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

NCERT
SOLUTIONS
CLASS - 11TH

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Class : 11th
Subject : Biology
Chapter : 10
Chapter Name : Cell Cycle and Cell Division

Q1 What is the average cell cycle span for a mammalian cell?

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

Page : 171 , Block Name : Exercise

Q2 Distinguish cytokinesis from karyokinesis.

Answer.

Page : 171 , Block Name : Exercise

Q3 Describe the events taking place during interphase.

Answer. Interphase involves a series of changes that a cell tor division. It is the period which the
ceu experiences growth and DNA replication in an orderly manner. Interphase is divided into three
phases.
(i) GI phase
(ii) S phase
(iii) 62 phase
GI phase — It is the stage during which the cell and prepares its DNA for replication. In this phase,
the cell is metabolically active. S phase — It is the stage during which DNA synthesis occurs. In
this phase, me amount of DNA (per cell) bubbles, but the chromosome number remains the same.

62 phase — In this phase, the cell continues to grow and prepares itself for division. The proteins
and RNA required for mitosis are synthesised during this stage.

Page : 171 , Block Name : Exercise

Q4 What is G (quiescent phase) ot cell cycle?
o

Answer. G or quiescent phase is me stage wherein cells remain metabolically active, but do not
o

proliferate unless called to do so. Such cells are used for replacing the cells lost during injury.

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Q5 Why is mitosis called equational division?

Answer. Mitosis is the process of cen divisbn wherein the chronusornes replicate and get ecpan
distributed into two daughter ceus. The chromosome number in each cell is equal to that in the
parent cell. i.e.. diploid. Hence. mitosis is known as equational division.

Q6 Name the stage of cell cycle at which one of the following events occur:
(i) Chromosomes are moved to spindle equator
(ii) Centromere splits and chromatids separate
(iii) pairing between homologous chromosomes takes place
(iv) Crossing over between homologous chromosomes takes place

Answer. (i) Metaphase
(ii) Anaphase
(iii) Zygotene of meiosis I
(iv) pachytene of meiosis I

Q7 Describe the following:
(a) synapsis
(b) bivalent
(c) chiasmata
Draw a diagram to illustrate your answer.

Answer. (a) Synapsis:
The pairing of homologous chromosomes is called synapsis. This occurs during the second stage of
prophase I or Zygotene.

(b) Bivalent
Bivalent or tetrad is a pair of synapsed homologous chromosomes. They are formed during the
zygotene stage of prophase I of meiosis.

(c) Chiasmata
Chiasmata is the site where two sister chromatids have crossed over. It represents the site of cross-
over. It is formed during the diplotene stage of prophase I of meiosis.

Q8 How does cytokinesis in plant cells differ from that in animal cells?

Answer.

Q9 Find examples where the four daughter cells from meiosis are equal in size and where they are
found unequal in size.

Answer. (a) Spermatogenesis or the formation of sperms in human beings occurs by the process of
meiosis. It results in formation of tour equal-sized daughter cells.
(b) Oogenesis or the formation of ovum in human beings occurs by the process of meiosis. It
results in the formation of tour daughter cells which are Unequal in size.

Q10 Distinguish anaphase of mitosis from anaphase I of meiosis.

Answer.

Q11 List the main differences between mitosis and meiosis.

Answer.

Q12 What is the signi cance of meiosis?

Answer. Meiosis is the process involving the reduction in the amount of genetic material. It
comprises two successive nuclear and cell divisions. with a single cycle of DNA replication. As a
result. at the end of meiosis II. tour haploid cells are Signi cance of meiosis
1. Meiosis maintains the chromosome number from generation to generation. It reduces the
chromosome number to halt so that the præess ot fertilisation restores the original number in the
zygote.
2. Variations are caused by the crossover and the random distribution of homologous
chromosomes between daughter cells. Variables play an important role in evolution.
3. Chromosomal mutations are brought about by the introduction ot certain abnormalities. These
chromosomal mutations may be advantageous for an individual.

Q13 Discuss with your teacher about
(i) haploid insects and lower plants where cell-division occurs, and
(ii) some haploid cells in higher plants where cell-division does not

Answer. (i) In some insects and lower plants, fertilization is immediately followed by zygotic
meiosis, which leads to the production of haploid organisms. This type ot lite cycle is known as
haplontic life cycle.
(ii) The phenomenon of polyploidy can be observed in some haploid cells in higher plants in which
cell division does not occur. polyploidy is a state in mich cells contain multiple pairs of
chromosomes than the basic set. polyploidy can be arti cially induced in plants by applying
colchicine to cell culture.

Q14 Can there be mitosis with 0LR DNA replicaüon in S phase?

Answer. Mitotic cell division cannot take place without DNA replication in S phase. Two important
events take place during S phase — one is the synthesis duplication of DNA and the other is the
duplication of the centriole. DNA duplication is important as it maintains the chromosome
number in the daughter cells. Mitosis is an equational division. Therefore. the duplication of DNA
is an important step.

Q15 Can there be DNA replication without cell division?

Answer. There can be DNA replication without cell division, During cell division. the parent cell
gets divided into two daughter cells. However, if there is a repeated replication of DNA without
any cell division, then this DNA will keep accumulating inside the cell. This would increase the
volume of the cell nucleus, thereby causing cell expansion. An example of DNA replication
without cell division is commonly observed in the salivary glands of Drosophila, The chromosome
undergoing repeated DNA duplication is known as polytene chromosome.

Q16 Analyse the events during every stage of cell cycle and notice how the following two
parameters change
(i) Number of chromosomes (N) per cell
(ii) Amount of DNA content (C) per cell

Answer. During meiosis, the number of chromosomes and the amount of DNA in a cell change.
(i) Number of chromosomes (N) per cell During anaphase I of the meiotic cycle, the homologous
chromosomes separate and start moving toward their respective poles. As a result, the bivalents
get divided into two sister chromatids and receive halt the chromosomes present in the parent
cell. Therefore, the number of chromosomes reduces in anaphase l.
(ii) Amount of DNA content (C) per cell During anaphase II of the meiotic cycle. the chromatids
separate as a result of the splitting of the centromere. It is the centromere that holds together the
sister chromatids of each chromosome. As a result, the chromatids move toward their respective
poles. Therefore, at each pole, a haploid
number of chromosomes and a haploid amount of DNA are present. During mitosis, the number of
chromosomes remains the same. The DNA duplicated in the S phase gets separated in the two
daughter cells during anaphase. As a result, the DNA content (C) of the two newly-formed
daughter cells remains the same.