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How do Organisms Reproduce?

NCERT Class 10 · Science Based on NCERT Class 10 Science textbook · Free CBSE study kit

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Chapter Notes

Overview
This chapter explains why reproduction is essential for the survival of species, not for the individual. It covers the basic mechanism of DNA copying and variation, and explores the diverse modes of reproduction—from simple asexual methods like fission and budding to complex sexual reproduction in flowering plants and humans. The one big idea is that reproduction ensures continuity of life and generates variations that are crucial for evolution and adaptation.
Key Concepts (32)
Reproduction
The process by which living organisms produce new individuals of the same species, ensuring the continuity of life and transfer of genetic information.
e.g. A mango tree producing seeds that grow into new mango trees.
DNA (Deoxyribo Nucleic Acid)
The genetic material present in the nucleus of a cell that contains information for inheritance of features from parents to the next generation and acts as a blueprint for body design.
e.g. DNA determines whether a child has curly or straight hair like their parents.
DNA Copying
The process by which a cell makes an exact copy of its DNA before cell division, ensuring that each daughter cell receives a complete set of genetic information.
e.g. During binary fission in Amoeba, DNA is copied before the cell splits into two.
Variation
The subtle differences that arise in DNA copies during reproduction due to inaccuracies in biochemical reactions, leading to diversity among individuals of a species.
e.g. Some bacteria in a population may be heat-resistant while others are not, due to variation.
Fission
A mode of asexual reproduction in unicellular organisms where the parent cell divides into two or more daughter cells.
e.g. Amoeba splitting into two equal halves.
Binary Fission
A type of fission in which the parent cell divides into two equal daughter cells; can occur in any plane (e.g., Amoeba) or in a definite orientation (e.g., Leishmania).
e.g. Leishmania undergoes binary fission in a definite orientation relative to its whip-like structure.
Multiple Fission
A type of fission in which the parent cell divides into many daughter cells simultaneously.
e.g. Plasmodium (malarial parasite) divides into many daughter cells at once.
Fragmentation
A mode of asexual reproduction in multicellular organisms with simple body organisation, where the body breaks into pieces and each piece grows into a new individual.
e.g. Spirogyra filaments break into fragments that grow into new filaments.
Regeneration
The ability of a fully differentiated organism to give rise to new individuals from its body parts when cut or broken, carried out by specialised cells that proliferate and differentiate.
e.g. A piece of Planaria cut into many pieces grows into a complete organism from each piece.
Budding
A mode of asexual reproduction in which a bud develops as an outgrowth due to repeated cell division at a specific site, matures, and detaches to become a new individual.
e.g. Hydra produces buds that develop into tiny individuals and detach from the parent.
Vegetative Propagation
A mode of asexual reproduction in plants where parts like root, stem, and leaves develop into new plants under appropriate conditions.
e.g. Potato tubers (stem) with buds (eyes) grow into new potato plants.
Spore Formation
A mode of asexual reproduction in which spores (cells covered by thick walls) are produced in sporangia and develop into new individuals when they land on a moist surface.
e.g. Rhizopus (bread mould) produces spores in sporangia that grow into new mould.
Asexual Reproduction
A mode of reproduction that involves only one parent and produces offspring that are genetically identical to the parent, without fusion of gametes.
e.g. Binary fission in Amoeba, budding in Hydra.
Sexual Reproduction
A mode of reproduction that involves two parents (male and female) and the fusion of their gametes to produce offspring with genetic variation.
e.g. Human reproduction involving sperm and egg fusion.
Gamete
A specialised germ-cell (male or female) that contains half the number of chromosomes and fuses with another gamete during fertilisation to form a zygote.
e.g. Sperm (male gamete) and egg (female gamete) in humans.
Meiosis
A type of cell division that reduces the number of chromosomes to half, producing gametes with half the DNA content of the parent cell.
e.g. In human testes, meiosis produces sperm with 23 chromosomes.
Pollination
The transfer of pollen grains from the anther (stamen) to the stigma (pistil) of a flower.
e.g. Bees carrying pollen from one flower to another.
Self-Pollination
The transfer of pollen grains from the anther to the stigma of the same flower.
e.g. In Hibiscus, pollen may fall on its own stigma.
Cross-Pollination
The transfer of pollen grains from the anther of one flower to the stigma of another flower of the same species, aided by wind, water, or animals.
e.g. Papaya flowers rely on insects for cross-pollination.
Fertilisation
The fusion of male and female gametes to form a zygote.
e.g. In flowering plants, the pollen tube carries the male gamete to fuse with the egg cell in the ovule.
Zygote
The cell formed by the fusion of male and female gametes during fertilisation, which develops into a new organism.
e.g. In humans, the zygote forms in the fallopian tube and implants in the uterus.
Puberty
The period during adolescence when the reproductive tissues begin to mature and the body undergoes sexual maturation, including changes like growth of hair, voice change, and menstruation.
e.g. Girls begin to menstruate and boys' voices crack during puberty.
Testes
