31 Years NEET Previous Year Questions: Cell Cycle & Division - 1 Free MCQ

• Mitosis is the process of cell division in eukaryotic cells that results in two genetically identical daughter cells. This process is crucial for growth, repair, and asexual reproduction in multicellular organisms.
• Spindle fibers are protein structures formed during cell division. They originate from the centrosomes (or spindle pole bodies in some organisms) and play a critical role in ensuring the chromosomes are properly aligned and separated.
• The spindle fibers attach to the centromeres of the chromosomes through structures called kinetochores. This attachment is essential for the correct segregation of chromosomes during mitosis.
• During metaphase, spindle fibers align the chromosomes along the metaphase plate (the center of the cell). This ensures that each daughter cell will receive an identical set of chromosomes.
• In anaphase, spindle fibers pull the sister chromatids apart by shortening, ensuring that one copy of each chromosome moves to opposite poles of the cell. This separation is important for maintaining the correct chromosome number in daughter cells.

During the process of mitosis, spindle fibers play a crucial role in the movement and alignment of chromosomes. These fibers are made of microtubules and are essential for the chromosomes to move accurately into position during cell division. Let's discuss when the spindle fibers attach to the kinetochores of the chromosomes:

• Prophase: This is the initial stage of mitosis, where the chromosomes condense and become visible. However, the spindle fibers have not yet attached to kinetochores. Spindle fibers begin to form, but their attachment to kinetochores happens later.
• Metaphase: This is the stage where the spindle fibers attach to the kinetochores of chromosomes. During metaphase, chromosomes align at the metaphase plate (also known as the equatorial plane of the cell). This alignment is facilitated by the spindle fibers, which pull the chromosomes into position for accurate separation into daughter cells.
• Anaphase: During anaphase, the sister chromatids separate and are pulled apart as the spindle fibers shorten. While spindle fibers are essential during this phase for pulling apart the chromatids, their initial attachment to kinetochores occurs earlier.
• Telophase: This is the final stage of mitosis, where the chromosomes reach the opposite poles of the cell, and the cell starts to re-form its nuclear membranes. Spindle fibers disassemble during this phase.

Based on the description of functions and timing within mitosis, the correct answer for when spindle fibers attach to kinetochores of chromosomes is Option B: Metaphase.

The two statements in question are related to specific stages of meiosis, specifically prophase I. Understanding these stages will help us decide the accuracy of the statements.

Statement I: Chromosomes become gradually visible under light microscope during leptotene stage.
Analysis of Statement I: During the leptotene stage of prophase I in meiosis, chromatin begins to condense, making chromosomes more visible under the light microscope. However, it is important to note that the chromosomes are not fully condensed at this stage. They continue condensing in the later stages (zygotene and pachytene). Therefore, the statement is largely accurate as it reflects the initial visibility of the chromosomes.

Statement II: The beginning of diplotene stage is recognized by dissolution of synaptonemal complex.
Analysis of Statement II: In the diplotene stage of prophase I, the previously formed synaptonemal complexes, which assist in the pairing of homologous chromosomes during the earlier zygotene and pachytene stages, start to dissolve. This dissolution marks the transition into diplotene and is a key event that defines this stage. So, this statement is true.
Given that the analysis shows both statements to be accurate in their description of events during meiosis, the most appropriate choice is:
Option A: Both Statement I and Statement II are true.

The correct sequence of stages of cell division is
 
The correct sequence will be

The matching between the sub-phases of Prophase I and their specific characters can be understood as follows:

List I (Sub Phases of Prophase I):

• Leptotene: This is the first stage where chromosomes start to become visible under the microscope as thin, thread-like structures. Hence, it is characterized by chromosomes looking like thin threads.
• Zygotene: During this stage, synapsis begins, which is the pairing of homologous chromosomes. This corresponds to the formation of the synaptonemal complex.
• Pachytene: At this stage, crossing over occurs where genetic material is exchanged between homologous chromosomes. Appearance of recombination nodules, which are essential for crossing over, characterizes this stage.
• Diakinesis: This is the final stage of prophase I where chiasmata (the site of crossing over) move to the ends of the chromosomes, a process known as terminalization. Hence, this phase is marked by the completion of terminalization of chiasmata.

List II (Specific Characters):

• I. Synaptonemal complex formation - Characteristic of Zygotene.
• II. Completion of terminalisation of chiasmata - Characteristic of Diakinesis.
• III. Chromosomes look like thin threads - Characteristic of Leptotene.
• IV. Appearance of recombination nodules - Characteristic of Pachytene.

