Selina Concise Solutions for ICSE Class 10 Biology Chapter 1 Cell Cycle Cell Division and Structure Of Chromosomes

Exercise 1

A. Multiple Choice Type

Question 1. The Chromatin material is formed forms of
(a) DNA only
(b) DNA and histones
(c) Histones only
(d) Nucleotides
Answer: (b) DNA and histones
In simple words: Chromatin is like a library where DNA (the books) is organized on histone proteins (the shelves) to keep everything neat and accessible.

📝 Teacher's Note: Use the analogy of DNA as thread and histones as spools to help students visualize how genetic material is packaged in the cell nucleus.

🎯 Exam Tip: Remember chromatin = DNA + histones. This combination is essential for proper chromosome structure and gene regulation.

Question 2. The term "chromosomes" literally means
(a) Inherited bodies
(b) Twisted threads
(c) Coloured bodies
(d) Shining threads
Answer: (c) Coloured bodies
In simple words: The word "chromosome" comes from Greek words meaning "colored body" because they take up stains and become visible under a microscope.

📝 Teacher's Note: Explain the etymology: "chromo" = color and "some" = body. Show students how chromosomes appear dark when stained during microscopy.

🎯 Exam Tip: Remember the literal meaning helps understand why chromosomes are only visible during cell division when they condense and can be stained.

B. Very Short Answer Type

Question. Name the following:
(a) The repeating components of each DNA strand lengthwise.
(b) The complex consisting of DNA strand and a core of histones.
(c) The type of bond which joins the complementary nitrogenous bases.
(d) The three components of a nucleotide.
Answer:
(a) Nucleotides.
(b) Nucleosome.
(c) Hydrogen Bond.
(d) Phosphate, Sugar and Nitrogenous base.
In simple words: These are the basic building blocks and structures that make up DNA - like LEGO pieces that connect in specific ways to form the genetic code.

📝 Teacher's Note: Draw the DNA structure step by step, starting with nucleotides as building blocks, then showing how they form strands and double helix.

🎯 Exam Tip: Learn the components in sequence: nucleotide → nucleosome → chromatin → chromosome. This hierarchy is frequently tested.

C. Short Answer Type

Question. What is the difference between chromatin fibre and chromosome?
Answer: Chromatin fibre is unfolded, uncondensed, extended DNA. It is only visible when cell under goes division whereas chromosomes are condensed DNA and they are visible when the cell is divided.
In simple words: Chromatin is like loose thread while chromosomes are like tightly coiled rope - same material, different packaging for different purposes.

📝 Teacher's Note: Use the analogy of a tangled earphone cord (chromatin) versus a neatly coiled rope (chromosome) to show the difference in organization.

🎯 Exam Tip: Focus on the key difference: chromatin is dispersed during interphase, chromosomes are condensed during division. Mention visibility under microscope.

Question. What are the rungs of the "DNA ladder" made of?
Answer: Rungs of DNA ladder is made of nitrogenous bases which includes Adenine (A), Guanine (G), Cytosine (C) and Thymine (T).
In simple words: The rungs are like the steps of a twisted ladder, made of four different chemical letters that spell out the genetic code.

📝 Teacher's Note: Emphasize the base pairing rules: A-T and G-C. Use the mnemonic "Apple Tree" and "Great Car" to help students remember.

🎯 Exam Tip: Always mention all four bases (A, T, G, C) and note that they form the rungs through hydrogen bonding between complementary pairs.

Question. Correct the following statements if there is any mistake.
(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:
(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.
In simple words: These corrections fix common mistakes - like replacing wrong chemical names with correct ones and understanding that each chromosome exists as a chromatin fiber during rest periods.

📝 Teacher's Note: Point out common errors: Thiamine vs Thymine, Adrenaline vs Adenine, sulphate vs phosphate, cysteine vs histone. Students often confuse these similar-sounding terms.

🎯 Exam Tip: Pay attention to precise terminology. Examiners often test knowledge of exact chemical names and structural components.

D. Long Answer Type

Question. What is a nucleosome?
Answer: Nucleosome is basic structural unit of DNA. Each strand of DNA winds around a core of eight histone molecules. This core can be imagined like a football, around which a long rope is wound with one or two loops. Each such complex structure is called a nucleosome. A single human chromosome may have about a million nucleosomes.
In simple words: A nucleosome is like DNA thread wrapped around a protein spool, creating a "beads on a string" structure that helps pack long DNA into tiny cell nuclei.

📝 Teacher's Note: Use physical models like thread around a spool or beads on a string to demonstrate nucleosome structure. Emphasize the 8 histone core.

🎯 Exam Tip: Mention the "beads on a string" appearance, 8 histone proteins, and the role in DNA packaging. Include the approximate number (million per chromosome).

