I. CELL CYCLE — OVERVIEW
A DEFINITION
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| FEATURE | DETAIL |
|---|---|
| Cell Cycle | The sequence of events by which a cell duplicates its genome, synthesises other constituents of the cell, and eventually divides into two daughter cells ⭐⭐ |
| Cell growth | Continuous process ⭐ |
| DNA synthesis | Occurs only during one specific stage (S phase) ⭐ |
| All events | Under genetic control ⭐ |
B DURATION
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| ORGANISM | DURATION |
|---|---|
| Typical eukaryotic cell (Human) | ~24 hours ⭐⭐ |
| Yeast | ~90 minutes ⭐ |
| Cell division proper (M phase) | Only ~1 hour (out of 24 hours) ⭐ |
| Interphase | >95% of total cell cycle duration ⭐⭐ |
⚡ EXAM TRAP
RE-NEET 2024, RE-NEET 2026 (reinforced): Human cell cycle = ~24 hours; Yeast = ~90 minutes. Interphase constitutes >95% of the duration. Correct sequence is G₁ → S → G₂ → M.
C TWO BASIC PHASES
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| PHASE | DETAIL |
|---|---|
| Interphase | Phase between two successive M phases; cell prepares for division (growth + DNA replication) ⭐ |
| M Phase (Mitosis phase) | Actual cell division occurs ⭐ |
🔑 NOTE: Interphase is often called 'resting phase' — but cell is NOT resting; it is actively preparing for division!
II. INTERPHASE — THREE SUB-PHASES
A G₁ PHASE (GAP 1)
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| FEATURE | DETAIL |
|---|---|
| Definition | Interval between mitosis and initiation of DNA replication ⭐ |
| Cell activity | Metabolically active; continuously grows ⭐⭐ |
| DNA replication | Does NOT replicate its DNA ⭐ |
| DNA content | Remains 2C ⭐ |
| Chromosome number | 2n (diploid) ⭐ |
| Organelle duplication | Most organelle duplication occurs ⭐ |
⚡ EXAM TRAP
NEET 2020, 2021, 2023, 2026 (reinforced): G₁ phase — metabolically active, continuously grows, NO DNA replication.
B S PHASE (SYNTHESIS)
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| FEATURE | DETAIL |
|---|---|
| Definition | Period during which DNA synthesis/replication takes place ⭐⭐⭐ |
| DNA content | Doubles: 2C → 4C ⭐⭐ |
| Chromosome number | Remains SAME (2n) — does NOT change! ⭐⭐⭐ |
| Centriole duplication | Centriole/Centrosome duplicates in the cytoplasm during S phase ⭐⭐ |
⚡ EXAM TRAP
NEET 2014, 2016, 2021, 2022, 2023, 2026 (reinforced): S phase — DNA replication; amount of DNA per cell doubles (2C→4C); chromosome number SAME (2n); centriole duplicates in cytoplasm.
🔑 KEY TRAP: If cell had 2n chromosomes at G₁ → after S phase → chromosome number is still 2n (NOT 4n!) — only DNA content doubles.
C G₂ PHASE (GAP 2)
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| FEATURE | DETAIL |
|---|---|
| Activity | Proteins synthesised in preparation for mitosis ⭐⭐ |
| Cell growth | Continues ⭐ |
| DNA content | Remains at 4C ⭐ |
| Chromosome number | Remains 2n ⭐ |
⚡ EXAM TRAP
NEET 2015, 2021, 2023, 2024, 2026 (reinforced): G₂ phase — proteins are synthesised for mitosis while cell growth continues.
D G₀ PHASE (QUIESCENT STAGE)
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| FEATURE | DETAIL |
|---|---|
| Entry | Cells that do not divide further exit G₁ phase to enter G₀ ⭐⭐ |
| Status | Remain metabolically active but no longer proliferate ⭐⭐⭐ |
| Can re-enter? | Yes — can re-enter cell cycle if called upon (depending on organism's requirement) ⭐ |
| Example | Heart cells (do not divide further) ⭐⭐ |
⚡ EXAM TRAP
NEET 2019, 2020, 2023, 2024: G₀ phase — cells exit G₁; metabolically ACTIVE but do NOT proliferate. NTA TRAP: G₀ cells are NOT inactive — they are metabolically active!
E CELL CYCLE SEQUENCE
🔑 SEQUENCE: G₁ → S → G₂ → M
⚡ EXAM TRAP
NEET 2024, RE-NEET 2026 (reinforced): Cell cycle sequence = G₁ → S → G₂ → M.
