CELL CYCLE AND CELL DIVISION

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DISCLAIMER:-

This chapter has been prepared for educational and examination-preparation purposes. The theory and original practice questions are intended to support students studying Cell Cycle and Cell Division for NEET-UG.

Previous-year question references are attributed to the examination name and year wherever identified. Some question wording may have been standardised for readability. Readers should consult the relevant official question papers and answer keys for authoritative verification.

Although reasonable care has been taken to ensure accuracy, the author does not guarantee that all information, question classifications or examination references are error-free. The author shall not be responsible for errors, omissions or any consequences arising from the use of this material.

This resource is independently prepared for educational purposes and is not affiliated with or endorsed by the National Testing Agency (NTA), the National Medical Commission (NMC) or any other examination authority.

Author: Shailendra Kumar Verma

PART 1: COMPLETE THEORY

1. INTRODUCTION TO CELL CYCLE AND CELL DIVISION

All living organisms are composed of cells. New cells arise from pre-existing cells through cell division. Cell division is essential for growth, repair, replacement of damaged cells, reproduction, and the continuity of life.

In unicellular organisms, cell division usually results in reproduction. In multicellular organisms, it supports growth, tissue repair, and the replacement of old or damaged cells.

The sequence of events through which a cell grows, duplicates its genetic material, and divides into daughter cells is called the cell cycle.

Cell division is broadly classified into two major types:

Mitosis: Produces daughter cells that generally retain the chromosome number of the parent cell.
Meiosis: Reduces the chromosome number by half and is involved in sexual reproduction.

Before examining the stages of cell division, it is essential to understand chromosomes, DNA replication, and the cell cycle.

2. IMPORTANT TERMS

Cell cycle: The ordered sequence of growth, DNA replication and division through which a cell passes.

Chromosome: A DNA-containing structure associated with proteins that carries genetic information.

Chromatin: The DNA-protein material that forms chromosomes. It is generally less condensed during interphase.

Chromatid: One of the DNA-containing copies of a replicated chromosome. Sister chromatids remain attached, especially at the centromere, until they separate.

Centromere: The chromosome region where sister chromatids remain associated and where the kinetochore forms.

Kinetochore: A protein structure assembled at the centromere that enables spindle microtubules to attach to the chromosome.

Spindle fibres: Microtubules involved in chromosome movement during cell division.

Cytokinesis: Division of the cytoplasm to form separate daughter cells.

Karyokinesis: Division of the nucleus.

Homologous chromosomes: A pair of chromosomes carrying the same types of genes at corresponding loci, one usually inherited from each parent in a diploid organism.

Synapsis: Pairing of homologous chromosomes during prophase I of meiosis.

Crossing over: Exchange of corresponding DNA segments between non-sister chromatids of homologous chromosomes during meiosis.

Chiasma (plural: chiasmata): The visible manifestation of a crossing-over connection between homologous chromosomes during later stages of prophase I.

Diploid (2n): A cell containing two sets of chromosomes.

Haploid (n): A cell containing one set of chromosomes.

Checkpoints: Regulatory mechanisms that help determine whether a cell can proceed to the next stage of the cell cycle.

3. THE CELL CYCLE

The cell cycle consists of two major phases:

Interphase
M phase (mitotic phase)

Interphase includes G1, S, and G2 phases. The M phase includes nuclear division and, usually, cytoplasmic division.

3.1 Interphase

Interphase is the period between successive cell divisions. It is traditionally divided into three phases:

G1 phase

S phase

G2 phase

Although chromosomes are not usually visible as distinct structures under an ordinary light microscope during interphase, the cell is metabolically active.

Interphase is not a resting phase. It involves growth, biosynthesis, DNA replication, and preparation for division.

3.2 G1 Phase (First Gap Phase)

G1 is the period between the completion of the previous M phase and the beginning of DNA replication.

Major events include:

Cell growth.

Synthesis of RNA and proteins.

Production of enzymes required for DNA replication.

Increase in the number of cell components and organelles.

Preparation for the S phase.

The cell assesses whether conditions are suitable for DNA replication and further division.

Some cells exit the active cell cycle and enter a non-dividing state called G0 phase.

Examples include many mature nerve cells and certain other differentiated cells. The ability to re-enter the cell cycle varies with cell type and physiological conditions.

3.3 S Phase (Synthesis Phase)

The S phase is the period during which DNA replication takes place.

Important events include:

Duplication of nuclear DNA.

Formation of sister chromatids for each replicated chromosome.

Duplication of the centrosome in typical animal cells.

Critical NEET concept: DNA content doubles during the S phase, but the chromosome number, counted by centromeres, generally remains unchanged until sister chromatids separate.

For example, in a diploid cell with 2n = 46 chromosomes:

Before S phase: 46 chromosomes and 46 chromatids.

After S phase: 46 replicated chromosomes and 92 sister chromatids.

The DNA content increases from 2C to 4C, where C represents the DNA content of one haploid genome.

3.4 G2 Phase (Second Gap Phase)

G2 follows DNA replication and precedes mitosis.

Major events include:

Further cell growth.

Synthesis of proteins required for mitosis.

Preparation of cellular structures for chromosome segregation.

Checking and repair of DNA damage where possible.

The cell prepares to distribute its replicated chromosomes accurately between daughter cells.

3.5 M Phase

The M phase is the phase of actual cell division.

It includes:

Karyokinesis.
Cytokinesis.

Karyokinesis is conventionally divided into four stages:

Prophase.

Metaphase.

Anaphase.

Telophase.

In many textbooks, the early stages of mitosis are described with additional detail, including prometaphase. For NEET preparation, the standard four-stage classification is especially important.

4. MITOSIS

Mitosis is a type of cell division in which one parent nucleus divides to form two daughter nuclei that generally contain the same chromosome number as the parent nucleus.

In a typical diploid somatic cell, mitosis produces two diploid daughter cells.

Mitosis is also called equational division because the chromosome number is maintained.

