Cell Biology, Genetics and Plant Science Notes for Competitive

Biology carries a large share of the General Science questions in SSC, railways, defence, police and state examinations. The topics of the cell, genetics and plant science are asked every year, and most of the questions are direct facts: who discovered what, which organelle does which function, what is the ratio, which hormone does what. This note explains these three topics in simple language with tables, so that you can learn and revise them quickly.

Part 1: The Cell

The cell is the basic structural and functional unit of life. Robert Hooke discovered the cell in 1665 while observing a thin slice of cork under a microscope. Anton van Leeuwenhoek first saw living cells, and Robert Brown discovered the nucleus in 1831. The cell theory was proposed by Schleiden and Schwann (1838-39), and Rudolf Virchow added that new cells arise from old cells. The study of cells is called cytology.

Prokaryotic and Eukaryotic Cells

Cell Organelles and Their Functions

Some record facts are also asked. The largest cell is the egg of an ostrich, the longest cell is the nerve cell (neuron), and the smallest cell is Mycoplasma. The mature red blood cells of mammals have no nucleus. Human cells have 46 chromosomes in 23 pairs. Plant cells have a cell wall, a large vacuole and chloroplasts, but animal cells do not have them.

Cell Division

Mitosis takes place in body (somatic) cells and gives two identical daughter cells with the same number of chromosomes; it is used for growth and repair. Its stages are prophase, metaphase, anaphase and telophase. Meiosis takes place in the reproductive organs and forms four cells with half the number of chromosomes, called gametes; it is also called reduction division. Crossing over, which gives variation, takes place in meiosis.

Part 2: Genetics

Genetics is the study of heredity and variation. Gregor Johann Mendel, who worked on pea plants, is called the Father of Genetics. A gene is a part of DNA that carries a trait. An allele is the alternate form of a gene. A dominant trait appears in the first generation, and a recessive trait is hidden. Genotype is the genetic make-up, and phenotype is the visible trait.

Mendel's Laws and Ratios

• Law of dominance: In a hybrid, only the dominant trait is expressed.
• Law of segregation: The two alleles of a gene separate during gamete formation.
• Law of independent assortment: Genes for different traits are inherited independently.
• In a monohybrid cross (Tt × Tt), the phenotypic ratio is 3:1 and the genotypic ratio is 1:2:1.
• In a dihybrid cross, the phenotypic ratio is 9:3:3:1.

DNA, RNA and Sex Determination

DNA (deoxyribonucleic acid) has a double helix structure, which was proposed by Watson and Crick in 1953. It has four bases: adenine (A), thymine (T), guanine (G) and cytosine (C), with A pairing with T and G pairing with C. RNA has uracil (U) in place of thymine and is single stranded. The three types of RNA are mRNA, tRNA and rRNA. Humans have 22 pairs of autosomes and one pair of sex chromosomes. A female has XX and a male has XY, so the sex of the child is decided by the father's chromosome.

Genetic Disorders and Biotechnology

DNA fingerprinting was developed by Alec Jeffreys in 1984, and in India the Centre for DNA Fingerprinting and Diagnostics (CDFD) is in Hyderabad. The Human Genome Project was completed in 2003. Dolly, the sheep, was the first mammal cloned from an adult cell, in 1996. Bt cotton is the only genetically modified crop approved for commercial cultivation in India, and CRISPR is a modern gene-editing tool. Blood groups were discovered by Karl Landsteiner.

Part 3: Plant Science

Botany is the study of plants, and Theophrastus is called the Father of Botany. Carl Linnaeus gave the system of binomial nomenclature, in which the genus name and the species name are used, such as Mangifera indica for mango. Plants are classified into algae, bryophytes (mosses), pteridophytes (ferns), gymnosperms (naked seeds, such as pine) and angiosperms (flowering plants, with seeds in fruits). Angiosperms are divided into monocots and dicots.

Photosynthesis, Respiration and Transport

Photosynthesis is the process in which green plants make food from carbon dioxide and water using sunlight and chlorophyll, and release oxygen: 6CO2 + 6H2O → C6H12O6 + 6O2. It takes place in the chloroplasts, and the gases enter through the stomata. Transpiration is the loss of water vapour through stomata. In the vascular tissues, the xylem carries water and minerals upwards, and the phloem carries food in all directions. Plants respire day and night, but photosynthesis takes place only in light.

Plant Hormones

Edible Parts of Some Plants

Potato is a stem tuber, carrot, radish, beetroot and sweet potato are roots, cabbage and spinach are leaves, cauliflower is an inflorescence (flower), onion and garlic are bulbs, ginger and turmeric are rhizomes (underground stems), and wheat, rice and peas are seeds. In a mango the fleshy part is the mesocarp, and in a coconut the edible part is the endosperm.

