Why is reproduction important?

1. It keeps the species going on earth (continuity of species) and keeps the population stable.
2. It creates variation in DNA, which gives stability to a species.

Do organisms make exact copies of themselves?

No, not always. Inside a cell, DNA (Deoxyribo Nucleic Acid) is the genetic material present in the nucleus. It carries the information to build a body. When a cell divides, it first makes a copy of its DNA. The copying is not perfect — small errors happen. These errors cause small changes, and those changes lead to variation.

How variation is born (step by step)

Example: all roses are roses, but they come in many colours and shapes.

Importance of variation

1. Survival when the environment changes.
2. Prevents extinction of a species → gives stability.
3. Helps in the evolution of species.
4. Variation in DNA gives different varieties of a species and the formation of new species.

Example: bacteria in water and global warming

6 modes of asexual reproduction

In unicellular organisms (single-celled), cell division or fission is the way of reproduction. It happens in many bacteria and protozoa.

🦠 Amoeba (unicellular) – Binary fission

1. Binary fission in amoeba.
2. Splitting of the cell can happen in any plane (any direction).
3. The parent cell divides into two daughter cells.

🦠 Leishmania (unicellular)

1. Has a whip-like structure (flagellum) at one end of the cell.
2. Binary fission happens in a fixed plane in relation to the whip-like structure — it is longitudinal fission.
3. Causes kala-azar (black fever).

🦠 Plasmodium (unicellular)

1. Divides by multiple fission (one cell splits into many daughter cells at once).
2. It is the malarial parasite (causes malaria).

Spirogyra is a green, thread-like (filamentous) multicellular alga found in ponds. When it matures, it breaks into smaller pieces (fragments). Each fragment grows into a new individual.

• If the individual is cut into many pieces, each piece can grow into a separate individual.
• Regeneration is carried out by specialised cells.
• Examples: Planaria (flatworm) and Hydra.

Hydra is a small aquatic (water) animal. It uses its regenerative cells for reproduction.

Steps of budding

Budding in Yeast

Yeast (a single-celled fungus) also reproduces by budding: a small bud grows out of the parent cell, grows bigger and then breaks off.

1. Rhizopus (bread mould) is a fungus.
2. Spores are covered by thick walls that protect them in unfavourable conditions.
3. In favourable conditions (moisture, warm temperature and nutrition) the spores germinate and grow into new Rhizopus.

Parts you should know (under the microscope)

Parts like roots, stem and leaves develop into new plants.

Natural methods

1. Stem – potato, ginger, onion
2. Root – sweet potato
3. Leaves – Bryophyllum (buds grow in the notches along the leaf edge, fall on the soil and grow into new plants)

Artificial methods

Advantages of vegetative propagation

1. Plants bear flowers and fruits earlier than plants grown from seeds.
2. A faster, easier and cheaper way to multiply plants.
3. Layering / grafting is used to grow plants like sugarcane, roses, grapes.
4. Makes it possible to grow plants that have lost the ability to make seeds (banana, orange, rose, jasmine).
5. New plants are genetically similar to the parent plant and keep its characteristics.
6. The desirable character of a fruit can be maintained.

Why do we need the sexual mode of reproduction?

1. It combines variations from two parents of the same species.
2. It increases the chances of survival by creating more variety in a population.

From gametes to a new organism

Chromosome numbers (why we need meiosis)

Human body cells have 23 pairs of chromosomes = 46 chromosomes.

The flower is the reproductive organ of a plant. Example: Hibiscus (China rose).

Bisexual and Unisexual flowers

♂ Stamen – male reproductive part

• Anther (the bag) – produces pollen grains, which contain the male gamete (sex cell). Pollen grains have a thick outer layer.
• Filament – the stalk that holds (bears) the anther.

♀ Pistil – female reproductive part

• Stigma – top part; sticky, so it receives pollen grains during pollination.
• Style – long tube-like part that connects stigma and ovary.
• Ovary – swollen bottom part of the pistil (later becomes the fruit).
• Ovule – present inside the ovary; contains the female gamete (egg); site of fertilisation; later becomes the seed. An ovary can have one or more ovules.

Steps

After fertilisation

Parts of a seed

• Seed coat – protects the seed.
• Cotyledon – stores food for the baby plant.
• Plumule – future shoot.
• Radicle – future root.

Agents of pollination: wind, water and animals (insects, birds).

• It usually happens between ages 10–14 for girls and 12–16 for boys.
• It causes physical changes and affects boys and girls differently.

Changes during puberty

Male gamete = sperm.   Sperm + fluid = semen.

Female gamete = egg cell / ovum.

What happens after sperm reaches the egg?

Male gamete vs Female gamete

Placenta

1. A disc-shaped tissue that develops inside the uterus during pregnancy.
2. It is attached to the uterus lining and to the umbilical cord.
3. It has villi which give a large surface area for the exchange of substances between mother and the developing foetus.
4. Placenta helps in:
   • Passing glucose and oxygen from mother to the developing embryo.
   • Passing wastes from the embryo to the mother through the umbilical cord.

Menstruation

• The ovary releases one egg every month.
• The uterus also prepares itself every month to receive a fertilised egg.
• So its lining becomes thick and spongy to nourish the embryo if fertilisation has taken place.

Life stages

Contraceptive methods help to prevent pregnancy. There are four types: