iii) (iv) (i) (ii)
(b) (iv) (iii) (ii) (i)
(c) (i) (ii) (iv) (iii)
(d) (ii) (iv) (iii) (i) 

Answer: B

Ques. Crossing over takes place between which chromatids and in which stage of the cell cycle ?
(a) Non-sister chromatids of non-homologous chromosomes at Zygotene stage of prophase I.
(b) Non-sister chromatids of homologous chromosomes at Pachytene stage of prophase I.
(c) Non-sister chromatids of homologous chromosomes at Zygotene stage of prophase I.
(d) Non-sister chromatids of non-homologous chromosomes at Pachytene stage of prophase I.

Answer: B

Ques. The stage during which separation of the paired homologous chromosomes begins is
(a) pachytene
(b) diplotene
(c) diakinesis
(d) zygotene. 

Answer: B

Ques. Match the stages of meiosis in column I to their characteristic features in column II and select the correct option using the codes given below.
     Column I                Column II
A. Pachytene         (i) Pairing of homologous chromosomes
B. Metaphase I      (ii) Terminalisation of chiasmata
C. Diakinesis        (iii) Crossing-over takes place
D. Zygotene         (iv) Chromosomes align at equatorial plate
(a) A-(iii), B-(iv), C-(ii), D-(i)
(b) A-(i), B-(iv), C-(ii), D-(iii)
(c) A-(ii), B-(iv), C-(iii), D-(i)
(d) A-(iv), B-(iii), C-(ii), D-(i) 

Answer: A

Ques. In meiosis crossing over is initiated at
(a) zygotene
(b) diplotene
(c) pachytene
(d) leptotene.

Answer: C

Ques. Arrange the following events of meiosis in correct sequence
(i) Crossing over (ii) Synapsis
(iii) Terminalisation of chiasmata
(iv) Disappearance of nucleolus
(a) (i), (ii), (iii), (iv) (b) (ii), (iii), (iv), (i)
(c) (ii), (i), (iv), (iii) (d) (ii), (i), (iii), (iv)

Answer: D

Read the following passage and answer the questions.

Mitosis is the type of division in which chromosomes replicate and become equally distributed both quantitatively and qualitatively into two daughter nuclei. Thus, it is also called equational division since the chromosome number of the parent is conserved in the daughter cell. On the basis of nuclear division, M-phase is divided into four stages: Prophase, Metaphase, Anaphase and Telophase. Mitosis starts with nuclear division followed by daughter chromosome separation and ends with division of cytoplasm. It is necessary for growth and development of a multicellular organism.

Question. Which among the following is the longest and shortest phase in mitosis?
(a) Telophase and Prophase
(b) Metaphase and Anaphase
(c) Prophase and Anaphase
(d) Anaphase and Prophase

Answer : C

Question. X phase is characterised by condensation of chromosomal material to form compact mitotic chromosomes X is
(a) prophase
(b) anaphase
(c) metaphase
(d) telophase.

Answer : A

Question. Which of the following is the best stage to study the morphology of chromosomes?
(a) Prophase
(b) Metaphase
(c) Anaphase
(d) Telophase

Answer : B

Question. How many mitotic divisions are required for a cell to make 512 cells?
(a) 8
(b) 6
(c) 7
(d) 9

Answer : D

Question. Read the given statements and select which ones are true (T) and which ones are false (F).
I. Germ cells are formed by mitosis.
II. Mitosis is capable to regenerate whole organism.
III. Mitosis involves multiplication of both unicellular and multicellular organisms.
IV. Mitosis increases the nucleocytoplasmic ratio.
I II III IV
(a) F F T F
(b) T T T F
(c) T F F F
(d) F T T F

Answer : D

Very Short Answer Type Questions

Question. Can there be DNA replication without cell division?

Answer. Yes, DNA replication takes place during the synthetic stage of interphase in the cell cycle. It is totally independent of cell division. After G2-phase a cell may or may not enter into the M-phase. 

Question. Write the name of the phase that occurs between the two successive mitotic divisions.

Answer. Interphase is the period between the two successive mitotic cell divisions.

Question. The diagram shows a bivalent at prophase-I of meiosis. Which of the four chromatids can cross over? (img 18)

Answer. The non-sister chromatids of homologous pair of chromosomes undergo crossing over.