Matching the sub-phases to their characters:
A (Diakinesis) matches with II (Completion of terminalisation of chiasmata).
B (Pachytene) matches with IV (Appearance of recombination nodules).
C (Zygotene) matches with I (Synaptonemal complex formation).
D (Leptotene) matches with III (Chromosomes look like thin threads).

Thus, the correct answer is Option C: A-II, B-IV, C-I, D-III.

• A. Chromosomes start pairing together to form synaptonemal complex → (II) Zygotene: During the zygotene stage of prophase I, homologous chromosomes begin to pair with each other to form the synaptonemal complex, which helps align the chromosomes for recombination.
• B. Chromosomes visible under light microscope and compaction continues → (IV) Leptotene: In leptotene, chromosomes start to condense and become visible under the light microscope, though they are not yet fully compacted.
• C. Four chromatids of bivalent chromosomes become distinct and recombinant nodules appear → (I) Pachytene: In pachytene, the homologous chromosomes are fully paired (bivalent), and the exchange of genetic material (crossing over) happens. The chromatids become clearly distinct, and recombinant nodules are visible.
• D. Dissolution of synaptonemal complex → (III) Diplotene: In diplotene, the synaptonemal complex dissolves, and the homologous chromosomes begin to separate, although they are still connected by chiasmata (sites of crossover).

Thus, the correct matching is:

• A → II (Zygotene)
• B → IV (Leptotene)
• C → I (Pachytene)
• D → III (Diplotene)

Therefore, the correct answer is (4) A-II, B-IV, C-I, D-III.

• A. G₁ phase (I): The G₁ phase is the first phase of the cell cycle, where the cell is metabolically active and grows continuously. During this phase, the cell prepares for DNA replication and other activities but does not replicate its DNA yet.
• B. S phase (II): The S phase is where DNA replication occurs. The cell synthesizes a complete copy of its DNA in preparation for cell division.
• C. G₀ stage (I): The G₀ stage is a resting phase where the cell is metabolically active but does not proliferate or divide. The cell is temporarily or permanently out of the cell cycle.
• D. G₂ phase (III): The G₂ phase is the period after DNA replication where the cell synthesizes proteins and prepares for mitosis. During this phase, the cell ensures that DNA replication is complete and checks for any errors.

Therefore, the correct matching is:

• A → IV (G1 phase → Cell is metabolically active and grows continuously)
• B → II (S phase → Replication of DNA)
• C → I (G0 stage → Cell is metabolically active but no proliferation)
• D → III (G2 phase → Synthesis of proteins)

Thus, the correct answer is (B) A-IV, B-II, C-I, D-III.

• A. Chiasmata → (III) Diplotene: Chiasmata are the visible points where homologous chromosomes exchange genetic material during crossing over. These chiasmata become visible during the diplotene stage of prophase I.
• B. Crossing over → (I) Pachytene: Crossing over occurs during the pachytene stage of prophase I, where homologous chromosomes exchange segments of genetic material, leading to genetic recombination.
• C. Synaptonemal complex formation → (IV) Zygotene: The synaptonemal complex forms during the zygotene stage of prophase I, where homologous chromosomes begin pairing up and aligning with each other.
• D. Terminalisation of chiasmata → (II) Diakinesis: In diakinesis, the terminalisation of chiasmata occurs, where the chiasmata move to the ends of the chromosomes as they prepare for metaphase.

Thus, the correct matching is:

• A → III (Chiasmata → Diplotene)
• B → I (Crossing over → Pachytene)
• C → IV (Synaptonemal complex formation → Zygotene)
• D → II (Terminalisation of chiasmata → Diakinesis)

Therefore, the correct answer is (A) A-III, B-I, C-IV, D-II.

Recombination, or crossing over, between homologous chromosomes occurs during pachytene, which is a stage of prophase I in meiosis. During this stage, homologous chromosomes pair up (synapsis) and exchange genetic material. This process results in recombination, leading to genetic diversity.

• Zygotene is the stage where homologous chromosomes begin pairing but crossing over has not yet occurred.

• Diplotene is when the synaptonemal complex dissolves, and the chiasmata (points of crossover) become visible.

• Diakinesis is the final stage of prophase I, where the chromosomes condense further, but recombination itself is already completed by the end of pachytene.

Thus, recombination between homologous chromosomes is completed by the end of pachytene. Therefore, the correct answer is (d) Pachytene.

