Question 1

Q1: What is the average cell cycle span for a mammalian cell?

Accepted Answer

The average cell cycle span for a mammalian cell, such as a human cell in culture, is approximately 24 hours. Of this time, the M phase (actual cell division) lasts for only about an hour, while the interphase lasts for more than 95% of the duration.

Question 2

Q10: Distinguish anaphase of mitosis from anaphase I of meiosis.

Accepted Answer

Question 3

Q11: List the main differences between mitosis and meiosis.

Accepted Answer

Question 4

Q12: What is the significance of meiosis?

Accepted Answer

Meiosis is a crucial process in sexually reproducing organisms and has two major points of significance: 1.  Conservation of Chromosome Number: Meiosis reduces the chromosome number from diploid (2n) to haploid (n) in the gametes. This is essential because during fertilization, the fusion of two hap...

Question 5

Q13: Discuss with your teacher about (i) haploid insects and lower plants where cell-division occurs, and (ii) some haploid cells in higher plants where ce...

Accepted Answer

(i) Haploid insects and lower plants where cell-division occurs:    Haploid Insects: An example mentioned in the text is the male honey bee (drone). Drones develop by parthenogenesis from unfertilized haploid eggs. Their bodies are composed of haploid cells, and they grow by mitosis. To produce game...

Question 6

Q14: Can there be mitosis without DNA replication in 'S' phase?

Accepted Answer

No, mitosis cannot occur without DNA replication in the S phase. The cell cycle is a sequence of events where the S phase (DNA synthesis) must precede the M phase (mitosis). The purpose of mitosis is to distribute the duplicated chromosomes equally into two daughter cells. If DNA replication does no...

Question 7

Q15: Can there be DNA replication without cell division?

Accepted Answer

Yes, DNA replication can occur without cell division. This phenomenon is known as endoreduplication, where a cell undergoes multiple rounds of S phase without entering mitosis or cytokinesis, leading to an increase in the number of chromosome sets (polyploidy). This occurs in certain specialized cel...

Question 8

Q16: Analyse the events during every stage of cell cycle and notice how the following two parameters change (i) number of chromosomes (N) per cell (ii) amo...

Accepted Answer

Let's analyze the changes for a diploid cell, where the initial chromosome number is 2n and the initial DNA content is 2C. Mitotic Cell Cycle: | Stage | Chromosome Number per cell | DNA Content per cell | | :--- | :--- | :--- | | $G_1$ | $2n$ | $2C$ | | S | $2n$ | $2C \rightarrow 4C$ | | $G_2$ | $2n...

Question 9

Q2: Distinguish cytokinesis from karyokinesis.

Accepted Answer

Karyokinesis and cytokinesis are two distinct events that occur during the M phase of the cell cycle.    Karyokinesis: This is the division of the nucleus. It involves the precise separation of the duplicated chromosomes into two daughter nuclei. It consists of four stages: prophase, metaphase, anap...

Question 10

Q3: Describe the events taking place during interphase.

Accepted Answer

Interphase is the preparatory phase between two successive M phases and is the longest phase of the cell cycle. It is divided into three further phases: 1.  $G_1$ phase (Gap 1): This is the post-mitotic phase where the cell is metabolically active and grows continuously. It synthesizes proteins and...

Question 11

Q4: What is $G_o$ (quiescent phase) of cell cycle?

Accepted Answer

The $G_0$ phase, or quiescent stage, is an inactive stage that some cells in adult animals enter when they exit the $G_1$ phase of the cell cycle. Cells in this stage do not divide further and are considered to be in a state of metabolic activity without proliferation. Examples include heart cells....

Question 12

Q5: Why is mitosis called equational division?

Accepted Answer

Mitosis is called equational division because the number of chromosomes in the progeny cells (daughter cells) is the same as that in the parent cell. During mitosis, the duplicated chromosomes are separated equally into two daughter nuclei, ensuring that each daughter cell receives an identical set...

Question 13

Q6: Name the stage of cell cycle at which one of the following events occur: (i) Chromosomes are moved to spindle equator. (ii) Centromere splits and chro...

Accepted Answer

(i) Metaphase: Chromosomes are moved to the spindle equator and align along the metaphase plate. (ii) Anaphase: The centromere of each chromosome splits, and the sister chromatids separate. (iii) Prophase I (Zygotene stage) of Meiosis: The pairing of homologous chromosomes, a process called synapsis...

Question 14

Q7: Describe the following: (a) synapsis (b) bivalent (c) chiasmata Draw a diagram to illustrate your answer.

Accepted Answer

(a) Synapsis: It is the process of pairing of homologous chromosomes that occurs during the zygotene stage of prophase I of meiosis. This precise alignment is facilitated by the formation of a proteinaceous structure called the synaptonemal complex between the homologous chromosomes. (b) Bivalent: A...

Question 15

Q8: How does cytokinesis in plant cells differ from that in animal cells?

Accepted Answer

Question 16

Q9: Find examples where the four daughter cells from meiosis are equal in size and where they are found unequal in size.

Accepted Answer

Equal Size: The four daughter cells from meiosis are equal in size in the formation of male gametes (spermatogenesis) in animals, such as humans. The process results in four equal-sized spermatids. Similarly, in the formation of microspores (pollen grains) in plants, the meiotic division of the micr...

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