4.1 Importance of Mitosis

Mitosis performs several essential functions:

Growth of multicellular organisms.
Replacement of old and damaged cells.
Wound healing and tissue repair.
Asexual reproduction in many organisms.
Maintenance of chromosome number across successive cell generations.
Genetic continuity between parent and daughter cells, barring mutations or other changes.

4.2 Prophase

Prophase is the first major stage of mitosis.

Important events include:

Chromatin condenses to form visible chromosomes.

Each replicated chromosome consists of two sister chromatids.

The nucleolus gradually disappears.

The mitotic spindle begins to form.

In typical animal cells, centrosomes move towards opposite poles.

As mitosis progresses, the chromosomes become more compact and easier to distinguish.

NEET point: Chromosome condensation begins during prophase.

4.3 Prometaphase

In the detailed description of mitosis, prometaphase follows prophase.

Major events include:

Breakdown of the nuclear envelope.

Attachment of spindle microtubules to kinetochores.

Movement of chromosomes towards the equatorial region.

Some textbooks include these events in the broader description of prophase.

NEET point: Spindle microtubules attach to kinetochores, not to telomeres.

4.4 Metaphase

Metaphase is the stage during which chromosomes align at the equatorial plane of the cell.

Major events include:

Chromosomes attain a highly condensed state.

Chromosomes become arranged at the metaphase plate.

Sister chromatids remain attached to each other.

Spindle microtubules connect with chromosome kinetochores.

The position of chromosomes at the equator allows their genetic material to be distributed accurately.

NEET point: The best stage for studying chromosome morphology is generally metaphase because chromosomes are maximally condensed and clearly visible.

4.5 Anaphase

Anaphase begins when sister chromatids separate.

Major events include:

Cohesion between sister chromatids is released.

Centromeric regions separate in typical mitosis.

Sister chromatids move towards opposite poles.

Each separated chromatid is now regarded as an individual chromosome.

The separation of sister chromatids ensures that each daughter nucleus receives one copy of each replicated chromosome.

Important distinction: During anaphase, the chromosome number within the undivided cell is temporarily counted differently because the separated sister chromatids are now individual chromosomes.

For example, in a human somatic cell, 46 replicated chromosomes are present before anaphase. Once sister chromatids separate, 92 individual chromosomes are present temporarily within the same cell, with 46 moving towards each pole.

4.6 Telophase

Telophase is the final major stage of mitosis.

Major events include:

Chromosomes reach opposite poles.

Chromosomes begin to decondense.

Nuclear envelopes re-form around the chromosome sets.

Nucleoli reappear.

The spindle apparatus disassembles.

At the end of telophase, two daughter nuclei have formed.

4.7 Cytokinesis

Cytokinesis divides the cytoplasm and usually follows nuclear division.

In animal cells:

A cleavage furrow develops.

A contractile ring helps constrict the cell membrane.

The parent cell separates into two daughter cells.

In plant cells:

The rigid cell wall prevents the formation of a typical cleavage furrow.

Golgi-derived vesicles gather in the centre of the cell.

These vesicles fuse to form a cell plate.

The cell plate expands outwards and develops into the new partition between daughter cells.

NEET comparison:

Feature	Animal cell	Plant cell
Cytokinesis	Cleavage furrow	Cell plate
Direction of development	Inward constriction	Generally from centre towards the periphery
Main mechanism	Contractile ring	Fusion of vesicles and cell plate formation

4.8 Significance of Mitosis

Mitosis maintains genetic continuity, supports organismal growth, replaces damaged cells, and enables asexual reproduction in many organisms.

However, mitosis alone does not create the normal reduction in chromosome number required for sexual reproduction.

5. Meiosis

Meiosis is a specialised type of cell division in which one round of DNA replication is followed by two successive nuclear divisions.

It usually produces four haploid products from one diploid parent cell, although the number of functional gametes or spores ultimately formed depends on the organism and its reproductive biology.

Meiosis consists of:

Meiosis I.
Meiosis II.

Meiosis I is reductional division because homologous chromosomes separate, reducing the chromosome-set number from diploid to haploid.

Meiosis II is equational division because sister chromatids separate without a further reduction in chromosome-set number.

5.1 Importance of Meiosis

Meiosis is essential because it:

Reduces the chromosome-set number in reproductive cells.

Helps maintain the characteristic chromosome number of a species across sexual generations.

Generates genetic variation through crossing over.

Produces new combinations of maternal and paternal chromosomes through independent assortment.

Supports sexual reproduction and evolution.

5.2 Meiosis I

Meiosis I is divided into prophase I, metaphase I, anaphase I, and telophase I.

Prophase I is especially important because homologous chromosomes pair and crossing over occurs.

5.3 Prophase I: Five Substages

Prophase I is divided into five substages:

Leptotene.
Zygotene.
Pachytene.
Diplotene.
Diakinesis.

A. Leptotene

During leptotene:

Chromosomes begin to condense.

Thin, thread-like chromosomes become visible.

Chromosomes are progressively organised into recognisable structures.

NEET point: Chromosomes become progressively visible during leptotene.

B. Zygotene

During zygotene:

Homologous chromosomes pair with each other.

This pairing is called synapsis.

The synaptonemal complex develops between paired homologous chromosomes.

A pair of synapsed homologous chromosomes is called a bivalent.

NEET point: Synapsis occurs during zygotene.

C. Pachytene

During pachytene:

Chromosomes become shorter and thicker.

Synapsis is generally complete.

Crossing over occurs between non-sister chromatids of homologous chromosomes.

Recombination nodules are associated with sites of genetic recombination.

A bivalent contains two homologous chromosomes and four chromatids; it is therefore also called a tetrad.

NEET point: Crossing over occurs during pachytene.

D. Diplotene

During diplotene:

The synaptonemal complex disassembles.

Homologous chromosomes begin to separate.

They remain connected at visible chiasmata, which are associated with previous crossing-over events.

In certain organisms, oocytes may remain arrested for extended periods at a stage of diplotene.

