GSEB Class 11 Biology Solutions Chapter 10 Cell Cycle and Cell Division

 

Question 1. What is the average cell cycle span for a mammalian cell?
Answer: A mammalian cell's average cell cycle lasts 24 hours.
In simple words: For a typical mammalian cell, the whole cycle, from start to end, takes about 24 hours.

Exam Tip: Remember specific numerical facts like the 24-hour cell cycle for mammalian cells as they are frequently tested for basic biological knowledge.

 

Question 2. Distinguish cytokinesis from karyokinesis.
Answer: The M phase begins with nuclear division, which is the separation of daughter chromosomes (karyokinesis). It typically finishes with the division of the cytoplasm, known as cytokinesis.
In simple words: Karyokinesis is when the nucleus splits. Cytokinesis is when the cell's main body (cytoplasm) divides, usually after the nucleus has split.

Exam Tip: Clearly define each term and state the order in which they occur during the M phase to show a complete understanding of the distinction.

 

Question 3. Describe the events taking place during the interphase.
Answer: During interphase, the cell gets ready for division by growing and copying its DNA in an organized way. This process has three main parts:

• G phase: In this stage, the cell is very active and grows steadily but does not copy its DNA.
• S phase (Synthesis): This is when DNA is made or copied. The amount of DNA in each cell doubles, but the chromosome count stays the same. For animal cells, DNA copying starts in the nucleus, and the centrioles also make copies in the cytoplasm.
• G2 phase: During this stage, proteins are created to help with mitosis, and the cell keeps growing.

In simple words: Interphase is the time before cell division where the cell grows, duplicates its DNA, and makes proteins to prepare for the split. It has three main phases: G1 (growth), S (DNA copying), and G2 (more growth and protein making).

Exam Tip: For descriptive questions about phases, name each sub-phase, list its key activities, and mention any changes in DNA amount or chromosome number.

 

Question 4. What is the Go (quiescent phase) of the cell cycle?
Answer: Some cells in adult animals, like heart cells, do not seem to divide, and many others divide only sometimes, usually to replace cells lost from damage or death. These cells stop dividing after the G1 phase and go into an inactive period called the quiescent stage (G0) of the cell cycle. Cells in this stage are still metabolically active but will not multiply unless the body needs them to.
In simple words: The G0 phase is a resting stage where cells are active but not dividing. They might re-enter the cell cycle if the body needs new cells for replacement or repair.

Exam Tip: When defining the G0 phase, emphasize that cells are metabolically active but non-proliferating, and provide relevant examples like heart cells.

 

Question 5. Why is mitosis called equational division?
Answer: The M phase is the most significant time in the cell cycle, involving a major restructuring of almost all cell parts. Because the count of chromosomes in the parent and new cells stays identical, it is also known as equational division. For ease, mitosis has been separated into four stages of nuclear division, but it's important to know that cell division is a continuous activity, and distinct boundaries between stages are not possible to draw. Mitosis includes these four stages:

• Prophase
• Metaphase
• Anaphase
• Telophase

In simple words: Mitosis is called equational division because the new cells it makes have the exact same number of chromosomes as the original cell, keeping the genetic information equal.

Exam Tip: The core reason for "equational division" is the conservation of chromosome number; always highlight this point clearly in your answer.

 

Question 6. Name the stage of the cell cycle at which one of the following events occur:
(1) Chromosomes are moved to the spindle equator.
(2) Centromere splits and chromatids separate.
(3) The pairing between homologous chromosomes takes place.
(4) Crossing over between homologous chromosomes takes place.
Answer:
(1) During the metaphase stage of the cell cycle, chromosomes move to the spindle equator and line up along the metaphase plate using microtubules towards both ends.
(2) In the anaphase stage of the cell cycle, centromeres divide, and chromatids move apart.
(3) The second part of prophase I is called zygotene. At this point, chromosomes begin to pair up, and this connection is known as synapsis. These paired chromosomes are called homologous chromosomes.
(4) Pachytene is the third part of prophase I. During this period, bivalent chromosomes are clearly seen as tetrads. This stage is defined by the presence of recombination nodules, which are the places where crossing over happens between non-sister chromatids of the homologous chromosomes. Crossing over involves the exchange of genetic material between two homologous chromosomes.
In simple words: (1) Chromosomes align in Metaphase. (2) Chromatids separate in Anaphase. (3) Homologous chromosomes pair up in Zygotene. (4) Crossing over occurs in Pachytene.

Exam Tip: To answer questions about cell cycle events, visualize each stage and associate its unique actions with the correct name (e.g., Metaphase for alignment, Anaphase for separation).