The male reproductive organs that produce sperms and the hormone testosterone; located outside the abdominal cavity in the scrotum for lower temperature.
e.g. In humans, testes are in the scrotum to maintain optimal temperature for sperm formation.
Vas Deferens
A tube that delivers sperms from the testes to the urethra, passing through glands that add seminal fluid.
e.g. During ejaculation, sperms travel through the vas deferens.
Seminal Vesicles and Prostate Gland
Glands that add secretions to the sperms to form a fluid that makes transport easier and provides nutrition.
e.g. The seminal fluid helps sperms swim towards the egg.
Ovaries
The female reproductive organs that produce eggs (ova) and hormones; present from birth with thousands of immature eggs.
e.g. One ovary releases one egg every month after puberty.
Oviduct (Fallopian Tube)
A thin tube that carries the egg from the ovary to the uterus; site of fertilisation.
e.g. The sperm meets the egg in the fallopian tube.
Uterus
An elastic bag-like structure where the embryo implants and develops into a foetus; its lining thickens monthly to receive a fertilised egg.
e.g. The foetus grows in the uterus for about nine months.
Placenta
A disc-like special tissue embedded in the uterine wall that contains villi on the embryo's side and blood spaces on the mother's side, allowing exchange of nutrients, oxygen, and waste.
e.g. The placenta provides glucose and oxygen to the growing foetus.
Menstruation
The monthly breakdown and shedding of the uterine lining (blood and mucous) through the vagina when the egg is not fertilised, lasting about two to eight days.
e.g. A girl experiences menstruation every month if pregnancy does not occur.
Contraception
Methods used to prevent pregnancy, including mechanical barriers (condoms), hormonal pills, intrauterine devices (copper-T), and surgical methods (blocking vas deferens or fallopian tubes).
e.g. Using a condom during sexual intercourse prevents sperm from reaching the egg.
Sexually Transmitted Diseases (STDs)
Diseases transmitted through sexual contact, including bacterial infections (gonorrhoea, syphilis) and viral infections (warts, HIV-AIDS).
e.g. HIV-AIDS can be transmitted through unprotected sexual contact.
Formulas & Equations (1)
No chemical equations or mathematical formulas are present in this chapter.
Reads as: Not applicable
Variables: Not applicable
Use when: Not applicable
⚠ Not applicable
How It Works
DNA Copying and Cell Division
  1. Step 1: The cell uses chemical reactions to build an exact copy of its DNA.
  2. Step 2: Two copies of DNA are created in the reproducing cell.
  3. Step 3: The cell creates additional cellular apparatus (organelles and cytoplasm).
  4. Step 4: The DNA copies separate, each with its own cellular apparatus.
  5. Step 5: The cell divides to give rise to two daughter cells.
💡 DNA copying is never 100% accurate; variations always arise, which is the basis for evolution.
Binary Fission in Amoeba
  1. Step 1: The nucleus of Amoeba divides by mitosis.
  2. Step 2: The cytoplasm divides, splitting the cell into two equal halves.
  3. Step 3: The splitting can occur in any plane.
  4. Step 4: Two daughter Amoebae are formed, each with a nucleus and cellular apparatus.
💡 Binary fission in Amoeba occurs in any plane, unlike Leishmania which has a definite orientation.
Binary Fission in Leishmania
  1. Step 1: The nucleus divides.
  2. Step 2: Cytoplasmic division occurs in a definite orientation relative to the whip-like structure at one end.
  3. Step 3: Two daughter cells are formed, each with the whip-like structure.
💡 Leishmania has a definite orientation for fission due to its body organisation.
Multiple Fission in Plasmodium
  1. Step 1: The nucleus of Plasmodium divides repeatedly without cytoplasmic division.
  2. Step 2: Many nuclei are formed within a single cell.
  3. Step 3: Cytoplasm divides simultaneously around each nucleus.
  4. Step 4: Many daughter cells are released at once.
💡 Multiple fission produces many offspring at once, unlike binary fission which produces two.
Fragmentation in Spirogyra
  1. Step 1: The filamentous body of Spirogyra matures.
  2. Step 2: The filament breaks into smaller pieces (fragments) naturally.
  3. Step 3: Each fragment grows into a new individual filament through cell division.
💡 Fragmentation works only in simple multicellular organisms like Spirogyra, not in complex organisms with specialised tissues.
Regeneration in Planaria
  1. Step 1: The body of Planaria is cut into pieces.
  2. Step 2: Specialised cells at the cut site proliferate and form a mass of cells.
  3. Step 3: Cells undergo changes to become various cell types and tissues in an organised sequence (development).
  4. Step 4: Each piece grows into a complete organism.
💡 Regeneration is not the same as reproduction because most organisms do not normally depend on being cut up to reproduce.
Budding in Hydra
  1. Step 1: Repeated cell division occurs at one specific site on the parent Hydra.
  2. Step 2: A bud develops as an outgrowth.
  3. Step 3: The bud grows into a tiny individual with its own mouth and tentacles.
  4. Step 4: When fully mature, the bud detaches from the parent body and becomes an independent individual.
💡 Budding uses regenerative cells but occurs at a specific site, not from any broken piece.
Vegetative Propagation in Potato
  1. Step 1: A potato tuber (stem) has notches or buds (eyes).
  2. Step 2: A piece of potato containing a bud is planted in moist soil.
  3. Step 3: The bud grows into a new shoot, and roots develop from the piece.
  4. Step 4: A new potato plant is formed.
💡 Only pieces with buds (eyes) can grow into new plants; pieces without buds do not grow.
Spore Formation in Rhizopus
  1. Step 1: The bread mould (Rhizopus) develops thread-like structures called hyphae.
  2. Step 2: Tiny blob-on-a-stick structures called sporangia form on some hyphae.