F DNA & CHROMOSOME NUMBER CHANGES
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| PHASE | DNA CONTENT | CHROMOSOME NUMBER |
|---|---|---|
| G₁ | 2C ⭐ | 2n ⭐ |
| After S | 4C ⭐ | 2n (SAME!) ⭐⭐ |
| G₂ | 4C ⭐ | 2n ⭐ |
| Prophase / Metaphase | 4C ⭐ | 2n ⭐ |
| After M phase (each daughter) | 2C ⭐ | 2n ⭐ |
⚡ EXAM TRAP
NEET 2014: DNA content at 4C during G₂ and early M phase (Prophase/Metaphase).
G SPECIAL CASES
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| FEATURE | DETAIL |
|---|---|
| In animals | Mitotic cell division seen only in diploid somatic cells ⭐ |
| Exception | Haploid cells divide by mitosis — e.g., Male Honey Bees ⭐⭐ |
| In plants | Mitotic divisions in BOTH haploid and diploid cells ⭐ |
⚡ EXAM TRAP
NEET 2017: In bacteria, DNA replication occurs prior to fission (no well-defined S phase).
⚡ EXAM TRAP
NEET 2016: Stalled DNA replication fork → G₁/S checkpoint predominantly activated.
III. M PHASE — MITOSIS (EQUATIONAL DIVISION)
A OVERVIEW
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| FEATURE | DETAIL |
|---|---|
| Also called | Equational division ⭐⭐ |
| Why "equational" | Number of chromosomes in parent and progeny cells is SAME ⭐ |
| M Phase starts with | Nuclear division (Karyokinesis) — separation of daughter chromosomes ⭐ |
| M Phase ends with | Cytoplasmic division (Cytokinesis) ⭐ |
⚡ EXAM TRAP
NEET 2023, 2026 (reinforced): Equational division = chromosome number in parent = progeny. Actual cell division occurs in M phase.
B KARYOKINESIS — FOUR STAGES
🔑 SEQUENCE: Prophase → Metaphase → Anaphase → Telophase
⚡ EXAM TRAP
NEET 2017: Sequence — Condensation → Nuclear membrane disassembly → Equator alignment → Centromere division → Segregation → Telophase.
C PROPHASE
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| FEATURE | DETAIL |
|---|---|
| Follows | S and G₂ phases of interphase ⭐ |
| Key event | Initiation of condensation of chromosomal material ⭐⭐ |
| Chromosomes | Seen as two chromatids attached together at the centromere ⭐ |
| Centrosome | Had duplicated during S phase → now begins to move towards opposite poles ⭐ |
| Asters | Microtubules radiated from each centrosome ⭐ |
| Mitotic apparatus | Two asters + Spindle fibres ⭐⭐ |
| Structures that disappear | Golgi complex, Endoplasmic reticulum, Nucleolus, Nuclear envelope — all absent at end of prophase ⭐⭐ |
⚡ EXAM TRAP
NEET 2017: Prophase — condensation begins; centrosome moves to poles; GC, ER, nucleolus, nuclear envelope disappear.
D METAPHASE
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| FEATURE | DETAIL |
|---|---|
| Start marked by | Complete disintegration of nuclear envelope ⭐ |
| Chromosomes | Condensation completed → can be observed clearly under microscope ⭐ |
| Best stage for | Studying morphology of chromosomes ⭐⭐ |
| Each chromosome | Made of two sister chromatids held by centromere ⭐ |
| Kinetochores | Small disc-shaped structures at surface of centromeres ⭐⭐ |
| Kinetochore function | Sites of attachment of spindle fibres to chromosomes ⭐⭐ |
| Chromosome position | All at the equator (metaphase plate) ⭐ |
| Spindle fibre attachment | Each chromatid connected via kinetochore to spindle fibres from opposite poles ⭐ |
⚡ EXAM TRAP
NEET 2013, 2016, 2020, 2022, 2024: Spindle fibres attach to KINETOCHORES (NOT centromere or telomere — TRAP!). NTA TRAP: Metaphase chromosomes = TWO sister chromatids (NOT four!).
E ANAPHASE
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| FEATURE | DETAIL |
|---|---|
| Key event | Centromeres split and chromatids separate ⭐⭐⭐ |
| Separated chromatids | Now called daughter chromosomes ⭐ |
| Movement | Daughter chromosomes migrate towards opposite poles ⭐ |
| Orientation | Centromere at leading edge (directed towards pole); arms trailing behind ⭐ |
⚡ EXAM TRAP
NEET 2022, 2023, 2025 (reinforced): Anaphase — centromere splits; chromatids separate. Main function of spindle fibres during mitosis is to separate the chromosomes!
