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Class 10 SELINA Solutions Biology Chapter 2 - Structure of Chromosomes, Cell Cycle and Cell Division

Structure of Chromosomes, Cell Cycle and Cell Division Exercise Ex. 1

Solution A.1

(b) DNA and Histones

Solution A.2

(c) Coloured bodies

Solution A.3

(c) both ovary and testis

Solution A.4

(c) Anaphase, telophase,  prophase, metaphase

Solution A.5

(c) DNA

Solution B.1

(a) - Nucleotides.

(b) - Nucleosome.

(c) - Hydrogen Bond.

(d) - Phosphate, Sugar and Nitrogenous base.

Solution B.2

Cell A: 2

Cell B: 4

Solution B.3

(a) - Metaphase.

(b) - Telophase.

(c) - Prophase.

(d) - Anaphase.

Solution B.4

(a) DNA replicates in the synthesis (S) phase of the cell cycle.  

(b) Mitosis occurs in our somatic (body) cells.

(c) Meiosis occurs only in reproductive cells.

(d) Modern humans have 46 chromosomes. Their sperms and eggs will have 23 chromosomes each.

(e) During the pairing of chromosomes in meiosis, the homologous chromosomes come to lie side by side.

(f) The two non-sister chromatids of a paired chromosome are attached to each other at chaisma during the process of crossing over.

Solution C.1

Chromatin fibers are long and thin, uncoiled structures found inside the nucleus while chromosomes are compact, thick, ribbon-like coiled structures seen prominently during cell division. In a non-dividing cell, DNA is present as chromatin fibres. Chromosomes are formed by coiling and supercoiling of chromatin fibres. They are most clearly visible during metaphase stage of mitotic cell division. Once cell division is over, the chromosomes uncoil back into chromatin fibres.

Solution C.2

Rungs of DNA ladder is made of nitrogenous bases which includes Adenine (A), Guanine (G), Cytosine (C) and Thymine (T).

Solution C.3

(a) The four nitrogenous bases in the DNA ladder are Guanine, Thymine, Adenine and Cytosine.

(b) Genes are specific sequences of nucleotides on a chromosome.

(c) A nucleotide is composed of a phosphate, sugar (pentose) and a nitrogenous base.

(d) Nucleosomes are groups of histone molecules surrounded by DNA strands.

(e) If there are 46 chromosomes in a cell there will be 46 chromatin fibres inside the nucleus during interphase.

Solution D.1

(a) False (F); Surface skin cells are continuously lost and replaced by the underlying cells.

(b) False (F); Nuclear membrane disappears in Prophase itself, however it reappears during Telophase.

(c) True (T); Mitotic cell division can be a mode of asexual reproduction in unicellular organisms like amoeba or yeast cell which divides into two daughter cells.

(d) True (T); While the maternal and paternal chromosomes are separating, the chromatid material gets exchanged between the two members of a homologous pair resulting in genetic recombination.

Solution D.2

(a) Chromosome: Chromosomes are highly condensed coiled chromatin fibres made of DNA which carry the hereditary material of the organisms.

(b) Gene: Genes are specific sequences of nucleotides on a chromosome that encode particular proteins which express in the form of some particular feature of the body.

(c) Cell division: Cell division is the method in which the cell divides and the duplicated chromosomes get evenly distributed into the daughter cells.

(d) Chromatid: Duplicated chromosomes consist of two identical strands, each of these is called a chromatid.

(e) Aster: Each centriole is surrounded by radiating rays and is termed aster.

Solution D.3

(a) Gametes must be produced by meiosis for sexual reproduction because the numbers of chromosomes are reduced to half during meiosis and then the normal diploid numbers of chromosomes are regained during the process of fertilization.

(b) Meiosis is referred to as 'reductional division' because the number of chromosomes are reduced to half i.e. out of the 23 pairs of chromosomes in humans, only single set of chromosomes are passed on to the sex cells.

(c) The mixing up or recombination of genes during meiotic division provides for the innumerable variations and diversity in the progeny. That is how, the children of the same parents, howsoever similar, are different from each other in certain aspects.

Solution D.4

(a) Differences between cytokinesis and karyokinesis:



1. It is the division of the cytoplasm.

1. It is the division of the nucleus.

2. It occurs after karyokinesis.

2. It is the first division.

3. It results in the formation of two daughter cells.

3. It results in the formation of two nuclei.


(b) Differences between DNA and RNA:



1. DNA is deoxyribonucleic acid.

1. RNA is ribonucleic acid.

2. It consists of four distinct bases: thymine, adenine, cytosine and guanine.

2. It consists of four distinct bases: uracil, adenine, cytosine and guanine.

3. It is located in the nucleus of a cell and in the mitochondria.

3. It is found in the cytoplasm, nucleus, and in the ribosome.

4. The DNA is a double-stranded molecule.

4. The RNA is a single-stranded molecule.


(c) Differences between nucleosome and nucleotide:



It is the complex that it is made up of DNA wrapped around histone proteins.

The chemical composition of nucleotide consists of a phosphate group, a sugar and a nitrogenous base.


(d) Differences between centrosome and centromere:



1. It is an organelle of the animal cell.

1. It is a non-stainable part of chromo-some at which two chromatids join.

2. It contains two centrioles which move towards the opposite poles and forms spindle fibres during cell division.

2. It provides attachment of spindle fibres during cell division.


(e) Differences between haploid and diploid:



1. It is a state of half the number of chromosomes than the original.

1. It is a state of full set of chromosomes.

2. It is denoted by n.

2. It is denoted by 2n.

3. This state is found during meiotic division.

3. This state is found during mitotic division.


Solution D.5

Changes which occur in the nucleus of the cell during mitosis:

During prophase:

 The nuclear membrane and the nucleolus disappear.

 The duplicated chromosomes begin to move towards the equator of the cell.

During anaphase:

 The two sister chromatids of each chromosome separate and are drawn apart towards the opposite poles.

Solution D.6

Stages of the cell cycle:

 Non-dividing interphase

 Dividing mitotic phase or M phase


In interphase, cells grow in size and volume and prepare for the next cell division.

It is divided into three phases:

 First growth phase (G1): RNA and proteins are synthesised, and the volume of the cytoplasm increases.

 Synthesis phase (S): DNA is synthesised and chromosomes are duplicated.

 Second growth phase (G2): RNA and proteins continue to be synthesised.


Mitosis is the division of somatic cells in which two identical daughter cells are produced by the division of one parent cell.

It consists of the following phases:

 Karyokinesis: It is the division of the nucleus during cell division.

 Cytokinesis: It is the division of the cytoplasm during cell division.


It occurs in four phases:

 Prophase: During prophase, chromatin fibres condense and thick chromosomes are visible. The nucleolus and nuclear membrane disappear. A pair of centrioles duplicates. The spindle apparatus starts forming.

 Metaphase: Chromosomes are arranged on the metaphase plate or equatorial plane.

 Anaphase: The centromere divides, and the sister chromatids separate from each other. Spindle fibres contract and pull chromatids towards opposite poles.

 Telophase: Spindle apparatus disappears. Chromosomes become thin and turn into chromatin fibres. Nuclear membranes and nucleoli reappear.


 The furrow continues to deepen in the cell, and it finally divides the cytoplasm forming two new daughter cells.

 In plant cells, a cell plate appears at the equatorial plane.

Solution E.1

(a) 2

(b) 2 on each strand

(c) 1- Phosphate,2- Sugar, 3- Bases,4- Hydrogen Bond,5 - Base


Solution E.2

B, C and A.

Solution E.3


1 - Centromere

2 - Spindle fibres

3 - Chromatids


b. The stage described in the diagram is the late anaphase of mitosis in an animal cell. The stage can be identified by the presence of separated chromatids which are found at the two poles of the cell. The appearance of the furrow in the cell membrane classifies the stage as the late anaphase.


c. The division is mitotic division and this kind of cell division occurs in all the cells of the body except for the reproductive cells.


d. The stage before anaphase is metaphase.


Solution E.4

Solution E.5

The exchange of chromatids between homologous chromosomes is called crossing-over. This is the process by which the two chromosomes of a homologous pair exchange equal segments with each other.

Crossing over occurs in the first division of meiosis. At that stage each chromosome has replicated into two strands called sister chromatids. The two homologous chromosomes of a pair synapse, or come together. While the chromosomes are synapsed, breaks occur at corresponding points in two of the non-sister chromatids, i.e., in one chromatid of each chromosome.

Since the chromosomes are homologous, breaks at corresponding points mean that the segments that are broken off contain corresponding genes, i.e., alleles. The broken sections are then exchanged between the chromosomes to form complete new units, and each new recombined chromosome of the pair can go to a different daughter sex cell. It results in recombination of genes found on the same chromosome, called linked genes that would otherwise always be transmitted together.

Solution E.6

(a) Late prophase. Because the nuclear membrane and nucleolus have disappeared.

(b) Centrioles.

(c) 1 - Centromere

2 - Chromatids.

3 - Spindle fibre.

(d) Metaphase. The centromeres of chromosomes are drawn to the equator by equal pull of two chromosomal spindle fibres that connects each centromere to the opposite poles, forming a metaphasic plate.




(i) Two daughter cells are produced.

(i) Four daughter cells are produced.

(ii) It is equational division i.e. the number of chromosome in the daughter cells or parent cells remains the same.

(ii) It is reductional division i.e. the number of chromosomes is reduced to half in the daughter cells.

Solution E.7

(a) Metaphase.

(b) 4.

(c) A - Animal

B - Animal

C - Plant

Solution E.8

(a) This is an animal cell because:

(i) The outline is circular (in plants it would be angular {rectangular or polygonal}) and cell wall is absent.

(ii) Centrosomes on centrioles are present. (These are found only in animal cells)

(b) Mitosis.

(c) B, C, D, A.

(d) Interphase.


Solution E.9

(a) It is a plant cell because centrioles are not shown in the diagram.

(b) Prophase

(c) Metaphase. Chromosomes arrange themselves on the metaphase or equatorial plate.

(d) Difference between mitosis and meiosis based on the chromosome number in daughter cells: 



Chromosome number remains the same as that of parent cells.

Daughter cells receive only half the number of chromosomes from parent cells.


(e) Duplicated chromosome:


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