Structure of Chromosomes, Cell Cycle and Cell Division

Multiple Choice Type

(Choose the best option out of the four alternatives a, b, c and d)

Question 1

The chromatin material is formed of

1. DNA only
2. DNA and Histones ✓
3. Histones only
4. Nucleotides

Question 2

The term “chromosomes” literally means

1. Inherited bodies
2. Twisted threads
3. Coloured bodies ✓
4. Shining threads

Question 3

The number of chromosomes in a certain type of cell division is halved. This kind of cell division occurs in

1. only testis
2. only ovary
3. both ovary and testis ✓
4. all body cells

Question 4

In which one of the following options the stages of mitosis have been given in correct sequence?

1. Prophase, metaphase, telophase, anaphase
2. Prophase, metaphase, anaphase, telophase ✓
3. Anaphase, telophase, prophase, metaphase
4. Telophase, anaphase, prophase, metaphase

Question 5

Synthesis phase in the cell cycle is called so for the synthesis of more of

1. RNA
2. RNA and proteins
3. DNA ✓
4. Glucose

Very Short Answer Type

Question 1

Name the following:

1. The repeating components of each DNA strand lengthwise.
2. The complex structure consisting of DNA strand and a core of histones.
3. The type of bond which joins the complementary nitrogenous bases.
4. The three components of nucleotide.

Answer

1. Nucleotides
2. Nucleosome
3. Hydrogen Bond
4. Phosphate, Sugar and Nitrogenous base.

Question 2

Imagine one cell (A) has undergone one mitotic division and another cell (B) has completed its meiotic division. How many cells would the two produce?
Cell A: ……………………….
Cell B: ………………………..

Answer

Cell A produces 2 cells
Cell B produces 4 cells

Question 3

Match the events given in column A with the phase in mitotic cell division in column B

Answer

Fill in the blanks

Question 4(a)

DNA replicates in the synthesis phase of the cell cycle.

Question 4(b)

Mitosis occurs in our somatic (body) cells.

Question 4(c)

Meiosis occurs only in reproductive cells.

Question 4(d)

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

Question 4(e)

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

Question 4(f)

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

Short Answer Type

Question 1

What is the difference between chromatin fibre and chromosome?

Answer

Question 2

What are the rungs of the “DNA ladder” made of?

Answer

The rungs of the DNA ladder are made of four types of nitrogenous bases. These are:

1. Adenine (A)
2. Guanine (G)
3. Cytosine (C)
4. Thymine (T)

Question 3

Correct the following statements for mistakes, if any.

(a) The four nitrogenous bases in the DNA are Guanine, Thiamine, Adrenaline and Cytosine.

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

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

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

(e) If there are 46 chromosomes in a cell, there will be 23 chromatin fibres inside the nucleus during interphase.

Answer

Corrected statements are as follows:

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

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

(c) A nucleotide is composed of a phosphate, a 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.

Descriptive Type

Question 1

Mention whether the following statements are true (T) or false (F). Give reason in support of your answer.

(a) As you grow from childhood to adulthood, your skin cells divide only to replace such cells that are lost from the surface.

Answer

False
Reason – During the growth period of a person from childhood to adulthood, the skin cells divide not only to replace such cells that are lost from the surface but also to produce more cells so that growth can occur.

(b) Nuclear membrane in a mitotically dividing cell remains intact up to the metaphase and disappears only in the telophase.

Answer

False
Reason – Nuclear membrane disappears in Prophase and reappears during Telophase.

(c) Mitotic cell division can be a mode of reproduction.

Answer

True
Reason – Mitotic cell division is a mode of asexual reproduction in unicellular organisms like amoeba or yeast cell which divides into two daughter cells.

(d) Crossing-over between chromatids can occur only between homologous chromosomes.

Answer

True
Reason – 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.

Question 2

Define the following terms:

(a) Chromosome

(b) Gene

(c) Cell division

(d) Chromatid

(e) Aster

Answer

(a) Chromosome — Chromosomes are formed of very long, highly coiled and condensed chromatin fibres which are made of DNA (about 40%) and histones (about 60%). They are present in the nucleus of the cell. They carry the chemical instructions for the reproduction of the cell.

(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. They are the units of heredity which are transferred from parents to offsprings and are responsible for some specific characteristics of the offspring.

(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. Before replication, one chromosome is composed of one DNA molecule. In replication, the DNA molecule is copied, and the two molecules are known as chromatids. During the later stages of cell division these chromatids separate longitudinally to become individual chromosomes.

(e) Aster — During mitosis in an animal cell, after the centrosome splits into two along with simultaneous duplication of the centrioles contained in it, each centriole is surrounded by radiating rays and is termed aster (meaning star).

Question 3

Give reason:

(a) Gametes must be produced by meiosis for sexual reproduction.

(b) Why is meiosis referred to as ‘reductional division’?

(c) The children of the same parents, howsoever similar, are different from each other in certain aspects.

Answer

(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. This is essential because when the male and female gametes fuse during fertilization, the normal double (diploid) number of chromosomes is reacquired. The diploid number, as a rule, is expressed as “2n” and the haploid number as “n”.

(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.

Question 4

Distinguish between the following pairs:

(a) Cytokinesis and Karyokinesis

(b) DNA and RNA

(c) Nucleosome and Nucleotide

(d) Centrosome and Centromere

(e) Haploid and Diploid

Answer

(a) Difference between Cytokinesis and Karyokinesis

(b) Difference between DNA and RNA

(c) Difference between Nucleosome and Nucleotide

(d) Difference between Centrosome and Centromere

(e) Difference between Haploid and Diploid

Question 5

Enumerate the various changes that occur in the nucleus of the cell during (a) prophase (b) anaphase of mitotic division.

Answer

(a) Prophase

1. Centrioles start moving apart and reach opposite poles.
2. Chromosomes become distinct.
3. Chromosomes are already duplicated as paired chromatids.
4. Sister chromatids attached to each other at a small region called centromere.
5. Spindle fibres appear between daughter centrioles forming the achromatic spindle.
6. Nuclear membrane and nucleolus disappear.

(b) Anaphase

1. Centromere attaching the two chromatids divides/splits.
2. The two sister chromatids of each chromosome separate and are drawn apart towards opposite poles pulled by shortening of spindle fibres.
3. A furrow starts in the cell membrane at the middle in animal cell.

Question 6

Name and explain the various stages of the cell cycle.

Answer

The cell cycle is a series of events that take place in a cell leading to the duplication of its DNA and the subsequent division of the cell to produce two daughter cells.

A cell cycle consists of two phases:

1. A non-dividing phase called the interphase, and
2. A dividing phase called the M-phase or simply mitosis.

Interphase

The two daughter cells produced from a mother cell are relatively small, with a full-sized nucleus but relatively little cytoplasm. These cells are said to be in interphase. The interphase itself has three phases —

1. First growth phase (G1) — RNA and proteins are synthesised, the volume of cytoplasm increases.
2. Synthesis phase (S) — More DNA is synthesised, the chromosomes are duplicated.
3. Second growth phase (G2) — This is a shorter growth phase in which RNA and proteins necessary for cell division continue to be synthesised.

Mitosis

Mitosis is the cell division in which one parent cell divides into identical daughter cells. Mitosis has two phases —

1. Karyokinesis — It is the division of the nucleus during cell division. It occurs in four phases:
   1. Prophase — Chromosomes have become short and thick and clearly visible inside the nucleus.
   2. Metaphase — The duplicate chromosomes arrange on the equatorial plane. Each chromosome gets attached to a spindle by its centromere.
   3. Anaphase — The centromere attaching the two chromatids divides and the two sister chromatids of each chromosome separate and are drawn apart towards opposite poles.
   4. Telophase — Spindle apparatus disappears. Chromosome become thinner and turns into a network of chromatin threads.
2. Cytokinesis — At the end of telophase, a furrow appears in the cell membrane in the middle, which deepens and finally splits the cytoplasm into two, thus producing two new cells.

Structured / Application / Skill Type

Question 1

Given below is a schematic diagram of a portion of DNA.

(a) How many strands are shown in the diagram?

(b) How many nucleotides have been shown in each strand?

(c) Name the parts numbered 1,2,3,4 and 5 respectively.

(d) Name the DNA unit constituted by the parts 1, 2 and 3 collectively.

Answer

(a) 2 strands are shown in the diagram.

(b) 2 on each strand.

(c) The parts are as follows:

• 1 → Phosphate
• 2 → Sugar
• 3 → Bases
• 4 → Hydrogen Bond
• 5 → Base

(d) The DNA unit constituted by the parts 1, 2 and 3 collectively is called Nucleotide.

