Chapter 10: Cell Cycle and Cell Division – Class 11 Biology

Cell Cycle and Cell Division, part of the NCERT Unit III – Cell Structure and Functions, is one of the most conceptually rich chapters in Class 11 Biology. For NEET aspirants, this chapter carries significant weight, particularly in understanding how cells grow, replicate, and pass on genetic material. Questions related to the cell cycle checkpoints, types of cell division (mitosis and meiosis), and chromosomal behavior are frequently asked in NEET, making a clear conceptual understanding crucial for scoring well. Every phase, process, and chromosomal detail in this chapter can be directly mapped to NEET MCQs, assertion-reasoning questions, or diagram-based items. This makes it essential for aspirants to focus on both the theoretical and application-based aspects of cell division.

Cell Cycle

The cell cycle is a highly regulated series of events that governs cell growth and reproduction. For NEET, questions often test the duration of each phase, the role of checkpoints, and DNA content transitions. A strong grasp of this topic is essential for understanding cancer biology, molecular genetics, and developmental processes.

Phases of Cell Cycle

• Interphase: NEET frequently tests both the duration and molecular significance of interphase, which accounts for around 95% of the entire cell cycle. This phase is crucial because it sets the stage for accurate and controlled cell division—any mistake here can have downstream effects, a point NEET often targets through MCQs and assertion-reason questions.
  • G1 Phase (Gap 1): This is the first phase after mitosis, where the cell grows in size, synthesizes mRNA, and produces proteins essential for DNA synthesis. NEET questions may include the enzymes active in this stage, the point of commitment (restriction point), and examples of metabolic activities.
  • S Phase (Synthesis): Here, DNA replication takes place, doubling the DNA content from 2C to 4C while keeping the chromosome number the same (2N). NEET often tests this concept with numerical or diagram-based questions. In addition, the centriole also duplicates, which is another NEET-relevant detail.
  • G2 Phase (Gap 2): This is the final checkpoint before the cell enters mitosis. NEET questions may assess the importance of centrosome duplication, ATP synthesis, and the activation of cyclin-dependent kinases (CDKs). Error-detection mechanisms for DNA integrity are also crucial and frequently questioned in NEET.
• G0 Phase: A special state where cells exit the cycle temporarily or permanently. Examples include neurons and cardiac cells which remain in G0. NEET may test on whether cells can return to the cycle, and what stimuli are required to do so.
• M Phase: This is the actual division phase where one cell becomes two. NEET focuses on the subdivision into karyokinesis (nuclear division) and cytokinesis (cytoplasmic division), and the distinct features that differentiate animal and plant cells at this stage. Understanding the regulation of this phase through spindle checkpoints is also key for NEET.

M Phase

The M phase, or mitotic phase, is one of the most straightforward yet frequently tested sections in NEET due to its predictable sequence and visual clarity. This is the phase where the cell divides, and NEET often includes diagram-based questions, sequence ordering, and functional understanding of each sub-stage. It includes two key processes—karyokinesis (division of the nucleus) and cytokinesis (division of the cytoplasm).

Karyokinesis (Nuclear Division)

1. Prophase: This is the longest stage of mitosis. Chromatin fibers condense into visible chromosomes, each consisting of two sister chromatids joined at a centromere. The nucleolus disappears, and the nuclear envelope disintegrates. Centrosomes migrate to opposite poles and initiate spindle formation. NEET questions often focus on chromosome visibility and disappearance of the nuclear envelope as stage identifiers.
2. Metaphase: Chromosomes align along the metaphase plate (the cell’s equatorial plane). Spindle fibers attach to the kinetochores on each chromatid. NEET MCQs may ask about the significance of this alignment and the role of kinetochore-spindle interactions. This is also the best stage to observe chromosomes for karyotyping, a commonly tested fact.
3. Anaphase: The shortest mitotic phase. The centromeres split, and the spindle fibers pull the sister chromatids toward opposite poles of the cell. This separation ensures each daughter cell will receive an identical set of chromosomes. NEET questions frequently test understanding of chromosome number and chromatid movement in this phase.
4. Telophase: Chromosomes begin to de-condense into chromatin, the nuclear envelope reforms around each set of chromosomes at the poles, and the nucleolus reappears. NEET may test the identification of this phase based on nuclear membrane reformation and chromatin state. This marks the end of nuclear division and prepares the cell for cytokinesis.