  3. Step 3: Sporangia contain cells called spores, covered by thick walls for protection.
  4. Step 4: When spores land on a moist surface, they germinate and grow into new Rhizopus individuals.
💡 Spores are protected by thick walls and can survive harsh conditions until they find a suitable moist surface.
Pollination and Fertilisation in Flowering Plants
  1. Step 1: Pollen grains are produced in the anther (stamen).
  2. Step 2: Pollen is transferred to the stigma (pistil) by self-pollination or cross-pollination (via wind, water, or animals).
  3. Step 3: A pollen tube grows out of the pollen grain and travels through the style to reach the ovary.
  4. Step 4: The male germ-cell from the pollen grain fuses with the female gamete (egg cell) in the ovule — this is fertilisation.
  5. Step 5: The zygote divides to form an embryo within the ovule.
  6. Step 6: The ovule develops a tough coat and becomes a seed; the ovary ripens into a fruit.
  7. Step 7: Petals, sepals, stamens, style, and stigma shrivel and fall off.
💡 Pollination is the transfer of pollen to stigma; fertilisation is the fusion of gametes inside the ovule. They are two different processes.
Human Male Reproductive System — Sperm Production and Delivery
  1. Step 1: Sperms are produced in the testes, located in the scrotum outside the abdominal cavity for lower temperature.
  2. Step 2: Testosterone hormone from testes regulates sperm formation and brings about puberty changes in boys.
  3. Step 3: Sperms are delivered through the vas deferens.
  4. Step 4: The vas deferens unites with a tube from the urinary bladder; the urethra is a common passage for sperms and urine.
  5. Step 5: Seminal vesicles and prostate gland add secretions to form seminal fluid, which provides nutrition and aids transport.
  6. Step 6: Sperms (with genetic material and a tail for movement) are ejaculated through the penis.
💡 Testes are outside the body because sperm formation requires a lower temperature than normal body temperature.
Human Female Reproductive System — Egg Production and Pregnancy
  1. Step 1: Ovaries produce eggs (ova) and hormones; at birth, ovaries contain thousands of immature eggs.
  2. Step 2: After puberty, one egg matures and is released every month from one ovary.
  3. Step 3: The egg is carried from the ovary to the uterus through the fallopian tube (oviduct).
  4. Step 4: During sexual intercourse, sperms enter through the vagina and travel upwards to the oviduct.
  5. Step 5: If a sperm meets the egg in the oviduct, fertilisation occurs, forming a zygote.
  6. Step 6: The zygote divides to form a ball of cells (embryo) that implants in the lining of the uterus.
  7. Step 7: The embryo develops into a foetus, receiving nutrition from the mother's blood via the placenta.
  8. Step 8: The placenta has villi on the embryo's side and blood spaces on the mother's side for exchange of glucose, oxygen, and waste.
  9. Step 9: The child develops for about nine months and is born through rhythmic contractions of the uterus.
💡 Fertilisation occurs in the fallopian tube, not in the uterus. The embryo implants in the uterus.
Menstrual Cycle
  1. Step 1: The ovary releases one egg every month.
  2. Step 2: The uterus prepares its lining (thickens and becomes spongy with blood supply) to receive a fertilised egg.
  3. Step 3: If the egg is not fertilised, it lives for about one day and then dies.
  4. Step 4: The thickened uterine lining is not needed, so it breaks down and comes out through the vagina as blood and mucous.
  5. Step 5: This cycle repeats roughly every month and lasts about two to eight days.
💡 Menstruation occurs only if fertilisation does not happen. It is the shedding of the uterine lining.
Contraceptive Methods
  1. Step 1: Mechanical barriers — condoms on the penis or coverings in the vagina prevent sperm from reaching the egg.
  2. Step 2: Hormonal pills — taken orally, they change hormonal balance to prevent egg release (ovulation).
  3. Step 3: Intrauterine devices (e.g., copper-T) — placed in the uterus to prevent pregnancy, but may cause side effects due to irritation.
  4. Step 4: Surgical methods — blocking the vas deferens in males or fallopian tubes in females prevents gamete transfer and fertilisation.
💡 Condoms also help prevent sexually transmitted diseases (STDs) to some extent; copper-T does not protect against STDs.
Exam Traps — Don't Make These Mistakes
⚠ Confusing Regeneration with Reproduction
✗ Regeneration is a mode of reproduction because it produces new individuals from body parts.
→ Regeneration is not the same as reproduction because most organisms do not normally depend on being cut up to reproduce. It is a repair mechanism that can incidentally produce new individuals, but it is not a primary reproductive strategy.
✓ Regeneration is the ability of an organism to grow lost body parts or new individuals from pieces, but it is not considered a mode of reproduction because it is not the normal way organisms increase their population.
⚠ Mixing Up Binary Fission and Multiple Fission
✗ Binary fission and multiple fission are the same; both produce two daughter cells.
→ Binary fission produces exactly two daughter cells, while multiple fission produces many daughter cells simultaneously. They are distinct processes.
✓ Binary fission (e.g., Amoeba) produces two daughter cells, while multiple fission (e.g., Plasmodium) produces many daughter cells at once.
⚠ Thinking Pollination and Fertilisation Are the Same
✗ Pollination is the fusion of male and female gametes in plants.
→ Pollination is only the transfer of pollen from anther to stigma; fertilisation is the fusion of male and female gametes inside the ovule. They are separate events.
