CBSE Class 11 Biology Cell Cycle and Cell Division Assignments

Very Short Answer Questions (1 mark each)

Question 1. What are kinetochores?
Answer: Kinetochores are small disc-shaped protein structures located on the outer surface of the centromeres of chromosomes, which serve as attachment sites for spindle fibers during cell division.
In simple words: Kinetochores act like handles on chromosomes that spindle fibers grip during mitosis and meiosis.

Exam Tip: Remember that kinetochores are distinct from centromeres; centromere is the region, while kinetochores are the protein plates attached to it.

Question 2. What is interkinesis?
Answer: Interkinesis is the short transitional stage that occurs between Meiosis I and Meiosis II. During this phase, the nuclear envelope may or may not reform, and no DNA replication takes place.
In simple words: Interkinesis is a brief pause between the first and second divisions of meiosis without any copying of DNA.

Exam Tip: Note that unlike interphase before meiosis, interkinesis lacks an S phase (no DNA synthesis).

Question 3. Why is mitosis called equational division?
Answer: Mitosis is called equational division because the chromosome number in the resulting daughter cells is exactly the same as that in the parent cell (e.g., diploid parent cell produces diploid daughter cells).
In simple words: It is called equational because the number of chromosomes remains equal before and after division.

Exam Tip: Contrast this with meiosis, which is reductional division.

Question 4. Name the stage of meiosis during which synaptonemal complex is formed.
Answer: The synaptonemal complex is formed during the zygotene stage of Prophase I of meiosis.
In simple words: Homologous chromosomes pair up and form the synaptonemal complex in zygotene.

Exam Tip: Associate Zygotene with pairing (synapsis) and formation of the synaptonemal complex.

Question 5. What is \(G_0\) phase of cell cycle?
Answer: The \(G_0\) phase (quiescent stage) is an inactive stage in which cells exit the \(G_1\) phase and stop dividing. Cells in this stage remain metabolically active but do not proliferate unless called upon to do so.
In simple words: It is a resting state where cells perform normal metabolic functions without dividing further.

Question 6. Where does mitosis take place in plants and animals?
Answer: In plants, mitosis takes place in meristematic tissues (such as root apices and shoot apices). In animals, it takes place in somatic cells throughout the body.
In simple words: Plant mitosis occurs in growth regions (meristems), while animal mitosis occurs in general body cells (somatic cells).

Exam Tip: Differentiate clearly between meristematic plant tissues and somatic animal cells.

Short Answer Questions-II (2 marks each)

Question 7. Differentiate between cytokinesis of plant and animal cell.
Answer: In animal cells, cytokinesis occurs by the appearance of a cleavage furrow in the plasma membrane that deepens centripetally and pinches the cytoplasm into two. In plant cells, cytokinesis occurs by the formation of a cell plate (formed from vesicles of the Golgi apparatus) that grows centrifugally from the center outward, forming the middle lamella.
In simple words: Animal cells divide by pinching inward (furrowing), while plant cells divide by building a wall down the middle (cell plate).

Exam Tip: Mention the terms 'cleavage furrow' for animals and 'cell plate / phragmoplast' for plants to secure full marks.

Question 8. What is chaismata? State its significance.
Answer: Chiasmata are X-shaped structures formed at the sites where crossing over has taken place between non-sister chromatids of homologous chromosomes during the diplotene stage of Prophase I. Significance: Chiasmata hold homologous chromosomes together after the dissolution of the synaptonemal complex and facilitate genetic recombination and variation.
In simple words: Chiasmata are the visible X-points where genetic material was exchanged, ensuring genetic diversity.

Exam Tip: Note the spelling and plural form (chiasma is singular, chiasmata is plural).

Question 9. Differentiate between chromatin and chromatid.
Answer:

In simple words: Chromatin is the loose thread-like DNA network, whereas a chromatid is one half of a duplicated chromosome.

Exam Tip: Always present structural differences in a tabular format for clarity.

Question 10. Give the terms for the following:
a) The period between 2 successive mitotic divisions
b) Cell division in which chromosome number is halved
c) Phase in cell cycle where DNA is synthesised
d) Division of nuclear material.
Answer:
a) Interphase
b) Meiosis
c) S phase (Synthesis phase)
d) Karyokinesis
In simple words: These are the standard biological terms for the respective cell cycle events.

Exam Tip: Be precise with terminology; write 'Karyokinesis' for nuclear division and 'Cytokinesis' for cytoplasmic division.

Question 11. What happens during S phase of interphase?
Answer: During the S (Synthesis) phase, replication of nuclear DNA takes place. The amount of DNA per cell increases from 2C to 4C (if the initial diploid content is 2C), although the chromosome number remains constant (\(2N\)). Additionally, centriole duplication occurs in the cytoplasm of animal cells.
In simple words: The cell makes an exact copy of its DNA during the S phase, doubling the DNA amount without changing chromosome count.

