NCERT Exemplar Class 11 Biology Solutions Chapter 10 Cell Cycle and Cell Division

Multiple Choice Questions:

Question:1

Meiosis in diploid organisms results in
(a) Production of gametes
(b) Reduction in the number of chromosomes
(c) All of the above
(d) Introduction of variation

Answer:

The answer is the option (c) All of the above
Explanation: Meiosis results in a reduction of the number of chromosomes and gametes are formed only after meiosis. Crossing over during meiosis leads to variations. Hence, all the options are correct.

Question:2

At which stage of meiosis does the genetic constitution of gametes is finally decided
(a) Metaphase I
(b) Anaphase I
(c) Metaphase II
(d) Anaphase II

Answer:

The answer is the option (b) Anaphase I
Explanation: In anaphase I of meiosis, the actual reduction occurs which decides the genetic constitution of the gamete. Every single homologous chromosome with its two chromatids and unsegregated centromere moves to the opposite poles of the cell.

Question:3

Meiosis occurs in organisms during
(a) Sexual reproduction
(b) Vegetative reproduction
(c) Both sexual and vegetative reproduction
(d) None of the above

Answer:

The answer is the option (c) Sexual reproduction
Explanation: Option ‘a’ is correct because there is no need for meiosis during vegetative reproduction.

Question:4

During anaphase-I of meiosis
(a) Non-sister chromatids separate
(b) Non-homologous autosomes separate
(c) Sister chromatids separate
(d) Homologous chromosomes separate

Answer:

(d) Homologous chromosomes separate
Explanation: Homologous chromosomes segregated during Anaphase I.

Question:5

Mitosis is characterised by
(a) Reduction division
(b) Both reduction and equal division
(c) Equal division
(d) Pairing of homologous chromosomes

Answer:

The answer is the option (c) Equal division
Explanation: There is no reduction in chromosomes, and hence mitosis is also called an equal division.

Question:6

A bivalent of meiosis-I consists of
(a) Four chromatids and two centromere
(b) Two chromatids and two centromere
(c) Two chromatids and one centromere
(d) Four chromatids and four centromere

Answer:

Ans. The answer is the option (a) Four chromatids and two centromere
Explanation: Bivalent is formed by pairing of two chromosomes. Hence, bivalent contains four chromatids and four centromeres.

Question:7

Cells which are not dividing are likely to be at
(a) G1
(b) G2
(c) S phase
(d) G0

Answer:

The answer is the option (d) G0
Explanation: Phase G0 is also called the quiescent phase, and no cell division takes place during this phase.

Question:8

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

Answer:

The answer is the option (a) Crossing over
Explanation: Crossing overtakes place only during prophase I of meiosis.

Question:9

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

Answer:

The answer is the option (b) Two cycles of DNA replication occur
Explanation: The number of chromosomes in daughter cells gets half, so there is no need for another cycle of DNA replication.

Question:10

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

Answer:

The answer is the option (c) DNA in the cell does not replicate
Explanation: The cell is metabolically active during the G1 phase and continues to grow and carries out the synthesis of macromolecules, but DNA synthesis does not take place in this phase.

Very Short Answer Type Questions:

Question:1

Between a prokaryote and a eukaryote, which cell has a shorter cell division time?

Answer:

Prokaryote has a shorter division time.

Question:2

Which of the phases of the cell cycle is of longest duration?

Answer:

Interphase is the longest phase of the cell cycle.

Question:3

Name a stain commonly used to colour chromosomes.

Answer:

Acetocarmine and Giemsa stain is used to colour chromosomes.

Question:4

Which tissue of animals and plants exhibits meiosis?

Answer:

Germ cells of male and female reproductive organs in animals and plants exhibit meiosis.

Question:5

Given that the average duplication time of E coli is 20 minutes how much time will two E coli cells take to become 32 cells.

Answer:
It will take them about 1 hour 20 minutes.
Number of cells after 20 minutes = 4 Number of cells after 40 minutes = 8 Number of cells after 60 minutes = 16 Number of cells after 80 minutes = 32

Question:6

Which part of the human body should one use to demonstrate stages in mitosis?

Answer:
To demonstrate stages of mitosis in humans beings, any part that contains somatic cells can be used.

Question:7

What attributes does a chromatid require to be classified as a chromosome?

Answer:
To be classified as a chromosome. There should be two identical sister chromatids attached at the center by the centromere.

Question:8

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

Answer:

In this situation, non-sister chromatids can cross over. Crossing over always takes place between two non-sister chromatids and never between two sister chromatids.

Question:9

If a tissue has at a given time 1024 cells, how many cycles of mitosis had the original parental single cell undergone?