NEET point: Chiasmata become clearly visible during diplotene.

E. Diakinesis

During diakinesis:

Chromosomes become highly condensed.

Chiasmata move towards the ends of chromosomes in a process called terminalisation.

The nucleolus disappears.

The nuclear envelope breaks down as the cell prepares for metaphase I.

NEET point: Terminalisation of chiasmata is characteristic of diakinesis.

5.4 Metaphase I

During metaphase I:

Homologous chromosome pairs align at the equatorial plane.

Each homolog is oriented towards an opposite pole.

The orientation of different pairs contributes to independent assortment.

Important distinction: Homologous chromosomes align as pairs during metaphase I. Individual replicated chromosomes align at the metaphase plate during mitotic metaphase.

5.5 Anaphase I

During anaphase I:

Homologous chromosomes separate.

One homolog from each pair moves towards each pole.

Sister chromatids generally remain attached.

Centromeres do not undergo the usual separation seen in anaphase of mitosis or anaphase II.

As a result, the chromosome-set number is reduced.

NEET point: Homologous chromosomes separate during anaphase I.

5.6 Telophase I

During telophase I:

Homologous chromosomes reach opposite poles.

Nuclear envelopes may re-form, depending on the organism.

Cytokinesis may occur.

Two haploid cells or nuclei are produced.

Each chromosome still consists of two sister chromatids.

5.7 Interkinesis

Interkinesis is the interval between meiosis I and meiosis II in organisms where such an interval occurs.

It is not equivalent to a complete interphase.

Critical point: No new round of DNA replication occurs between meiosis I and meiosis II.

5.8 Meiosis II

Meiosis II resembles mitosis in the way sister chromatids separate, but it begins with haploid cells whose chromosomes are still replicated.

Prophase II

Chromosomes condense if they had partially decondensed.

The spindle apparatus forms.

Nuclear envelopes, if present, break down.

Metaphase II

Chromosomes align individually at the equatorial plane.

Sister chromatids are oriented towards opposite poles.

Anaphase II

Sister chromatids separate.

Centromeric cohesion is released.

The separated chromatids move towards opposite poles.

NEET point: Centromeres separate during anaphase II.

Telophase II

Chromosomes reach the poles.

Nuclear envelopes re-form where applicable.

Chromosomes decondense.

Cytokinesis produces the final cellular products.

Meiosis typically generates four haploid products, with genetic differences resulting from recombination and chromosome assortment.

6. Mitosis Versus Meiosis

Feature	Mitosis	Meiosis
Number of nuclear divisions	One	Two
DNA replication before division	One round	One round before meiosis I
Synapsis of homologous chromosomes	Absent	Present during prophase I
Crossing over	Not a normal feature of mitosis	Occurs during prophase I
Homologous chromosome separation	Not the defining event	Anaphase I
Sister chromatid separation	Anaphase	Anaphase II
Chromosome-set number	Generally maintained	Reduced during meiosis I
Typical products	Two daughter cells	Four haploid products
Genetic similarity	Usually very similar	Usually genetically different
Major biological role	Growth and repair	Sexual reproduction and genetic variation

7. Chromosome Number and DNA Content

This is one of the most important numerical concepts in this chapter.

Let a diploid cell contain 2n = 4 chromosomes.

Stage	Chromosome number	Chromatid status	DNA content
G1 phase	4	Four unreplicated chromosomes	2C
After S phase	4	Eight sister chromatids	4C
Metaphase of mitosis	4	Eight sister chromatids	4C
Anaphase of mitosis, within the undivided cell	8	Sister chromatids have separated	4C
Each daughter cell after mitosis	4	Four unreplicated chromosomes	2C
After S phase before meiosis I	4	Eight sister chromatids	4C
Metaphase I	4	Eight sister chromatids	4C
Each product after meiosis I	2	Four sister chromatids	2C
Each product after meiosis II	2	Two unreplicated chromosomes	1C

Here, C represents the DNA content of one haploid genome.

The table assumes normal division without errors and uses chromosome number counted by centromeres. It describes a typical diploid cell and should not be applied without adjustment to organisms with unusual life cycles.

Remember these rules:

DNA replication doubles DNA content but does not immediately double chromosome number.

Separation of sister chromatids increases the chromosome count within the still-undivided cell.

Meiosis I separates homologous chromosomes.

Meiosis II separates sister chromatids.

DNA does not replicate again between meiosis I and meiosis II.

8. REGULATION OF THE CELL CYCLE

Cell-cycle progression is controlled by regulatory proteins and checkpoints.

8.1 G1 Checkpoint

The G1 checkpoint assesses cell size, nutrients, growth signals, and DNA integrity before the cell proceeds towards DNA replication.

8.2 G2 Checkpoint

The G2 checkpoint helps ensure that DNA replication has been completed and that significant DNA damage is addressed before mitosis.

8.3 Spindle Assembly Checkpoint

This checkpoint helps prevent the cell from proceeding into chromosome separation until chromosomes have achieved appropriate spindle attachment.

Defects in cell-cycle regulation may contribute to uncontrolled cell proliferation and cancer.

9. QUICK REVISION: HIGH-YIELD NEET FACTS

Interphase consists of G1, S, and G2 phases.
DNA replication occurs during the S phase.
G0 is a quiescent state outside the active cell-cycle sequence.
Mitosis includes prophase, metaphase, anaphase, and telophase.
Chromosomes are highly condensed during metaphase.
Spindle microtubules attach to kinetochores.
Animal-cell cytokinesis occurs through cleavage-furrow formation.
Plant-cell cytokinesis generally involves cell-plate formation.
Synapsis occurs during zygotene.
Crossing over occurs during pachytene.
Chiasmata become visible during diplotene.
Terminalisation of chiasmata is associated with diakinesis.
Homologous chromosomes separate during anaphase I.
Sister chromatids separate during anaphase II.
No DNA replication occurs between meiosis I and meiosis II.
Meiosis I is reductional division.
Meiosis II is equational division.
Mitosis generally preserves chromosome number.
Meiosis contributes to genetic variation.
DNA content and chromosome number are not interchangeable measurements.