• A. Cells are metabolically active and proliferate → (II) G₁ phase: In the G₁ phase, cells are metabolically active, grow, and prepare for DNA replication. This phase involves cell proliferation as the cell carries out its normal functions and grows.
• B. DNA replication takes place → (IV) S phase: The S phase (Synthesis phase) is when DNA replication occurs. During this phase, the cell makes an identical copy of its DNA in preparation for cell division.
• C. Proteins are synthesized → (I) G₂ phase: During the G₂ phase, the cell continues to grow and synthesizes the necessary proteins and organelles in preparation for mitosis (M phase).
• D. Quiescent stage with metabolically active cells → (III) G₀ phase: The G₀ phase is a resting stage where cells are not dividing but are still metabolically active. Cells in this phase have temporarily or permanently exited the cell cycle.

Thus, the correct matching is:

• A → II (G₁ phase)
• B → IV (S phase)
• C → I (G₂ phase)
• D → III (G₀ phase)

Therefore, the correct answer is (D) A-II, B-IV, C-I, D-III.

During meiosis, the division of the centromere occurs in Anaphase II. At this stage, the sister chromatids of each chromosome (which are attached at the centromere) are pulled apart and move toward opposite poles of the cell. This is similar to what happens in anaphase of mitosis. In contrast, during Anaphase I of meiosis, homologous chromosomes are separated but the centromeres do not divide, meaning the sister chromatids stay together.

Recombination nodules appear during the pachytene stage of prophase I in meiosis. These nodules are thought to be involved in crossing over, a process where homologous chromosomes exchange genetic material. This leads to genetic recombination, which is a significant source of genetic variation in sexually reproducing organisms.

In eukaryotes, DNA replication takes place during the S phase (Synthesis phase) of the cell cycle. During this phase, the entire genome is replicated, resulting in two copies of each chromosome to ensure that both daughter cells receive a complete set of genetic information during cell division.

• M phase or mitosis is the phase where the actual cell division occurs. Mitosis is also called equational division.
• During G₂ phase DNA synthesis stops but cell synthesis RNA, proteins, etc. for next phase.
• Quiescent stage is inactive phase in which non-dividing cells enters.
• G₁ phase is the interval between mitosis and initiation of DNA replication.

Therefore, option (c) is correct.

• Statement I is incorrect. The G0 phase is a state in the cell cycle in which cells exist in a quiescent or dormant stage. However, this does not mean that the cell is metabolically inactive. Rather, the cell maintains its normal functions but is not preparing for cell division.
• Statement II is correct. During the S phase (synthesis phase) of interphase, DNA replication occurs, and the centrosome, which plays a key role in cell division, also duplicates. This prepares the cell for the later stages of the cell cycle, where the cell divides into two daughter cells.

1. Tetrad formation is seen during the Zygotene stage : During the Zygotene stage of Prophase I in meiosis, homologous chromosomes pair up, forming tetrads or bivalents.
2. During Anaphase, the centromeres split and chromatids separate : In Anaphase, the centromeres divide, and the sister chromatids are pulled apart towards opposite poles of the cell.
3. Terminalization of chiasmata takes place during Diakinesis : Diakinesis is the final stage of Prophase I in meiosis. During this stage, terminalization occurs, which is the process in which chiasmata, the points of crossing over between non-sister chromatids, move toward the ends of the chromosomes.
4. Nucleolus, Golgi complex, and ER are reformed during Telophase : During Telophase, the nuclear envelope starts to reassemble around the separated chromosomes, and the nucleolus, Golgi complex, and endoplasmic reticulum (ER) are reformed.
5. Crossing over takes place between non-sister chromatids of homologous chromosomes : This process occurs during the Pachytene stage of Prophase I in meiosis, leading to the exchange of genetic material between the non-sister chromatids, which increases genetic diversity in the resulting gametes.

• The synaptonemal complex is a structure that forms during the zygotene stage of prophase I in meiosis. It helps in the pairing of homologous chromosomes.
• The dissolution of the synaptonemal complex occurs during the diplotene stage of prophase I. In this stage, homologous chromosomes begin to separate, but they are still held together at the points of crossover (chiasmata).
• The pachytene stage is when crossing over and recombination occur, and the synaptonemal complex is still intact.
• The diakinesis is the final stage of prophase I, where chromosomes are fully condensed, and chiasmata are terminalized, but the synaptonemal complex is already dissolved by the time this stage starts.
• The leptotene is the very early stage of prophase I, where chromosomes start to condense, but synaptonemal complex formation is yet to occur.

Thus, the correct answer is (b) Diplotene.

