What is the difference between mitosis and meiosis?

Mitosis: Cell division producing 2 daughter cells with same chromosome number as parent (diploid). Used for growth, repair, and asexual reproduction. Meiosis: Cell division producing 4 daughter cells with half the chromosome number (haploid). Used for sexual reproduction — forms gametes (sperm and egg). Meiosis involves two divisions; mitosis involves one.

What are Mendel's Laws of Inheritance?

Mendel's Three Laws: (1) Law of Dominance: When two alleles are present, one (dominant) masks the other (recessive). (2) Law of Segregation: Each individual has two alleles for each trait; they separate during gamete formation — each gamete gets one. (3) Law of Independent Assortment: Genes for different traits are inherited independently of each other (for genes on different chromosomes).

What is DNA and what is its function?

DNA (Deoxyribonucleic Acid) is the genetic material that carries hereditary information. Structure: Double helix (Watson and Crick, 1953) made of nucleotides — each nucleotide has a sugar (deoxyribose), phosphate group, and a base (A, T, G, or C). A pairs with T; G pairs with C (complementary base pairing). DNA → RNA → Protein (Central Dogma of molecular biology).

Biology — Cell, Genetics and Heredity — TNPSC Group II/IIA Science

Overview

Cell biology and genetics form a significant part of TNPSC Group 2/2A science questions. Understanding cell structure, types of cell division, DNA structure, and inheritance patterns (Mendelian genetics) is essential. Focus on the differences between cell types and between mitosis and meiosis.

The Cell — Basic Unit of Life

Cell Theory

1. All living organisms are composed of cells 2. The cell is the basic structural and functional unit of life 3. All cells arise from pre-existing cells (Virchow, 1855)

Discoverers

Robert Hooke (1665): First observed cells in cork; coined term "cell"
Anton van Leeuwenhoek: First observed living microorganisms
Schleiden (1838): All plants made of cells
Schwann (1839): All animals made of cells

Cell Structure

Cell Membrane (Plasma Membrane)

Present in ALL cells
Controls what enters and exits
Semi-permeable; fluid mosaic model
Made of phospholipid bilayer + proteins

Cell Wall

Only in plant cells, fungi, and bacteria (NOT animal cells)
Plant cell wall: Made of cellulose
Bacterial cell wall: Peptidoglycan
Fungal cell wall: Chitin
Provides structural support and protection

Nucleus

"Control centre" of the cell
Contains DNA (chromosomes)
Nuclear membrane (nuclear envelope): Double layer; has pores
Nucleolus: Makes ribosomal RNA (rRNA)
Chromatin: DNA + proteins in non-dividing cells
Chromosomes: Condensed chromatin during cell division

Mitochondria

"Powerhouse of the cell"
Site of cellular respiration (ATP production)
Has its own DNA (maternal inheritance)
Double membrane
ATP (Adenosine Triphosphate): Energy currency of cell

Chloroplasts

Only in plant cells and algae
Site of photosynthesis
Contains chlorophyll (green pigment)
Has its own DNA
Double membrane + thylakoids (where light reactions occur)

Ribosomes

Site of protein synthesis
Found in all cells
Two types: 70S (prokaryotes) and 80S (eukaryotes)
Can be free or attached to ER

Endoplasmic Reticulum (ER)

Network of membranes
Rough ER (rER): Has ribosomes; makes proteins for secretion
Smooth ER (sER): No ribosomes; makes lipids; detoxification

Golgi Apparatus

"Post office" of cell — processes and packages proteins
Sends proteins to different destinations (secretion, lysosomes, cell membrane)

Lysosomes

"Suicide bags" of cell
Contain hydrolytic enzymes
Digest worn-out organelles, cellular debris, bacteria
Only in animal cells (and some fungi)

Vacuoles

Plant cells: Large central vacuole — maintains turgor pressure; stores water/sap
Animal cells: Small vacuoles or none

Centrioles

Only in animal cells (and lower plants)
Help in cell division (mitosis/meiosis)
Absent in plant cells (plants divide without centrioles)

Differences: Plant Cell vs Animal Cell

Feature	Plant Cell	Animal Cell
Cell Wall	Present (cellulose)	Absent
Chloroplasts	Present	Absent
Vacuole	Large central vacuole	Small or absent
Centrioles	Absent	Present
Lysosomes	Rare	Common
Shape	Generally rectangular	Generally irregular/round
Storage	Starch	Glycogen

DNA and RNA

DNA (Deoxyribonucleic Acid)

Feature	Details
Structure	Double helix (Watson-Crick, 1953)
Sugar	Deoxyribose
Bases	Adenine (A), Thymine (T), Guanine (G), Cytosine (C)
Base pairing	A–T (two H bonds); G–C (three H bonds)
Location	Nucleus (mainly); also mitochondria, chloroplasts
Function	Stores and transmits genetic information

Nobel Prize: Watson, Crick, Wilkins (1962) for DNA structure Rosalind Franklin: Key X-ray diffraction work used by Watson and Crick

RNA (Ribonucleic Acid)

Type	Function
mRNA (messenger RNA)	Carries genetic code from DNA to ribosomes
rRNA (ribosomal RNA)	Makes up ribosomes
tRNA (transfer RNA)	Carries amino acids during protein synthesis

Difference from DNA: RNA is single-stranded; uses Uracil (U) instead of Thymine (T); sugar is Ribose