PART 2: NEET PREVIOUS-YEAR QUESTIONS (PYQs)

Question 1. DNA Synthesis During the Cell Cycle

Examination: NEET-UG 2016, Phase 2
Topic: Interphase

During which phase of the cell cycle does DNA synthesis take place?

A. G1 phase
B. S phase
C. G2 phase
D. M phase

Correct Answer: B. S phase

Explanation: DNA replication occurs during the synthesis (S) phase of interphase. The DNA content doubles, but the chromosome number generally remains unchanged because the replicated sister chromatids remain attached.

NEET takeaway: S phase = DNA synthesis.

Question 2. Attachment of Spindle Fibres

Examination: NEET-UG 2016, Phase 1
Topic: Chromosome structure and mitosis

During mitosis, spindle fibres attach to which chromosome structure?

A. Telomere
B. Kinetochore
C. Nucleolus
D. Nuclear envelope

Correct Answer: B. Kinetochore

Explanation: The kinetochore is a protein structure assembled at the centromeric region of a chromosome. Spindle microtubules attach to kinetochores and help position and separate chromosomes during cell division.

Telomeres are protective structures at chromosome ends and are not the usual attachment sites for spindle microtubules.

NEET takeaway: Spindle microtubules attach to kinetochores.

Question 3. Stages of Meiotic Prophase I

Examination: NEET-UG 2016, Phase 2
Topic: Meiosis

Match the meiotic stages in Column I with their characteristic events in Column II.

Column I

a. Pachytene
b. Metaphase I
c. Diakinesis
d. Zygotene

Column II

i. Pairing of homologous chromosomes
ii. Terminalisation of chiasmata
iii. Crossing over
iv. Alignment of homologous chromosome pairs at the equator

Choose the correct option.

A. a–iii, b–iv, c–ii, d–i
B. a–i, b–iii, c–iv, d–ii
C. a–ii, b–iv, c–iii, d–i
D. a–iv, b–ii, c–i, d–iii

Correct Answer: A. a–iii, b–iv, c–ii, d–i

Explanation:

Pachytene: Crossing over occurs.

Metaphase I: Homologous chromosome pairs align at the equator.

Diakinesis: Terminalisation of chiasmata occurs.

Zygotene: Homologous chromosomes pair through synapsis.

NEET takeaway: Learn the sequence of prophase I stages and their defining events.

Question 4. Separation of Centromeres During Meiosis

Examination: NEET-UG 2021
Topic: Meiosis II

During which stage of meiosis do centromeres normally separate, allowing sister chromatids to move towards opposite poles?

A. Metaphase I
B. Anaphase I
C. Anaphase II
D. Telophase I

Correct Answer: C. Anaphase II

Explanation: During anaphase I, homologous chromosomes separate, but sister chromatids generally remain attached. During anaphase II, sister chromatids separate and move towards opposite poles.

NEET takeaway:

Anaphase I: Homologous chromosomes separate.

Anaphase II: Sister chromatids separate.

Question 5. Matching Cell-Cycle Phases

Examination: NEET-UG 2021
Topic: Cell-cycle regulation and interphase

Match the cell-cycle phases in Column I with their appropriate descriptions in Column II.

Column I

a. S phase
b. G2 phase
c. G0 phase
d. G1 phase

Column II

i. Synthesis of proteins required for cell division
ii. Quiescent state
iii. Interval between mitosis and the initiation of DNA replication
iv. DNA replication

Choose the correct option.

A. a–iii, b–ii, c–i, d–iv
B. a–iv, b–i, c–ii, d–iii
C. a–ii, b–iv, c–iii, d–i
D. a–i, b–iii, c–iv, d–ii

Correct Answer: B. a–iv, b–i, c–ii, d–iii

Explanation:

S phase: DNA replication.

G2 phase: Preparation for mitosis, including synthesis of required proteins.

G0 phase: Quiescent state outside the active cell-cycle sequence.

G1 phase: Interval between mitosis and the beginning of DNA replication.

NEET takeaway: G1, S, G2, and G0 have different functions.

Question 6. Synapsis During Meiosis

Examination: NEET-UG 2020, Phase 2
Topic: Prophase I

During which stage of meiotic prophase I does synapsis of homologous chromosomes occur?

A. Leptotene
B. Zygotene
C. Pachytene
D. Diplotene

Correct Answer: B. Zygotene

Explanation: Synapsis is the pairing of homologous chromosomes. It occurs during zygotene, when the synaptonemal complex develops between the homologous chromosomes.

During pachytene, crossing over occurs between non-sister chromatids.

NEET takeaway: Zygotene = synapsis; pachytene = crossing over.

Question 7. Chromosome Number After DNA Replication

Examination: NEET-UG 2021
Topic: Chromosome number and DNA replication

A fruit fly has eight chromosomes in each diploid cell. If a cell contains eight chromosomes during the G1 phase, how many chromosomes will it normally contain after completing the S phase?

A. 4
B. 8
C. 16
D. 32

Correct Answer: B. 8

Explanation: DNA replication occurs during S phase. Each chromosome forms two sister chromatids, but the sister chromatids remain attached.

Therefore:

Before S phase: 8 chromosomes and 8 chromatids.

After S phase: 8 chromosomes and 16 chromatids.

The DNA content doubles, but the chromosome number remains eight.

NEET takeaway: DNA replication does not immediately double chromosome number.

Question 8. Terminalisation of Chiasmata

Examination: NEET-UG 2021
Topic: Prophase I

During which stage of meiotic prophase I is terminalisation of chiasmata characteristically observed?

A. Leptotene
B. Zygotene
C. Diplotene
D. Diakinesis

Correct Answer: D. Diakinesis

Explanation: During diakinesis, chromosomes become highly condensed and chiasmata move towards the ends of chromosomes. This movement is known as terminalisation.