Question. What are genes?
Answer: Gene is a structural and functional unit of heredity and variations. 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. In other words, gene is the DNA segment of the chromosome and it controls the expression of characteristics.
In simple words: Genes are like recipes in a cookbook - specific instructions written in DNA that tell the cell how to make proteins for different body features.

📝 Teacher's Note: Connect genes to observable traits students can see - eye color, height, blood type. Explain the DNA → RNA → protein → trait pathway simply.

🎯 Exam Tip: Define genes as DNA sequences that code for proteins. Link structure (nucleotide sequences) with function (trait expression).

E. Structured / Application / Skill Type

Question. Given below is a schematic diagram of a portion of DNA.
(a) How many strands are shown in the pic?
(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, 3 collectively.
Answer:
(a) 2
(b) 2 on each strand
(c) 1- Phosphate, 2- Sugar, 3- Nitrogen Base, 4- Hydrogen Bond, 5 - Base
(d) Nucleotide
In simple words: This diagram shows the double helix structure with two strands connected by bases, like a ladder twisted into a spiral.

📝 Teacher's Note: Practice labeling DNA diagrams regularly. Students often confuse the sugar-phosphate backbone with the nitrogenous bases.

🎯 Exam Tip: Remember the nucleotide = phosphate + sugar + base. Count carefully and identify hydrogen bonds between complementary bases.

Question. The three sketch (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: B, C and A.
In simple words: DNA replication follows a sequence where the double helix unwinds, separates, and then each strand serves as a template to make new complementary strands.

📝 Teacher's Note: Walk through DNA replication step-by-step using diagrams. Emphasize the semiconservative nature where each new molecule has one old and one new strand.

🎯 Exam Tip: Follow the logical sequence: intact double helix → unwinding → separation → new strand synthesis. Each step builds on the previous one.

Exercise 2

Multiple Choice Type

Question 1. The number of chromosomes in a certain type of cell division is halved. This kind of cell division occurs in.
(a) Only testis
(b) Only ovary
(c) Both ovary and testis
(d) All body cells
Answer: (c) Both ovary and testis
In simple words: Meiosis happens in reproductive organs (testis and ovary) to make gametes with half the chromosomes for sexual reproduction.

📝 Teacher's Note: Distinguish meiosis (gamete formation, chromosome reduction) from mitosis (body cell division, chromosome maintenance). Use examples of sperm and egg formation.

🎯 Exam Tip: Remember meiosis = reduction division = reproductive organs. Mitosis = same chromosome number = all body cells.

Question 2. In which one of the following options the two stages of mitosis have been given in correct sequence?
(a) Prophase, anaphase
(b) Metaphase, telophase
(c) Anaphase, telophase
(d) Telophase, anaphase
Answer: (c) Anaphase, telophase
In simple words: Mitosis follows a specific order: chromosomes line up, then separate, then the cell divides - like an organized dance with set steps.

📝 Teacher's Note: Teach the PMAT sequence (Prophase, Metaphase, Anaphase, Telophase) using memory aids like "Please Meet At Twelve" or visual mnemonics.

🎯 Exam Tip: Learn the mitosis sequence: P-M-A-T. Anaphase (chromosome separation) always comes before telophase (nuclear reformation).

Question 3. Synthesis phase in the cell cycle is called so for the synthesis of more of.
(a) RNA
(b) RNA and proteins
(c) DNA
(d) Glucose
Answer: (c) DNA
In simple words: The S-phase is when the cell makes an exact copy of all its DNA before dividing, like photocopying important documents.

📝 Teacher's Note: Emphasize that S-phase (synthesis phase) specifically refers to DNA replication. Connect this to the need for identical genetic material in daughter cells.

🎯 Exam Tip: S-phase = DNA synthesis phase. This is the preparation step before cell division to ensure each daughter cell gets complete genetic information.

B. Very Short Answer Type

Question. 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?
Answer: Cell A: 2, Cell B: 4
In simple words: Mitosis makes 2 identical cells while meiosis makes 4 different gametes - like one copy machine versus one shuffling machine.

📝 Teacher's Note: Use visual diagrams showing 1→2 for mitosis and 1→4 for meiosis. Emphasize that meiosis involves two successive divisions.

🎯 Exam Tip: Remember: Mitosis = 1 becomes 2 (identical). Meiosis = 1 becomes 4 (genetically different). This is a fundamental difference.

Question. Match the events given in column A with the phase in mitotic cell division in column B
Answer:
(a) Metaphase
(b) Anaphase
(c) Prophase
(d) Telophase
In simple words: Each phase of mitosis has specific events - like acts in a play where chromosomes prepare, line up, separate, and then the cell splits.

📝 Teacher's Note: Create a matching exercise with key events and phases. Use action words to help students remember: condense, align, separate, divide.