During metaphase, chromosomes align at the metaphase plate, and the sister chromatids are attached by centromeres. If mitosis is disrupted at this stage, the chromosomes may fail to separate properly, resulting in a doubling of the chromosome number because each daughter cell receives a full set of chromosomes from the division process.

• In mitosis, the centromere splits during anaphase, allowing the sister chromatids to separate and move to opposite poles of the cell.
• In meiosis, during anaphase II, the centromere of each chromosome splits, allowing the sister chromatids of the chromosomes in the second meiotic division to separate.

The G₀ phase is the quiescent or resting phase in the cell cycle. Cells in this phase are not actively dividing and may remain in this state temporarily or permanently, depending on the cell type and environmental factors. It's essentially a phase where the cell is metabolically active but not proliferating.

The sequence of prophase I substages in meiosis follows this order:

• Leptotene (D): This is the first substage of prophase I, where the chromosomes begin to condense and become visible under a microscope, but they are still thin and long. Homologous chromosomes do not yet pair.
• Zygotene (A): During this substage, homologous chromosomes begin to pair up through a process called synapsis, forming a structure called the synaptonemal complex.
• Pachytene (B): In this substage, homologous chromosomes are fully paired (bivalents), and crossing over (genetic recombination) occurs between non-sister chromatids of homologous chromosomes.
• Diplotene (E): In this substage, the synaptonemal complex dissolves, and homologous chromosomes begin to separate slightly, but they remain connected at points called chiasmata (sites of crossing over).
• Diakinesis (C): This is the final substage of prophase I, where the chromosomes condense further, the nuclear envelope breaks down, and the chiasmata move toward the ends of the chromosomes (terminalization). The cell is now ready to move on to metaphase I.

Thus, the correct order is: Leptotene → Zygotene → Pachytene → Diplotene → Diakinesis, corresponding to option (c).

In oocytes of some vertebrates, diplotene lasts for months or years. This stage is referred as dictyotene stage.

Correct sequence of events during Prophase I of meiosis is : (b) → (a) → (c) → (e) → (d)
(b) Chromosomes become gradually visible under microscope.
(a) Synapsis of homologues chromosomes.
(c) Crossing over between non-sister chromatids of homologous chromosomes.
(e) Dissolution of synaptonemal complex.
(d) Terminalisation of chiasmata.

Bivalent or tetrad formation is called synapsis which is accompanied by the formation of complex structure called synaptonemal complex.

The question asks which statement is incorrect with respect to the metaphase stage of cell division.

Option D is incorrect because metaphase chromosomes are not made up of four sister chromatids held together by the centromere. Instead, metaphase chromosomes consist of two sister chromatids held together by the centromere. The sister chromatids are identical copies of a single chromosome that have been replicated during the S phase of the cell cycle.

Now, let's look at the other options, which are correct :

• Option A : Chromosomes lie at the equator of the cell - During metaphase, the chromosomes align along the equatorial plane (also known as the metaphase plate) in the middle of the cell. This arrangement ensures that each daughter cell will receive an equal number of chromosomes during cell division.
• Option B : Complete disintegration of the nuclear envelope takes place - In both mitosis and meiosis, the nuclear envelope breaks down during prophase and is completely disintegrated by the time the cell reaches metaphase. This allows the spindle fibers to attach to the chromosomes and facilitate their alignment at the equator.
• Option C : Chromosomes are highly condensed - During metaphase, the chromosomes are at their highest level of condensation, which allows them to be more easily visualized under a microscope. This condensation also aids in the proper alignment and segregation of chromosomes during cell division.

Pachytene stage of meiosis is characterized by the appearance of recombination nodules, the sites at which crossing over occurs between non sister chromatids of homologous chromosomes.

• Meiosis involves two sequential cycles of nuclear and cell division called meiosis-I and meiosis-II but only single cycle of DNA replication.
• The stage between two meiotic divisions is called interkinesis and is generally short lived and involves no DNA replication.

• Spindle fibres attach to the kinetochores of chromosomes.
• Kinetochores are the disc shaped structures present on sides of primary constriction or centromere of chromosomes.

In metacentric chromosome, centromere is in the middle of the chromosomes. Acrocentric chromosome has centromere close to the end of the chromosome. In submetacentric chromosome, centromere is slightly away from the middle of the chromosome. Telocentric chromosome has terminal centromere.

• Pairing of homologous chromosomes occurs during prophase I of meiosis.
• Coiling and condensation of chromatids, spindle fibres attachment to the kinetochores and movement of centrioles towards opposite poles occur in both mitosis and meiosis.