Chiasmata become visible during diplotene, whereas their terminalisation is associated with diakinesis.

NEET takeaway: Diplotene = visible chiasmata; diakinesis = terminalisation.

Question 9. Chromosomes That Do Not Pair During Mitosis

Examination: NEET-UG 2022
Topic: Mitosis versus meiosis

Which of the following events does not normally occur during mitotic cell division?

A. Attachment of spindle microtubules to kinetochores
B. Chromosome condensation
C. Pairing of homologous chromosomes
D. Separation of sister chromatids

Correct Answer: C. Pairing of homologous chromosomes

Explanation: Pairing of homologous chromosomes, called synapsis, is characteristic of prophase I of meiosis. It is not a normal feature of mitosis.

Chromosome condensation, kinetochore attachment, and sister chromatid separation are all associated with mitosis.

NEET takeaway: Synapsis is characteristic of meiosis I, not mitosis.

Question 10. Recombination Nodules

Examination: NEET-UG 2022
Topic: Crossing over

The appearance of recombination nodules on homologous chromosomes during meiosis is associated with which process?

A. Terminalisation of chiasmata
B. Crossing over
C. Separation of sister chromatids
D. Cytokinesis

Correct Answer: B. Crossing over

Explanation: Recombination nodules are protein-containing structures associated with the machinery responsible for genetic recombination during meiosis. Crossing over occurs between non-sister chromatids of homologous chromosomes during pachytene.

This process produces new combinations of genetic material.

NEET takeaway: Recombination nodules are associated with crossing over during pachytene.

Question 11. Quiescent Stage of the Cell Cycle

Examination: NEET-UG 2023, Manipur paper
Topic: G0 phase

Which of the following represents the quiescent stage of the cell cycle?

A. G1 phase
B. S phase
C. G2 phase
D. G0 phase

Correct Answer: D. G0 phase

Explanation: Cells in G0 have exited the active cell-cycle sequence. Depending on the cell type and physiological conditions, they may remain quiescent temporarily or for extended periods.

Many mature nerve cells remain outside the active cell cycle under normal conditions.

NEET takeaway: G0 is the quiescent state.

Question 12. Attachment of Spindle Fibres During Mitosis

Examination: NEET-UG 2024
Topic: Mitotic spindle

Spindle fibres attach to the kinetochores of chromosomes during which stage of mitosis?

A. Prophase
B. Prometaphase
C. Telophase
D. Cytokinesis

Correct Answer: B. Prometaphase

Explanation: In the detailed description of mitosis, the nuclear envelope breaks down during prometaphase, allowing spindle microtubules to access the chromosomes and attach to their kinetochores.

Some simplified descriptions include these events within prophase. For examination purposes, distinguish the detailed prometaphase description from the broader four-stage classification of mitosis.

NEET takeaway: Kinetochore attachment becomes established during prometaphase.

Question 13. Main Function of Spindle Fibres

Examination: NEET-UG 2025
Topic: Mitosis

What is the main function of spindle fibres during mitosis?

A. Synthesis of DNA
B. Production of ATP
C. Movement and segregation of chromosomes
D. Formation of ribosomes

Correct Answer: C. Movement and segregation of chromosomes

Explanation: The mitotic spindle is made of microtubules and associated proteins. It helps position chromosomes and separate sister chromatids into the two daughter-cell regions.

DNA replication occurs during S phase, while ribosome production involves structures and processes associated with the nucleolus and other cellular components.

NEET takeaway: The mitotic spindle ensures accurate chromosome segregation.

PART 3: ADDITIONAL NEET-STYLE PRACTICE QUESTIONS

Practice Question 14. DNA Content in G2

A diploid cell contains 2C DNA in G1. What will its DNA content normally be during G2?

A. C
B. 2C
C. 4C
D. 8C

Correct Answer: C. 4C

Explanation: DNA replication occurs during S phase, doubling the DNA content from 2C to 4C. The DNA content remains 4C during G2.

Examination name and year: Not applicable — original practice question.

Practice Question 15. Reductional Division

Which stage of meiosis is directly responsible for reducing the chromosome-set number?

A. Prophase II
B. Metaphase II
C. Anaphase I
D. Anaphase II

Correct Answer: C. Anaphase I

Explanation: Homologous chromosomes separate during anaphase I. This leads to the reduction from two chromosome sets to one in the resulting cells.

Examination name and year: Not applicable — original practice question.

Practice Question 16. Plant-Cell Cytokinesis

Which structure develops during cytokinesis in a typical plant cell?

A. Cleavage furrow
B. Cell plate
C. Synaptonemal complex
D. Recombination nodule

Correct Answer: B. Cell plate

Explanation: Golgi-derived vesicles gather and fuse in the middle of the dividing plant cell. Their fusion produces the cell plate, which expands and forms the new partition between daughter cells.

Examination name and year: Not applicable — original practice question.

Practice Question 17. Crossing Over

Crossing over during normal meiosis occurs between:

A. Sister chromatids of the same chromosome
B. Non-sister chromatids of homologous chromosomes
C. Two unrelated chromosomes
D. Two centromeres

Correct Answer: B. Non-sister chromatids of homologous chromosomes

Explanation: During pachytene of prophase I, non-sister chromatids of homologous chromosomes exchange corresponding DNA segments, producing genetic recombination.

Examination name and year: Not applicable — original practice question.

Practice Question 18. DNA Replication Between Meiotic Divisions

Which statement about DNA replication between meiosis I and meiosis II is correct?

A. DNA replication occurs twice.
B. DNA replication occurs immediately before meiosis II.
C. DNA replication does not occur between meiosis I and meiosis II.
D. DNA replication occurs during anaphase I.

Correct Answer: C. DNA replication does not occur between meiosis I and meiosis II.

Explanation: One round of DNA replication takes place before meiosis I. Meiosis II follows without another S phase, allowing sister chromatids to separate.

