Reproduction Class 9 Notes NCERT

Introduction: What is reproduction?

Reproduction is the biological process by which living organisms produce new individuals of their own kind. It helps maintain the continuity of a species from one generation to the next.

Every organism has a life cycle. It is born, grows, reaches maturity, and eventually dies. Reproduction ensures that the species can continue even after individual organisms die.

Examples:

• Mango trees produce seeds that can grow into new mango plants.

• Yeast reproduces by budding.

• Many bacteria reproduce by binary fission.

• Humans reproduce sexually and give birth to babies.

Why is reproduction important?

1. Continuity of species: It prevents a species from disappearing simply because existing individuals die.

2. Transfer of hereditary information: Offspring receive genetic information from their parent or parents.

3. Variation: Sexual reproduction creates differences among offspring, helping populations adapt to changing environments over many generations.

4. Maintenance of populations: Reproduction produces new individuals to replace those that die.

Important: Reproduction is essential for the continuation of a species, but it is not necessary for the survival of an individual organism.

1. Modes of reproduction

Living organisms reproduce mainly by two methods:

Reproduction

Asexual reproduction

Usually one parent; no fusion of gametes

A. Asexual reproduction

Asexual reproduction is a type of reproduction in which a new organism develops from a single parent without the fusion of male and female gametes.

The offspring are generally genetically very similar to the parent, although mutations can introduce differences.

Characteristics

• Usually involves one parent.

• Does not involve fertilisation.

• Often occurs rapidly under favourable conditions.

• Produces offspring with little genetic variation, apart from changes such as mutations.

• Common in many microorganisms, some simple animals and plants.

Types of asexual reproduction

1. Binary fission

In binary fission, one parent cell divides to form two daughter cells.

Example: Amoeba and many bacteria.

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Process:

1. The genetic material is copied.

2. The nucleus or genetic material divides.

3. The cytoplasm divides.

4. Two daughter cells are formed.

2. Multiple fission

In multiple fission, the parent cell produces several daughter cells in a single reproductive episode after repeated nuclear division.

Example: Plasmodium under suitable conditions.

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3. Budding

In budding, a small outgrowth called a bud develops on the parent. The bud grows and may separate to form a new individual.

Examples: Yeast and Hydra.

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In yeast: A small bud forms on the parent cell, grows, and separates or may remain attached temporarily.

In Hydra: A bud develops through cell division on the body wall and eventually separates as a young Hydra.

4. Fragmentation

Fragmentation occurs when an organism breaks into pieces and each suitable fragment grows into a new individual.

Example: Spirogyra, a filamentous green alga.

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This method works in organisms whose body structure allows the fragments to develop into complete individuals.

5. Regeneration

Regeneration is the ability to regrow lost or damaged body parts. In certain organisms, a body fragment can develop into a complete new individual.

Examples: Planaria and certain other simple animals.

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Remember: Regeneration and reproduction are not identical in every organism. The ability to regrow a lost part does not always mean that a whole new organism can form.

6. Spore formation

In spore formation, an organism produces tiny reproductive structures called spores. Under favourable conditions, these spores can germinate and develop into new organisms.

Example: Rhizopus, commonly called bread mould.

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Process:

1. Spores develop inside structures called sporangia.

2. The sporangia release spores.

3. Spores spread through air or other agents.

4. Under suitable conditions of moisture, temperature and nutrients, spores germinate.

The thick protective covering of many spores helps them survive unfavourable conditions.

B. Vegetative propagation in plants

Vegetative propagation is a form of asexual reproduction in which new plants develop from vegetative parts such as stems, roots or leaves rather than from seeds.

Examples:

Artificial methods of vegetative propagation

1. Cutting: A piece of stem or another suitable plant part is planted so it can produce roots and shoots. Examples include rose and sugarcane.

2. Layering: A branch is encouraged to develop roots while still attached to the parent plant. It can then be separated and planted independently.

3. Grafting: A shoot or bud from one plant, called the scion, is joined to a rooted plant, called the stock. They grow together as one plant.

4. Tissue culture: Small pieces of plant tissue are grown under sterile, controlled conditions on a nutrient medium to produce new plants.

Advantages of vegetative propagation

• Many plants can be produced relatively quickly.

• Desirable characteristics of the parent plant can be maintained.

• It is useful for plants that produce few viable seeds or do not reproduce reliably through seeds.

• Some plants propagated this way flower or bear fruit earlier than seed-grown plants.

Limitation: Since the plants are genetically very similar, a disease or environmental change that affects one may affect many of them.

C. Sexual reproduction

Sexual reproduction involves the formation of male and female reproductive cells, called gametes, and usually their fusion during fertilisation.

The fertilised cell is called a zygote. It develops into an embryo and eventually a new organism.