Answer:
2¹⁰ = 1024
The above equation proves that a cell must undergo 10 rounds of mitosis to produce 1024 cells.

Question:10

An anther has 1200 pollen grains. How many pollen mother cells must have been there to produce them?

Answer:

1200 / 4 = 300 (one mother cell produces four daughter cells after the process of meiosis). Therefore, there must be 300 pollen mother cells to produce 1200 pollen grains.

Question:11

At what stage of the cell cycle does DNA synthesis take place?

Answer:

DNA synthesis takes place during the S phase.

Question:12

It is said that the one cycle of cell division in human cells (eukaryotic cells) takes 24 hours. Which phase of the cycle, do you think occupies the maximum part of the cell cycle?

Answer:

Interphase is the phase of the cycle that occupies the maximum part of the cell cycle.

Question:13

It is observed that heart cells do not exhibit cell division. Such cells do not divide further and exit _____ phase to enter an inactive stage called _____ of cell cycle.

Fill in the blanks.

Answer:

G1 phase, quiescent phase

Question:14

In which phase of meiosis are the following formed? Choose the answers from hint points given below.
(a) Synaptonemal complex _____
(b) Recombination nodules _____
(c) Appearance/activation of ____ enzyme recombinase
(d) Termination of chiasmata ____
(e) Interkinesis _____
(f) Formation of dyad of cells _____

Answer:
(a) Zygotene
(b) Pachytene
(c) Telophase - I/After Meiosis – I
(d) Diakinesis
(e) After Telophase - I/before Meiosis – II
(f) Pachytene

Short Answer Type Questions:

Question:1

State the role of centrioles other than spindle formation.

Answer:

Apart from its role in spindle formation, Centrioles also forms the basal body of cilia and flagella. They also aid in the formation of microtubules and sperm tails.

Question:2

Mitochondria and plastids have their own DNA (genetic material). What is known about their fate during nuclear division like mitosis?

Answer:
The DNA in mitochondria and plastids are extra-genomic DNA in nature. They have no correlation with the nuclear division during mitosis. Thus, karyokinesis during cell division has no effect on extra-genomic DNAs.

Question:3

Label the diagram and also determine the stage at which this structure is visible

Answer:

Transition from prophase to metaphase during mitosis.

Question:4

A cell has 32 chromosomes. It undergoes mitotic division. What will be the chromosome number (N) during metaphase? What would be the DNA content (C) during anaphase?

Answer:

The number will remain 32 because the number of chromosomes does not change during metaphase or anaphase. DNA replication takes place during the S phase and hence, the DNA content gets doubled after the S phase; and will remain so during mitosis.

Cell Cycle and Cell Division Exercise: 1.4

Question:5

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 cells with 16 chromosomes could have arisen from cells with 32 chromosomes or vice versa?

Answer:

The difference in the chromosome numbers can be attributed to Mosaicism. It is the phenomenon that describes the existence of two types of chromosome numbers in a different cell of the body. This situation arises due to lag of anaphase or non-disjunction of chromosomes.

Question:6

The following events occur during the various phases of the cell cycle, Name the phase against each of the events.
(a) Disintegration of nuclear membrane
(b) Appearance of nucleolus
(c) Division of centromere
(d) Replication of DNA

Answer:

(a) Prophase
(b) Telophase
(c) Anaphase
(d) S-phase

Question:7

Mitosis results in producing two cells which are similar to each other. What would be the consequence if each of the following irregularities occur during mitosis?

(a) Nuclear membrane fails to disintegrate
(b) Duplication of DNA does not occur
(c) Centromeres do not divide
(d) Cytokinesis does not occur.

Answer:

(a) The spindle fibres would not be able to reach chromosomes if the nuclear membrane fails to disintegrate. Because of this, the chromosomes would not be able to reach the opposite poles of the cell.

(b) The cell may not be able to reach the M phase if the DNA duplication does not take place. This shall result in the cessation of the cell cycle.

(c) If the division of centromeres does not take place, then one of the daughter cells will get a complete pair of chromosomes, and another daughter cell will get none. This may result in trisomy.

(d) If cytokinesis does not occur, then a cell with the multinucleate condition would be formed.

Question:8

Both unicellular and multicellular organisms undergo mitosis. What are the differences, if any, observed in the process between the two?

Answer:

In unicellular organisms, the cells are divided into two parts. This type of cell division is called amitosis. In multicellular organisms, karyokinesis takes place before the division of cytoplasm. In amitosis, duplication of DNA content takes place simultaneously with the division of the nucleus. In the case of multicellular organisms, there is a separate S phase for duplication of DNA.

Question:9

Name the pathological condition when uncontrolled cell division occurs.

Answer:

Malignancy is the pathological condition when uncontrolled cell division occurs.