Examination name and year: Not applicable — original practice question.

PART 4: REPEATED QUESTIONS AND CONCEPTS

Concept	Examination years represented in this selection	Key point
DNA synthesis during the cell cycle	2016	S phase
Spindle attachment	2016, 2024	Kinetochores
Synapsis	2016, 2020	Zygotene
Crossing over	2016, 2022	Pachytene
Terminalisation of chiasmata	2016, 2021	Diakinesis
Chromosome number after DNA replication	2021	Chromosome number remains unchanged during S phase
Centromere separation in meiosis	2021	Anaphase II
Cell-cycle phases and G0	2021, 2023	G1, S, G2 and G0 have distinct functions
Spindle function	2024, 2025	Chromosome positioning and segregation

PART 5: ADVANCED THEORY

10. Cell-Cycle Duration

The duration of the cell cycle varies among organisms and cell types.

A rapidly dividing cell may complete its cycle relatively quickly, whereas other cells may divide infrequently or remain in G0 for extended periods.

The duration of the cell cycle can be expressed as:

Total cell-cycle duration = Duration of G1 + Duration of S + Duration of G2 + Duration of M

This equation describes the conventional active cell cycle. If a cell enters G0, the time spent in that state is not ordinarily included in the duration of one active cycle.

Important Points

G1 is often the most variable phase.
DNA replication takes place during S phase.
G2 prepares the cell for mitosis.
M phase includes nuclear division and usually cytokinesis.
Cells that have entered G0 may remain there temporarily or for an extended period.
The duration of each phase depends on the organism, cell type, and environmental conditions.

11. CELL-CYCLE CHECKPOINTS

Checkpoints help regulate the progression of cells through the cell cycle.

11.1 G1/S Checkpoint

This checkpoint helps determine whether the cell is ready to begin DNA replication.

Factors assessed include:

Cell size.

Nutrient availability.

Growth signals.

DNA damage.

If conditions are unsuitable, the cell may delay progression.

11.2 G2/M Checkpoint

This checkpoint helps ensure that DNA replication has been completed and that DNA damage is addressed before mitosis begins.

11.3 Spindle Assembly Checkpoint

The spindle assembly checkpoint helps ensure that chromosomes are appropriately attached to the spindle before sister chromatids separate.

Failure of chromosome segregation can lead to daughter cells with abnormal chromosome numbers.

11.4 Cyclins and Cyclin-Dependent Kinases

Cell-cycle progression is regulated by proteins called cyclins and enzymes known as cyclin-dependent kinases (CDKs).

Cyclin concentrations change during the cell cycle, helping control the activity of CDKs. Together, these regulators help coordinate DNA replication, mitosis, and other cell-cycle events.

NEET focus: Understand the roles of checkpoints and cell-cycle regulation without confusing them with the physical stages of mitosis.

12. SIGNIFICANCE OF CROSSING OVER

Crossing over occurs during pachytene of prophase I of meiosis.

It involves the exchange of corresponding DNA segments between non-sister chromatids of homologous chromosomes.

Importance

Produces new combinations of alleles.
Increases genetic variation among meiotic products.
Contributes to differences among offspring.
Helps establish connections between homologous chromosomes that are important for their proper segregation in meiosis I.

Crossing over does not normally change the chromosome number. Instead, it changes the genetic combinations carried by chromosomes.

13. Independent Assortment

During metaphase I, homologous chromosome pairs align at the equator.

The orientation of each pair is independent of the orientation of other pairs, subject to the usual assumptions of chromosome assortment.

Consequently, maternal and paternal homologues can be distributed into different combinations in the resulting cells.

For an organism with n pairs of homologous chromosomes, the number of possible chromosome combinations arising from independent assortment alone is:

Number of combinations = 2ⁿ

For example, in humans, n = 23.

The number of possible chromosome combinations from independent assortment alone is:

2²³ = 8,388,608

This calculation excludes additional variation caused by crossing over and other sources of genetic diversity.

14. Abnormalities During Cell Division

Errors during cell division may result in abnormal chromosome numbers or chromosome structures.

14.1 Nondisjunction

Nondisjunction occurs when chromosomes or sister chromatids fail to separate properly.

It may occur during meiosis I, meiosis II, or mitosis.

During meiosis I: Homologous chromosomes fail to separate properly.

During meiosis II: Sister chromatids fail to separate properly.

Nondisjunction can produce cells or gametes with abnormal chromosome numbers.

14.2 Aneuploidy

Aneuploidy refers to the gain or loss of one or more individual chromosomes rather than an entire chromosome set.

Examples include:

Monosomy: 2n − 1.

Trisomy: 2n + 1.

These conditions may arise from errors in chromosome segregation.

14.3 Polyploidy

Polyploidy refers to the presence of more than two complete chromosome sets.

For example:

Triploidy: 3n.
Tetraploidy: 4n.

Polyploidy is particularly important in plant evolution and plant breeding.

14.4 Uncontrolled Cell Division

Disruption of cell-cycle regulation can contribute to uncontrolled cell proliferation.

Cancer can develop when genetic and regulatory changes allow abnormal cells to survive and proliferate despite normal control mechanisms.

15. Visual Revision of Mitosis

The following sequence illustrates the progression from DNA replication to the formation of two daughter cells.

PART 6: ADDITIONAL PREVIOUS-YEAR QUESTIONS

Question 19. Dissolution of the Synaptonemal Complex

Examination: NEET-UG 2023, Manipur paper
Topic: Prophase I of meiosis

The dissolution of the synaptonemal complex occurs during which stage of meiotic prophase I?

A. Leptotene
B. Zygotene
C. Pachytene
D. Diplotene

Correct Answer: D. Diplotene

Explanation: The synaptonemal complex develops during the pairing of homologous chromosomes. It begins to disassemble during diplotene, when homologous chromosomes start separating but remain connected at chiasmata.

Key point: Diplotene is associated with the dissolution of the synaptonemal complex.