Plant Reproduction and Growth

Plants reproduce in two ways. In asexual or vegetative reproduction, a new plant grows from a part of the parent, such as a stem cutting of rose, a tuber of potato, a bud of Bryophyllum leaf or a runner of grass; the new plant is a copy of the parent. In sexual reproduction, the flower is the reproductive organ. The male part is the stamen (anther and filament), and the female part is the pistil (stigma, style and ovary). Pollination is the transfer of pollen from the anther to the stigma, and it can be done by wind, water, insects or birds. After this, fertilisation takes place, the ovule becomes the seed and the ovary becomes the fruit. Angiosperms show a special double fertilisation. When a seed gets water, air and a suitable temperature, it germinates, and the first root that comes out is the radicle, while the first shoot is the plumule. The growth of a plant towards a stimulus is called a tropism: a shoot moves towards light (positive phototropism), a root moves towards gravity (geotropism) and towards water (hydrotropism).

Common Mistakes in Biology Questions

Students often confuse similar terms, and examiners use this confusion to set the options. The nucleus was discovered by Robert Brown, but the cell was discovered by Robert Hooke. Mendel is the Father of Genetics, but the word gene was given by Johannsen. Mitosis gives identical cells and meiosis gives variation. DNA has thymine and RNA has uracil. Genotype is the gene combination and phenotype is the visible character. Xylem moves water and phloem moves food. Chloroplast is the site of photosynthesis, and mitochondria are the site of respiration. Auxin makes the plant grow towards light, while abscisic acid closes the stomata. Write these pairs on a single page, and revise them before every mock test.

Practice Questions With Answers

(1) Who discovered the cell? Robert Hooke. (2) Which organelle is the powerhouse of the cell? Mitochondria. (3) Who is the Father of Genetics? Gregor Mendel. (4) What is the monohybrid phenotypic ratio? 3:1. (5) Which hormone ripens fruits? Ethylene. (6) Which tissue carries water in plants? Xylem. (7) Down syndrome is caused by which chromosome? Chromosome 21. (8) Where does photosynthesis take place? In the chloroplasts.

Quick Revision

• Hooke discovered the cell; Brown discovered the nucleus; mitochondria make ATP.
• Mitosis gives two identical cells; meiosis gives four cells with half the chromosomes.
• Mendel: 3:1 and 9:3:3:1; Watson and Crick: DNA double helix (1953).
• Xylem carries water, phloem carries food; ethylene ripens fruits.

Read the tables twice a week, and link every fact with a name or a number to remember it for a long time.

Related Notes to Read Next

Continue your preparation with these related notes from the same subject:

• Atomic Structure, Carbon Compounds and Fuels Notes
• SI Units and Measurement Notes for Competitive Exams
• Periodic Table and Periodic Classification Notes
• All General Science notes

Frequently Asked Questions (FAQ)

Who discovered the cell?

Robert Hooke discovered the cell in 1665 while observing a slice of cork under a microscope.

Why are mitochondria called the powerhouse of the cell?

Mitochondria produce ATP, the energy currency of the cell, and so they are called the powerhouse of the cell.

What is the difference between prokaryotic and eukaryotic cells?

Prokaryotic cells have no true nucleus or membrane-bound organelles, while eukaryotic cells have a true nucleus and organelles.

What is the difference between mitosis and meiosis?

Mitosis gives two identical cells with the same chromosome number for growth, while meiosis gives four cells with half the chromosomes for reproduction.

Who is the Father of Genetics?

Gregor Johann Mendel, who worked on pea plants, is called the Father of Genetics.

What is the phenotypic ratio of a monohybrid cross?

The phenotypic ratio of a monohybrid cross is 3:1 and the genotypic ratio is 1:2:1.

Who proposed the double helix structure of DNA?

James Watson and Francis Crick proposed the double helix structure of DNA in 1953.

Who decides the sex of a child?

The father decides the sex of the child because he can give either an X or a Y chromosome, while the mother always gives an X.

What causes Down syndrome?

Down syndrome is caused by an extra copy of chromosome 21, which is called trisomy 21.

What is photosynthesis?

Photosynthesis is the process in which green plants make glucose from carbon dioxide and water in the presence of sunlight and chlorophyll, releasing oxygen.

What is the difference between xylem and phloem?

Xylem carries water and minerals from the roots to the leaves, and phloem carries the prepared food from the leaves to the other parts.

Which plant hormone is responsible for fruit ripening?

Ethylene is the gaseous plant hormone that causes fruit ripening.