Introduction to Cell Division
All living organisms grow and reproduce. New cells are formed only from pre-existing cells. Cell division is the process by which a parent cell divides to produce two or more daughter cells. It is fundamental to growth, repair, and reproduction.
There are two main types of cell division:
- Mitosis: Equational division producing 2 genetically identical diploid daughter cells. Occurs in somatic (body) cells.
- Meiosis: Reductional division producing 4 genetically unique haploid daughter cells. Occurs in cells of reproductive organs to form gametes.
Cell Cycle: Overview and Phases
The cell cycle is the ordered sequence of events a cell undergoes from one division to the next. In a rapidly dividing human cell, the cycle takes about 24 hours.
The cell cycle has two main phases:
- Interphase (I phase): G1 + S + G2 phases. Preparation for division; takes up ~90-95% of total cell cycle time.
- M phase (Mitotic phase): The actual nuclear division (karyokinesis) followed by cytoplasmic division (cytokinesis). Takes ~1-2 hours.
G0 phase (quiescent stage): Some cells exit the active cell cycle from G1 and enter G0. These cells are metabolically active but not dividing. Mature neurons are permanently in G0; liver cells are in G0 but can re-enter the cycle when stimulated.
Cell cycle checkpoints are control points that ensure each phase is completed correctly before the cell moves on:
- G1 checkpoint (restriction point): Checks cell size, nutrient availability, and growth factor signals. Cells failing this exit to G0.
- G2 checkpoint: Ensures DNA replication (S phase) is complete and error-free. Prevents entry into M phase if DNA is damaged.
- Spindle assembly checkpoint (M checkpoint): Ensures all chromosomes are attached to spindle fibers before anaphase begins.
Cell cycle: phases, events, and DNA content
Click each phase to explore its events, duration, and DNA content changes.
G1 Phase
Duration: ~8–11 hours (longest single phase)
Key events: Cell grows in size; proteins, enzymes and RNA are synthesised; organelles increase in number; G1 checkpoint checks nutrient and growth-factor availability
DNA content: 2C (unchanged)
NEET focus: G1 is the first gap phase. DNA replication has NOT started. Cells that are going to divide pass the G1 restriction point (checkpoint). Cells that fail the checkpoint exit to G0.
Try this
- NEET trap: After S phase, chromosome number = 2N (unchanged), but DNA content = 4C (doubled). The chromosome number only changes after chromatid separation in anaphase.
Interphase: G1, S, and G2 Phases
G1 Phase (First Gap Phase)
G1 is the longest phase of the active cell cycle. The cell grows in volume, synthesizes proteins and RNA, and increases the number of organelles. The G1 checkpoint (restriction point) determines whether the cell will proceed to S phase, arrest in G1, or exit to G0.
- DNA content: 2C (unreplicated)
- Chromosome number: 2N (unchanged)
- Active RNA and protein synthesis
S Phase (Synthesis Phase)
DNA replication occurs during S phase. Each chromosome is duplicated, producing two identical sister chromatids joined at the centromere. Histones are synthesized during S phase. Centrosome replication also begins.
- DNA content: increases from 2C to 4C
- Chromosome number: STILL 2N (unchanged; sister chromatids are joined, not separate)
- Each chromosome now consists of 2 sister chromatids
NEET trap: After S phase
After S phase: chromosome number = 2N (same as before), DNA content = 4C (doubled). Do not confuse! The chromosome number does not change after S phase because sister chromatids are still joined at the centromere and counted as ONE chromosome.
G2 Phase (Second Gap Phase)
G2 follows S phase and precedes M phase. The cell continues to grow, duplicates organelles (mitochondria, chloroplasts), and synthesizes tubulin for spindle fibers. The G2 checkpoint verifies that DNA replication is complete and error-free.
- DNA content: 4C (unchanged from end of S phase)
- Spindle proteins (tubulin) synthesized
- Organelle duplication
- G2 checkpoint operates here
Mitosis: Prophase
Mitosis (M phase karyokinesis) is divided into four stages: prophase, metaphase, anaphase, and telophase (PMAT). Prophase is the first and longest stage of mitosis.