Question 20. Doubling of Chromosome Number Through Disrupted Division

Examination: NEET-UG 2023, Manipur paper
Topic: Mitosis and chromosome number

Doubling of the chromosome-set number in a cell can result if mitotic division is disrupted after chromosome duplication but before the cell completes division. Which outcome is most directly associated with failure of cytokinesis?

A. Formation of a cell with two nuclei or an increased chromosome complement
B. Immediate reduction of the chromosome number by half
C. Formation of four haploid cells
D. Crossing over between homologous chromosomes

Correct Answer: A. Formation of a cell with two nuclei or an increased chromosome complement

Explanation: If nuclear division occurs but cytokinesis fails, the cell may retain two nuclei. Depending on subsequent cell-cycle events, this can contribute to an increased chromosome complement.

Failure of cytokinesis is not the same as normal meiosis, which reduces the chromosome-set number.

Question 21. Centromere Separation in Mitosis and Meiosis

Examination: NEET-UG 2023, Manipur paper
Topic: Chromosome segregation

During which stages of mitosis and meiosis, respectively, do centromeres normally separate?

A. Anaphase I and anaphase II
B. Anaphase and anaphase II
C. Metaphase and metaphase I
D. Telophase and telophase I

Correct Answer: B. Anaphase and anaphase II

Explanation: In mitosis, sister chromatids separate during anaphase. In meiosis, sister chromatids remain attached during anaphase I and separate during anaphase II.

Key point: Centromere separation normally occurs during anaphase of mitosis and anaphase II of meiosis.

Question 22. Meiotic Arrest in Oocytes

Examination: NEET-UG 2022, Phase 2
Topic: Diplotene

Which stage of meiosis may last for months or years in the oocytes of some vertebrates?

A. Leptotene
B. Zygotene
C. Pachytene
D. Diplotene

Correct Answer: D. Diplotene

Explanation: In many vertebrates, primary oocytes remain arrested in the diplotene stage of prophase I for an extended period. This arrest can last for months or years, depending on the species.

Question 23. Correct Sequence of Prophase I

Examination: NEET-UG 2022, Phase 2
Topic: Stages of meiosis

Arrange the following events in the correct sequence:

Synapsis of homologous chromosomes.
Chromosomes become gradually visible.
Crossing over between non-sister chromatids.
Terminalisation of chiasmata.
Dissolution of the synaptonemal complex.

A. 2 → 1 → 3 → 5 → 4
B. 1 → 2 → 5 → 3 → 4
C. 3 → 2 → 1 → 4 → 5
D. 2 → 3 → 1 → 5 → 4

Correct Answer: A. 2 → 1 → 3 → 5 → 4

Explanation:

Leptotene: Chromosomes become progressively visible.

Zygotene: Synapsis occurs.

Pachytene: Crossing over occurs.

Diplotene: The synaptonemal complex dissolves.

Diakinesis: Terminalisation of chiasmata occurs.

Question 24. Bivalent or Tetrad Formation

Examination: NEET-UG 2022, Phase 2
Topic: Prophase I

Bivalent or tetrad formation is a characteristic feature of which meiotic stage?

A. Leptotene
B. Zygotene
C. Pachytene
D. Anaphase I

Correct Answer: B. Zygotene

Explanation: Homologous chromosomes pair during zygotene to form bivalents. Each bivalent contains two homologous chromosomes. Since each chromosome consists of two sister chromatids, the paired structure contains four chromatids and is also called a tetrad.

The formation of the bivalent begins with synapsis during zygotene, while crossing over occurs during pachytene.

Key point: Synapsis and initial bivalent formation are associated with zygotene.

Question 25. Correct Sequence of Cell-Cycle Events

Examination: NEET-UG 2024
Topic: Cell cycle

Arrange the following events in the correct sequence:

A. Gap 2 phase
B. Cytokinesis
C. Synthesis phase
D. Karyokinesis
E. Gap 1 phase

Choose the correct option.

E → C → A → D → B
C → E → D → A → B
E → B → D → A → C
B → D → E → A → C

Correct Answer: Option 1 — E → C → A → D → B

Explanation: The correct sequence is:

G1 → S → G2 → Karyokinesis → Cytokinesis.

G1: Cell growth and preparation for DNA replication.

S: DNA replication.

G2: Preparation for mitosis.

Karyokinesis: Nuclear division.

Cytokinesis: Cytoplasmic division.

Question 26. Chromosomes Become Visible

Examination: NEET-UG 2024
Topic: Prophase I

Consider the following statements.

Statement I: Chromosomes become gradually visible under a light microscope during the leptotene stage.

Statement II: The beginning of diplotene is recognised by the dissolution of the synaptonemal complex.

Choose the correct answer.

A. Both statements are correct.
B. Both statements are incorrect.
C. Statement I is correct, but Statement II is incorrect.
D. Statement I is incorrect, but Statement II is correct.

Correct Answer: A. Both statements are correct.

Explanation:

Statement I is correct because chromosomes progressively condense and become visible during leptotene.

Statement II is also correct because the synaptonemal complex begins to disassemble during diplotene, allowing homologous chromosomes to start separating while remaining connected at chiasmata.

Question 27. Recombination Between Homologous Chromosomes

Examination: NEET-UG 2024, Re-Examination
Topic: Prophase I

Recombination between homologous chromosomes is completed by the end of which stage of meiotic prophase I?

A. Leptotene
B. Zygotene
C. Pachytene
D. Diakinesis

Correct Answer: C. Pachytene

Explanation: Crossing over between non-sister chromatids of homologous chromosomes occurs during pachytene. This process produces new combinations of alleles.

Diplotene follows pachytene and is characterised by the dissolution of the synaptonemal complex and the visibility of chiasmata.

Question 28. Events During Prophase I

Examination: NEET-UG 2024, Re-Examination
Topic: Substages of prophase I

Match List I with List II.

