Cell Cycle and Cell Division Module

Q1. The process of appearance of recombination nodules occurs at which sub-stage of prophase I in meiosis?

1. Diakinesis
2. Zygotene
3. Pachytene
4. Diplotene

Answer – 3, Pachytene
Explanation: The process of the appearance of recombination nodules occurs during the Pachytene sub-stage of prophase I in meiosis. It is important to note that this is a very complex stage of meiosis and requires a thorough understanding to answer this question accurately.

Q2. Among eukaryotes, replication of DNA takes place in

1. G2 Phase
2. M phase
3. S phase
4. G1 phase

Answer – 3, S phase
Explanation: Replication of DNA takes place in the S phase of the cell cycle. The G2 phase follows the S phase and prepares cells for division in the M phase, while the G1 phase is the initial stage in which cells prepare themselves and grow.

Q3. Which one of the following never occurs during mitotic cell division?

1. Coiling and condensation chromatids
2. Spindle fibers attach to Kinetochores of chromosomes
3. Movement of centrioles toward opposite poles
4. The pairing of homologous chromosomes

Answer – 4, Pairing of homologous chromosomes
Explanation: The pairing of homologous chromosomes. During mitotic cell division, chromatins coil and condense, spindle fibers attach to kinetochores of chromosomes, and centrioles move toward opposite poles. However, the pairing of homologous chromosomes does not occur during this process.

Q4. Regarding meiosis, which statement is incorrect?

1. Four haploid cells form at the end of meiosis-II
2. In meiosis, there are two stages: meiosis I and II
3. DNA replication occurs in the S phase of meiosis-II
4. The pairing of homologous and recombination chromosomes occurs during meiosis-1

Answer – 3, DNA replication occurs in the S phase of meiosis -II
Explanation: DNA replication occurs during the S phase of meiosis-I, not meiosis II. Meiosis-I happens when homologous chromosomes pair up, undergo recombination (crossing over), and divide into two daughter cells. The daughter cells then undergo meiosis II without undergoing DNA replication again.

In sexually reproducing organisms, meiosis is a specialized cell division that produces four haploid cells (gametes) from a diploid cell through two rounds of division, known as meiosis I and meiosis II.

During meiosis-I, the pairing of homologous chromosomes occurs (synapsis), followed by crossing over (recombination) between non-sister chromatids of homologous chromosomes. Genetic variation is a result of these genetic exchanges. Upon separation of homologous chromosomes, two daughter cells are formed, each containing a pair of replicated chromosomes.

Similar to mitosis, meiosis-II involves the separation of sister chromatids. The two daughter cells from meiosis-I enter meiosis II. Meiosis-II separates sister chromatids, which results in four haploid cells (gametes) with unique genetic combinations.

There are two stages of meiosis: meiosis-I and meiosis-II. DNA replication occurs during meiosis-I. The pairing of homologous chromosomes and recombination occur during meiosis-I.

Q5. When do the chromosomes pair during meiosis I?

1. Leptotene
2. Zygotene
3. Pachytene
4. Diplotene

Answer – 2, Zygotene
Explanation: During this stage of meiosis I, homologous chromosomes pair together and form tetrads. This pairing ensures that each daughter cell receives a complete set of chromosomes.

Q6. What is true about the mitotic spindle?

1. It is composed of actin and myosin microfilaments.
2. It includes the kinetochore at the metaphase plate.
3. It is composed of microtubules, which separate chromosomes at opposite poles of the cell.
4. It originates only from centrioles of centrioles.

Answer- 3, It is composed of microtubules, which separate chromosomes at opposite poles of the cell.
Explanation: The mitotic spindle is a structure essential for cell division, which organizes the separation and distribution of chromosomes to daughter cells. This crucial apparatus comprises protein filaments called microtubules, which attach to the centromeres of the chromosomes via kinetochores and align them at the metaphase plate. As mitosis progresses, the microtubules shorten, exerting forces that pull sister chromatids in opposite directions until each daughter cell gets its share of chromosomes. While actin and myosin microfilaments are essential for other cellular processes, they do not make up the spindle. Centrosomes, with their centrioles, have involvement in organizing the spindle’s microtubules, yet it is possible to form these filaments and fulfill their purpose without them. Thus, asserting that the entire process depends on centrioles alone is inaccurate.

Q7. The synapsis occurs between

1. male gamete and female gamete
2. mRNA and ribosomes
3. spindle fiber and centromere
4. two homologous chromosomes

Answer – 4, two homologous chromosomes
Explanation: The synopsis is a step in meiosis that occurs during prophase I, where two homologous chromosomes come together to form a bivalent or tetrad. The chromosomes align side by side and are held together by the synaptonemal complex, which facilitates crossing over – an exchange of genetic material between non-sister chromatids. This process of genetic recombination is integral to generating new combinations of alleles, ultimately contributing to genetic diversity.

Q8. The appearance of recombination nodules on homologous chromosomes during meiosis characterizes:

1. Personalization
2. Synaptonemal complex
3. Bivalent
4. Sites at which crossing over occurs

Answer – 4, Sites at which crossing over occurs
Explanation: Crossing over, or genetic recombination occurs at specific locations along the homologous chromosomes, known as recombination nodules or chiasmata. This phenomenon is an essential part of meiosis, leading to the shuffling and exchanging of genetic information. The synaptonemal complex helps to facilitate this transfer of material, though it is not necessarily related to the appearance of recombination nodules. Bivalents consist of two homologous chromosomes and four chromatids; the presence of recombination nodules is a characteristic feature. Terminalization refers to the orientation of these nodules towards the ends of chromosomes in later stages but does not have any connection with their emergence.