- Chromatin fibres condense into distinct chromosomes (each = 2 sister chromatids)
- Nuclear envelope disintegrates (breaks down)
- Nucleolus disappears
- Centrosomes (containing centrioles in animal cells) move to opposite poles
- Asters form around centrosomes; spindle fibers extend toward chromosomes
- Spindle fibers attach to kinetochores (protein structures at centromeres)
Mitosis: stage-by-stage explorer
Click each stage to explore what happens, key NEET distinctions, and a diagram.
Prophase
•
Chromatin fibres condense into visible chromosomes
•
Each chromosome consists of 2 sister chromatids joined at centromere
•
Nuclear envelope disintegrates (breaks down)
•
Nucleolus disappears
•
Centrosomes migrate to opposite poles; spindle fibres (aster) form
•
Spindle fibres begin to attach to kinetochores
NEET focus: The nuclear envelope breaks down in PROPHASE (not metaphase). Nucleolus also disappears in prophase. Chromosomes are first visible as distinct structures in prophase.
Try this
- PMAT mnemonic: Prophase, Metaphase, Anaphase, Telophase. In MITOSIS anaphase: sister chromatids separate. In MEIOSIS I anaphase: homologous chromosomes separate. This distinction is a classic NEET question.
Mitosis: Metaphase
Metaphase is characterized by the alignment of chromosomes at the metaphase plate (equatorial plate), a plane equidistant from both poles.
- Chromosomes are maximally condensed (shortest, thickest form)
- This is the best stage to count and study chromosomes (karyotyping)
- Each chromosome is attached to spindle fibers from BOTH poles via sister kinetochores
- The spindle assembly checkpoint operates here: all chromosomes must be properly attached before anaphase begins
Mitosis: Anaphase
Anaphase begins when all chromosomes split simultaneously: the centromeres split and sister chromatids are pulled to opposite poles by shortening spindle fibers.
- Centromeres split; sister chromatids separate
- Each chromatid is now called a chromosome
- Chromosomes move toward opposite poles (V or J shape, centromere leading)
- Cell elongates as polar microtubules lengthen
- Each pole has a full set of chromosomes (2N)
NEET: Anaphase distinction
In MITOSIS anaphase: SISTER CHROMATIDS separate (centromere splits). In MEIOSIS I anaphase: HOMOLOGOUS CHROMOSOMES separate (centromere does NOT split). In MEIOSIS II anaphase: sister chromatids separate (like mitosis). This is one of the most frequently tested distinctions.
Mitosis: Telophase and Cytokinesis
Telophase
Telophase is the reverse of prophase. Nuclear envelopes reform around each set of chromosomes, nucleoli reappear, chromosomes decondense back to chromatin, and the spindle apparatus disassembles.
Cytokinesis
Cytokinesis is the division of the cytoplasm. It begins in late anaphase and completes after telophase.
- Animal cells: Cleavage furrow forms at the equator (actin-myosin contractile ring); ingresses inward until the cell pinches into two.
- Plant cells: Cell plate forms at the phragmoplast (remnants of spindle in the centre) using Golgi vesicles; grows outward toward the parent cell wall.
Significance of Mitosis
- Growth: All multicellular organisms grow by mitosis; daughter cells are identical to the parent.
- Repair and regeneration: Damaged or worn-out cells (skin, gut lining, liver) are replaced by mitosis.
- Asexual reproduction: Many organisms (bacteria via binary fission, yeasts by budding, planaria by fission) reproduce asexually using mitosis.
- Maintains chromosome number: Diploid chromosome number (2N) is preserved in all somatic cells.
- Genetic stability: Daughter cells are genetically identical to each other and to the parent cell.
Meiosis: Overview and Significance
Meiosis is a specialized type of cell division that occurs in the gonads (reproductive organs) to produce gametes (sperm and eggs). It involves TWO successive divisions (meiosis I and meiosis II) from ONE cell, producing FOUR haploid (N) cells.
Significance of meiosis:
- Chromosome number halving: Reduces chromosome number from 2N to N, ensuring that fertilization (N + N) restores the species diploid number (2N).
- Genetic variation: Crossing over during prophase I and independent assortment of chromosomes during metaphase I produce genetically unique gametes. This is the basis of genetic variation in sexually reproducing populations.
- Evolution: Genetic variation provides the raw material for natural selection and evolution.
Mitosis vs meiosis: comparison for NEET
Side-by-side comparison of all key differences. Toggle to show all or only the most important NEET distinctions.
9 rows shown
| Feature | Mitosis | Meiosis |
|---|---|---|
| ★Number of divisions | One (1) | Two (meiosis I + II) |
| ★Daughter cells produced | 2 cells | 4 cells |
| ★Ploidy of daughters | Diploid (2N) | Haploid (N) |
| ★Genetic composition | Genetically identical to parent | Genetically unique (variation) |
| ★Purpose | Growth, repair, asexual reproduction | Gamete formation, sexual reproduction |
| ★Crossing over | Does NOT occur | Occurs in pachytene of prophase I |
| ★Chromosome number change | Same as parent (2N to 2N) | Halved (2N to N) |
| ★Anaphase event | Sister chromatids separate | Homologs separate (meiosis I); sister chromatids (meiosis II) |
| ★Interphase between divisions | N/A (only one division) | Interkinesis (NO DNA replication) |
★ = High-priority NEET distinction
3 most-tested NEET distinctions:
1.
Mitosis: 2 diploid identical daughters. Meiosis: 4 haploid unique daughters.
2.
Crossing over in MEIOSIS (pachytene of prophase I). NEVER in mitosis.
3.
Anaphase of MITOSIS: sister chromatids separate. Anaphase I of MEIOSIS: homologs separate.
Try this
- NEET trap: "Between meiosis I and meiosis II, does DNA replication occur?" Answer: NO. The cell goes through interkinesis (a brief gap) with NO S phase. This is why meiosis II starts with haploid cells.
Meiosis I: Reductional Division
Meiosis I is called the reductional division because the chromosome number is halved (2N to N). It is the more complex of the two meiotic divisions due to the lengthy prophase I with its 5 sub-stages.
Prophase I: 5 Sub-stages (LZPDD)
- Leptotene: Chromosomes begin to condense and become visible. Each chromosome consists of 2 sister chromatids.
- Zygotene: Synapsis occurs: homologous chromosomes pair along their entire length. The synaptonemal complex (protein scaffold) forms. Paired structure = bivalent (tetrad) = 2 chromosomes = 4 chromatids.
- Pachytene: Chromosomes thicken further. Crossing over occurs between non-sister chromatids of homologous chromosomes. Recombination nodules are the sites of crossing over.
- Diplotene: Synaptonemal complex dissolves; homologs begin to separate. Chiasmata (sites of crossing over) become visible. Chromatids remain attached at chiasmata.
- Diakinesis: Maximum condensation. Chiasmata move toward chromosome ends (terminalisation). Nuclear envelope breaks down. Nucleolus disappears. Spindle forms.
Meiosis I: prophase sub-stages and key events
Click each sub-stage to learn the events, key distinctions, and NEET traps.
Prophase I sub-stages (click each):
Remaining stages of meiosis I:
Leptotene
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Chromosomes begin to condense and become visible under a microscope
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Each chromosome consists of 2 sister chromatids (DNA replicated in S phase)
•
Homologous chromosomes not yet paired
•
"Lept" = thin (Greek): chromosomes are thin and thread-like at this stage
NEET focus: Leptotene is the FIRST stage of prophase I. Chromosomes are visible but homologs are NOT yet paired. Remember: "Leptotene = thin threads visible, no pairing yet."
Prophase I sub-stages at a glance:
Leptotene:
Chromosomes visible (thin threads)
Zygotene:
Synapsis + synaptonemal complex
Pachytene:
Crossing over occurs
Diplotene:
Chiasmata visible, homologs begin to separate
Diakinesis:
Terminalisation, nuclear envelope breaks, spindle forms
Mnemonic: LZPDD (Lazy Zebras Pick Delicious Dates)
Try this
- NEET classic: "In which sub-stage of meiosis does crossing over occur?" Answer: Pachytene. The resultant sites of crossing over are called chiasmata, which become visible in DIPLOTENE.
Crossing over: process, significance, and NEET terms
Explore the mechanism of crossing over, why it matters, and the key NEET vocabulary.
What happens step by step:
1.
During pachytene, homologous chromosomes are held together by the synaptonemal complex.
2.