🎯 Exam Tip: Learn the key event for each phase: Prophase (condensation), Metaphase (alignment), Anaphase (separation), Telophase (reformation).

Question 3. Fill in the blanks:
(a) Mitosis occurs in our ______ cells
(b) Mitosis produces two daughter cells whereas meiosis produces ______ daughter cells.
(c) Meiosis occurs only in __________ cells.
(d) Humans have 46 chromosomes: Their sperms and eggs will have ______ chromosomes each.
(e) During the pairing of chromosomes in meiosis, the ________ chromosomes come to lie side by side.
(f) The _______ (s) are surrounded by radiating rays called aster.
Answer:
(a) Somatic (body)
(b) Four
(c) Reproductive
(d) 23 and 23
(e) Homologous
(f) centriole
In simple words: Mitosis happens in all body cells and makes identical copies, while meiosis happens only in reproductive cells and makes gametes with half the chromosomes.

📝 Teacher's Note: Use diagrams to show mitosis making 2 identical cells and meiosis making 4 different gametes. Students often confuse which type of division happens where.

🎯 Exam Tip: Remember "Mitosis = body cells, same number" and "Meiosis = sex cells, half number" for quick recall in exams.

C. Short Answer Type:

Question 1. State the difference between:
(a) Chromosome and chromatid:
(b) Centrosome and centromere:
(c) Aster and spindle fibres:
(d) Haploid and diploid:
Answer:
(a) A chromosome is an organized structure of DNA and protein found in cells. It is a single piece of coiled DNA containing many genes, regulatory elements and other nucleotide sequences whereas a chromatid is one of the two copies of DNA making up a duplicated chromosome, which are joined at their centromeres, for the process of cell division (mitosis or meiosis).
(b) The centrosome is an area in the cell where microtubules are produced. Within an animal cell centrosome, there is a pair of small organelles called the centrioles. During animal cell division, the centrosome divides and the centrioles replicate (make new copies) whereas each chromosome in its condensed form consists of two chromatids joined at some point along the length. This point of attachment is called centromere.
(c) An aster is a cellular structure shaped like a star, formed around each Centrosome during mitosis in an animal cell whereas spindle fibers are aggregates of microtubules that move chromosomes during cell division.
(d) A haploid cell is a cell that contains one complete set of chromosomes. Gametes are haploid cells that are produced by meiosis whereas a diploid cell is a cell that contains two sets of chromosomes. One set of chromosomes is donated from each parent.
In simple words: Think of chromosome as a book and chromatid as a photocopy of that book stuck together. Centrosome is like a factory making threads, centromere is like a clip holding two papers together.

📝 Teacher's Note: Use analogies like "chromosome = book, chromatid = photocopied pages" to help students visualize these structures. Draw simple diagrams on the board.

🎯 Exam Tip: Always mention "joined at centromere" when defining chromatids, and specify "one set vs two sets" for haploid vs diploid.

Question 2. "First meiotic division is the reduction division" what does the word 'reduction' refer to in this statement?
Answer: In this statement, reduction means that 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.
In simple words: "Reduction" means cutting the chromosome number in half - from 46 to 23 in humans.

📝 Teacher's Note: Use the example of 46→23 in humans to make this concrete. Emphasize this is why it's called "reduction" division.

🎯 Exam Tip: Always mention specific numbers (46→23 in humans) when explaining reduction division for full marks.

Question 3. "Gametes must be produced by meiosis for sexual reproduction". Why is it so?
Answer: 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.
In simple words: If gametes had full chromosome sets, babies would have double the normal number after fertilization - that would be a disaster!

📝 Teacher's Note: Use math: 23+23=46 (normal), but if 46+46=92 (wrong!). This helps students understand why reduction is necessary.

🎯 Exam Tip: Mention both "reduction to half" AND "restoration during fertilization" for complete answer.

Question 4. Mention whether the following statements are (T) Or (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.
(b) The unfertilized human egg has half the number of chromosomes of the body cells.
(c) Nuclear membrane in mitotically dividing cells remains intact up to the metaphase and disappears only in the telophase.
(d) Mitotic cell division can be a mode of reproduction.
(e) Crossing over between chromatids can occur only between homologous chromosomes.
Answer:
(a) False - Surface skin cells are continuously lost and replaced by the underlying cells.
(b) True - All types of human cells, have 46 chromosomes. The only type of cell which does not have 46 chromosomes are the sex cells, which have only half of the number, so they have 23 chromosomes. The egg cell is a sex cell (found in female). So it must have 23 chromosomes.
(c) False - Nuclear membrane disappears in Prophase itself, however it reappears during Telophase.
(d) True - Mitotic cell division can be a mode of asexual reproduction in unicellular organisms like amoeba or yeast cell which divides into two daughter cells.
(e) True - 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.
In simple words: Skin cells are constantly renewing (not just replacing lost ones), egg cells have half chromosomes like all gametes, nuclear membrane vanishes early in cell division, and some organisms reproduce just by splitting in two.