Question 2

The three sketches given below (A, B and C) are intended to represent the replication of DNA. What should be their correct sequence starting with the first and ending with the last?

Answer

The correct sequence is :
B, C and A

Question 3

The diagram below represents a stage during cell division. Study the same and then answer the questions that follow:

a. Name the parts labelled 1, 2 and 3.

b. Identify the above stage and give a reason to support your answer.

c. Mention the type of cells in our body where this type of cell division occurs.

d. Name the stage prior to this stage and draw a diagram to represent the same.

Answer

(a) The parts are as follows:

• 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 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. Below diagram shows metaphase:

Question 4

Draw a labelled diagram to show the metaphase stage of mitosis in an animal cell having “6” chromosomes.

Answer

Below diagram shows the metaphase stage of mitosis in an animal cell having 6 chromosomes:

Question 5

The diagram given below represents a certain phenomenon which occurs during meiosis. Name and explain the phenomenon by using the terms – homologous chromosomes, chromatids, and crossing-over.

Answer

The phenomenon shown in the diagram is termed Crossing-Over. It is the exchange of chromatid material between the two members of a homologous pair of chromosomes while the maternal and paternal chromosomes are separating.

Question 6

Given below is a diagram representing a stage during mitotic cell division in an animal cell. Examine it carefully and answer the questions which follow.

(a) Identify the stage. Give one reason in support of your answer.

(b) Name the cell organelle that forms the ‘aster’.

(c) Name the parts labelled 1, 2 and 3.

(d) Name the stage that follows the one shown here. How is that stage identified?

(e) Mention two points of difference between mitosis and meiosis with regard to:

1. The number of daughter cells produced.
2. The chromosome number in the daughter cells.

Answer

(a) By observing the given figure we can say that it is the late prophase stage because the nuclear membrane and nucleolus have disappeared.

(b) Centrioles is the cell organelle that form the aster.

(c) The parts are as follows:

• 1 → Centromere
• 2 → Chromatids
• 3 → Spindle fibre

(d) The stage that follows the one shown here is Metaphase. In 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.

(e) Difference between mitosis and meiosis :

Question 7

Given below are three diagrammatic sketches (A, B and C) of one and the same particular phase during mitotic type of cell division.

(a) Identify the phase.

(b) What is the diploid number of chromosomes shown in them?

(c) Identify whether these are animal cells or plant cell? Give reasons.

Answer

(a) It is Metaphase.

(b) The diploid number of chromosomes shown in them is 4.

(c) Categorisation of cell A, B and C are as given below:

• Cell A – It is an animal cell as the centrosome has split into two centrioles and the centrioles have formed asters. Also, no cell wall is present.
• Cell B – It is an animal cell as the centrosome has split into two centrioles and the centrioles have formed asters. Also, no cell wall is present.
• Cell C – It is a plant cell as no centrioles are seen and cell wall is present.

Question 8

Shown below are four stages (A, B, C, D) (not in sequence) of a certain kind of cell division.

(a) Is it a plant cell or an animal cell? Give two reasons.

(b) Is it undergoing mitosis or meiosis?

(c) What should be the correct sequence of these four stages among themselves?

(d) Name the stage that should precede the earliest of these stages.

(e) Draw the stage named above inside the blank space provided.

Answer

(a) This figure is of the animal cell because:

1. Centrosomes on centrioles are present.
2. Cell wall is absent

(b) It is undergoing mitosis.

(c) The correct sequence is B, C, D and A

(d) The stage that should precede the earliest of these stages is interphase.

(e) Below diagram shows interphase stage of mitosis:

Question 9

Given below is a diagram representing a stage during mitotic cell division. Study it carefully and answer the questions that follow:

(a) Is it a plant cell or an animal cell? Give a reason to support your answer.

(b) Identify the stage shown.

(c) Name the stage that follows the one shown here. How is that stage identified?

(d) How will you differentiate between mitosis and meiosis on the basis of the chromosome number in the daughter cells?

(e) Draw a duplicated chromosome and label its parts.

Answer

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

(b) The stage shown is Prophase.

(c) Metaphase follows Prophase. In Metaphase, Chromosomes line up in one plane at equator.

(d) In Mitosis, full set of chromosomes is passed on to each daughter cell. This is the diploid (2n) number of chromosomes whereas in Meiosis, only half the number of chromosomes is passed on to each daughter cell. This is the haploid (n) number of chromosomes.

(e) Below is the labelled diagram of a duplicated chromosome: