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

Gujarat Board GSEB Textbook Solutions Class 11 Biology Chapter 10 Cell Cycle and Cell Division Textbook Questions and Answers.

Gujarat Board Textbook Solutions Class 11 Biology Chapter 10 Cell Cycle and Cell Division

GSEB Class 11 Biology Cell Cycle and Cell Division Text Book Questions and Answers

Question 1.
What is the average cell cycle span for a mammalian cell?
Answer:
24 hours is the average cell cycle for mammalian cell.

Question 2.
Distinguish cytokinesis from karyokinesis.
Answer:
The M phase starts with the nuclear division, corresponding to the separation of daughter chromosomes (karyokinesis), and usually ends with the division of cytoplasm (cytokinesis).

Question 3.
Describe the events taking place during the interphase.
Answer:
During interphase, the cell prepares for division by undergoing both cell growth and DNA replication in an orderly manner. It’s divided into three-phase.

• G phase: In this phase the cell is metabolically active and continuously grows but does not replicate its DNA.
• S phase /Synthesis): In this phase DNA synthesis or replication takes place. During this time the amount of DNA per cell doubles but the chromosome number remains the same. In animal cells, DNA replication begins in the nucleus and the centrioles duplicate in the cytoplasm.
• G₂ phase: In this phase proteins are synthesised in preparation for mitosis and cell growth continues.

Question 4.
What is the G₀ (quiescent phase) of the cell cycle?
Answer:
Some cells in the adult animals do not appear to exhibit division (e.g. heart cells) and many other cells divide only occasionally, as needed to replace cells that have been lost because of injury or cell death. These cells that do not divide further exit the G₁ phase to enter an inactive stage called the quiescent stage (G₀) of the cell cycle. Cells in this stage remain metabolically active but no longer proliferate unless called on to do so depending on the requirement of the organism.

Question 5.
Why is mitosis called equational division?
Answer:
M phase is the most dramatic period of the cell cycle involving a major reorganization of virtually all components of the cell. Since the number of chromosomes in the parent and progeny cells is the same it is also called equational division. Though for convenience mitosis has been divided into four stages of nuclear division, it is very essential to understand that cell division is a progressive process and very clear-cut lines cannot be drawn between various stages. Mitosis is divided into the following four stages:

• Prophase
• Meta phase
• Anaphase
• Telophase

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. In the metaphase stage of the cell cycle, chromosomes are moved to the spindle equator and get aligned along with the metaphase plate through microtubules to both poles.

2. In the anaphase stage of the cell cycle, centromeres split and chromatids separate.

3. The second stage of prophase I am called zygotene. During this stage, chromosomes start pairing together and this process of association is called synapsis. Such paired chromosomes are called homologous chromosomes

4. Pachytence stage is the third stage of prophase I. During this stage, bivalent chromosomes now clearly appear as tetrads. This stage is characterized by the appearance of recombination nodules, the sites at which crossing over occurs between non-sister chromatids of the homologous chromosomes. Crossing over is the exchange of genetic material between two homologous chromosomes.

Question 7.
Describe the following briefly :

1. Synapsis
2. bivalent
3. Chiasmata

Answer:

1. Synapsis: In the zygotene stage, homologous chromosomes start pairing together and this process is called synapsis
2. Bivalent: The complex formed by a pair of the synapsed homologous chromosomes is called a bivalent or a tetrad.
3. Chiasmata: In diplotene, the dissolution of the synaptonemal complex and the tendency of the recombined homologous chromosome of the bivalents to separate from each other except at the sites of cross-overs is recognized. These X-shaped structures are called chiasmata.

Question 8.
How does cytokinesis in plant cells differ from that in animal cells?
Answer:
In an animal cell, this is achieved by the appearance of a furrow in the plasma membrane. The furrow gradually deepens and ultimately joins in the center dividing the cell cytoplasm into two. Plant cells however are enclosed by a relatively inextensible cell wall, therefore they undergo cytokinesis by a different mechanism. In-plant Celis, wall formation starts in the center of the cell and grows outward to meet the existing lateral walls.

The formation of the new cell wall begins with the formation of a simple precursor, called the cell-plate that represents the middle lamella between the walls of two adjacent cells. At the time of cytoplasmic division, organelles like mitochondria and plastids get distributed between the two daughter cells. In some organisms, karyokinesis is not followed by cytokinesis as a result of which multinucleate condition arises leading to the formation of the syncytium.

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 forming equal daughter cells – Spermatogenesis
• Meiosis forming unequal daughter cells – Oogenesis.

Question 10.
Distinguish anaphase of mitosis from anaphase I of meiosis.
Answer:
Differences between anaphase of mitosis from anaphase I of meiosis:
Anaphase of mitosis:

1. Each chromosome arranged at the metaphase plate is split simultaneously and the two daughter chromatids, now referred to as chromosomes of the future daughter nuclei, begin their migration towards the two opposite poles.

2. As each chromosome moves away from the equatorial plane, the centromere of each chromosome is towards the pole and hence at the leading edge, with the arms of the chromosome trailing behind.

