Cell division turns one cell into two, and eventually a single zygote into a ten-trillion-cell human. Mitosis handles growth and repair; meiosis makes gametes. The differences are a NEET favourite. Get the stages and checkpoints right and this is a guaranteed scoring topic.
The entire chapter boils down to one question: when and how does a cell copy its DNA and split into two? The cell cycle gives the timing. Mitosis gives the mechanism for identical copies. Meiosis gives the mechanism for gamete formation with genetic variation. If you understand the DNA content changes at each stage, every numerical and conceptual question becomes straightforward.
Core Concepts
Phases of the cell cycle
The cell cycle is the sequence of events from one division to the next:
| Phase | Events | Duration (typical) |
|---|---|---|
| G1 (Gap 1) | Cell grows, synthesises proteins, organelles increase | 10-12 hours |
| S (Synthesis) | DNA replicates — each chromosome now has 2 chromatids | 6-8 hours |
| G2 (Gap 2) | Preparation for division — centriole replication, protein synthesis | 3-4 hours |
| M (Mitosis) | Nucleus divides (karyokinesis) + cytoplasm divides (cytokinesis) | ~1 hour |
| G0 (Quiescent) | Cell exits cycle, does not divide (neurons, mature RBCs) | Indefinite |
DNA content changes: In G1, each cell has 2n chromosomes, 2C DNA. After S phase, it has 2n chromosomes, 4C DNA (each chromosome is now two sister chromatids joined at the centromere). After mitosis, each daughter cell returns to 2n, 2C.
Chromosome number (2n) stays the same throughout mitosis. DNA content (C) doubles during S and halves at division.
Stages of mitosis
Chromatin condenses into visible chromosomes (each with 2 chromatids). Nuclear envelope begins to break down. Centrosomes move to opposite poles, forming the spindle apparatus. Nucleolus disappears.
Chromosomes align at the metaphase plate (cell equator). Each chromosome is attached to spindle fibres via its kinetochore. This is the best stage for counting and photographing chromosomes (they are maximally condensed).
Centromeres split. Sister chromatids separate and move to opposite poles, pulled by shortening spindle fibres. Once separated, each chromatid is now an individual chromosome. The cell elongates.
Chromosomes reach the poles and begin to decondense. Nuclear envelope reforms around each set. Nucleolus reappears. The spindle disassembles.
Cytokinesis: In animal cells — cleavage furrow (contractile ring of actin) pinches the cell in two. In plant cells — cell plate forms from the centre outward (Golgi vesicles fuse to build a new cell wall).
Stages of meiosis
Two successive divisions — meiosis I (reductional) and meiosis II (equational).
Meiosis I separates homologous chromosomes:
Prophase I has five sub-stages (remember: LZPDD):
- Leptotene — chromosomes become visible as thin threads
- Zygotene — homologous chromosomes pair up (synapsis), forming bivalents (tetrads)
- Pachytene — crossing over occurs between non-sister chromatids of homologues. Recombination nodules appear. Genetic variation is generated here.
- Diplotene — homologues begin to separate but remain connected at chiasmata (sites of crossing over)
- Diakinesis — chiasmata shift to chromosome ends (terminalisation). Nuclear envelope breaks down. Bivalents are fully condensed and ready for metaphase I.
Metaphase I — bivalents line up at the metaphase plate (not individual chromosomes). Independent assortment occurs here — each bivalent orients randomly, so maternal and paternal chromosomes are distributed randomly.
Anaphase I — homologous chromosomes separate (NOT sister chromatids). Each pole gets one chromosome from each homologous pair → reduction from 2n to n.
Telophase I — nuclear envelope may reform. A brief interkinesis (NO DNA replication) may occur.
Meiosis II — essentially a mitosis of haploid cells. Sister chromatids separate in anaphase II. Result: 4 haploid cells from one diploid cell.
Significance of meiosis
- Halves the chromosome number — essential so that fertilisation restores 2n
- Generates genetic variation through:
- Crossing over (pachytene) — shuffles alleles between homologues
- Independent assortment (metaphase I) — random orientation of bivalents
- Together, these ensure that no two gametes are genetically identical
With 23 chromosome pairs in humans, independent assortment alone creates possible gamete combinations. Add crossing over, and the diversity is essentially unlimited.
Checkpoints and cancer
The cell cycle has built-in quality control:
| Checkpoint | Location | Checks for |
|---|---|---|
| G1/S (Restriction point) | End of G1 | Is the cell large enough? Is DNA undamaged? Are growth signals present? |
| G2/M | End of G2 | Is DNA fully and correctly replicated? |
| Spindle checkpoint | Metaphase | Are all chromosomes attached to spindle fibres? |
Key regulatory proteins: cyclins (fluctuate through the cycle) and CDKs (cyclin-dependent kinases — active only when bound to cyclins). Tumour suppressors like p53 and Rb act as brakes. Mutations in p53 remove the G1/S brake → cell divides with damaged DNA → cancer.
The role of p53 as a tumour suppressor is tested in NEET. Key fact: p53 is mutated in over 50% of human cancers. It is called the “guardian of the genome” because it stops the cell cycle when DNA damage is detected, allowing repair or triggering apoptosis (programmed cell death).
Worked Examples
A human cell (2n = 46) enters meiosis. After S phase: 46 chromosomes, each with 2 chromatids (92 chromatids total, 4C DNA).
After meiosis I: each cell has 23 chromosomes (each still with 2 chromatids) = n, 2C.