✓ Pollination is the transfer of pollen grains from the anther to the stigma, while fertilisation is the fusion of the male germ-cell with the female egg cell in the ovule.
⚠ Confusing the Site of Fertilisation in Humans
✗ Fertilisation occurs in the uterus.
→ Fertilisation occurs in the fallopian tube (oviduct), not in the uterus. The zygote then travels to the uterus for implantation.
✓ Fertilisation in humans takes place in the fallopian tube (oviduct).
⚠ Assuming Copper-T Protects Against STDs
✗ Copper-T prevents both pregnancy and sexually transmitted diseases.
→ Copper-T is an intrauterine device that prevents pregnancy by irritating the uterus, but it does not provide any protection against sexually transmitted diseases.
✓ Copper-T prevents pregnancy but does not protect against sexually transmitted diseases. Condoms can help prevent STDs to some extent.
⚠ Thinking All Multicellular Organisms Can Regenerate
✗ All multicellular organisms can reproduce by regeneration if cut into pieces.
→ Only simple multicellular organisms like Hydra and Planaria have the ability to regenerate into complete individuals. Complex multicellular organisms have specialised tissues and organs that cannot regenerate into a whole organism from a piece.
✓ Regeneration is possible only in simple multicellular organisms like Hydra and Planaria, not in complex organisms with specialised tissues and organs.
Compare & Contrast
Number of daughter cells produced
Binary Fission
Multiple Fission
Binary fission produces two daughter cells (e.g., Amoeba), while multiple fission produces many daughter cells simultaneously (e.g., Plasmodium).
Source of pollen
Self-Pollination
Cross-Pollination
Self-pollination involves transfer of pollen from anther to stigma of the same flower, while cross-pollination involves transfer from one flower to another flower of the same species, aided by wind, water, or animals.
Number of parents and genetic variation
Asexual Reproduction
Sexual Reproduction
Asexual reproduction involves a single parent and produces genetically identical offspring (no variation), while sexual reproduction involves two parents and produces offspring with genetic variation due to combination of DNA from two individuals.
Mechanism and normal occurrence
Fragmentation
Regeneration
Fragmentation is a natural mode of reproduction where the body breaks into pieces that grow into new individuals (e.g., Spirogyra), while regeneration is the ability to grow lost parts or new individuals from pieces but is not the normal mode of reproduction (e.g., Planaria).
Exam Focus
Marks pattern: 2 MCQs (1 mark each), 1 Very Short Answer (1 mark), 2 Short Answer (3 marks each), 1 Long Answer (5 marks). Total approximately 13-15 marks.
Most asked: DNA copying and its importance · Variation and its significance for survival of species · Modes of asexual reproduction (fission, budding, fragmentation, spore formation, vegetative propagation) · Sexual reproduction in flowering plants (pollination, fertilisation, seed and fruit formation) · Human male and female reproductive systems (organs and their functions) · Menstruation and its cause · Contraceptive methods and their types · Reproductive health and sexually transmitted diseases
Don't skip: Draw a labelled diagram of the human male or female reproductive system and explain the function of each part (5 marks).; Explain the process of fertilisation and development of embryo in humans (5 marks).; Compare and contrast asexual and sexual reproduction with examples (3 marks).; Describe the various contraceptive methods and their advantages/disadvantages (3 marks).
One-Line Revision (read aloud before the exam)
  1. Reproduction is not essential for an individual's survival but is essential for the continuity of the species.
  2. DNA copying creates variations due to inaccuracies in biochemical reactions, which is the basis for evolution.
  3. Variation is beneficial for the survival of a species when environmental conditions change.
  4. Binary fission in Amoeba can occur in any plane, while in Leishmania it occurs in a definite orientation.
  5. Multiple fission in Plasmodium produces many daughter cells simultaneously.
  6. Fragmentation occurs in simple multicellular organisms like Spirogyra.
  7. Regeneration in Hydra and Planaria is carried out by specialised cells that proliferate and differentiate.
  8. Budding in Hydra involves an outgrowth at a specific site that detaches when mature.
  9. Vegetative propagation allows plants like banana, rose, and jasmine to reproduce even if they have lost the ability to produce seeds.
  10. Spores in Rhizopus are covered by thick walls that protect them until they find a moist surface.
  11. Sexual reproduction involves combining DNA from two individuals, creating new combinations of variants.
  12. Meiosis produces gametes with half the number of chromosomes.
  13. Pollination is the transfer of pollen from anther to stigma; fertilisation is the fusion of gametes in the ovule.
  14. In humans, testes are located in the scrotum outside the body for lower temperature required for sperm formation.
  15. The seminal vesicles and prostate gland add fluid to sperms for nutrition and easier transport.
  16. Fertilisation in humans occurs in the fallopian tube, not in the uterus.
  17. The placenta provides glucose and oxygen to the embryo and removes waste through the mother's blood.
  18. Menstruation occurs when the egg is not fertilised, and the uterine lining is shed as blood and mucous.
  19. Condoms help prevent both pregnancy and sexually transmitted diseases to some extent.
  20. Copper-T prevents pregnancy but does not protect against sexually transmitted diseases.
  21. Surgical methods block the vas deferens in males or fallopian tubes in females to prevent fertilisation.
  22. Prenatal sex determination is prohibited by law to maintain a healthy female-male sex ratio.