Cytokinesis

• In Animal Cells: Cytokinesis occurs through the formation of a cleavage furrow, which begins as an indentation in the plasma membrane at the equator of the cell. This furrow deepens gradually as a contractile ring composed of actin and myosin filaments constricts the cell membrane. Eventually, the furrow pinches the cell into two daughter cells, each with an identical nucleus and cytoplasm. NEET often includes questions comparing the mechanism of cleavage and the proteins involved in the process.
• In Plant Cells: Due to the presence of a rigid cell wall, plant cells cannot form a cleavage furrow. Instead, cytokinesis proceeds via the formation of a cell plate. Vesicles from the Golgi apparatus accumulate at the center of the cell (the phragmoplast region) and fuse to form the cell plate. This plate gradually expands outward until it fuses with the existing plasma membrane, thereby separating the two daughter cells. NEET questions frequently test understanding of this mechanism with diagram-based comparisons between plant and animal cytokinesis.

Significance of Mitosis

Mitosis is a high-frequency NEET topic because of its role in fundamental biological processes and its clinical relevance. Students are often tested on its function in growth, development, and repair mechanisms, as well as in pathological contexts such as tumor formation.

• It maintains genetic stability by ensuring equational division—daughter cells receive the exact number of chromosomes as the parent cell (2N → 2N), which is a frequently asked NEET concept.
• Mitosis plays a vital role in growth by increasing cell number, and it supports asexual reproduction in unicellular organisms and somatic growth in multicellular organisms. NEET often tests examples like binary fission in amoeba or budding in yeast.
• NEET questions also assess knowledge of abnormal mitotic activity. When mitosis is unregulated due to failure of cell cycle checkpoints, it can lead to tumor formation and cancer—topics commonly linked to checkpoint proteins like p53 or CDKs.

Meiosis

Meiosis has a high NEET weightage due to its central role in genetics, heredity, and the generation of variation—core themes in the NEET Biology syllabus. A deep understanding of this process helps in tackling questions about chromosome behavior, genetic recombination, and gametogenesis.

Meiosis I (Reductional Division)

• Prophase I: This is the most extended and complex stage of meiosis, further divided into five sub-stages—leptotene, zygotene, pachytene, diplotene, and diakinesis. NEET questions often test the chronological order of these stages or the events specific to each.
  • In leptotene, chromosomes begin to condense.
  • In zygotene, homologous chromosomes pair up through synapsis, forming bivalents.
  • During pachytene, crossing over occurs between non-sister chromatids at structures called chiasmata—an extremely common NEET question.
  • Diplotene marks the beginning of bivalent separation while chiasmata remain visible.
  • Diakinesis shows terminalization of chiasmata and preparation for metaphase.
• Metaphase I: Homologous chromosome pairs align at the equatorial plate. Spindle fibers attach to centromeres. Unlike mitosis, sister chromatids remain together—this difference is often highlighted in NEET.
• Anaphase I: Homologous chromosomes (not chromatids) move toward opposite poles, reducing the chromosome number to half. NEET often tests this “reductional division” and the concept of ploidy change.
• Telophase I: Chromosomes may or may not decondense. Nuclear membranes may reform. Cells enter interkinesis before proceeding to meiosis II. Students should recognize this as the stage where haploid cells begin to form—a key NEET takeaway.