⚡ EXAM TRAP
NEET 2017: If APC (Anaphase Promoting Complex) is defective → chromosomes will NOT segregate.
F TELOPHASE
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| FEATURE | DETAIL |
|---|---|
| Chromosomes | Decondense and lose their individuality ⭐ |
| Chromatin material | Collects at each of two poles ⭐ |
| Structures that REFORM | Nuclear envelope, Nucleolus, Golgi complex, Endoplasmic reticulum ⭐⭐⭐ |
⚡ EXAM TRAP
NEET 2013, 2022, 2023: Telophase — decondensation; ER, GC, Nucleolus, Nuclear envelope REFORM.
G CYTOKINESIS
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| FEATURE | DETAIL |
|---|---|
| Definition | Division of cytoplasm ⭐ |
| Follows | Karyokinesis ⭐ |
| FEATURE | ANIMAL CELLS | PLANT CELLS |
|---|---|---|
| Method | Cell furrow method (cleavage furrow) ⭐⭐ | Cell plate method ⭐⭐ |
| Mechanism | Furrow in plasma membrane → gradually deepens → joins in centre → divides cytoplasm ⭐ | Wall formation starts in centre → grows outward to meet existing lateral walls ⭐ |
| Cell plate | — | Represents middle lamella between walls of two adjacent cells ⭐⭐ |
⚡ EXAM TRAP
NEET 2013: Cytokinesis — Animal cell = furrow method; Plant cell = cell plate method.
Additional Facts ⭐
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| ADDITIONAL FACTS | DETAIL |
|---|---|
| Organelle distribution | Mitochondria and plastids get distributed between two daughter cells ⭐ |
| Syncytium | When karyokinesis is NOT followed by cytokinesis → multinucleate condition ⭐⭐ |
| Example of syncytium | Liquid endosperm of coconut ⭐⭐ |
| Polyploidy | If cytokinesis is disrupted after anaphase/telophase → chromosome doubling → polyploidy ⭐ |
⚡ EXAM TRAP
NEET 2016, 2023: Disrupted cytokinesis → polyploidy. Syncytium = multinucleate coconut endosperm.
IV. SIGNIFICANCE OF MITOSIS
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| SIGNIFICANCE | DETAIL |
|---|---|
| 1 | Production of diploid daughter cells with identical genetic complement ⭐ |
| 2 | Growth of multicellular organisms ⭐ |
| 3 | Restores nucleo-cytoplasmic ratio (cell growth disturbs ratio between nucleus & cytoplasm) ⭐ |
| 4 | Cell repair — e.g., cells of upper layer of epidermis, lining of gut, blood cells (constantly replaced) ⭐⭐ |
| 5 | Mitotic divisions in meristematic tissues — apical and lateral cambium → continuous growth in plants ⭐ |
What NEVER Occurs in Mitosis? ⭐⭐⭐
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| FEATURE | DETAIL |
|---|---|
| Pairing of homologous chromosomes (Synapsis) | NEVER happens in mitosis — exclusive to Meiosis ⭐⭐⭐ |
⚡ EXAM TRAP
NEET 2016, 2022: Synapsis NEVER occurs in mitosis — only in meiosis.
V. MEIOSIS — OVERVIEW
A DEFINITION & FEATURES
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| FEATURE | DETAIL |
|---|---|
| Definition | Cell division that reduces chromosome number by half → production of haploid daughter cells ⭐⭐ |
| Also called | Reduction division ⭐ |
| Occurs in | Diploid cells destined to form gametes (gametogenesis) ⭐ |
| Meiosis | Ensures production of haploid phase ⭐ |
| Fertilisation | Restores diploid phase ⭐ |
| DNA replication | ONCE (before Meiosis I) ⭐⭐ |
| Nuclear + Cell division | TWICE (Meiosis I + Meiosis II) ⭐⭐ |
| End product | Four haploid daughter cells ⭐⭐ |
⚡ EXAM TRAP
NEET 2022: DNA replication occurs only ONCE (before Meiosis I); does NOT occur before Meiosis II!
B KEY FEATURES
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| FEATURE | DETAIL |
|---|---|
| Initiated after | Parental chromosomes have replicated to produce identical sister chromatids at S phase ⭐ |
| Involves | Pairing of homologous chromosomes (synapsis) ⭐ |
| Recombination | Between non-sister chromatids of homologous chromosomes ⭐ |
VI. MEIOSIS I
A PROPHASE I — LONGEST & MOST COMPLEX
🔑 FIVE SUB-STAGES (Le-Zy-Pa-Di-Di): Leptotene → Zygotene → Pachytene → Diplotene → Diakinesis
⚡ EXAM TRAP
NEET 2023, 2024: Know all five sub-stages of Prophase I in order.
