Cell Biology and Genetics: Cell Organelles, Cell Division, Mendel's Laws, DNA and Genetic Disorders for SSC
Rahul Kumar
SSC Exam Expert & Content Editor
In this note
- 1.Cell Theory
- 2.Prokaryotic and Eukaryotic Cells
- 3.Cell Organelles and Functions
- 4.Plant Cell and Animal Cell
- 5.Cell Division
- 6.DNA and RNA
- 7.Mendel’s Laws
- 8.Sex Determination in Humans
- 9.Genetic Disorders
- 10.Solved Examples
- 11.Quick Tips
- 12.Quick Revision Points
- 13.Practice Questions With Answers
- 14.How to Practise This Chapter
- 15.Frequently Asked Questions
PDF Notes: Cell Biology and Genetics: Cell Organelles, Cell Division, Mendel's Laws, DNA and Genetic Disorders for SSC
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The cell is the basic unit of life, and genetics explains how traits pass from parents to children. Questions on organelles, DNA, Mendel and genetic disorders are asked every year in SSC general science. This guide on cell biology and genetics for SSC covers the cell theory, organelles, cell division, DNA, Mendel’s laws and genetic disorders, with practice questions and FAQs.
Cell Theory
- The cell was discovered by Robert Hooke in 1665 in a slice of cork.
- Schleiden and Schwann (1838–39) proposed that all living things are made of cells.
- Rudolf Virchow (1855) said that new cells come from pre-existing cells.
- Anton van Leeuwenhoek first saw living cells (bacteria and protozoa).
- The nucleus was discovered by Robert Brown in 1831.
Prokaryotic and Eukaryotic Cells
| Feature | Prokaryotic | Eukaryotic |
|---|---|---|
| Nucleus | No true nucleus (nucleoid) | True nucleus with a membrane |
| Organelles | No membrane-bound organelles | Membrane-bound organelles present |
| Size | Small (1–10 µm) | Large (10–100 µm) |
| Examples | Bacteria, blue-green algae | Plants, animals, fungi, protists |
Cell Organelles and Functions
| Organelle | Function |
|---|---|
| Plasma membrane | Controls the entry and exit of substances; selectively permeable |
| Cell wall | Rigid outer covering in plant cells made of cellulose; gives shape and protection |
| Nucleus | Control centre; contains DNA and chromosomes |
| Mitochondria | Powerhouse of the cell; produce ATP through respiration; have their own DNA |
| Ribosomes | Protein synthesis |
| Endoplasmic reticulum (ER) | Rough ER makes proteins; smooth ER makes lipids and detoxifies |
| Golgi apparatus | Packing, modification and secretion of proteins; discovered by Camillo Golgi |
| Lysosomes | Suicide bags; digest waste and dead cells |
| Plastids (chloroplasts) | Photosynthesis; contain chlorophyll; present only in plants |
| Vacuole | Storage of water and nutrients; large in plant cells |
| Centrosome (centrioles) | Cell division in animal cells |
Plant Cell and Animal Cell
| Feature | Plant Cell | Animal Cell |
|---|---|---|
| Cell wall | Present | Absent |
| Plastids | Present | Absent |
| Vacuole | Large and central | Small or absent |
| Centrioles | Absent (generally) | Present |
| Shape | Fixed (rectangular) | Round or irregular |
Cell Division
| Feature | Mitosis | Meiosis |
|---|---|---|
| Occurs in | Somatic (body) cells | Reproductive cells (gametes) |
| Divisions | One | Two |
| Daughter cells | 2, identical to the parent | 4, with half the chromosomes |
| Chromosome number | Remains the same | Halved |
| Purpose | Growth and repair | Formation of gametes |
DNA and RNA
- DNA is the genetic material; its double-helix structure was proposed by Watson and Crick in 1953.
- DNA has four bases: adenine (A), thymine (T), guanine (G) and cytosine (C); A pairs with T and G pairs with C.
- RNA has uracil (U) instead of thymine, and is single-stranded.
- Types of RNA: mRNA, tRNA and rRNA.
- The human genome has about 3 billion base pairs; humans have 46 chromosomes (23 pairs).