Meiosis II (Equational Division)

• This division is very similar to mitosis but occurs in a haploid cell. NEET frequently includes questions comparing it directly to mitosis.
• It includes prophase II, metaphase II, anaphase II, and telophase II.
  • Chromosomes line up individually at the metaphase plate.
  • Sister chromatids finally separate during anaphase II.
  • Telophase II results in four genetically distinct haploid cells.
• NEET questions often focus on the behavior of chromatids and the exact number of chromosomes and chromatids at each stage. Understanding this division is crucial for solving NEET problems related to spermatogenesis and oogenesis.

Significance of Meiosis

NEET repeatedly tests the biological significance of meiosis because it is fundamental to sexual reproduction, heredity, and evolution. Questions often revolve around chromosomal consistency, variation, and genetic mechanisms that result from meiosis.

• Essential for maintaining chromosomal number across generations: Meiosis ensures that gametes are haploid (n), so that when fertilization occurs, the resulting zygote restores the diploid (2n) condition. This chromosomal stability is often tested through ploidy-based numerical NEET questions.
• Introduces variation—key for evolution, a common NEET Biology theme: Genetic variation is achieved through mechanisms such as crossing over and independent assortment. This variation forms the basis of natural selection and evolution, which NEET tests in conceptual and assertion-reason formats.
• Crossing over and random orientation are emphasized in NEET: Crossing over during pachytene and the random orientation of bivalents during metaphase I lead to genetically distinct gametes. These events increase genetic diversity and are commonly illustrated through NEET diagrams and process-based MCQs.

NEET-Focused Highlights

NEET Illustrative Questions

1. Assertion and Reason:
   • Assertion: DNA replication occurs in the S phase of interphase.
   • Reason: DNA content doubles from 2C to 4C while chromosome number remains constant.
   • Answer: Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
2. Diagram-Based Question:
   • Identify the correct stage of mitosis where chromosomes are aligned at the equator with spindle fibers attached to kinetochores.
   • Answer: Metaphase
3. Multiple Choice Question:
   • In which sub-stage of Prophase I does crossing over occur? a) Leptotene
     b) Zygotene
     c) Pachytene
     d) Diakinesis
   • Answer: c) Pachytene
4. Match the Following:
   • Column A (Events) | Column B (Phases)
     • a) Synapsis | i) Zygotene
     • b) Terminalization of chiasmata | ii) Diakinesis
     • c) Bivalent formation | iii) Zygotene
     • d) Separation of homologous chromosomes | iv) Anaphase I
   • Answer: a-i, b-ii, c-iii, d-iv
5. Numerical MCQ:
   • A diploid cell (2n = 16) undergoes mitosis. What will be the number of chromosomes and chromatids in metaphase?
   • Answer: Chromosomes = 16, Chromatids = 32
6. Conceptual Question:
   • Why is meiosis I called a reductional division?
   • Answer: Because it reduces the chromosome number from diploid (2n) to haploid (n) by separating homologous chromosomes.

FAQs

1. What is the difference between mitosis and meiosis?

This is one of NEET’s most commonly asked questions. Mitosis produces two genetically identical diploid cells, while meiosis results in four genetically diverse haploid cells.

2. Why is interphase important in NEET?

It’s during interphase that the cell prepares for division by growing and replicating DNA. NEET often tests what occurs in each sub-phase.

3. What happens if cell cycle checkpoints fail?

NEET asks this to assess understanding of cancer development. Uncontrolled cell division due to failed checkpoints can lead to tumor formation.

4. Which stage does crossing over occur in meiosis?

Crossing over—a crucial NEET concept—takes place during the pachytene stage of Prophase I.

5. What are some NEET-expected questions from this chapter?

• Identify metaphase by chromosome alignment.
• Define synapsis and mention where it occurs.
• Diagram-based questions on cytokinesis differences.

Conclusion

From a NEET perspective, mastering the cell cycle and cell division is non-negotiable. These foundational processes connect to higher-level topics like genetics, cancer biology, and reproduction. Diagrams, sequence-based stages, and checkpoint regulation are heavily tested in NEET, making this chapter not just important but indispensable. Strengthening your basics here sets the stage for scoring high in NEET Biology.