Short & Long Question Answers with Solution: Cell Cycle and Cell Division | Science for ACT PDF Download

Short Answer Type Questions

Q1. What is the role of centrioles apart from spindle formation?
Ans: Within the centrosome, two centrioles are arranged in a perpendicular orientation, forming a distinctive cartwheel-like pattern. In addition to their role in generating spindle fibers during animal cell division, these centrioles serve as the foundation for the cilia and flagella found in both animal and plant cells. Furthermore, they play a vital role in the development of sperm tails and microtubules.

Q2. Why is the process of Meiosis necessary in sexually reproducing organisms?
Ans: The process of meiosis is necessary for sexually reproducing organisms because of the following reasons:

• Firstly, it constantly maintains the number of chromosomes in the generation process, as Meiosis is also called reductional division.
• Secondly, it causes variations among the progeny because crossing over occurs during Meiosis. This variation is important for evolution.

Q3. A cell having 32 chromosomes undergoes mitotic division. During metaphase, what will the chromosome number (N) of the cell? During anaphase, what will the DNA content of the cell be?
Ans: Mitosis is a process that takes place in the somatic cells of organisms. Throughout mitosis, the number of chromosomes remains constant between the parent and daughter cells, remaining unchanged even during anaphase and metaphase. However, the DNA content doubles during the interphase or synthetic phase. Chromosome division occurs during anaphase, yet the total number of chromosomes remains consistent.

Q4. Describe the importance of mitosis.
Ans: Mitosis is an important process because –

• through the process of mitosis, genetic stability occurs.
• It helps in the growth and development of multicellular organisms.
• Many plants and animals multiply with mitosis, i.e., asexual reproduction occurs to regenerate the whole organism.
• It helps regenerate the lost parts of an animal’s body by regenerating the new cells in place of dead and worn-out cells.

Q5. Which tissue of animals and plants exhibit meiosis?
Ans: Meiosis exclusively occurs within the tissues responsible for generating gametes. In animals, this process unfolds within the germ cells found in the body's reproductive organs, encompassing both the testes and ovaries. Meanwhile, in plants, meiosis takes place within the male reproductive structures known as the Androecium and the female reproductive structures known as the Gynoecium.

Q6. What do you understand by the term cell reproduction?
Ans:
Cell reproduction: Reproduction is an essential phenomenon in the continuity of life. Rudolf Virchow discovered that new cells arise by the division of pre-existing cells.
Reproduction is mainly classified into two types:

• Sexual reproduction
• Asexual reproduction.

The growth and development of the living being depend on cell division. The single-celled zygotes, by the process of cell division, develop into an adult having many cells.

Q7. Name the stages of the cell cycle at which these events occur:

1. Chromosomes are moved to the spindle equator.
2. Centromere splits apart, and chromatids separate.
3. Pairing between homologous chromosomes occurs.
4. Crossing overtakes place between the homologous chromosomes.

Ans:

1. Chromosomes move to the spindle equator in the Metaphase stage.
2. Centrosomes split apart, and the chromatids separate in the Anaphase
3. Pairing occurs between the homologous chromosomes in the Zygotene stage of prophase 1 in Meiosis
4. Crossing over occurs between homologous chromosomes and takes place during the Pachytene stage of prophase 1 in Meiosis

Q8. Which is the cell that is captured in the diplotene phase for months and years? How does it complete its cell cycle?
Ans: Diplotene can persist for extended periods, spanning months or even years, within the oocytes of select vertebrates.
During the diplotene phase, diplotene chromosomes are observed in the oocytes of certain amphibians or frogs.
Lampbrush chromosomes become noticeable during the meiotic prophase. These chromosomes tend to return to their normal state after growth, thereby concluding the cell cycle.

Q9. Is there mitosis without DNA replication in the ‘S’ phase, and can there be DNA replication without cell division?
Ans:
No, without DNA replication, mitosis does not occur. Because during the S phase, DNA synthesis or replication of DNA takes place and DNA replication is essential for cell division.
Yes, DNA replication can take place without the cell division process. To prepare for cell division, DNA replication is necessary. Cell division is the next logical step that occurs post-cell division.

Q10. Which tissue of plants and animals exhibits Meiosis?
Ans: The process of Meiosis takes place only in the tissues that produce gametes. So, in animals, Meiosis occurs within the body’s reproductive cells or germ cells. This also includes the cells of the ovaries and testes. In plants, Meiosis occurs in the Androecium (male reproductive) and Gynoecium (female reproductive) parts of the plant.