Characteristics

• Involves the formation of gametes.

• Fertilisation usually involves the fusion of two gametes.

• In humans and many animals, male and female individuals produce the gametes.

• In flowering plants, male gametes are carried in pollen and the female gamete is present in the ovule.

• Offspring inherit genetic information from both gametes, creating genetic variation.

Asexual vs sexual reproduction

2. Cell division and variation

Cell division is important for growth, repair and reproduction.

Mitosis

Mitosis is a type of cell division in which one parent cell produces two daughter cells with essentially the same chromosome number and, ordinarily, the same genetic information.

It is important in:

• Growth and development.

• Replacement of worn-out cells.

• Tissue repair.

• Many forms of asexual reproduction.

Meiosis

Meiosis is a specialised type of cell division that reduces the chromosome number by half to produce haploid cells.

It is important in sexual reproduction because it helps maintain the characteristic chromosome number of a species across generations when gametes fuse.

During meiosis, genetic material is reshuffled, creating new combinations of genes.

Why is variation important?

It means differences in characteristics among individuals of the same species.

Variation can arise through:

• Changes in DNA called mutations.

• The reshuffling of genetic material during meiosis.

• The combination of genetic material from two gametes during fertilisation.

It may help some individuals survive when environmental conditions change. Over many generations, natural selection acting on heritable variation can contribute to evolution.

Key point: Variation does not guarantee that an organism will survive. Its usefulness depends on the environment.

3. Sexual reproduction in flowering plants

Flowers are the reproductive structures of flowering plants. Many flowers contain both male and female reproductive parts, although some flowers have only male or only female parts.

A. Structure of a flower

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A typical flower has four main groups of parts.

Parts of the stamen

• Anther: Produces pollen grains.

• Filament: Supports the anther.

Parts of the pistil (carpel)

• Stigma: Receives pollen.

• Style: Connects the stigma to the ovary.

• Ovary: Contains one or more ovules.

• Ovule: Contains the female reproductive cell and supporting tissues.

B. Pollination

Pollination is the transfer of pollen grains from the anther to the stigma of a flower.

There are two major types.

1. Self-pollination

Pollen is transferred to the stigma of the same flower or another flower on the same plant, depending on the type of self-pollination.

2. Cross-pollination

Pollen is transferred from a flower on one plant to a flower on another plant of the same species.

Agents of pollination

Pollination may occur through:

• Wind: Carries pollen grains from one flower to another.

• Water: Transfers pollen in certain aquatic plants.

• Insects: Bees, butterflies and other insects carry pollen while visiting flowers.

• Other animals: Birds and bats can pollinate some flowering plants.

Flowers pollinated by insects often have attractive petals, scents or nectar. Wind-pollinated flowers commonly produce abundant, lightweight pollen.

C. Fertilisation in flowering plants

Fertilisation is the fusion of male and female gametes to form a zygote.

The process occurs as follows:

1. A pollen grain lands on a compatible stigma.

2. The pollen grain germinates and forms a pollen tube.

3. The pollen tube grows through the style towards an ovule.

4. A male gamete travels through the pollen tube.

5. In the ovule, the male gamete fuses with the female gamete.

6. A zygote forms and develops into an embryo.

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D. What happens after fertilisation?

In a typical flowering plant:

• The zygote develops into an embryo.

• The ovule develops into a seed.

• The ovary usually develops into a fruit.

• The seed contains the embryo and a protective covering; stored food or nutritive tissue supports the embryo in many seeds.

Exam tip: Do not confuse pollination with fertilisation. Pollination is the transfer of pollen; fertilisation is the fusion of gametes.

E. Seed dispersal

Seed dispersal is the movement of seeds away from the parent plant. It reduces overcrowding and competition for sunlight, water, space and nutrients.

Seeds generally germinate when conditions such as moisture, oxygen and a suitable temperature are available. Some seeds also require particular light conditions or a period of dormancy to end.

4. Sexual reproduction in animals

In many animals, the male reproductive system produces sperm and the female reproductive system produces ova (eggs).

A. Gametes

Sperm: The male reproductive cell. It is generally small and motile in humans.

Ovum: The female reproductive cell. It is larger than a sperm cell and contains stored materials that support early development.

Gametes are haploid, meaning they contain one set of chromosomes. When a sperm and an ovum fuse, the resulting zygote typically has two sets.

B. Fertilisation

There are two broad types of fertilisation in animals.

External fertilisation: Fusion of gametes occurs outside the female’s body, commonly in water.

Examples: Many fish and amphibians, such as frogs.

Internal fertilisation: Fusion of gametes occurs inside the female reproductive tract.

Examples: Humans, birds, reptiles and many other mammals.