📝 Teacher's Note: For (c), emphasize the correct sequence: prophase (disappears) → metaphase → anaphase → telophase (reappears). Students often mix this up.

🎯 Exam Tip: For True/False questions, always give a specific reason. Don't just state True or False without explanation.

D. Structured / Application/ Skill Type:

Question 1. Draw a labelled diagram to show the metaphase stage of mitosis in an animal cell having '6' chromosomes.
Answer: [Diagram shows metaphase with chromosomes aligned at equatorial plane, spindle fibers, centrioles with aster rays, and all parts properly labeled]
In simple words: In metaphase, all chromosomes line up in the middle like soldiers in a row, ready to be pulled apart.

📝 Teacher's Note: Emphasize that chromosomes must be at the center (equatorial plane) and spindle fibers attach to centromeres. Use the analogy of a tug-of-war with equal pulling from both sides.

🎯 Exam Tip: Always label centrioles, spindle fibers, chromosomes at equator, and aster rays for full marks in metaphase diagrams.

Question 2. The diagram below represents a stage during cell division. Study the same and then answer the question 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 where in the body this type of cell division occurs.
(d) Name the stage prior to this stage and draw a diagram to represent the same.
Answer:
(a) 1 – Centrosome, 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.
In simple words: This shows late anaphase where chromosomes have been pulled apart to opposite ends, and the cell is starting to pinch in the middle.

📝 Teacher's Note: Point out the cell furrow forming - this is key to identifying late anaphase vs early anaphase. Also emphasize chromatids are now at opposite poles.

🎯 Exam Tip: For stage identification, mention both the position of chromosomes AND any membrane changes (like furrow formation) for complete answer.

Question 3. The diagram given below represents a certain phenomenon which occurs during meiosis. Name and explain the phenomenon by using the terms – homologous chromosomes, chromatids, crossing over.
Answer: 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.
In simple words: Crossing over is like two people swapping parts of their similar necklaces - they end up with mixed versions that create genetic variety.

📝 Teacher's Note: Use pipe cleaners or colored strings to physically demonstrate crossing over. Students need to see the physical exchange to understand the concept.

🎯 Exam Tip: Always mention "non-sister chromatids" and "genetic recombination" when explaining crossing over for full credit.

Question 4. 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 above, how is that stage identified?
(e) Mention two differences between mitosis and meiosis with regards to:
(i) The number of daughter cells produced.
The chromosome number in the daughter cells.
Answer:
(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.
(e)

In simple words: Late prophase is when the cell's "control center" (nucleus) breaks down to let chromosomes move freely, and mitosis makes copies while meiosis makes halves.

📝 Teacher's Note: Emphasize that nuclear membrane disappearance is the key marker for late prophase. Students often confuse early and late prophase stages.

🎯 Exam Tip: For comparing mitosis and meiosis, always mention both number of cells AND chromosome number changes for complete comparison.

Question. 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 cells?
(d) Which of these is/are shown in correct direction?
(i) Only A
(ii) Only B
(iii) Only A and C
(iv) All the three
Answer:
(a) Metaphase
(b) 4
(c) A – Animal, B – Animal, C – Plant
(d) (iv) All the three
In simple words: This is metaphase where chromosomes line up in the middle of the cell. All three diagrams show this correctly, with A and B being animal cells (round shape) and C being a plant cell (rectangular shape).

📝 Teacher's Note: Use the visual cues to help students distinguish cell types - animal cells are round while plant cells are rectangular due to the rigid cell wall. The chromosome alignment at the cell's center is the key identifying feature of metaphase.

🎯 Exam Tip: When identifying mitotic phases, look for chromosome position first - scattered (prophase), lined up in middle (metaphase), or moving apart (anaphase/telophase). Cell shape helps distinguish plant vs animal cells.

Question. 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 names above inside the blank space provided.
Answer:
(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 or centrioles are present. (These are found only in animal cells)
(b) Mitosis
(c) B, C, D, A
(d) Interphase
(e) [Diagram showing labeled stages]
In simple words: This shows an animal cell going through mitosis - you can tell it's an animal cell because it's round and has centrioles (structures that plant cells don't have). The stages happen in order: B, C, D, A.

📝 Teacher's Note: Emphasize the two key differences between plant and animal cells during division - shape (round vs rectangular) and presence of centrioles. Have students practice identifying the sequence by following chromosome movement and cell shape changes.

🎯 Exam Tip: Always give two clear reasons when asked to distinguish plant from animal cells. The most reliable markers are cell shape and presence/absence of centrioles and cell walls.