3. Anaphase stage is characterized by the following key events:

• Centromeres split and chromatids separate.
• Chromatids move to opposite poles.

Anaphase I of meiosis:
1. The spindle fibers contract and pull the centromeres of homologous chromosomes towards the opposite poles.

2. The centromere is not divided. Thus half of the chromosomes of the parent nucleus go to one pole and the remaining half to the opposite pole.

3. The homologous chromosomes separate, while sister chromatids remain associated at their centromeres.

Question 11.
List the main differences between mitosis and meiosis.
Answer:
Main differences between mitosis and meiosis:
Mitosis:

1. It takes place in somatic cells
2. It produced two cells.
3. The number of chromosomes remains the same after mitosis.
4. Prophase is simple and short.
5. No crossing over.
6. The two nuclei formed after mitosis are similar to the nucleus of a parent.
7. Chromosomes behave independently of each other.
8. Mitosis is the basis of growth and repair.

Meiosis:

1. It takes place in reproductive cells
2. It produced four haploid cells.
3. The number of chromosomes is reduced to one half after meiosis.
4. Prophase I is complicated and long.
5. Crossing over takes place.
6. The four nuclei formed after meiosis differs from the parent nucleus due to crossing over.
7. Homologous chromosomes get paired (synapsis).
8. Meiosis is the basis of sexual reproduction as well as providing variations in the progeny.

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

• Conservation of specific chromosome number of each species across generations is achieved by meiosis.
• Due to crossing over, the hereditary characters from male and female parents get mixed. So genetic variability in the population of organisms from one generation to the next occurs which is most important for the process of evolution.
• Gametes are produced by Meiosis.
•  Meiosis results in reproduction and is similar among different organisms.

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 lower plants and in some social insects haploid cells also divide by mitosis. It is very essential to understand the significance of this division in the life of an organism. Mitosis results in the production of diploid daughter cells with identical genetic complement usually. The growth of multicellular organisms is due to mitosis. Cell growth results in disturbing the ratio between the nucleus and the cytoplasm.

2. The plants can show mitotic divisions in both haploid and diploid cells.

Question 14.
Can there be mitosis without DNA replication in the ‘S’ phase?
Answer:
S or synthesis phase marks the period during which DNA synthesis or replication takes place. During this time the amount of DNA per cell doubles. If the initial amount of DNA is denoted as 2C then it increases to 4C. However, there is no increase in the chromosome number; if the cell had diploid or 2n number of chromosomes at G, even after the S phase the number of chromosomes remains the same i.e. 2n.

Question 15.
Can there be DNA replication without cell division?
Answer:
In animal cells, during the S phase, as DNA replication begins in the nucleus, the centrioles, initiate replication in the cytoplasm.

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:

• Gt Phase: DNA and chromosome content do not change.
• S Phase: Amount of DNA content doubles eg: from ‘2c’ to ‘4c’.
  Number of chromosome remains the same.
• G2 phase: Amount of DNA is twice that of the original cell.
• M phase: Chromosome number per cell remains the same after M phase. DNA content is same as the original cell.

G₁ phase corresponds to the interval between mitosis and initiation of DNA replication. During G₁ phase the cell is metabolically active and continuously grows but does not replicate its DNA. S phase marks the period during which DNA synthesis or replication takes place. During this time the amount of DNA per cell doubles. If the initial amount of DNA is denoted as 2C then it increases to 4 C. There is no increase in the chromosome number. During the G, phase proteins are synthesized in preparation for mitosis while cell growth continues.

M phase is the most dramatic period of the cell cycle, involving a major reorganization of virtually all components of the cell. Mitosis is divided into the following four stages:

1. Pm phase
2. Metaphase
3. Anaphase
4. Telophase

The completion of prophase can thus be marked by the following characteristic events:

1. Chromosomal material condenses to find compact mitotic chromosomes. Chromosomes are seen to be composed of two chromatids attached together at the centromere.
2. Initiation of the assembly of the mitotic spindle, the microtubules, the proteinaceous components of the cell cytoplasm help in the process.

The key features of metaphase are:

1. Microtubules attach to the kinetochores of chromosomes.
2. Chromosomes are moved to the spindle equator and get aligned along with the metaphase plate through microtubules to both poles.

The anaphase stage is characterized by the following key events:

1. Centromeres split and chromatids separate.
2. Chromatids move to opposite poles.

Telophase is the stage that shows the following key events:

1. Chromosomes cluster at opposite spindle poles and their identity is lost as discrete elements.
2. The nuclear envelope assembles around the chromosome clusters.
3. Nucleolus, golgi complex and ER reform.

The key features of meiosis are as follows:

1. Meiosis involves two sequential cycles of nuclear and cell division, called meiosis I and meiosis II but only a single cycle of DNA replication.
2. Meiosis I is initiated after the parental chromosomes have replicated to produce identical sister chromatids at the S phase.
3. Meiosis involves the pairing of homologous chromosomes and recombination between them.
4. Four haploid cells are formed at the end of meiosis II.