Long Answer Type Questions

Q1. Describe the different phases occurring in meiotic prophase – I. Also, mention the chromosomal events during each stage.
Ans: During the meiotic prophase – I, entities like genetic recombination and variation in sexually reproducing take place. Also, the chromosomal events during each stage are-

Leptotene stage: In this stage, the chromosomes are long, thin and slender, where the chromatin network gets exposed and threads appear clear.
Also, it contains a diploid number of chromosomes.

Zygotene stage: In this stage, the similar chromosomes turn intimately associated with the
Synapse. And as the synapses are exact, pairing occurs between the chromosomes and corresponding individual units.

These chromosomes appear thicker and shorter.

Pachynema Stage
In this stage, the synaptic chromosomes become intimately related to each other. There are thick and short pairs of chromosomes crossed over one another, and the chiasmata are visible clearly.

Diplotene Stage
The homologous chromosomes start detaching from each other and the Chiasmata tend to shift away, resulting in the terminalization of chiasmata. Also, the chromosomes detach and result in incomplete separation. The nucleolus and nuclear membrane also start to fade.

Diakinesis
In this stage, the bivalents are distributed randomly after further condensation.
Also, the paired chromosomes separate, and the terminalisation of chiasmata is concluded with the disappearance of the nucleolus and nuclear membrane.

Q2. Telophase is the reverse of prophase. Elucidate the statement.
Ans: Prophase commences with the condensation of chromosomal material. As this chromatin condenses, it undergoes a process of untangling. In the final stage of mitosis, which is telophase, as the chromosomes reach their respective poles, they de-condense and lose their distinct structure. When viewed through a microscope, cells at the conclusion of prophase lack Golgi complexes, nucleoli, endoplasmic reticulum, and the nuclear envelope. During the telophase stage, the nuclear envelope reforms around the cluster of chromosomes, and the Golgi complex, endoplasmic reticulum, and nucleolus reassemble.

Q3. Telophase is said to be the reverse of prophase. Describe the statement.
Ans: The condensation of the chromosomal material initiates the prophase. During this process, chromatin condensation occurs, and the chromosomal material untangles.
Also, at the start of the final stage of mitosis, that is, the telophase phase, the chromosomes arrive at the respective poles and then de-condense and lose their individuality. When observed under a compound microscope, the cells at the end of the prophase stage do not show organelles like  golgi complexes, nucleolus, endoplasmic reticulum, and the nuclear envelope. The nuclear envelope then gathers around the chromosomes cluster at the telephone stage: the Golgi complex, ER, and nucleolus reform.

Q4. Describe the different phases of meiotic prophase – I. Mention the chromosomal events during each stage.
Ans: During the prophase – I, genetic recombination and variation in sexually reproducing entities takes place due to the events of this stage.

Leptotene

• Chromosomes are long, thin and slender
• Chromatin network exposes and threads appear clear
• The diploid number of chromosomes

Zygotene

• Similar chromosomes turn intimately associated
• Synapse is exact hence pairing is not just between chromosomes, but corresponding individual units.
• Chromosomes appear thicker and shorter

Pachynema

• Synaptic chromosomes become intimately related
• Thick and short pair of chromosomes
• Cross over occurs, Chiasmata visible clearly

Diplotene

• Homologous chromosomes start detaching from each other.
• Chiasmata tends to shift away from chromosomes, termed as thermalization of chiasmata
• Chromosomes detach out, but it is an incomplete separation
• Nucleolus and nuclear membrane start to fade.

Diakinesis

• The bivalents are randomly distributed after further condensation
• The paired chromosomes separate completely
• Terminalisation of chiasmata is almost concluded
• The disappearance of the nucleolus and nuclear membrane

Q5. Describe the following events that take place during interphase.
Ans: Interphase involves a series of changes that occur during the preparation of a cell for division. It is said to be the period during which the cell experiences growth and DNA replication in a definite and orderly manner. Interphase is majorly divided into three phases.
(i) G1 phase
(ii) G2 phase
(iii) S phase

G1 phase: The G1 phase is the stage during which the cell grows and prepares the DNA for its replication. Also, in this phase, the cell becomes metabolically active.

S phase: S phase is the stage during which DNA synthesis occurs. In other words, during this phase, the amount of DNA doubles, but the number of chromosomes remains the same.

G2 phase: In the G2 phase, the cell continues to grow and prepares itself for division. The proteins and RNA required for mitosis are synthesised during this stage.