1. LEPTOTENE ⭐⭐
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| FEATURE | DETAIL |
|---|---|
| Chromosomes | Become gradually visible under light microscope (look like thin threads) ⭐ |
| Compaction | Of chromosomes continues throughout leptotene ⭐ |
⚡ EXAM TRAP
NEET 2024: Leptotene — chromosomes visible under light microscope.
2. ZYGOTENE ⭐⭐⭐
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| FEATURE | DETAIL |
|---|---|
| Key event | Chromosomes start pairing together ⭐⭐ |
| Pairing process | Called SYNAPSIS ⭐⭐⭐ |
| Paired chromosomes | Called homologous chromosomes ⭐ |
| Complex formed | Synaptonemal complex ⭐⭐ |
| Pair of synapsed homologous chromosomes | Called BIVALENT or TETRAD ⭐⭐⭐ |
| Clearly visible | At next stage (Pachytene) ⭐ |
⚡ EXAM TRAP
NEET 2012, 2013, 2015, 2016, 2020, 2022, 2023: Zygotene — Synapsis; Synaptonemal complex; Bivalent/Tetrad = pair of synapsed homologous chromosomes.
3. PACHYTENE ⭐⭐⭐
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| FEATURE | DETAIL |
|---|---|
| Duration | Longest among first two stages of Prophase I ⭐ |
| Bivalent / Tetrad | Clearly visible — four chromatids of each bivalent distinct ⭐ |
| Recombination nodules | Appear at this stage — sites at which crossing over occurs ⭐⭐⭐ |
| Crossing over | Occurs between non-sister chromatids of homologous chromosomes ⭐⭐⭐ |
| Crossing over = | Exchange of genetic material between two homologous chromosomes ⭐ |
| Enzyme | Recombinase ⭐⭐ |
| Crossing over is | Enzyme-mediated process ⭐ |
| Recombination | Completed by end of pachytene ⭐⭐ |
| After recombination | Chromosomes linked at sites of crossing over ⭐ |
⚡ EXAM TRAP
NEET 2014, 2015, 2016, 2020, 2022, 2023, 2024, RE-NEET 2024: Pachytene — crossing over; recombination nodules; non-sister chromatids; recombinase enzyme; recombination completed.
4. DIPLOTENE ⭐⭐⭐
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| FEATURE | DETAIL |
|---|---|
| Recognised by | Dissolution of synaptonemal complex ⭐⭐⭐ |
| Homologous chromosomes | Begin to separate from each other ⭐⭐ |
| Remain connected at | Sites of crossovers ⭐ |
| X-shaped structures | Called CHIASMATA ⭐⭐⭐ |
| Special fact | In oocytes of some vertebrates, diplotene can last for months or years (Dictyotene stage) ⭐⭐ |
⚡ EXAM TRAP
NEET 2018, 2020, 2023, 2024, RE-NEET 2024: Diplotene — synaptonemal complex dissolves; chiasmata (X-shaped); can last months/years in oocytes.
5. DIAKINESIS ⭐⭐⭐
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| FEATURE | DETAIL |
|---|---|
| Position | Final stage of Prophase I ⭐ |
| Key event | Terminalisation of chiasmata ⭐⭐⭐ |
| Chromosomes | Fully condensed ⭐ |
| Meiotic spindle | Assembled to prepare homologous chromosomes for separation ⭐ |
| Nucleolus | Disappears ⭐ |
| Nuclear envelope | Breaks down ⭐ |
| Represents | Transition to Metaphase I ⭐ |
⚡ EXAM TRAP
NEET 2016, 2020, 2021, 2023, 2026, RE-NEET 2024, RE-NEET 2026 (reinforced): Diakinesis — terminalisation of chiasmata; chromosomes are fully condensed at end of Prophase I and meiotic spindle is assembled for separation of homologous chromosomes.
B METAPHASE I
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| FEATURE | DETAIL |
|---|---|
| Bivalent chromosomes | Align on equatorial plate ⭐⭐ |
| Spindle fibres | Attached to kinetochores of homologous chromosomes ⭐ |
| Note | Microtubules from opposite poles attach to kinetochore of homologous chromosomes ⭐ |
⚡ EXAM TRAP
NEET 2016: Metaphase I — bivalents align at equatorial plate.