Q9. Where does the synaptonemal complex appear? a. Between homologous chromosomes b. In the zygotene stage c. Composed of DNA + protein d. All the above

Answer – 4, All of the above
Explanation: The synaptonemal complex forms during the zygotene stage of prophase I of meiosis and is comprised of proteins and DNA. It helps ensure the alignment and stabilization of the chromosomes, enabling genetic material to be exchanged between non-sister chromatids. This complex plays a crucial role in providing accurate recombination and genetic diversity.

Q10. Which of the following precedes nuclear envelope reformation during the M phase of the cell cycle?

1. Chromosome decondensation and reassembly of the nuclear lamina
2. Transcription from chromosomes and nuclear leaf recombination
3. Contraction ring formation and phragmoplast. formation
4. Contraction ring formation and transcription from chromosomes.

Answer – 1, Decondensation from chromosomes and reassembly of the nuclear lamina
Explanation: During the M-phase (mitosis) of the cell cycle, the nuclear envelope breaks down to allow the separation of replicated chromosomes and subsequent division. When the chromosomes have been divided during the M-phase and the spindle fibers, have retracted, the nuclear envelope reforms around these separate sets.

The nuclear envelope must undergo several events before being rebuilt, including decondensation and reassembly of the nuclear lamina. During the prophase of mitosis, chromosomes condense to facilitate alignment and separation, but during subsequent steps of mitosis, they de-condense and return to their extended, non-condensed state. Decondensation allows condensed chromatin structures to relax and disperse.

At the same time, the nuclear lamina, which supports the atomic envelope structurally, undergoes disassembly during early mitosis. In contrast, as the chromosomes de-condense and the nuclear envelope reforms during late mitosis, the nuclear lamina reassembles during late mitosis, helping to rebuild the nuclear envelope structure.

The correct answer is Decondensation from chromosomes and reassembly of the nuclear lamina.

Q11. As there occurs more and more condensation of chromatin during cell division, there occurs

1. Increase in heterochromatin
2. Increase in euchromatin
3. Differentiation of heterochromatin and euchromatin decreases
4. Differentiation of heterochromatin and euchromatin increases

Answer- 4, Differentiation of heterochromatin and euchromatin increases
Explanation: As cell division progresses and cells enter the mitotic phase, there is an increase in chromatin condensation. This increased condensation is associated with the formation of distinct regions of heterochromatin and euchromatin within the chromosomes.

Heterochromatin refers to the densely packed and transcriptionally inactive regions of chromatin, while euchromatin refers to the less condensed and transcriptionally active parts. During mitosis, the chromatin undergoes significant condensation to facilitate proper chromosome segregation and organization.

Therefore, the correct answer is Differentiation of heterochromatin and euchromatin increases.

Q12. If a diploid cell is treated with colchicine, it becomes.

1. triploid
2. tetraploid
3. diploid
4. Monoploid

Answer – 2, tetraploid
Explanation: Colchicine is a drug that inhibits the formation of spindles during cell division, thus preventing chromosome separation and resulting in a tetraploid cell with four sets of chromosomes.

Q13. What results in the formation of chiasmata?

1. Exchange of parts of paired homologous chromosome
2. Exchange of part of unpaired non-homologous chromosome
3. Duplication of parts of paired homologous chromosome
4. Loss of parts of unpaired non-homologous chromosome

Answer – 1, Exchange of parts of paired homologous chromosome
Explanation: Exchange of parts of paired homologous chromosomes. This process, known as crossing over, forms chiasmata, which hold homologous chromosomes together during meiosis and allow them to separate and form haploid gametes.

Q14. During mitosis ER and nucleolus begin to disappear at.

1. late prophase
2. early metaphase
3. late metaphase
4. early prophase

Answer – 4, early prophase
Explanation: During mitosis, the endoplasmic reticulum (ER) and nucleolus disappear at early prophase.

During early prophase, the chromatin condenses, and the nuclear envelope breaks down. As the nuclear envelope breaks down, the ER, which is continuous with the nuclear membrane, also starts to disassemble—the disassembly of the ER results in its disappearance from the cytoplasm during early prophase.

Similarly, during early prophase, the nucleolus, which is responsible for ribosome assembly, also starts to disappear. As the chromatin condenses and the nuclear envelope breaks down, the nucleolus undergoes reorganization and disperses within the nucleus.

Q15. The stage which serves as a connecting link between meiosis 1 and meiosis 2

1. Interphase
2. Interphase
3. Interkinesis
4. None of the above

Answer- 3, Interkinesis
Explanation: Interkinesis is the correct answer. Interkinesis serves as a connecting link between meiosis 1 and meiosis 2. During this stage, chromosome structure is not altered, but spindle fibers form and the nucleus dissolves. This provides an important pause in the division process before each of the two daughter cells can enter into meiosis 2.

Q16. ________ is the best stage to observe the shape, size, and number of chromosomes

1. interphase
2. metaphase
3. prophase
4. Telophase

Answer – 2, Metaphase
Explanation: Metaphase is the best time to observe chromosome shape, size, and number.

At the metaphase plate, a region in the center of the cell, the replicated chromosomes align for mitosis or meiosis. The chromosomes at metaphase are highly condensed and tightly packed, making it easy to observe their individual structures. They are also aligned in one plane, making it convenient to compare and analyze. Chromosomes are now clearly visible under a microscope, appearing as rod-like structures, and their number, shape, and size can be analyzed.

In contrast, during interphase, the chromosomes are in a more extended and diffuse state, making it difficult to observe individual chromosomes. It is less possible to examine chromosomes’ shape, size, and number precisely during prophase and telophase when they are either condensing or decondensing, respectively.