List I: Substage	List II: Characteristic event
A. Diakinesis	I. Synaptonemal complex formation
B. Pachytene	II. Terminalisation of chiasmata
C. Zygotene	III. Chromosomes appear as thin threads
D. Leptotene	IV. Appearance of recombination nodules

Choose the correct matching.

A–II, B–IV, C–I, D–III
A–I, B–II, C–III, D–IV
A–III, B–I, C–IV, D–II
A–IV, B–III, C–II, D–I

Correct Answer: Option 1 — A–II, B–IV, C–I, D–III

Explanation:

Diakinesis: Terminalisation of chiasmata.

Pachytene: Recombination nodules are associated with crossing over.

Zygotene: Synaptonemal complex formation and synapsis.

Leptotene: Chromosomes appear as thin threads.

Question 29. Chromosome and Chromatid Count

A diploid cell has 2n = 12 chromosomes. Immediately after DNA replication, how many chromosomes and chromatids are normally present?

A. 12 chromosomes and 12 chromatids
B. 24 chromosomes and 24 chromatids
C. 12 chromosomes and 24 chromatids
D. 24 chromosomes and 12 chromatids

Correct Answer: C. 12 chromosomes and 24 chromatids

Explanation: DNA replication produces two sister chromatids per chromosome. The chromosome number remains 12 because the sister chromatids remain attached.

Question 30. DNA Content After Meiosis I

A diploid cell has a DNA content of 4C after S phase. What is the DNA content of each daughter cell immediately after meiosis I, assuming normal division?

A. C
B. 2C
C. 4C
D. 8C

Correct Answer: B. 2C

Explanation: Meiosis I separates homologous chromosomes. Each resulting haploid cell retains chromosomes consisting of two sister chromatids, so its DNA content is 2C.

After meiosis II, each resulting cell has a DNA content of C.

Question 31. Mitosis and Genetic Continuity

Which statement best describes the usual genetic outcome of mitosis?

A. Four genetically different haploid cells are formed.
B. Two daughter cells generally retain the parent cell’s chromosome number.
C. Homologous chromosomes undergo synapsis.
D. The chromosome number is necessarily reduced by half.

Correct Answer: B. Two daughter cells generally retain the parent cell’s chromosome number.

Explanation: Mitosis distributes replicated chromosomes between two daughter nuclei. It normally preserves chromosome number and genetic continuity, although mutations or chromosome-segregation errors may produce differences.

Question 32. Independent Assortment

Independent assortment of homologous chromosomes primarily results from:

A. DNA replication during G1
B. Random orientation of homologous chromosome pairs during metaphase I
C. Separation of sister chromatids during prophase I
D. Cytokinesis during telophase II

Correct Answer: B. Random orientation of homologous chromosome pairs during metaphase I

Explanation: The orientation of each homologous chromosome pair at metaphase I influences which homolog moves to each pole. The independent orientation of different pairs creates different combinations of maternal and paternal chromosomes.

Question 33. Meiosis and Genetic Variation

Which pair of processes contributes directly to genetic variation during meiosis?

A. DNA replication and cytokinesis only
B. Crossing over and independent assortment
C. Chromosome condensation and telophase
D. Cleavage-furrow formation and DNA repair

Correct Answer: B. Crossing over and independent assortment

Explanation: Crossing over exchanges DNA between non-sister chromatids of homologous chromosomes. Independent assortment creates different combinations of homologous chromosomes in meiotic products.

Question 34. Failure of Chromosome Segregation

Failure of homologous chromosomes to separate during meiosis I is an example of:

A. Synapsis
B. Nondisjunction
C. Cytokinesis
D. Crossing over

Correct Answer: B. Nondisjunction

Explanation: Nondisjunction is the failure of chromosomes or sister chromatids to segregate correctly. When homologous chromosomes fail to separate during meiosis I, the resulting products may contain abnormal chromosome numbers.

Question 35. Chromosome Number During Anaphase of Mitosis

A diploid cell has 2n = 10 chromosomes. How many individual chromosomes are present within the undivided cell after sister chromatids separate during anaphase of mitosis?

A. 5
B. 10
C. 20
D. 40

Correct Answer: C. 20

Explanation: Before anaphase, the cell has 10 replicated chromosomes, each containing two sister chromatids. When the sister chromatids separate, each becomes an individual chromosome. Thus, 20 individual chromosomes are temporarily present in the undivided cell, with 10 moving towards each pole.

PART 8: FINAL REVISION TABLE

Question topic	Essential answer
DNA replication	S phase
Cell growth before DNA replication	G1 phase
Preparation for mitosis	G2 phase
Quiescent state	G0 phase
Chromosome condensation at the beginning of meiosis	Leptotene
Synapsis	Zygotene
Crossing over	Pachytene
Dissolution of the synaptonemal complex	Diplotene
Visible chiasmata	Diplotene
Terminalisation of chiasmata	Diakinesis
Homologous chromosome separation	Anaphase I
Sister chromatid separation in meiosis	Anaphase II
Sister chromatid separation in mitosis	Anaphase
Animal-cell cytokinesis	Cleavage furrow
Plant-cell cytokinesis	Cell plate
Reductional division	Meiosis I
Equational division	Mitosis and meiosis II
Genetic variation	Crossing over and independent assortment
Failure of chromosome segregation	Nondisjunction
Gain or loss of individual chromosomes	Aneuploidy

PART 9: ADVANCED MULTIPLE-CHOICE QUESTIONS

Question 36. Duration of Interphase

Examination: Original NEET-style practice question — no previous-year attribution.

Which statement about interphase is correct?

A. It is a metabolically inactive period.
B. It includes G1, S, and G2 phases.
C. DNA replication occurs during G2.
D. Chromosome segregation occurs throughout interphase.

Correct Answer: B. It includes G1, S, and G2 phases.

Explanation: Interphase consists of G1, S, and G2 phases. During this period, the cell grows, replicates its DNA, synthesises proteins, and prepares for division. It is metabolically active rather than a resting period.

Question 37. Chromosome Number in Metaphase of Mitosis