C ANAPHASE I
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| FEATURE | DETAIL |
|---|---|
| Key event | Homologous chromosomes SEPARATE ⭐⭐⭐ |
| Sister chromatids | Remain associated at their centromeres (do NOT separate!) ⭐⭐⭐ |
| Centromeres | Do NOT split in Anaphase I ⭐⭐⭐ |
⚡ EXAM TRAP
NEET 2012, 2015: Anaphase I — homologous chromosomes separate; sister chromatids remain attached; centromeres do NOT split.
KEY DIFFERENCE ⭐⭐⭐
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| EVENT | ANAPHASE (MITOSIS) | ANAPHASE I (MEIOSIS I) |
|---|---|---|
| Centromere splits? | YES ⭐ | NO ⭐⭐ |
| What separates? | Sister chromatids ⭐ | Homologous chromosomes ⭐ |
| Sister chromatids? | Separate ⭐ | Stay together ⭐ |
D TELOPHASE I
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| FEATURE | DETAIL |
|---|---|
| Nuclear membrane | Reappears ⭐ |
| Nucleolus | Reappears ⭐ |
| Cytokinesis follows | Called a DYAD of cells ⭐⭐ |
| Each pole receives | Half the chromosome number of parent cell ⭐ |
E INTERKINESIS
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| FEATURE | DETAIL |
|---|---|
| Definition | Stage between two meiotic divisions ⭐ |
| Duration | Generally short-lived ⭐ |
| DNA replication | ABSENT (NO DNA replication during interkinesis!) ⭐⭐⭐ |
⚡ EXAM TRAP
NEET 2022: No DNA replication during interkinesis.
VII. MEIOSIS II
🔑 KEY FACT: Meiosis II resembles a normal mitosis ⭐⭐; initiated immediately after cytokinesis of Meiosis I.
A PROPHASE II
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| FEATURE | DETAIL |
|---|---|
| Simpler than | Prophase I ⭐ |
| Chromosomes | Become compact again ⭐ |
| Nuclear membrane | Disappears by end of Prophase II ⭐ |
B METAPHASE II
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| FEATURE | DETAIL |
|---|---|
| Chromosomes | Align at equator ⭐ |
| Microtubules | From opposite poles attach to kinetochores of sister chromatids ⭐ |
C ANAPHASE II
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| FEATURE | DETAIL |
|---|---|
| Key event | Simultaneous splitting of centromere of each chromosome ⭐⭐⭐ |
| Sister chromatids | SEPARATE → move towards opposite poles ⭐⭐ |
| Now called | Daughter chromosomes ⭐ |
⚡ EXAM TRAP
NEET 2015, 2021, 2023: Anaphase II — centromere splits; sister chromatids separate.
KEY MATCH ⭐⭐⭐
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| EVENT | WHEN CENTROMERE SPLITS |
|---|---|
| Mitosis | Anaphase ⭐ |
| Meiosis | Anaphase II (NOT Anaphase I!) ⭐⭐ |
⚡ EXAM TRAP
NEET 2023: Mitosis Anaphase = Meiosis Anaphase II (both involve centromere splitting).
D TELOPHASE II
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| FEATURE | DETAIL |
|---|---|
| Meiosis ends with | Telophase II ⭐ |
| Chromosomes | Enclosed by nuclear envelope again ⭐ |
| Cytokinesis follows | Results in TETRAD of cells ⭐ |
| End product | Four haploid daughter cells ⭐⭐ |
VIII. SIGNIFICANCE OF MEIOSIS
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| SIGNIFICANCE | DETAIL |
|---|---|
| 1 | Conservation of specific chromosome number of each species across generations in sexually reproducing organisms ⭐⭐ |
| 2 | Increases genetic variability in the population from one generation to next ⭐⭐ |
| 3 | Variations are very important for the process of evolution ⭐ |
IX. RAPID REVISION — KEY COMPARISON TABLES
TABLE 1: Interphase Sub-Phases ⭐⭐⭐
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| PHASE | KEY EVENT | DNA CONTENT | CHROMOSOME NO. |
|---|---|---|---|
| G₁ | Growth; metabolically active; NO DNA replication | 2C | 2n |
| S | DNA replication; centriole duplicates | 2C → 4C | 2n (SAME) |
| G₂ | Protein synthesis for mitosis; growth continues | 4C | 2n |
| G₀ | Quiescent; metabolically active; no proliferation | 2C | 2n |
TABLE 2: Stages of Mitosis ⭐⭐⭐
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| STAGE | KEY EVENTS |
|---|---|
| Prophase | Condensation begins; centrosome moves to poles; mitotic apparatus forms; GC, ER, Nucleolus, Nuclear envelope disappear |