Question:10

Two key events take place, during S phase in animal cells, DNA replication and duplication of the centriole. In which parts of the cell do events occur?

Answer:

While the process of the duplication of the centriole takes place in the cytoplasm, the process of DNA replication occurs in the nucleus

Question:11

Comment on the statement - Meiosis enables the conservation of specific chromosome number of each species even though the process per se, results in a reduction of chromosome number

Answer:

During fertilization, when two haploid cells (gametes) fuse, the formation of diploid zygote takes place. In case of absence of meiosis, production of two diploid gametes would take place, result in the formation of tetraploid zygote. Meiosis is extremely important to conserve the chromosome number

Question:12

Name a cell that is found arrested in diplotene stage for months and years. Comment in 2-3 lines how it completes the cell cycle?

Answer:

The cell that get arrested in the diplotene stage is the oocyte of mammalian females. It grows and forms secondary follicle, then subsequently its growth is arrested at the diplotene stage. The first meiosis gets completed when the woman gains sexual maturity. The meiosis takes place one by one in the egg that is released from the ovary.

Question:13

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

Answer:

Long Answer Type Questions:

Question:1

Comment on the statement - Telophase is reverse of prophase.

Answer:

Following contrasting features between prophase and telophase prove that telophase is reverse of prophase.
Prophase -

1. Mitochondria, Golgi Complex, ER, etc. disappear in the prophase.

2. Nuclear membrane starts to fall apart during the prophase.

3. Chromosomes become clearly visible in the prophase.

Telophase -

1. Mitochondria, ER, Golgi Complex, etc. reappear in the telophase.

2. Nuclear membrane starts to integrate during the telophase.

3. Chromosomes start to lose their visibility in this telophase.

Question:2

What are the various stages of meiotic prophase-I? Enumerate the chromosomal events during each stage?

Answer:

These are the different stages of meiotic prophase:

(a) Leptotene: During the stage of leptotene, the chromosomes are slowly visible under a light microscope. The compression of chromosomes continues until the end of this stage. The sister chromatids which are tightly bound so that they cannot be differentiated from each other.

(b) Zygotene: Zygotene is the stage where the chromosomes start forming into pairs.This process of formation is called synapsis. Formation of synapsis goes along with the formation of synaptonemal complexes. Each pair formed in this process is called a bivalent or tetrad.

(c) Pachytene: In the stage of pachytene, the bivalent chromosomes get detached.Reconnection of the nodules starts to appear on the tetrads. The recombination nodule is the site at which the crossing over of the non-sister chromatid takes place. Crossing over happens between non-sister chromatids of the homologous chromosomes.Crossing over helps in the exchange of genes between two homologous chromosomes.

(d) Diplotene: In the stage of diplotene, the synaptonemal complex starts to break down. The reconnected chromosomes begin to segregate from each other. But they are connected at the site of crossing over. This makes an X-shaped structure called chiasmata.

(e) Diakinesis: In the Diakinesis stage, the Chiasmata is terminated. The nucleolus disappears in this process, and so does the nuclear membrane.

Question:3

Differentiate between the events of mitosis and meiosis.

Answer:

Question:4

Write a brief note on the following
(a) Synaptonemal complex
(b) Metaphase plate

Answer:

(a) Synaptonemal complex consists of protein. It is made between two homologous chromosomes in the process of meiosis. It is the site of matching, synapsis and recombination. During prophase–I of meiosis, the homologous chromosomes get matched through the synaptonemal complex. Through this complex, the exchange of genes takes place. Hence, reconnection or recombination of genetic characters happens through a synaptonemal complex.

(b) The metaphase plate is an imaginary line which is at equal distances from the two centrosome poles. During the process of metaphase, the chromosomes get together at the metaphase plate. The even alignment of chromosomes happens because of the facing kinetochore microtubules.

Question:5

Write briefly the significance of mitosis and meiosis in multicellular organisms.

Answer:
Significance of Mitosis:

Mitosis facilitates the development of single-cell zygote into a full-grown organism. It helps growth of an organism. It helps repair worn out or damaged tissues. It helps vegetative propagation in flowering plants. It helps regeneration in some animals.

Significance of Meiosis:

Meiosis makes sure that the number of chromosomes becomes diploid in gametes. This aids in making sure that the conservation of particular chromosomal numbers in species. Meiosis helps genetic variability in populations. This happens due to crossing over during prophase–I.

Question:6

An organism has two pairs of chromosomes (i.e., chromosome number = 4). Diagrammatically represent the chromosomal arrangement during different phases of meiosis-II.

Answer:

The following diagram shows different phases of meiosis—II of an organism which has two pairs of chromosomes.