MCQs — 10 Questions with Answers

Q1. A student observes under a microscope that an Amoeba cell is dividing into two equal halves. Which of the following best explains why the two daughter cells are similar but not absolutely identical?

  • A. DNA copying is not 100% accurate, leading to slight variations in the DNA copies each time ✓
  • B. The Amoeba deliberately creates variations to ensure survival in changing environments
  • C. The two halves divide unequally, causing one cell to have more DNA than the other
  • D. Variations only occur when environmental conditions are unfavorable

Answer: A — The chapter explicitly states that no biochemical reaction is absolutely reliable, so DNA copying has inherent variations; students often misconceive that reproduction always creates identical copies (option C) or that variations are intentional (option B).

Q2. In a laboratory, a student grows yeast in sugar solution at 37°C for 2 hours and observes budding. If the same culture is suddenly exposed to 50°C, most yeast cells die, but a few survive and multiply. This observation best demonstrates which concept?

  • A. Variation in a population provides some individuals the ability to survive environmental changes ✓
  • B. Yeast can only reproduce through budding at optimal temperatures
  • C. DNA copying stops when temperature increases, preventing reproduction
  • D. Heat-resistant yeast deliberately evolve to handle higher temperatures immediately

Answer: A — The survival of heat-resistant variants illustrates variation's evolutionary advantage; students often wrongly assume all organisms respond identically to stress (option B) or that evolution is intentional (option D).

Q3. Assertion (A): Variation during reproduction is disadvantageous for an individual organism in a stable environment. Reason (R): Variations in DNA copies may result in altered proteins that could make the organism less suited to its current niche. Choose the correct option:

  • A. Both A and R are true and R is the correct explanation of A ✓
  • B. Both A and R are true but R is not the correct explanation of A
  • C. A is true but R is false
  • D. A is false but R is true

Answer: A — Both statements are true: variations can harm individuals in stable niches (A), and the reason is that altered proteins may reduce fitness in that niche (R), making R the correct explanation; students often think variation is always beneficial (misconception).

Q4. A biologist studies a population of bacteria in a temperate lake. When global warming increases water temperature, 99% of the bacteria die within days. Based on the chapter's discussion of variation and niches, what most likely enabled the surviving 1% to persist?

  • A. The surviving bacteria quickly evolved new heat-resistant genes during the temperature change
  • B. Pre-existing genetic variations in the population made a few bacteria naturally heat-tolerant ✓
  • C. The bacteria altered their DNA copying mechanism to produce heat-resistant offspring
  • D. Only the oldest bacteria had accumulated enough mutations to survive heat

Answer: B — Variation exists in populations before environmental stress; survivors possessed pre-existing traits suited to the new niche, not newly evolved genes (option A) or altered mechanisms (option C).