| Metaphase | Nuclear envelope completely disintegrated; chromosomes fully condensed; align at equator; spindle fibres attach to kinetochores; best for chromosome morphology study |
| Anaphase | Centromere splits; chromatids separate → daughter chromosomes; migrate to opposite poles |
| Telophase | Chromosomes decondense; Nuclear envelope, Nucleolus, GC, ER reform |
| Cytokinesis | Cytoplasmic division; Animal = furrow; Plant = cell plate |
TABLE 3: Five Sub-stages of Prophase I (Meiosis I) ⭐⭐⭐
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| SUB-STAGE | KEY EVENT | MNEMONIC |
|---|---|---|
| Leptotene | Chromosomes become visible (thin threads); compaction continues | Looks visible |
| Zygotene | Synapsis; Synaptonemal complex; Bivalent/Tetrad formed | Zipping (pairing) |
| Pachytene | Crossing over (non-sister chromatids); Recombination nodules; Recombinase; Recombination completed | Pach = Exchange |
| Diplotene | Synaptonemal complex dissolves; Chiasmata (X-shaped); Can last months/years in oocytes | Dissolve |
| Diakinesis | Terminalisation of chiasmata; Fully condensed; Nucleolus & Nuclear envelope disappear; Transition to Metaphase I | Done (final) |
TABLE 4: Anaphase Comparison (HIGH-YIELD) ⭐⭐⭐
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| FEATURE | ANAPHASE (MITOSIS) | ANAPHASE I (MEIOSIS I) | ANAPHASE II (MEIOSIS II) |
|---|---|---|---|
| What separates? | Sister chromatids | Homologous chromosomes | Sister chromatids |
| Centromere splits? | YES | NO | YES |
| Sister chromatids? | Separate | Remain associated | Separate |
| Result | 2n → 2n | 2n → n (per pole) | n → n |
TABLE 5: Mitosis vs Meiosis ⭐⭐⭐
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| FEATURE | MITOSIS | MEIOSIS |
|---|---|---|
| Also called | Equational division | Reduction division |
| Divisions | ONE | TWO (Meiosis I + II) |
| DNA replication | Once (S phase) | Once (before Meiosis I only) |
| Daughter cells | 2 diploid | 4 haploid |
| Genetic identity | Identical to parent | Different (due to crossing over) |
| Synapsis | NEVER | Yes (Zygotene) |
| Crossing over | NEVER | Yes (Pachytene) |
| Chiasmata | NEVER | Yes (Diplotene) |
| Bivalent / Tetrad | NEVER | Yes (Zygotene / Pachytene) |
| Occurs in | Somatic cells (mostly diploid) | Reproductive cells (diploid → haploid) |
| Centromere splits at | Anaphase | Anaphase II |
| Significance | Growth, repair | Gamete formation, genetic variation |
TABLE 6: Cytokinesis — Animal vs Plant ⭐⭐⭐
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| FEATURE | ANIMAL CELL | PLANT CELL |
|---|---|---|
| Method | Cell furrow (cleavage furrow) | Cell plate method |
| Direction | Deepens from outside → inward | Starts from centre → grows outward |
| Cell plate | Not formed | Forms → represents middle lamella |
TABLE 7: What Disappears and Reappears ⭐⭐⭐
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| STRUCTURE | DISAPPEARS AT | REAPPEARS AT |
|---|---|---|
| Nuclear envelope | End of Prophase | Telophase |
| Nucleolus | End of Prophase | Telophase |
| Golgi complex | End of Prophase | Telophase |
| Endoplasmic reticulum | End of Prophase | Telophase |
TABLE 8: Key Numbers — Quick Reference ⭐⭐⭐
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| PARAMETER | VALUE |
|---|---|
| Human cell cycle duration | ~24 hours |
| Yeast cell cycle | ~90 minutes |
| M phase duration | ~1 hour |
| Interphase = | >95% of cell cycle |
| G₁ DNA content | 2C |
| After S phase DNA | 4C |
| G₂ DNA content | 4C |
| After M phase DNA (each daughter) | 2C |
| Chromosome number after S phase | SAME (2n → 2n) |
| Mitosis daughter cells | 2 (diploid) |
| Meiosis daughter cells | 4 (haploid) |
| DNA replication in Meiosis | Once only |
| Cell divisions in Meiosis | Twice |
| Prophase I sub-stages | 5 |
| Meiosis I result | Dyad of cells |
| Meiosis II result | Tetrad of cells |
X. COMMON EXAM TRAPS — QUICK REFERENCE
NEET 2025, 2026 & RE-NEET 2026 REINFORCED TRAPS
RE-NEET 2026: Correct sequence of cell cycle is G₁ → S → G₂ → M. Human cell cycle = ~24 hours; Yeast = ~90 minutes.
