Cell Biology — Concept Core
Prokaryote vs Eukaryote
| Feature | Prokaryote | Eukaryote |
|---|
| Nucleus | Absent (nucleoid) | Present (membrane-bound) |
| Membrane organelles | Absent | Present |
| Ribosomes | 70S (50S+30S) | 80S (60S+40S) |
| Cell wall | Peptidoglycan (bacteria) | Cellulose (plants), absent (animals) |
| DNA | Circular, no histone | Linear, with histone proteins |
| Examples | Bacteria, Cyanobacteria, Mycoplasma | All other organisms |
Cell Membrane (Fluid Mosaic Model)
Proposed by Singer and Nicolson (1972). Phospholipid bilayer with embedded proteins. Lipid bilayer = amphipathic molecules (hydrophilic head outside, hydrophobic tail inside). Cholesterol maintains membrane fluidity.
Proteins: Peripheral (loosely attached) and Integral (embedded). Functions: transport, receptors, enzymes, structural.
Key Organelles
MITOCHONDRIA
Double membrane. Powerhouse of cell. ATP synthesis (oxidative phosphorylation). Contains own DNA + 70S ribosomes. Self-replicating.
CHLOROPLAST
Double membrane + thylakoids + stroma. Photosynthesis. Own DNA + 70S ribosomes. Grana = stacked thylakoids.
GOLGI COMPLEX
Cis face (receives from ER), trans face (exports). Glycosylation, packaging, secretion. Forms lysosomes.
RIBOSOMES
70S in prokaryotes + mitochondria + chloroplasts. 80S in eukaryotic cytoplasm. Site of protein synthesis.
LYSOSOMES
Single membrane. Hydrolytic enzymes (pH 5). Suicidal bags. Autophagy (self-digestion), phagocytosis. Formed by Golgi.
CENTROSOME
Animal cells only (absent in plant cells). Contains centrioles. Forms aster and spindle fibres during cell division.
NUCLEOLUS
rRNA synthesis. Disappears during cell division. Not membrane-bound. May have multiple nucleoli.
ENDOPLASMIC RETICULUM
Rough ER: ribosomes, protein synthesis. Smooth ER: lipid synthesis, detoxification. Connected to nuclear envelope.
Mitosis — Stages
PMAT: Prophase → Metaphase → Anaphase → Telophase
Prophase: Chromatin condenses into chromosomes. Nucleolus disappears. Spindle formation begins (asters visible in animals).
Metaphase: Chromosomes align at metaphase plate (equatorial plane). Centromeres on equatorial plate. Best stage to count chromosomes.
Anaphase: Centromeres split. Sister chromatids pulled to poles. V, J, L, I shapes of chromosomes depending on centromere position.
Telophase: Nuclear envelope reforms. Chromosomes decondense. Nucleolus reappears. Followed by cytokinesis.
Meiosis — Key Events
Meiosis I (Reductional division): Homologous chromosomes separate. Produces 2 haploid cells.
Meiosis II (Equational division): Sister chromatids separate (like mitosis).
Prophase I sub-stages (LZPDD):
Leptotene (chromosomes condense) → Zygotene (synapsis begins, synaptonemal complex) → Pachytene (crossing over at chiasmata) → Diplotene (repulsion, chiasmata visible) → Diakinesis (chromosomes maximally condensed, terminalization of chiasmata)
Cell Biology — Worked Examples
Q1. Which stage of meiosis is best for karyotyping, and during which sub-stage of meiosis I does crossing over occur?
A) Metaphase I; Zygotene
B) Metaphase II; Pachytene
C) Metaphase I; Pachytene
D) Anaphase I; Diplotene
Karyotyping is best at Metaphase (chromosomes are most condensed and distinct). For meiosis, Metaphase II produces individual chromatids best for counting. Crossing over occurs at PACHYTENE (Prophase I) where chiasmata form between non-sister chromatids of homologous chromosomes. Remember: LZPDD — crossing over = Pachytene.
Q2. A cell has 2n = 46. After S phase of mitosis and after meiosis II, the chromosome numbers are respectively:
A) 92 and 46
B) 92 and 23
C) 46 and 23
D) 46 and 46
After S phase: chromosome NUMBER stays 46 (chromosomes are still connected at centromere), but DNA CONTENT doubles (92 chromatids total, as each chromosome has 2 sister chromatids). After meiosis II: 23 chromosomes (haploid gametes). Key distinction: chromosome number ≠ chromatid number after S phase.
Q3. Which of the following is NOT found in prokaryotes?
A) 70S ribosomes
B) Plasmid DNA
C) Mitochondria
D) Cell membrane
Prokaryotes lack membrane-bound organelles: no mitochondria, no chloroplasts, no Golgi, no ER, no nucleus. They DO have: 70S ribosomes, plasmids, cell membrane, cell wall (peptidoglycan in bacteria), mesosome (functions like mitochondria for respiration). Mitochondria evolved from bacterial endosymbiosis (endosymbiont theory).
Cell Biology — Mistake DNA
❌ "Ribosomes in mitochondria and chloroplasts are 80S (eukaryotic type)"
✅ WRONG. Mitochondria and chloroplasts have 70S ribosomes (prokaryotic type) because they evolved from ancient bacteria (endosymbiont theory). Cytoplasmic ribosomes of eukaryotes are 80S. This is a classic trick question in NEET — ribosomes in organelles = 70S.
❌ "Centrosome is present in all eukaryotic cells"
✅ WRONG. Centrosome (with centrioles) is present in ANIMAL cells and lower plant cells (bryophytes, pteridophytes). It is ABSENT in higher plant cells (angiosperms, gymnosperms). Higher plants still undergo mitosis using astral spindle formed differently.
❌ "Chromosomes are visible throughout the cell cycle"
✅ WRONG. Chromosomes are only visible during cell division (mitosis/meiosis). During interphase, DNA is in the form of chromatin (diffuse, extended). The condensation into visible chromosomes happens in Prophase. That's why interphase cells in microscopy show diffuse staining, not distinct chromosomes.
❌ "In meiosis II, the chromosome number is reduced from 2n to n"
✅ WRONG. Chromosome number reduction (2n → n) happens in MEIOSIS I (first division = reductional division). Meiosis II is equational — it separates sister chromatids, similar to mitosis, but starting with n chromosomes. After meiosis II, cells still have n chromosomes.
Cell Biology — Chapter Intelligence
AVG QUESTIONS
5–6
Consistent across all recent NEET papers
NCERT CHAPTERS
Ch 8: Cell — Unit of Life, Ch 10: Cell Cycle and Cell Division
TOP QUESTION TYPES
Organelle function identification, Mitosis vs Meiosis comparison, Meiosis sub-stage identification
DIFFICULTY
Easy-Medium (mostly conceptual, some application)
2025Synaptonemal complex formation stage, cell plate formation in plant cell division, nucleosome structure (8 histones)
2024Fluid mosaic model (Singer & Nicolson), differences between prokaryote and eukaryote ribosome types, meiosis significance
2023Crossing over stage (pachytene), chromosome count after meiosis I vs meiosis II, centromere position types
2022Lysosome origin (Golgi body), chloroplast features (self-replicating, own DNA), G0 phase significance
🎯 High-Probability 2026 Topics
✦ Meiosis Prophase I sub-stage identification (LZPDD mnemonic)
✦ Comparison of organelle features (presence/absence in prokaryotes)
✦ Chromosome number/DNA content at different stages of cell cycle
✦ Role of specific organelles (Golgi → lysosomes, ER → protein/lipid synthesis)