Cell Division

Mitosis

Feature	Details
Result	2 daughter cells; same chromosome number as parent
Chromosome number	Diploid → Diploid (2n → 2n)
Phases	Prophase → Metaphase → Anaphase → Telophase → Cytokinesis
Purpose	Growth, repair, asexual reproduction
Occurs in	Somatic (body) cells
Genetically	Identical cells

Meiosis

Feature	Details
Result	4 daughter cells (gametes); half the chromosome number
Chromosome number	Diploid → Haploid (2n → n)
Divisions	Two divisions: Meiosis I + Meiosis II
Purpose	Sexual reproduction; formation of gametes
Occurs in	Gonads (testes/ovaries)
Genetically	Different cells (variation due to crossing over)
Crossing over	In Prophase I; exchange of DNA between homologous chromosomes

Human Chromosomes

Human cells: 46 chromosomes (23 pairs)
Sex chromosomes: XX (female), XY (male)
Autosomes: 22 pairs (non-sex chromosomes)
Gametes: 23 chromosomes (haploid)

Genetics and Heredity

Mendel's Experiments (Gregor Mendel, 1866)

Studied pea plants (Pisum sativum)
Worked with 7 traits: Seed shape, seed colour, pod shape, pod colour, flower colour, flower position, plant height
Each trait had two contrasting forms

Key Terminology

Term	Definition
Gene	Unit of heredity; sequence of DNA coding for a trait
Allele	Alternative forms of a gene
Dominant	Allele that is expressed when two different alleles are present
Recessive	Allele that is masked by dominant allele
Genotype	Genetic makeup (e.g., Tt, TT, tt)
Phenotype	Physical appearance (e.g., Tall, Short)
Homozygous	Two identical alleles (TT or tt)
Heterozygous	Two different alleles (Tt)

Mendel's Laws

1. Law of Dominance:

In a cross between two homozygotes with contrasting traits, F1 shows only dominant trait
TT (tall) × tt (short) → Tt (all tall) — tall is dominant

2. Law of Segregation:

Each organism has two alleles for each gene
They segregate during gamete formation — each gamete gets one allele
F2 ratio: 3:1 (dominant : recessive) in monohybrid cross

Monohybrid Cross (Tt × Tt):

  T  t
T TT Tt
t Tt tt

Genotype ratio: 1TT : 2Tt : 1tt Phenotype ratio: 3 Tall : 1 Short

3. Law of Independent Assortment:

Genes for different traits assort independently during gamete formation
F2 ratio: 9:3:3:1 in dihybrid cross

Blood Groups (ABO System)

Blood Group	Antigen on RBC	Antibody in Plasma	Can Donate to	Can Receive from
A	A antigen	Anti-B	A, AB	A, O
B	B antigen	Anti-A	B, AB	B, O
AB	A and B antigens	None	AB only	All (Universal recipient)
O	None	Anti-A and Anti-B	All (Universal donor)	O only

Rh Factor: Rh+ most common; Rh− rare; important in blood transfusion and pregnancy

Key Facts for TNPSC Group II/IIA

Robert Hooke coined term "cell"; Schleiden and Schwann proposed Cell Theory
Mitochondria: "Powerhouse of cell"; ATP production; own DNA
Chloroplasts: Only in plant cells; photosynthesis; own DNA
Cell wall: Cellulose in plants; chitin in fungi; absent in animal cells
Lysosomes: "Suicide bags"; only in animal cells
Watson and Crick (1953): Double helix structure of DNA
DNA: A-T, G-C base pairs; double stranded; deoxyribose sugar
Mitosis: 2 identical diploid cells; for growth and repair
Meiosis: 4 haploid gametes; for sexual reproduction
Mendel: 3 laws; pea plants; dominant-recessive; 3:1 ratio in F2
Blood Group O: Universal donor; Blood Group AB: Universal recipient

Previous Year MCQ Patterns

Q: The "powerhouse of the cell" is: a) Nucleus b) Ribosome c) Mitochondria d) Chloroplast

Answerc) Mitochondria — Mitochondria are called the "powerhouse of the cell" because they produce ATP (Adenosine Triphosphate) through cellular respiration. ATP is the energy currency of cells. Mitochondria have their own DNA and are inherited maternally.

Q: In Mendel's monohybrid cross, what is the phenotypic ratio in F2 generation? a) 1:2:1 b) 3:1 c) 9:3:3:1 d) 1:1

Answerb) 3:1 — In a monohybrid cross (e.g., Tt × Tt), the F2 phenotypic ratio is 3 dominant : 1 recessive (3:1). The genotypic ratio is 1TT : 2Tt : 1tt. The 9:3:3:1 ratio is for dihybrid crosses.

Q: Which blood group is known as the "Universal Donor"? a) A b) B c) AB d) O

Answerd) O — Blood Group O (O negative in particular) is called the Universal Donor because O group red blood cells have no A or B antigens on their surface, so they can be given to any blood group without causing an immediate immune reaction. AB is the Universal Recipient (can receive all blood groups).

Frequently Asked Questions

Plant cells have: cell wall (cellulose), chloroplasts (for photosynthesis), large central vacuole, and no centrioles. Animal cells have: centrioles (for cell division), no cell wall, no chloroplasts, small vacuoles or none, and lysosomes. Both have nucleus, mitochondria, ribosomes, cell membrane, endoplasmic reticulum, and Golgi apparatus.