Question. Distinguish cytokinesis from karyokinesis?

Answer. Cytokinesis is the division of cytoplasm whereas karyokinesis is the division of nucleus of the cell.

Question. What is the average cell cycle span for a mammalian cell?

Answer. The average cell cycle span for a mammalian cell is 24 hours.

Short Answer Type Questions

Question. Analyse the events during every stage of cell cycle and briefly explain in relation with chromosome number and DNA content.

Answer. (i) During the complete cell cycle, the number of chromosomes (n) per cell remains same. It does not show any changes if the division is of mitosis type. (ii) The overall DNA content will remains same however it becomes doubles during the S-phase and afterwards become half during the dividing phase.

Question. There occurs a process in which division of nucleus takes place. Identify the process and also write about its different phases.

Answer. Karyokinesis is a series of uninterrupted changes before forming two daughters nucleus. Though karyokinesis is a continuous process, it has been divided into four phases. They are prophase, metaphase, anaphase and telophase.

Question. While examining the mitotic stage in a tissue, one finds some cells with 16 chromosomes and some with 32 chromosomes. What possible reasons could you assign to this difference in chromosome number. Do you think that cell with 16 chromosomes number could have arisen from cell with 32 chrmosomes or vice-versa?

Answer. Difference in chromosome number during mitotic division might be due to polyploidy, i.e., having more than two sets of chromosomes. It occurs due to failure of chromosomes to separate during anaphase or due to non-disjunction. This change in chromosome number can be due to mutations. Cells with 32 chromosomes might have arisen from cells with 16 chromosomes.

Question. Mention the significance of meiosis.

Answer. Meiosis is the mechanism by which conservation of specific chromosome number of each species is achieved across generations in sexually reproducing organisms. It also increases the genetic variability in the population of organism from one generation to the next. Variations are very important for the process of evolution.

Question. Differentiate between S-phase and G2-phase.

Answer. Differences between S-phase and G2-phase are :

Question. Name the stage of cell cycle at which one of the following events occur?
(i) Chromosomes move to spindle equator.
(ii) Centromere splits and chromatids separate.
(iii) Pairing between homologous chromosomes take place.
(iv) Crossing over between homologous chromosomes take place.

Answer. (i) Metaphase
(ii) Anaphase
(ii) Zygotene stage of meiosis-I
(iv) Pachytene stage

Question. The diagram below represents the changes in the number of chromosomes during several processes that occur in an animal. (img 19)
(i) Name the process of cell division occurring at X and Y.
(ii) State two differences in the behaviour of chromosomes between X and Y.

Answer. (i) X-Mitosis Y-Meiosis
(ii) (a) In X, there is no association of homologous chromosomes but in Y, homologous chromosomes pair up together to form bivalents during prophase-I. (b) In X, there is no crossing over as there is no formation of chiasmata but in Y, crossing over occurs at the chiasmata whereby some genes are swapped between homologous chromosomes.

Question. Meiosis is advantageous than mitosis as it produces variations, so it plays important role in evolution. What are the stages during which chances of recombination of genes are increased during meiosis?

Answer. Stages during which recombination of genes occurs in meiosis are
(i) Crossing over is exchange of genes between the nonsister chromatids of homologous chromosomes. It occurs at recombination nodules during pachytene stage of prophase-I of meiosis-I and is regulated by recombinase enzyme.
(ii) Arrangement of bivalents at the equator of the spindle during metaphase-I of meiosis.

Long Answer Type Questions

Question. Describe the following words
(i) Synapsis (ii) Bivalent (ii) Chiasmata

Answer. (i) Synapsis : During meiosis-I, the process of pairing of two homologous chromosomes is known as synapsis. It is so exact that pairing is not merely between corresponding chromosomes but between corresponding individual units.
(ii) Bivalent : The complex formed by a pair of synapsed homologous chromosomes is called a bivalent or a tetrad. It consists of four chromatids.
(iii) Chiasmata : The chiasmata formation is the indication of completion of crossing over and beginning of separation of chromosomes. The chiasma is formed when the chromosomal parts begin to repel each other except in the region where these are in contact. Thus, chiasmata formation is necessary for the separation of homologous chromosomes.

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