 

Question 7. Describe the following briefly :
(1) Synapsis
(2) bivalent
(3) Chiasmata
Answer:
(1) Synapsis: In the zygotene stage, homologous chromosomes begin to pair up, and this process is known as synapsis.
(2) Bivalent: The structure created by two paired homologous chromosomes in synapsis is called a bivalent or a tetrad.
(3) Chiasmata: In diplotene, the synaptonemal complex starts to break down, and the connected homologous chromosomes in the bivalents tend to move apart, except at the points where crossing-over occurred. These X-shaped structures are recognized as chiasmata.
In simple words: Synapsis is the pairing of homologous chromosomes. A bivalent is the pair of these connected homologous chromosomes. Chiasmata are the X-shaped places where homologous chromosomes still touch after pairing, showing where crossing over happened.

Exam Tip: Focus on linking each term directly to its definition and the specific stage of meiosis I where it occurs, such as synapsis in zygotene and chiasmata in diplotene.

 

Question 8. How does cytokinesis in plant cells differ from that in animal cells?
Answer: In animal cells, cytokinesis occurs through a furrow that forms in the plasma membrane. This furrow gets deeper and eventually meets in the middle, splitting the cell's cytoplasm into two. Plant cells, however, are surrounded by a strong cell wall, so they divide their cytoplasm differently. In plant cells, wall formation begins in the cell's center and spreads outwards to meet the existing side walls.
The new cell wall starts as a simple structure called the cell-plate, which becomes the middle lamella between two neighboring cell walls. During cytoplasmic division, organelles such as mitochondria and plastids are shared between the two new cells. In some organisms, if karyokinesis (nuclear division) is not followed by cytokinesis, a multinucleated state occurs, forming a syncytium.
In simple words: Animal cells divide their cytoplasm by pinching in from the outside with a furrow. Plant cells, with their stiff cell wall, build a new cell wall from the inside out using a cell plate to split.

Exam Tip: Highlight the key difference: animal cells use a contractile ring to form a cleavage furrow, while plant cells form a cell plate from the center outwards.

 

Question 9. Find examples where the four daughter cells from meiosis are equal in size and where they are found unequal in size.
Answer:

• Meiosis creates equal daughter cells in spermatogenesis.
• Meiosis generates unequal daughter cells in oogenesis.

In simple words: In males (spermatogenesis), meiosis makes four equal sperm cells. In females (oogenesis), it makes one large egg cell and smaller polar bodies, so the cells are unequal.

Exam Tip: Remember that spermatogenesis produces four functional gametes of equal size, whereas oogenesis produces one large functional ovum and smaller polar bodies.

 

Question 10. Distinguish anaphase of mitosis from anaphase I of meiosis.
Answer: Differences between anaphase of mitosis and anaphase I of meiosis:
Anaphase of mitosis:
1. Each chromosome aligned at the metaphase plate splits at the same time, and the two daughter chromatids, now called chromosomes for the future nuclei, start moving towards the two opposite poles.
2. As each chromosome travels from the center plane, its centromere faces the pole, acting as the leading edge, with the chromosome's arms following behind.
3. The anaphase stage is marked by these main happenings:

• Centromeres divide, and chromatids move apart.
• Chromatids travel to opposite poles.

Anaphase I of meiosis:
1. The spindle fibers shrink and pull the centromeres of homologous chromosomes towards the different poles.
2. The centromere does not divide. Therefore, half of the parent nucleus's chromosomes go to one pole, and the other half goes to the opposite pole.
3. Homologous chromosomes move apart, while sister chromatids stay connected at their centromeres.
In simple words: In mitotic anaphase, sister chromatids separate, and the centromeres split. In meiotic anaphase I, homologous chromosomes separate, but sister chromatids remain attached, and centromeres do not split.

Exam Tip: The key distinction lies in what separates: sister chromatids in mitotic anaphase versus homologous chromosomes in anaphase I of meiosis.

 

Question 11. List the main differences between mitosis and meiosis.
Answer: Main differences between mitosis and meiosis:
Mitosis:
1. It happens in somatic cells (body cells).
2. It creates two cells.
3. The chromosome count stays identical after mitosis.
4. Prophase is straightforward and brief.
5. No crossing over occurs.
6. The two nuclei formed after mitosis are just like the parent nucleus.
7. Chromosomes act separately from each other.
8. Mitosis supports growth and tissue repair.
Meiosis:
1. It happens in reproductive cells (germ cells).
2. It creates four haploid cells.
3. The number of chromosomes is cut in half after meiosis.
4. Prophase I is intricate and extended.
5. Crossing over occurs.
6. The four nuclei formed after meiosis are different from the parent nucleus because of crossing over.
7. Homologous chromosomes pair up (synapsis).
8. Meiosis is essential for sexual reproduction and for adding variations in offspring.
In simple words: Mitosis occurs in body cells, making two identical cells for growth and repair, with no change in chromosome number. Meiosis occurs in reproductive cells, making four genetically different haploid cells for sexual reproduction and genetic variation, halving the chromosome number.

Exam Tip: Create a table in your mind or on scratch paper comparing key aspects like cell type, number of daughter cells, ploidy, and genetic variation for effective recall.

 

Question 12. What is the significance of meiosis?
Answer:

• Meiosis helps keep the specific chromosome count of each species stable over generations.
• Because of crossing over, genetic traits from male and female parents combine. This leads to genetic variety in a population from one generation to the next, which is very important for evolution.
• Gametes (sex cells) are made through meiosis.
• Meiosis enables reproduction and is alike across various organisms.

In simple words: Meiosis is important because it maintains the correct chromosome number across generations, creates genetic diversity through crossing over, and produces sex cells (gametes) needed for reproduction.

Exam Tip: When discussing the significance of meiosis, always include its roles in maintaining chromosome number, generating genetic variation, and producing gametes.

 

Question 13. Discuss with your teacher about.
(1) haploid insects and lower plants where cell-division occurs, and
(2) some haploid cells in higher plants where cell-division does not occur.
Answer:
(1) In some simpler plants and certain social insects, haploid cells also undergo division by mitosis. It is extremely important to grasp the meaning of this division for the organism's existence. Mitosis usually creates diploid daughter cells with the same genetic information. The expansion of multicellular organisms is supported by mitosis. Cell growth can upset the balance between the nucleus and the cytoplasm.
(2) Plants are capable of mitotic divisions in both haploid and diploid cells.
In simple words: For certain lower plants and insects, haploid cells divide by mitosis, which helps in their growth and life cycles. However, in higher plants, some haploid cells do not divide at all.

Exam Tip: Be aware that while mitosis commonly involves diploid cells, there are exceptions in organisms like haploid insects and lower plants where haploid cells also divide mitotically for growth.

 

Question 14. Can there be mitosis without DNA replication in the 'S' phase?
Answer: The S or synthesis phase is when DNA synthesis or copying happens. During this period, the amount of DNA per cell becomes twice. If the original DNA amount is called 2C, it then becomes 4C. However, the number of chromosomes does not go up; if a cell started with a diploid or 2n set of chromosomes at G1, it still has 2n chromosomes even after the S phase.
In simple words: No, mitosis usually needs DNA replication in the S phase first. Without this, the daughter cells would not get a full set of genetic material.

Exam Tip: Emphasize that DNA replication in the S phase is a prerequisite for proper mitosis, ensuring genetic integrity in daughter cells.

 

Question 15. Can cell division?
Answer: No, regular cell division (mitosis or meiosis) cannot happen without earlier DNA copying in the S phase. DNA replication is vital to make sure each new cell gets a full and exact set of genetic information. Without copying, daughter cells would have less or incomplete DNA, which would harm their ability to work and live. DNA creation guarantees genetic consistency and correct distribution of chromosomes during cell division.
In simple words: No, proper cell division cannot happen without DNA replication first. DNA copying ensures that each new cell gets all the necessary genetic material to function correctly.

Exam Tip: Always connect DNA replication (S phase) to the necessity of maintaining genetic continuity and providing complete genetic information to daughter cells during division.

 

Question 16. Analyze the events during every stage of the cell cycle and notice how the following two parameters change
(1) Number of chromosomes (N) per cell
(2) Amount of DNA content (C) per cell
Answer: During the cell cycle, the quantity of chromosomes (N) and the amount of DNA (C) alter in these ways:

1. G1 Phase:

• Chromosome count (N): Stays constant (e.g., 2n).
• DNA amount (C): Stays constant (e.g., 2C). The cell gets bigger, but DNA is not copied.

2. S Phase (Synthesis Phase):

• Chromosome count (N): Stays constant (e.g., 2n).
• DNA amount (C): Becomes double (e.g., from 2C to 4C) because of DNA replication.

3. G2 Phase:

• Chromosome count (N): Stays constant (e.g., 2n).
• DNA amount (C): Is twice the starting quantity (e.g., 4C). The cell keeps growing and making proteins.

4. M Phase (Mitosis):

• Prophase & Metaphase:
  
  • Chromosome count (N): Stays constant (e.g., 2n, but each chromosome now has two chromatids).
  • DNA amount (C): Stays at 4C.
• Anaphase:
  
  • Chromosome count (N): Temporarily doubles (e.g., from 2n to 4n) as sister chromatids move apart, becoming separate chromosomes.
  • DNA amount (C): Stays at 4C.
• Telophase & Cytokinesis:
  
  • Chromosome count (N): Goes back to the initial amount (e.g., 2n) in each new cell.
  • DNA amount (C): Goes back to the initial amount (e.g., 2C) in each new cell.

In simple words: In G1, N and C stay the same. In S, N stays the same, but C doubles. In G2, N stays the same, and C is still double. In M phase (mitosis), N temporarily doubles during anaphase, then returns to normal, and C halves by the end of telophase/cytokinesis.

Exam Tip: Clearly delineate changes in 'N' (chromosome number) and 'C' (DNA content) for each phase (G1, S, G2, M), noting temporary shifts like the doubling of 'N' in anaphase and the halving of 'C' after cytokinesis.