After meiosis II: each cell has 23 chromosomes (each with 1 chromatid) = n, C.
Four haploid gametes from one diploid cell. If any of these fuse with another gamete (also n), the zygote is restored to 2n = 46.
Mitosis keeps chromosome number constant (2n → 2n). If gametes were made by mitosis, each would be 2n. Fertilisation would double the number: 2n + 2n = 4n. In the next generation: 4n + 4n = 8n. The chromosome number would increase exponentially with each generation. Meiosis halves it (2n → n), so fertilisation restores the original (n + n = 2n), keeping the species number stable.
If a somatic cell has 10 pg of DNA in G1:
- After S phase: 20 pg (DNA has replicated)
- At metaphase of mitosis: 20 pg (not yet divided)
- After mitosis: 10 pg per daughter cell
- After meiosis I: 10 pg per cell (chromosome number halved but each chromosome still has 2 chromatids)
- After meiosis II: 5 pg per gamete
Without crossing over, alleles on the same chromosome would always be inherited together (complete linkage). Crossing over breaks this linkage, creating new combinations of alleles. This recombination is the raw material for natural selection — it increases the genetic diversity of a population, giving evolution more variants to work with.
Common Mistakes
Saying crossing over happens in mitosis. It happens only in meiosis I (pachytene of prophase I). Mitosis produces identical cells — introducing variation would defeat its purpose.
Confusing chromosome number with chromatid number. After S phase, a cell has 2n chromosomes but 4C DNA — each chromosome has 2 chromatids. The chromosome count does not change until anaphase separates them.
Writing that meiosis produces two daughter cells. It produces four. Two divisions (meiosis I and II) mean 1 → 2 → 4 cells.
Mixing up anaphase I and anaphase II. Anaphase I separates homologous chromosomes (one from each pair to each pole). Anaphase II separates sister chromatids (like anaphase of mitosis). The distinction is the most tested point in meiosis.
Thinking DNA replication occurs between meiosis I and meiosis II. It does not. Interkinesis is a brief gap phase with no S phase. DNA content halves after meiosis I and halves again after meiosis II.
Exam Weightage and Strategy
Cell Cycle and Cell Division carries 4-5 marks in CBSE Class 11 boards. NEET asks 1-2 questions per year, typically on chromosome counting, meiosis stages, or crossing over significance. The chapter is highly visual — drawing diagrams helps both understanding and exam answers.
Draw a combined meiosis I and II diagram on a single page with DNA content (C value) marked at each stage. Add a parallel column for chromosome number. Revise only that page — it handles every counting question. For the cell cycle, memorise the three checkpoints and p53’s role.
Practice Questions
Q1. A cell has 20 chromosomes. How many chromosomes and chromatids are present at metaphase I of meiosis?
At metaphase I, homologous pairs (bivalents) line up. Chromosomes: 20 (still 2n — reduction has not happened yet). Chromatids: 40 (each of the 20 chromosomes has 2 sister chromatids). Bivalents: 10 (20/2 = 10 homologous pairs). After anaphase I: 10 chromosomes per cell (n), each with 2 chromatids.
Q2. What is the significance of the spindle checkpoint?
The spindle checkpoint (at metaphase) ensures all chromosomes are properly attached to spindle fibres via their kinetochores before anaphase begins. If even one chromosome is unattached, the checkpoint halts the cell cycle. Without this checkpoint, chromosomes could be unequally distributed → aneuploidy (wrong chromosome number) → cell death or diseases like Down syndrome (trisomy 21).
Q3. Why are neurons in G0?
Most mature neurons permanently exit the cell cycle and enter G0. They are highly specialised and need to maintain stable connections. Dividing would disrupt the neural circuits they are part of. This is why brain and spinal cord damage is largely irreversible — dead neurons are not replaced (with minor exceptions in the hippocampus). Some cells like liver hepatocytes are in G0 but can re-enter the cycle if stimulated (e.g., after partial liver removal).
Q4. How does a cell with a mutant p53 contribute to cancer?
Normal p53 detects DNA damage at the G1/S checkpoint and stops the cycle, allowing repair. If repair fails, p53 triggers apoptosis (cell death). A mutant p53 cannot perform these functions → cells with damaged DNA proceed to S phase → mutations accumulate → eventually oncogenes are activated and tumour suppressors are lost → uncontrolled growth = cancer. This is why p53 mutations are found in over 50% of all human cancers.
FAQs
What is the difference between karyokinesis and cytokinesis?
Karyokinesis is division of the nucleus (including chromosome separation). Cytokinesis is division of the cytoplasm. Both usually happen together, but not always — in some organisms (like Syncytium in Drosophila embryo), karyokinesis occurs multiple times without cytokinesis, creating a multinucleate cell.
Can mitosis occur in haploid cells?
Yes. In haploid organisms (like yeast in the haploid phase), mitosis produces identical haploid daughter cells. Mitosis simply duplicates whatever chromosome complement the cell has — it does not change ploidy.
What is non-disjunction?
Failure of chromosomes to separate properly during anaphase (either meiosis I or meiosis II). One daughter cell gets an extra chromosome and the other gets one fewer. If a gamete with an extra chromosome 21 is fertilised, the result is trisomy 21 (Down syndrome). Non-disjunction is more common in older maternal age.
If you understand the timing — when DNA replicates versus when chromosomes separate — every PYQ on this topic falls into place.