NEET 2026: G₁ phase is metabolically active and continuously grows but does not replicate DNA.
NEET 2026: Synthesis of DNA occurs in S phase and amount of DNA per cell doubles.
NEET 2026: Proteins are synthesized while cell growth continues in G₂ phase.
NEET 2026: Actual cell division occurs in M phase.
NEET 2025: Main function of spindle fibers during mitosis is to separate the chromosomes.
RE-NEET 2026: Chromosomes are fully condensed at the end of Prophase I and meiotic spindle is assembled for separation of homologous chromosomes.
CONSOLIDATED PYQ Q&A TABLE
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| TRAP / QUESTION | CORRECT ANSWER |
|---|---|
| Cell cycle definition? | Sequence of events: genome duplication → synthesis → division into two daughter cells ⭐ |
| Human cell cycle duration? | ~24 hours ⭐ |
| Yeast cell cycle? | ~90 minutes ⭐ |
| M phase lasts? | ~1 hour ⭐ |
| Interphase = % of cell cycle? | >95% ⭐ |
| Is interphase a 'resting phase'? | Called so, but cell is ACTIVELY preparing (growth + DNA replication) ⭐ |
| Cell cycle sequence? | G₁ → S → G₂ → M ⭐ |
| G₁ phase — DNA replication? | NO ⭐ |
| G₁ phase — metabolically active? | YES ⭐ |
| S phase — what happens? | DNA replication (2C → 4C) ⭐ |
| S phase — chromosome number change? | NO! Remains 2n (TRAP!) ⭐⭐ |
| Centriole duplicates during? | S phase (in cytoplasm) ⭐ |
| G₂ phase — what synthesised? | Proteins for mitosis ⭐ |
| G₂ DNA content? | 4C ⭐ |
| G₀ phase — cells exit from? | G₁ phase ⭐ |
| G₀ cells metabolically active? | YES (TRAP — they're not inactive!) ⭐ |
| G₀ cells divide? | NO — no longer proliferate (unless called upon) ⭐ |
| G₀ example? | Heart cells ⭐ |
| Mitotic division in animals seen in? | Diploid somatic cells only ⭐ |
| Exception — haploid mitosis? | Male Honey Bees ⭐ |
| Plants — mitotic division in? | Both haploid AND diploid cells ⭐ |
| Mitosis = ? | Equational division (parent = progeny chromosomes) ⭐ |
| Four stages of karyokinesis? | Prophase, Metaphase, Anaphase, Telophase ⭐ |
| Prophase follows which phases? | S and G₂ ⭐ |
| Prophase — what condenses? | Chromosomal material ⭐ |
| Chromosomes at prophase = ? | Two chromatids attached at centromere ⭐ |
| Mitotic apparatus = ? | Two asters + Spindle fibres ⭐ |
| Structures absent at end of prophase? | GC, ER, Nucleolus, Nuclear envelope ⭐ |
| Metaphase start? | Complete disintegration of nuclear envelope ⭐ |
| Best stage for chromosome morphology? | Metaphase ⭐ |
| Spindle fibres attach to? | KINETOCHORES (NOT centromere, NOT telomere — TRAP!) ⭐⭐ |
| Kinetochores are? | Small disc-shaped structures at surface of centromeres ⭐ |
| Metaphase — chromosomes at? | Equator (metaphase plate) ⭐ |
| Metaphase chromosomes have how many chromatids? | TWO sister chromatids (NOT four — TRAP!) ⭐ |
| Anaphase — what splits? | Centromere ⭐ |
| Anaphase — what separates? | Chromatids (now called daughter chromosomes) ⭐ |
| Centromere orientation in Anaphase? | Leading edge (towards pole); arms trailing ⭐ |
| If APC defective? | Chromosomes will NOT segregate ⭐ |
| Telophase — chromosomes? | Decondense; lose individuality ⭐ |
| Telophase — what reforms? | Nuclear envelope, Nucleolus, GC, ER ⭐ |
| Cytokinesis = ? | Division of cytoplasm ⭐ |
| Animal cell cytokinesis? | Cell furrow (cleavage furrow) method ⭐ |
| Plant cell cytokinesis? | Cell plate method ⭐ |
| Cell plate direction? | Centre → outward ⭐ |
| Cell plate represents? | Middle lamella ⭐ |