Q5. During an experiment, a student prepares two yeast cultures: Culture A in sugar solution at room temperature and Culture B in the same solution at 5°C. After 24 hours, Culture A shows visible growth with many budding cells, while Culture B shows minimal growth. What does this suggest about the relationship between reproduction and metabolic conditions?

  • A. Yeast can only reproduce at room temperature and cannot survive at low temperatures
  • B. The rate of DNA copying and cell division depends on the metabolic activity enabled by appropriate environmental conditions ✓
  • C. Yeast at 5°C produces different DNA copies than at room temperature
  • D. Cold temperatures prevent variation in DNA, so cells cannot divide

Answer: B — Metabolic activity (driven by temperature) affects the rate of reproduction, not whether it occurs; students often conflate inability to grow with inability to survive (option A) or assume DNA copying changes with temperature (option C).

Q6. Assertion (A): The creation of DNA copies during reproduction is more important than the separation of these copies into different cells. Reason (R): A DNA copy pushed out of a cell without cellular apparatus cannot maintain life processes. Choose the correct option:

  • A. Both A and R are true and R is the correct explanation of A
  • B. Both A and R are true but R is not the correct explanation of A
  • C. A is false but R is true ✓
  • D. Both A and R are false

Answer: C — R is true (explained in chapter), but A is false because both DNA copying AND cell division with apparatus are equally essential for successful reproduction; students often prioritize one process over the other.

Q7. A student observes mould growing on wet bread kept in a dark place for a week. The mould spreads across the bread surface forming visible colonies. What does this observation indicate about the organism's reproductive strategy?

  • A. Moulds reproduce only through binary fission like bacteria
  • B. Moulds can reproduce asexually, creating many individuals from a single parent, allowing rapid population growth ✓
  • C. Moulds require sexual reproduction to colonize new areas on food
  • D. The mould spreads because water prevents its DNA from copying

Answer: B — Rapid mould spread indicates asexual reproduction creating many offspring from one parent; students often confuse mould reproduction with bacterial fission (option A) or assume sexual reproduction is required for colonization (option C).

Q8. Assertion (A): All variations arising during DNA copying in a reproducing cell are beneficial for the survival of the species. Reason (R): Some variations are so drastic that the newborn cell cannot function and dies. Choose the correct option:

  • A. Both A and R are true and R is the correct explanation of A
  • B. Both A and R are true but R is not the correct explanation of A
  • C. A is false but R is true ✓
  • D. A is true but R is false

Answer: C — A is false because harmful variations exist; R is true because lethal variations kill cells; R actually contradicts A rather than supporting it, showing that not all variations are beneficial.

Q9. In a classroom activity, students dissolve sugar in water, add yeast, and observe budding after 2 hours. If a student then adds a chemical that blocks DNA copying in yeast cells, what would most likely happen to the culture after several hours?

  • A. Yeast cells would immediately turn into bacteria to continue reproduction
  • B. Budding would stop because new cells cannot be formed without DNA copies in each daughter cell ✓
  • C. Yeast cells would reproduce faster to compensate for the blocked DNA copying
  • D. The sugar would be consumed faster since energy is not used for reproduction

Answer: B — DNA copying is essential for creating two complete DNA-containing cells; without it, cell division cannot produce functional daughter cells; students often think cells can bypass this requirement (option C) or switch reproduction modes (option A).

Q10. A researcher studying a population of bacteria finds that after several generations of reproduction, the population contains more variation than initially observed. Based on the chapter's explanation of DNA copying, what is the most accurate interpretation?

  • A. Bacteria deliberately increase variations to improve survival chances
  • B. Accumulation of multiple small variations over successive DNA copying events increases diversity in the population ✓
  • C. All variations appear in the first generation of reproduction
  • D. Variation only increases when environmental stress is present

Answer: B — Each DNA copying has small variations; over many generations, these accumulate, increasing population diversity; students often assume variations are intentional (option A) or appear all at once (option C).

Flashcards

Why is DNA copying the basic event in reproduction?

DNA contains the blueprint for body design, and copying it ensures offspring inherit similar features from parents.

What is binary fission?

A mode of asexual reproduction where a unicellular organism divides into two equal daughter cells.

Why are offspring similar but not identical to parents?

DNA copying is not 100% accurate; small variations occur during the copying process, creating subtle differences.

How does variation help a species survive environmental change?

If a population has genetic variants, some individuals may possess traits allowing survival in altered conditions while others perish.

What is the role of cellular apparatus in reproduction?

The cellular apparatus (organelles, cell membrane, etc.) is necessary to maintain life processes in newly created daughter cells.

What is multiple fission?

A mode of asexual reproduction where a unicellular organism divides simultaneously into many daughter cells, not just two.