Non-sister chromatids (one from each homolog) break at the same position.
3.
The broken ends rejoin with the other chromatid (strand exchange).
4.
This produces 2 recombinant chromatids with new allele combinations.
5.
The parental chromatids remain unchanged.
6.
Result: each bivalent has 2 parental + 2 recombinant chromatids.
Try this
- Crossing over occurs in PACHYTENE but chiasmata become VISIBLE in DIPLOTENE. NEET often asks which stage chiasmata are visible: the answer is diplotene, not pachytene.
Metaphase I, Anaphase I, Telophase I
- Metaphase I: Bivalents (not individual chromosomes) align at the metaphase plate. Each spindle pole attaches to one chromosome of each bivalent. Random orientation of bivalents = independent assortment.
- Anaphase I: Homologous chromosomes move to opposite poles. Centromere does NOT split; each chromosome still has 2 chromatids. Chromosome number halves at each pole.
- Telophase I: Chromosomes reach poles. Nuclear envelopes may reform. Brief interkinesis (NO DNA replication). Cytokinesis I produces 2 haploid cells (N) each with 2C DNA.
Meiosis II: Equational Division
Meiosis II is called the equational division because chromosome number does not change (N to N); it is structurally similar to mitosis. The key difference is that meiosis II starts with haploid (N) cells.
- No DNA replication occurs between meiosis I and meiosis II (interkinesis has no S phase).
- Prophase II: Chromosomes condense, nuclear envelope breaks down, spindle forms.
- Metaphase II: Individual chromosomes (not bivalents) align at the metaphase plate.
- Anaphase II: Centromeres split; sister chromatids separate and move to opposite poles (like mitosis anaphase).
- Telophase II + Cytokinesis II: Nuclear envelopes reform; 4 haploid (N) cells produced with 1C DNA each.
Meiosis II: equational division step by step
Meiosis II is similar to mitosis. Click each step to see events, DNA content, and NEET distinctions.
DNA content
2C per cell (after meiosis I)
Chromosome status
N (haploid, but each chromosome has 2 chromatids)
Interkinesis
•
Brief gap between meiosis I and meiosis II
•
NO DNA replication occurs (critical distinction)
•
May or may not have a short interphase depending on species
•
Cells proceed directly to meiosis II
NEET focus: NO DNA replication occurs between meiosis I and meiosis II. This is a key NEET distinction. The cell proceeds from meiosis I directly to meiosis II without duplicating DNA.
Meiosis summary: chromosome and DNA count
| Stage | Chromosome # | DNA content |
|---|---|---|
| G1 (before meiosis) | 2N | 2C |
| After S phase | 2N | 4C |
| After Meiosis I | N | 2C |
| After Meiosis II | N | 1C |
Try this
- Meiosis II is called the EQUATIONAL division because chromosome number does not change during it (N to N). Meiosis I is the REDUCTIONAL division (2N to N). Meiosis II resembles mitosis but starts with haploid cells.
Chromosome and DNA content tracker through cell division
Set the diploid number and follow chromosome count and DNA content through mitosis or meiosis.
Diploid chromosome number (2N): 46
G1 (before S phase)
Starting state: diploid chromosome count, unreplicated DNA
After S phase
DNA doubled; chromosomes = same count but each has 2 chromatids
After G2
G2 does not change DNA or chromosome count
Prophase / Metaphase / early Anaphase
Chromosomes still paired as chromatids; total DNA = 4C
After Anaphase (chromatids separated)
Chromatids separate → each becomes a chromosome (count doubles transiently)
Each daughter cell (end of mitosis)
Each daughter: same chromosome count as parent, half the DNA
Key rule to remember:
After S phase: chromosome count stays the same (each chromosome = 2 chromatids joined at centromere), but DNA doubles. Chromosome count only changes when centromeres split (mitosis anaphase, meiosis II anaphase) or when homologs separate (meiosis I anaphase).
Try this
- For human cells (2N = 46): After S phase = 46 chromosomes, 4C DNA. After meiosis I = 23 chromosomes, 2C DNA. After meiosis II = 23 chromosomes, 1C DNA. Use this tracker to see any organism!
