NEET Cell Cycle and Cell Division Previous Year Questions with Solutions

The cell cycle is divided into two basic phases: Interphase (which occupies over 95% of cycle duration) and M Phase (mitosis). Interphase is divided into three sequential periods: G₁ phase, S phase, and G₂ phase.

• • G₁ (Gap 1): Interval between mitosis and initiation of DNA replication.
• • S (Synthesis): DNA replication doubles DNA content from 2C to 4C while chromosome number remains 2n.
• • G₂ (Gap 2): Proteins are synthesized in preparation for mitosis while cell growth continues.

Following G₂, the cell enters the M phase (mitosis), where sister chromatids are partitioned equally into two daughter cells through prophase, metaphase, anaphase, telophase, and cytokinesis.

• • Matches Option (3).

Option (1) places G₂ before S (physically impossible as DNA must be copied in S before G₂). Option (2) places M between G₁ and G₂. Option (4) scrambles the sequence completely.

• • Matches official NTA final answer key Option C / (3).

Prophase I of Meiosis I is divided into five substages: Leptotene, Zygotene, Pachytene, Diplotene, and Diakinesis. During Diakinesis, chiasmata undergo terminalisation, the chromosomes achieve full condensation, the nucleolus disappears, and the nuclear envelope breaks down.

• • Prepares homologous bivalents for alignment on the equatorial plate in Metaphase I.

Meiosis I is fundamentally distinct from mitosis: it involves synapsis of homologous pairs, formation of synaptonemal complexes, genetic crossing over, and disjunction of homologous chromosome pairs without centromere division (reducing chromosome number from diploid 2n to haploid n). In contrast, Meiosis II involves centromeric division and separation of sister chromatids, exactly resembling a standard mitosis.

• • Statement II falsely claims that Meiosis I resembles mitosis. Hence, Statement II is FALSE.

Statement I is correct, but Statement II is false. This corresponds directly to Option (3).

• • Matches official NTA final answer key Option C / (3).

During prophase and prometaphase, microtubule organizing centers (centrosomes) assemble the bipolar mitotic spindle:

• • Spindle fibers are polymeric microtubules composed of $\alpha$- and $\beta$-tubulin heterodimers.
• • Kinetochore microtubules bind specifically to the proteinaceous trilaminar kinetochores assembled on centromeres of condensed chromosomes.
• • During metaphase, dynamic tension from opposing spindle poles congresses all chromosomes to the equatorial plane (metaphase plate).

Driving chromosome segregation into daughter nuclei:

• • At the onset of anaphase, cohesin complexes holding sister chromatids together are cleaved.
• • Depolymerization of kinetochore microtubules at their plus ends generates mechanical pulling forces.
• • These forces pull the split sister chromatids (now individual daughter chromosomes) toward opposite centrosomal poles.
• • This guarantees equal and faithful genetic distribution into both newly forming daughter cells.

The fundamental and primary function of spindle fibers during mitosis is to separate the chromosomes.

The complete disintegration of the nuclear envelope marks the start of the second phase of mitosis, metaphase. Hence the chromosomes are spread through the cytoplasm of the cell.

Small disc-shaped structures at the surface of the centromeres are called kinetochores. These structures serve as the sites of attachment of spindle fibres formed by spindle microtubules.

NCERT explicitly states that metaphase is characterized by spindle fibres attaching to kinetochores of chromosomes and chromosomes moving to the spindle equator.

During leptotene stage, the chromosomes become gradually visible under the light microscope. The compaction of chromosomes continues throughout leptotene. Thus, Statement I is completely true.

The beginning of diplotene is recognized by the dissolution of the synaptonemal complex and the tendency of the recombined homologous chromosomes of the bivalents to separate from each other except at the sites of crossovers (forming X-shaped chiasmata). Thus, Statement II is completely true.

Both Statement I and Statement II are true statements, corresponding to Option (3).

During leptotene, chromosomes become gradually visible under light microscopy appearing like thin individual threads (D -> III). During zygotene, homologous chromosomes begin pairing (synapsis) accompanied by the formation of the complex structure called synaptonemal complex (C -> I).

During pachytene, bivalents show four chromatids and recombination nodules appear at sites where crossing over occurs between non-sister chromatids (B -> IV). The final stage of meiotic prophase I is diakinesis, marked by the completion of terminalisation of chiasmata (A -> II).

A-II, B-IV, C-I, D-III corresponds to Option (1).

Interphase, though called the resting phase, is the time during which the cell is preparing for division by undergoing both cell growth and DNA replication in an orderly manner. It is divided into three phases: - G1 phase (Gap 1): Cell is metabolically active and continuously grows (Stage E). - S phase (Synthesis): DNA replication takes place (Stage C). - G2 phase (Gap 2): Proteins are synthesised in preparation for mitosis while cell growth continues (Stage A).