Question 12. Distinguish between metaphase of mitosis and metaphase I of meiosis.
Answer:

In simple words: In mitosis, single chromosomes line up on the equator, whereas in metaphase I, pairs of homologous chromosomes line up.

Exam Tip: Highlight the alignment of single chromosomes versus bivalents.

Question 13. What will be the DNA content of a cell at \(G_1\), after S and \(G_2\), if the content after M phase is 2C?
Answer:
- DNA content at \(G_1\) phase = 2C
- DNA content after S phase = 4C
- DNA content at \(G_2\) phase = 4C
In simple words: DNA content starts at 2C in \(G_1\), doubles to 4C during S phase, and remains 4C through \(G_2\) until mitosis divides it again.

Exam Tip: Remember that DNA replication occurs exclusively in the S phase, so \(G_1\) is 2C, while both S and \(G_2\) maintain 4C before cell division.

Short Answer Questions-I (3 marks each)

Question 14. Differentiate between mitosis and meiosis.
Answer:

In simple words: Mitosis creates two identical body cells, while meiosis creates four varied sex cells with half the chromosomes.

Exam Tip: Mention 4 key contrasting points: type of cells, number of daughter cells, chromosome number, and crossing over.

Question 15. List the significance of mitosis.
Answer:
1. Growth: Addition of new cells enables the growth and development of multicellular organisms from a single zygote.
2. Maintenance of chromosome number: Ensures that every daughter cell receives an identical complement of chromosomes and genetic material.
3. Repair and regeneration: Replaces dead, worn-out, or injured tissues (e.g., healing of wounds and skin regeneration).
4. Maintenance of cell surface-to-volume ratio: Keeps cells within an optimal functional size.
5. Reproduction: Serves as a mode of asexual reproduction in unicellular organisms.
In simple words: Mitosis is essential for growth, tissue repair, maintaining exact genetic copies, and asexual reproduction.

Exam Tip: List at least 3 to 4 points with brief explanations.

Question 16. Describe the following:
(a) Synapsis
(b) Bivalent
(c) Leptotene
Answer:
(a) Synapsis: The pairing process of homologous chromosomes that occurs during the zygotene stage of Prophase I of meiosis.
(b) Bivalent: A complex formed by a pair of synapsed homologous chromosomes during Prophase I, which clearly appears as a tetrad of four chromatids under an electron microscope.
(c) Leptotene: The initial phase of Prophase I during which individual chromosomes gradually become visible as long, thin, single-stranded structures with continued compaction.
In simple words: Synapsis is chromosome pairing, a bivalent is the paired chromosome unit, and leptotene is the starting stage of Prophase I where chromosomes first become visible.

Question 17. With the help of labelled diagram, explain the following:
(a) Diplotene
(b) Anaphase of mitosis
(c) Prophase I
Answer:
(a) Diplotene: Characterized by the dissolution of the synaptonemal complex. The homologous chromosomes begin to separate from each other except at the sites of crossing over, forming distinct X-shaped structures called chiasmata.
(b) Anaphase of mitosis: The centromere of each chromosome splits longitudinally, separating the two sister chromatids. The newly separated chromatids (now daughter chromosomes) move toward opposite poles pulled by shortening spindle fibers.
(c) Prophase I: The longest and most complex phase of meiosis, subdivided into five distinct sub-stages: Leptotene, Zygotene, Pachytene, Diplotene, and Diakinesis, marked by chromosomal condensation, synapsis, crossing over, and terminalization.
In simple words: Diplotene shows chiasmata, mitotic anaphase pulls chromatids to opposite poles, and Prophase I involves complex chromosome pairing and crossing over.

Exam Tip: For 5-mark questions, provide clear explanations for each sub-part and draw neat, well-labelled diagrams of mitotic anaphase and meiotic prophase stages.

Question 18. What is cell cycle? Explain the events occuring in this cycle.
Answer: The cell cycle is the ordered series of events by which a cell duplicates its genome, synthesizes other cell constituents, and eventually divides into two daughter cells. The cell cycle is broadly divided into two main phases:
1. Interphase: The non-dividing growth phase during which the cell prepares for division. It is subdivided into:
- \(G_1\) phase (Gap 1): Period of active growth and normal metabolic activity without DNA synthesis.
- S phase (Synthesis): Period when DNA replication occurs, increasing DNA content from 2C to 4C.
- \(G_2\) phase (Gap 2): Synthesis of proteins and organelles in preparation for mitosis.
2. M Phase (Mitosis/Meiosis Phase): The actual division phase, comprising karyokinesis (nuclear division) and cytokinesis (cytoplasmic division). Karyokinesis proceeds through Prophase, Metaphase, Anaphase, and Telophase.
In simple words: The cell cycle is the life cycle of a cell consisting of a growth period (Interphase) followed by a division period (M phase).

Exam Tip: Structure your answer by defining the cell cycle, outlining Interphase (\(G_1\), S, \(G_2\)), and detailing the M phase stages with proper subheadings.