Worked NEET Problems
NEET-style problem · Meiosis I prophase sub-stages
Question
Solution
NEET-style problem · DNA content after cell cycle phases
Question
Solution
NEET-style problem · Anaphase distinction
Question
Solution
NEET-style problem · Crossing over and chiasmata
Question
Solution
NEET-style problem · Cytokinesis
Question
Solution
Cheat Sheet
- Cell cycle: G1 (growth) + S (DNA replication) + G2 (organelle duplication) + M (division)
- G0: quiescent state; neurons permanently G0; liver cells reversibly G0
- After S phase: chromosome number = 2N (same), DNA = 4C (doubled)
- Mitosis: 1 division, 2 diploid (2N) identical daughters
- Meiosis: 2 divisions, 4 haploid (N) unique daughters
- Prophase I sub-stages: Leptotene, Zygotene, Pachytene, Diplotene, Diakinesis (LZPDD)
- Synapsis (homolog pairing): ZYGOTENE; synaptonemal complex forms
- Crossing over: PACHYTENE; between non-sister chromatids of homologs
- Chiasmata visible: DIPLOTENE; synaptonemal complex dissolves
- Terminalisation of chiasmata: DIAKINESIS
- Bivalent = 2 homologs = 4 chromatids (tetrad)
- Metaphase I: bivalents align; metaphase of mitosis: individual chromosomes align
- Anaphase I: homologs separate (centromere does NOT split)
- Anaphase of mitosis and anaphase II: sister chromatids separate (centromere splits)
- No DNA replication between meiosis I and II (interkinesis)
- After meiosis I: N chromosomes, 2C DNA per cell
- After meiosis II: N chromosomes, 1C DNA per cell
- Plant cytokinesis: cell plate (centre outward); animal: cleavage furrow (periphery inward)
- Karyokinesis without cytokinesis: endosperm formation, polyploidy (colchicine)
Cell Cycle and Cell Division, NEET quiz
Question 1 of 12 · Topic: Cell cycle
Which phase of the cell cycle takes the LONGEST time in a typical mammalian cell?
A.
G1 phase
B.
S phase
C.
M phase
D.
G2 phase
0 answered
Frequently asked questions
How frequently does Cell Cycle and Cell Division appear in NEET?
Cell Cycle and Cell Division is a High Weightage chapter with about 5 questions per NEET exam. Questions focus on identifying phases of mitosis/meiosis, events in interphase, significance of divisions, crossing over, and chromosome number changes.
What is the difference between mitosis and meiosis?
Mitosis produces 2 genetically identical diploid (2N) daughter cells; it occurs in somatic cells for growth and repair. Meiosis produces 4 genetically unique haploid (N) cells; it occurs in reproductive organs to form gametes. Meiosis has two rounds of division (meiosis I and II); mitosis has only one.
What happens during the S phase of interphase?
DNA replication (synthesis) occurs during S phase. The DNA content doubles from 2N to 4N (in a diploid cell), but the chromosome number stays the same. Each chromosome now consists of two identical sister chromatids joined at the centromere.
What is crossing over and when does it occur?
Crossing over is the exchange of genetic material between non-sister chromatids of homologous chromosomes. It occurs during pachytene of prophase I of meiosis. The site of crossing over is called a chiasma (plural: chiasmata). Crossing over produces recombinant chromosomes, which is the main source of genetic variation in sexually reproducing organisms.
What is the significance of meiosis?
Meiosis (1) halves the chromosome number from diploid (2N) to haploid (N), maintaining the species chromosome number across generations; (2) generates genetic variation through crossing over during prophase I and independent assortment of homologous chromosomes; and (3) produces gametes for sexual reproduction.
What is the G0 phase?
G0 (quiescent phase) is a resting state outside the active cell cycle. Cells in G0 have exited G1 and are not dividing. Some cells (like mature neurons) are permanently in G0. Others (like liver cells) can re-enter the cycle when stimulated. G0 cells are metabolically active but not preparing for division.
What is the key difference between anaphase of mitosis and anaphase I of meiosis?
In anaphase of mitosis, sister chromatids separate (centromere splits; sister chromatids move to opposite poles). In anaphase I of meiosis, homologous chromosomes separate (the centromere does NOT split; each chromosome still consists of two chromatids moving together). Sister chromatids separate only in anaphase II of meiosis.
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