Why is consistency in DNA copying important for maintaining a niche?

Consistent DNA copying preserves body design features that allow an organism to survive and function in its specific ecological role.

What happens if DNA variations are too drastic during reproduction?

The new cell cannot function properly with its inherited cellular apparatus and will die.

How are organisms in the same species usually identified?

By their similar appearance and body design, which result from inherited DNA blueprints from parents.

What is the relationship between reproduction and population stability?

Reproduction maintains populations of species by creating new individuals; consistency in DNA copying stabilizes population traits.

Important Board Questions

What is the importance of DNA copying in reproduction? [2 marks]

State that DNA is the blueprint for body design and contains inheritance information. Explain that accurate copying ensures offspring maintain similar features to parents and survive in their ecological niche.

Explain why variation is beneficial to species even though it may not always benefit individual organisms. [3 marks]

Explain that variation creates genetic diversity within a population. When environmental conditions change, some variants possess traits allowing survival while others perish—only variations ensure species persistence over time, not immediate individual advantage.

A population of bacteria adapted to temperate water is exposed to drastically increased water temperature. Using the concept of variation, explain how some bacteria might survive this change and what would happen to the overall population over time. [5 marks]

Explain that within any population, small genetic variations exist due to inevitable errors in DNA copying. If heat-resistant variants pre-exist among bacteria, they will survive while heat-sensitive ones die. Over time, reproduction of surviving heat-resistant variants will shift the population toward heat-resistance. This demonstrates how variation—seemingly small differences—provides evolutionary advantage when environmental niches change, ensuring species adaptation and survival.

Assertion-Reason Questions for How do Organisms Reproduce?

Q1. Assertion (A):

DNA copying is the most basic and essential event in reproduction.

Reason (R):

DNA contains all the information required for inheritance of body design features from parents to offspring.

Answer: A — Both A and R are true, and R is the correct explanation of A — Both statements are true, and the reason correctly explains why DNA copying is essential. DNA copying is basic because DNA carries hereditary information, making it the foundation of reproduction.

Q2. Assertion (A):

Variation in populations is more important for species survival than consistency in DNA copying.

Reason (R):

Both genetic consistency and variation are important for species survival in different environmental conditions.

Answer: B — Both A and R are true, but R is NOT the correct explanation of A — Both statements are true. Consistency maintains body design features suited to current niches, while variation allows adaptation to changing environments. However, the reason doesn't fully explain why variation alone would be more important than consistency; both are equally critical.

Q3. Assertion (A):

Yeast and Amoeba use identical modes of reproduction.

Reason (R):

Yeast reproduces by budding while Amoeba reproduces by binary fission in any plane.

Answer: D — A is false but R is true — Both the assertion and reason are false. They do not use identical modes—budding and binary fission are different processes. The reason correctly describes each organism's mode, proving the assertion is false.

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Case-Based Questions (CBQ) for How do Organisms Reproduce?

Case Study:

A microbiology student observed bacterial cultures in a laboratory. In Petri dish A, normal Escherichia coli bacteria were grown at 37°C in nutrient broth. In Petri dish B, the same bacteria were exposed to a sudden temperature increase to 50°C. After 24 hours, Petri dish A showed dense bacterial growth covering the entire surface, while Petri dish B showed only a few isolated colonies scattered across the plate. When the surviving colonies from Petri dish B were isolated and cultured, they grew normally even at 50°C, suggesting they had acquired heat-resistant variations.

Q1. What does the difference in bacterial growth between dishes A and B demonstrate?

  • A. All bacteria can survive temperature changes
  • B. Environmental stress eliminates most organisms but selects for variants with advantageous variations ✓
  • C. Temperature has no effect on bacterial reproduction
  • D. Heat always increases bacterial reproduction

Answer: B

Q2. Why did the surviving colonies from dish B maintain heat resistance in subsequent cultures?

  • A. They learned to survive heat
  • B. Their DNA variations conferring heat resistance were copied during reproduction ✓
  • C. Heat changed their environmental preferences permanently
  • D. Temperature alone provides resistance without genetic changes

Answer: B

Q3. This experiment best illustrates which principle from the reproduction chapter?

  • A. All organisms reproduce identically
  • B. DNA copying is always 100% accurate
  • C. Variation is beneficial for species survival when environments change ✓
  • D. Reproduction requires no energy consumption

Answer: C

Q4. If the student continued culturing the heat-resistant bacteria at 37°C for many generations, what would most likely occur?

  • A. They would gradually lose heat resistance ✓
  • B. They would become more heat-resistant
  • C. Heat resistance would become permanent regardless of temperature
  • D. No change would occur in their characteristics

Answer: A

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Previous Year Questions (PYQ) for How do Organisms Reproduce?

Real CBSE board exam questions from past papers — free to attempt.