The M Phase starts with the nuclear division, corresponding to the separation of daughter chromosomes (karyokinesis) (Stage D) and usually ends with division of cytoplasm (cytokinesis) (Stage B).

The correct progression is E -> C -> A -> D -> B, which corresponds to Option (2).

Chromosome dynamics differ fundamentally between the two meiotic anaphase stages:

NCERT defines Anaphase II: "It begins with the simultaneous splitting of the centromere of each chromosome (which was holding the sister chromatids together), allowing them to move toward opposite poles of the cell."

Upon splitting of the centromere in Anaphase II, each sister chromatid officially becomes an independent individual daughter chromosome.

Prophase I is exceptionally prolonged and categorized into five consecutive stages based on chromosomal behavior:

Recombination nodules are protein-rich multicomponent enzymatic complexes (containing recombinase, DNA endonucleases, and ligases) that assemble along the synaptonemal complex between non-sister chromatids of homologous chromosomes at pachytene.

NCERT states verbatim: "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."

Interphase is subdivided into three distinct sequential phases:

NCERT states verbatim: "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."

Crucially, while the DNA content doubles from $2C$ to $4C$, the chromosome number remains unchanged at $2n$ because sister chromatids remain physically united at the centromere until anaphase.

Correlating each cell cycle phase with its verbatim NCERT definition:

NCERT explicitly states: "These cells that do not divide further exit G1 phase to enter an inactive stage called quiescent stage (G0) of the cell cycle. Cells in this stage remain metabolically active but no longer proliferate."

A-IV is present in Options (3) and (4). B-I uniquely identifies Option (4), while Option (3) incorrectly assigns B to II.

Cells that suspend division (e.g., heart cells, neurons) exit $G_1$ to enter the quiescent stage ($G_0$). These cells do not replicate DNA or divide, but carry out full cellular respiration, protein synthesis, and basal metabolism:

During the synthesis ($S$) phase in animal cells, nuclear events and cytoplasmic events proceed in parallel: 1. Nuclear event: Nuclear DNA replication ($2C \to 4C$). 2. Cytoplasmic event: Centrosome (centriole pair) duplication in the cytoplasm.

NCERT states verbatim: - "Cells in this stage [G0] remain metabolically active but no longer proliferate unless called on to do so." (Statement I is FALSE) - "In animal cells, during the S phase, DNA replication begins in the nucleus, and the centriole duplicates in the cytoplasm." (Statement II is TRUE)

Analyzing the mitotic and meiotic cytological events according to NCERT: - Statement B: At the onset of anaphase, each chromosome arranged at the metaphase plate splits simultaneously at its centromere, and sister chromatids separate toward opposite poles. (Statement B is TRUE) - Statement D: Telophase reverses prophase events: chromosomes decondense, the nuclear envelope assembles, and the nucleolus, Golgi complex, and ER reform. (Statement D is TRUE)

Analyzing the specific errors planted in the distractors: - Statement A: Leptotene is characterized by progressive condensation of chromatin. Bivalents/tetrads form during zygotene and are clearly visible as tetrads during pachytene. - Statement C: Terminalization of chiasmata is the defining hallmark of the final stage of prophase I, diakinesis, not pachytene. - Statement E: Crossing over occurs between non-sister chromatids of homologous chromosomes. Recombination between identical sister chromatids would yield no genetic variation.

Only Statements B and D are factually correct. Option (3) is the required answer.

The behavior of centromeres is the single defining difference between meiotic stages:

Meiosis II closely resembles a standard mitotic division. At Anaphase II, the cohesin proteins at the centromeric region are cleaved, the centromere divides, and the kinetochore microtubules pull sister chromatids toward opposite spindle poles.

Interphase represents over 95% of cell cycle duration and is divided into three distinct stages along with the quiescent exit phase:

Matching (a)-(iv), (b)-(i), (c)-(ii), (d)-(iii) leads uniquely to Option (3).

During interphase of the mitotic cell cycle, the cell transitions through $G_1$, $S$, and $G_2$ phases: - In $G_1$ phase: The chromosome number is diploid ($2n = 8$) and the DNA content is $2C$. - In $S$ phase (Synthesis phase): Semi-conservative replication of nuclear DNA occurs. The DNA quantity doubles from $2C$ to $4C$. - However, the newly formed sister chromatids remain attached to each other at the same centromere. Because chromosome count is defined strictly by the number of functional centromeres, the chromosome number does not increase.

For Drosophila melanogaster with $2n = 8$: - Chromosomes in $G_1$: 8 - Chromosomes in $S$: 8 (each consisting of two identical sister chromatids) - Chromosomes in $G_2$: 8 - The physical separation of sister chromatids into individual daughter chromosomes only occurs later during mitotic Anaphase.