| Organelles distributed? | Mitochondria and plastids ⭐ |
| What is syncytium? | Multinucleate condition (karyokinesis without cytokinesis) ⭐ |
| Syncytium example? | Liquid endosperm of coconut ⭐ |
| Disrupted cytokinesis → ? | Polyploidy ⭐ |
| What NEVER happens in mitosis? | Synapsis (pairing of homologous chromosomes) ⭐⭐ |
| Cell repair examples? | Epidermis, Gut lining, Blood cells ⭐ |
| Meristematic tissues? | Apical and Lateral cambium ⭐ |
| Meiosis = ? | Reduction division (chromosome number halved) ⭐ |
| Meiosis produces? | Haploid daughter cells ⭐ |
| DNA replication in Meiosis? | ONCE (before Meiosis I only) ⭐ |
| Nuclear + cell division in Meiosis? | TWICE ⭐ |
| DNA replication before Meiosis II? | NO ⭐⭐ |
| End product of Meiosis? | FOUR haploid cells ⭐ |
| Meiosis occurs in? | Diploid cells → gametes ⭐ |
| Meiosis ensures? | Haploid phase ⭐ |
| Fertilisation restores? | Diploid phase ⭐ |
| Prophase I sub-stages? | Leptotene → Zygotene → Pachytene → Diplotene → Diakinesis ⭐ |
| Leptotene — key event? | Chromosomes visible under light microscope; compaction continues ⭐ |
| Zygotene — key event? | Synapsis (pairing); Synaptonemal complex; Bivalent/Tetrad formed ⭐ |
| Synapsis = ? | Pairing of homologous chromosomes ⭐ |
| Synaptonemal complex = ? | Complex formed during chromosome synapsis ⭐ |
| Bivalent = Tetrad = ? | Pair of synapsed homologous chromosomes ⭐ |
| Pachytene — key event? | Crossing over (non-sister chromatids); Recombination nodules; Recombinase enzyme ⭐ |
| Crossing over = ? | Exchange of genetic material between homologous chromosomes ⭐ |
| Crossing over occurs between? | Non-sister chromatids of HOMOLOGOUS chromosomes ⭐ |
| Recombination completed at? | End of Pachytene ⭐ |
| Diplotene — recognised by? | Dissolution of synaptonemal complex ⭐ |
| Chiasmata = ? | X-shaped structures at crossover sites ⭐ |
| Diplotene can last? | Months or years in oocytes ⭐ |
| Diakinesis — key event? | Terminalisation of chiasmata ⭐ |
| Diakinesis — chromosomes? | Fully condensed ⭐ |
| Diakinesis — what disappears? | Nucleolus & Nuclear envelope ⭐ |
| Diakinesis represents? | Transition to Metaphase I ⭐ |
| Metaphase I — what aligns? | Bivalent chromosomes at equatorial plate ⭐ |
| Anaphase I — what separates? | Homologous chromosomes ⭐ |
| Anaphase I — sister chromatids? | Remain ASSOCIATED at centromeres (do NOT separate!) ⭐⭐ |
| Anaphase I — centromere splits? | NO! ⭐⭐ |
| Telophase I + Cytokinesis = ? | Dyad of cells ⭐ |
| Interkinesis — DNA replication? | ABSENT (NO replication) ⭐⭐ |
| Interkinesis — duration? | Generally short-lived ⭐ |
| Meiosis II resembles? | Normal mitosis ⭐ |
| Prophase II — nuclear membrane? | Disappears ⭐ |
| Metaphase II — chromosomes? | At equator ⭐ |
| Metaphase II — microtubules attach to? | Kinetochores of sister chromatids ⭐ |
| Anaphase II — what splits? | Centromere ⭐⭐ |
| Anaphase II — what separates? | Sister chromatids ⭐ |
| Centromere splits in Mitosis at? | Anaphase ⭐ |
| Centromere splits in Meiosis at? | Anaphase II ⭐⭐ |
| Mitosis Anaphase = Meiosis ? | Anaphase II ⭐ |
| Telophase II — what forms? | Nuclear envelope around chromosomes ⭐ |
| Final product of Meiosis II? | Tetrad (4 haploid cells) ⭐ |
| Significance of Meiosis? | (1) Conservation of chromosome number; (2) Genetic variability; (3) Important for evolution ⭐ |
| Stalled replication fork activates? | G₁/S checkpoint ⭐ |
| Bacteria DNA replication? | Prior to fission; no well-defined S phase ⭐ |