After fertilisation, the zygote divides repeatedly and develops into an embryo. Further development eventually produces a young organism.

5. Reproduction in human beings

Humans reproduce sexually. The male reproductive system produces sperm, while the female reproductive system produces ova and supports the development of the embryo and foetus during pregnancy.

Puberty is the stage when the body undergoes changes that lead to reproductive maturity. The timing of puberty varies among individuals.

A. Male reproductive system

Main parts and functions:

• Testes: Produce sperm and the hormone testosterone.

• Scrotum: Holds the testes outside the main body cavity and helps maintain a suitable temperature for sperm production.

• Sperm ducts (vas deferens): Carry sperm.

• Seminal vesicles and prostate gland: Add fluids that contribute to semen and support sperm function.

• Urethra: Passage through which urine leaves the body; in males, it also carries semen during ejaculation, at a different time.

• Penis: Organ through which semen is released during ejaculation.

Sperm production begins at puberty and can continue throughout much of adult life, although fertility may change with age and health.

B. Female reproductive system

Main parts and functions:

• Ovaries: Produce ova and hormones, including oestrogen and progesterone.

• Oviducts (fallopian tubes): Carry the ovum towards the uterus; fertilisation commonly occurs here.

• Uterus: Muscular organ where an embryo can implant and develop.

• Cervix: Narrow lower part of the uterus that opens into the vagina.

• Vagina: Muscular canal connecting the cervix to the outside of the body; it receives sperm during sexual intercourse and serves as the birth canal.

C. Puberty and reproductive maturity

Puberty involves hormonal changes that lead to reproductive maturity and the development of secondary sexual characteristics.

In boys, changes may include:

• Growth of facial and body hair.

• Deepening of the voice.

• Increased muscle development.

• Growth of the reproductive organs.

In girls, changes may include:

• Breast development.

• Growth of pubic and underarm hair.

• Changes in body shape.

• Beginning of menstruation.

Both boys and girls experience growth spurts, changes in body composition and emotional development. These changes occur at different rates in different people.

6. The menstrual cycle

The menstrual cycle is a series of cyclic changes in the ovaries and uterus. Its purpose is to prepare the body for a possible pregnancy.

A cycle is often described as lasting about 28 days, but normal cycles can be shorter or longer and may vary, especially during adolescence.

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Main stages of the menstrual cycle

1. Menstruation

If pregnancy has not occurred, the uterine lining breaks down and is discharged through the vagina along with blood and tissue.

2. Follicular phase

A follicle in an ovary develops around an egg. Oestrogen levels rise, and the uterine lining begins to thicken.

3. Ovulation

An ovary releases an egg. In a typical 28-day cycle, this occurs around day 14, but the timing varies.

4. Luteal phase

The structure left after ovulation produces progesterone, which helps maintain the uterine lining. If pregnancy does not occur, hormone levels fall, and the next menstruation begins.

What happens if fertilisation occurs?

If a sperm fertilises an ovum, a zygote forms. As it develops, it travels towards the uterus. Implantation occurs when the developing embryo attaches to the uterine lining. Hormonal changes then help maintain the pregnancy.

If fertilisation does not occur, the uterine lining is usually shed during menstruation.

Important: Menstruation is not a disease. It is a normal biological process associated with the reproductive cycle.

7. Fertilisation, pregnancy and development

Steps from fertilisation to birth

Placenta and umbilical cord

The placenta develops during pregnancy and forms an interface between the mother’s blood supply and the developing foetus.

Its functions include:

• Supplying oxygen and nutrients to the foetus.

• Allowing carbon dioxide and many waste products to pass from the foetus to the mother.

• Producing hormones that help maintain pregnancy.

The umbilical cord connects the foetus to the placenta. Maternal and foetal blood normally do not mix directly, although substances can pass between their circulations through the placenta.

8. Reproductive health and responsible choices

Reproductive health involves physical, mental and social well-being in matters related to reproduction.

Important concepts include:

• Understanding puberty and normal body changes.

• Maintaining personal hygiene.

• Respecting personal boundaries, consent and privacy.

• Knowing that pregnancy can be prevented through contraception.

• Understanding that condoms help reduce the risk of many sexually transmitted infections (STIs) as well as pregnancy.

• Seeking accurate health information and qualified medical advice when needed.

Methods of contraception

Contraception refers to methods used to prevent pregnancy.

No method is suitable for everyone. Correct use, effectiveness, health considerations and access to medical advice matter when making choices.

Sexually transmitted infections

STIs are infections that can spread through sexual contact. Some examples include HIV infection, syphilis, gonorrhoea and chlamydia.

Prevention includes correct condom use, testing when appropriate, vaccination against preventable infections such as HPV and hepatitis B, and timely medical care. Many STIs can be treated, while some require ongoing management.