The synthesis ($S$) phase of interphase is characterized by coordinated replication of genetic and spindle-organizing structures in animal cells: - In the Nucleus: Semi-conservative DNA replication occurs, doubling the DNA content from $2C$ to $4C$. - In the Cytoplasm: The centrosome, containing a pair of orthogonally oriented centrioles, duplicates so that two centrosomes (each with two centrioles) are formed. - This duplication is indispensable because each centrosome will later migrate to opposite poles during prophase to organize the mitotic spindle apparatus.

- $G_1$ phase: Gap 1 involves metabolic growth, RNA and protein synthesis; centrioles do not replicate. - $G_2$ phase: Gap 2 involves synthesis of tubulin and preparations for mitosis. - Prophase: Centrosomes (already duplicated during S-phase) begin their bipolar migration; aster rays and spindle fibers assemble. - Metaphase: Chromosomes align on the equatorial plate.

Meiosis I Prophase I is an extended, intricate phase subdivided into five successive stages: 1. Leptotene: Chromatin condenses; chromosomes become progressively visible under light microscopy. 2. Zygotene: Homologous chromosomes undergo pairing (synapsis), mediated by the formation of the tripartite nucleoprotein synaptonemal complex to produce bivalents (tetrads). 3. Pachytene: Non-sister chromatids of homologous chromosomes exchange genetic segments (crossing over) at recombination nodules, catalyzed by the enzyme recombinase. 4. Diplotene: Dissolution of the synaptonemal complex occurs; paired homologs separate except at crossover sites, revealing X-shaped chiasmata. 5. Diakinesis: The terminal sub-stage characterized by terminalisation of chiasmata, where cross-over contact points slide toward telomeric ends like unzipping a zipper. The nucleolus disappears, and the nuclear envelope disintegrates.

NCERT explicitly defines Diakinesis as: "The final stage of meiotic prophase I is diakinesis. This is marked by terminalisation of chiasmata." Thus, Diakinesis is the precise answer.

Interphase accounts for >95% of cell cycle duration, and G1 phase corresponds to the temporal interval between the completion of telophase (mitosis) and the initiation of chromosomal DNA replication: - Metabolic Activity: High rates of transcription (mRNA, tRNA, rRNA) and translation occur. Enzymes required for DNA synthesis (DNA polymerases, kinases, ribonucleotide reductase) are synthesized. - Cellular Growth: The volume of the cytoplasm increases, and cytoplasmic organelles (mitochondria, ribosomes, endoplasmic reticulum) proliferate. - Absence of DNA Replication: The nuclear DNA content remains strictly at the basal diploid complement ($2C$), and chromosomes remain un-replicated monads. Replication of DNA occurs exclusively during the subsequent S (Synthesis) phase.

- (1) DNA synthesis takes place: False; occurs exclusively in the S phase. - (2) Reorganisation of cell components: False; occurs in early Prophase / Mitosis. - (4) Nuclear division: False; karyokinesis is a hallmark of the M phase.

During Zygotene, chromosomes start pairing together through a process called synapsis, facilitated by the formation of a proteinaceous synaptonemal complex forming bivalents.

During Pachytene, four chromatids of each bivalent chromosome become distinct as tetrads. Crossing over occurs between non-sister chromatids of homologous chromosomes, mediated by enzyme recombinase.

The beginning of Diplotene is recognized by the dissolution of the synaptonemal complex and tendency of homologous chromosomes to separate, except at sites of crossovers, creating X-shaped structures called chiasmata.

The final stage of meiotic prophase I is Diakinesis, which is marked by the terminalization of chiasmata as chromosomes fully condense and nucleolus disappears.

Cells that do not continue dividing exit the regular progression of the cell cycle and enter a specialized non-proliferative resting state designated as the quiescent stage (G0).

NCERT states verbatim: "These cells that do not divide further exit G1 phase to enter an inactive stage called quiescent stage (G0) of the cell cycle."

Cells in G0 stage remain metabolically active, maintaining basic cellular functions and tissue homeostasis, but no longer proliferate unless specifically stimulated (e.g., hepatocytes following partial hepatectomy).

M phase is followed by cytokinesis, delivering daughter cells into G1; S phase is committed DNA replication; G2 phase prepares for mitosis. The commitment decision to divide or withdraw occurs in G1.

Prophase I of meiosis is subdivided into five successive stages based on chromosomal behavior: Leptotene, Zygotene, Pachytene, Diplotene, and Diakinesis.

NCERT states verbatim: "The beginning of diplotene is recognised by the dissolution of the synaptonemal complex and the tendency of the recombined homologous chromosomes of the bivalents to separate from each other except at the sites of crossovers."

These remaining X-shaped crossover connection structures are called chiasmata, which become prominently visible in diplotene precisely because the synaptonemal complex has dissolved elsewhere along the chromosome arms.

In Leptotene chromosomes condense; in Zygotene synaptonemal complex forms; in Pachytene crossing over occurs; in Diakinesis chiasmata terminalize. Thus, dissolution uniquely marks Diplotene.