CBSE 2026 · Set 31/4/1 · 1 mark

If the part 'P' of the flower is removed, which of the following processes will not take place?

  • Formation of pollens
  • Formation of flower
  • Formation of fruit
  • Formation of seed

CBSE 2026 · Set 31/4/1 · 1 mark

Assertion (A): Variations are not often seen in off-springs produced by asexual reproduction. Reason (R): DNA molecule generated by replication is not similar to the original DNA.

  • Both Assertion (A) and Reason (R) are true and Reason (R) is the correct explanation of the Assertion (A).
  • Both Assertion (A) and Reason (R) are true, but Reason (R) is not the correct explanation of the Assertion (A).
  • Assertion (A) is true, but Reason (R) is false.
  • Assertion (A) is false, but Reason (R) is true.

CBSE 2026 · Set 31/4/2 · 1 mark

Assertion (A): Reproduction, like any other life process, is essential to maintain the life of an individual organism. Reason (R): Reproduction involves creation of a DNA copy and additional cellular apparatus by the cell involved in the process.

  • Both Assertion (A) and Reason (R) are true and Reason (R) is the correct explanation of the Assertion (A).
  • Both Assertion (A) and Reason (R) are true, but Reason (R) is not the correct explanation of the Assertion (A).
  • Assertion (A) is true, but Reason (R) is false.
  • Assertion (A) is false, but Reason (R) is true.

CBSE 2026 · Set 31/5/1 · 1 mark

In human beings, the implantation of fertilised egg takes place in which part of female reproductive system?

  • Oviduct
  • Cervix
  • Uterus
  • Vagina

CBSE 2026 · Set 31/5/1 · 5 marks

(A) (i) In an angiospermic flower, fertilization is said to take place when male gametes carried in the pollen tube fuse with the female gamete, present in the embryo sac. This results in lot of changes in different parts of the flower. State the changes taking place in the following structures of flower post fertilization: (a) Zygote (b) Ovule (c) Ovary (d) Sepals (ii) Define germination.

CBSE 2026 · Set 31/5/2 · 1 mark

In human beings, the implantation of fertilised egg takes place in which part of female reproductive system ?

  • Oviduct
  • Cervix
  • Uterus
  • Vagina

CBSE 2026 · Set 31/5/2 · 1 mark

Yeast multiplies by which of the following methods ?

  • Fragmentation
  • Binary Fission
  • Multiple Fission
  • Budding

CBSE 2026 · Set 31/5/2 · 1 mark

Assertion (A) : Bacteria produced as a result of asexual reproduction would be similar. Reason (R) : There would be only minor differences between the bacteria generated due to small inaccuracies in DNA replication.

  • Both Assertion (A) and Reason (R) are true and Reason (R) is the correct explanation of the Assertion (A).
  • Both Assertion (A) and Reason (R) are true, but Reason (R) is not the correct explanation of the Assertion (A).
  • Assertion (A) is true, but Reason (R) is false.
  • Assertion (A) is false, but Reason (R) is true.

CBSE 2026 · Set 31/5/2 · 5 marks

(A) (i) Sugarcane does not produce seeds so name the process through which it will be able to reproduce. (ii) List any two advantages of this method. (iii) Besides sugarcane, give two more examples of plants that reproduce by this method. (iv) Why regeneration is not possible in all the types of animals ?

CBSE 2026 · Set 31/5/3 · 5 marks

(A) (i) Sugarcane does not produce seeds so name the process through which it will be able to reproduce. (ii) List any two advantages of this method. (iii) Besides sugarcane, give two more examples of plants that reproduce by this method. (iv) Why regeneration is not possible in all the types of animals?

CBSE 2026 · Set 31/2/1 · 1 mark

Spirogyra, despite being a multi-cellular organism, still shows a simple reproductive method. Select the correct option with regard to reproduction in Spirogyra:

  • Asexual reproduction by division of cells into two cells
  • Sexual reproduction by division of cells into many cells
  • Asexual reproduction by breaking up of Spirogyra filaments into smaller parts
  • Sexual reproduction by the formation of gametes after meiosis of cells of filaments followed by fusion of male and female gametes

CBSE 2026 · Set 31/2/1 · 5 marks

(a) (i) Name an organ that helps in the formation of male germ cells in humans and also mention the condition that favours the formation of male germ cells. (ii) In male reproductive system, write the pathway followed to deliver male germ cells to the site of fertilisation. (iii) Mention the main physical feature of the male germ cell that helps it move towards the female germ cell.

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Class 10 Science Formula Sheet

Every formula and chemical equation from the 10 chapters on one page, checked against the NCERT textbook and the CBSE 2026-27 syllabus. Free PDF download.

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Class 10 Science Previous Year Questions, Chapter-wise

All 875 CBSE board questions from 2022–2026, sorted by chapter and question type. Free PDF download.

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