Mendel’s Laws
Gregor Mendel, the Father of Genetics, experimented on garden pea plants (Pisum sativum) in 1856–63.
| Law | Statement |
|---|---|
| Law of dominance | In a cross of pure parents, only the dominant trait appears in the F1 generation |
| Law of segregation | The two alleles of a gene separate during gamete formation |
| Law of independent assortment | Genes for different traits are inherited independently |
- Monohybrid cross: phenotypic ratio 3 : 1 and genotypic ratio 1 : 2 : 1 in F2.
- Dihybrid cross: phenotypic ratio 9 : 3 : 3 : 1 in F2.
- Test cross: crossing an organism with a homozygous recessive to find its genotype.
Sex Determination in Humans
Females have two X chromosomes (XX) and males have one X and one Y (XY). The sex of the child depends on the father’s sperm: X sperm gives a girl and Y sperm gives a boy.
Genetic Disorders
| Disorder | Cause |
|---|---|
| Down syndrome | Extra chromosome 21 (trisomy 21) |
| Turner syndrome | Only one X chromosome in a female (XO) |
| Klinefelter syndrome | Extra X chromosome in a male (XXY) |
| Haemophilia | X-linked recessive disorder; blood does not clot |
| Colour blindness | X-linked recessive disorder |
| Sickle cell anaemia | Mutation in the haemoglobin gene |
| Thalassemia | Defective haemoglobin production |
Solved Examples
Example 1. Which organelle is called the powerhouse of the cell?
Mitochondria.
Example 2. If a person has 47 chromosomes with an extra chromosome 21, which disorder is this?
Down syndrome.
Example 3. In a monohybrid cross of Tt × Tt, what is the ratio of tall to dwarf plants?
3 : 1.
Quick Tips
- Learn the function of every organelle.
- Remember that plastids and cell walls are present only in plant cells.
- Remember 9:3:3:1 for the dihybrid cross.
- Learn the genetic disorders with their cause.
Quick Revision Points
- Robert Hooke discovered the cell in 1665.
- Mitochondria are the powerhouse of the cell.
- Mitosis gives 2 identical cells; meiosis gives 4 cells with half the chromosomes.
- Mendel is the Father of Genetics.
- Down syndrome is trisomy 21.
Practice Questions With Answers
- Who discovered the cell?
Answer: Robert Hooke - Which organelle is called the suicide bag?
Answer: Lysosome - Which organelle makes proteins?
Answer: Ribosome - How many chromosomes does a human cell have?
Answer: 46 - Who is the Father of Genetics?
Answer: Gregor Mendel - What is the phenotypic ratio of a dihybrid cross?
Answer: 9 : 3 : 3 : 1 - Which chromosomes are present in a male?
Answer: XY - Which disorder is caused by trisomy 21?
Answer: Down syndrome - Where does photosynthesis take place?
Answer: Chloroplast - Which type of cell division produces gametes?
Answer: Meiosis
How to Practise This Chapter
Practise 10 questions every day and write the steps on paper. Then attempt timed sets from our practice papers and the previous year papers page. See all topics on the General Science notes page, read the previous topic Current Electricity and Magnetism and the next topic Communicable and Non-Communicable Diseases.
Frequently Asked Questions
Who discovered the cell?
Robert Hooke in 1665.
What is the powerhouse of the cell?
The mitochondria.
Which organelle is present only in plant cells?
Chloroplast (plastids) and the cell wall.
What is the function of ribosomes?
Protein synthesis.
What is the difference between mitosis and meiosis?
Mitosis makes two identical cells for growth; meiosis makes four cells with half the chromosomes for gametes.
What is DNA?
The genetic material that carries information of heredity.
What are the four bases of DNA?
Adenine, thymine, guanine and cytosine.
Who is the Father of Genetics?
Gregor Mendel.
What are Mendel’s laws?
The laws of dominance, segregation and independent assortment.
How is the sex of a child determined?
By the father’s sperm, which carries either X or Y.
What is Down syndrome?
A genetic disorder caused by an extra chromosome 21.
Is this topic asked in SSC?
Yes. Cell and genetics questions are asked in SSC general science every year.
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Human Diseases: Communicable and Non-Communicable Diseases, Causes, Vectors and Vaccines for SSC General Science
Rahul Kumar
SSC Exam Expert & Content Editor
Rahul is a senior SSC exam strategist with 8+ years of experience helping aspirants crack CGL, CHSL and MTS exams. He writes in-depth notification breakdowns, exam pattern guides and preparation strategies.