10.4 Reproduction

10.4 Reproduction

Why It Matters

All living things eventually die, so they must produce new individuals of their own kind to keep the species alive. This process is called reproduction. There are two main types: asexual (one parent; offspring are genetically identical — clones) and sexual (two parents; offspring show variation). The balance between speed (asexual) and variety (sexual) is a key theme in RRB questions.

Asexual Reproduction

A single parent produces offspring that are genetically identical to itself (clones). It is fast and useful in stable environments.

Vegetative propagation is very useful agriculturally — banana, jasmine, orange and rose are commonly grown from cuttings, buds or grafts to preserve desirable qualities. Tissue culture (growing new plants from small pieces of tissue in a nutrient medium) is a modern commercial method based on this.

Sexual Reproduction

Sexual reproduction involves the fusion of two specialised cells called gametes — a small, motile male gamete (sperm in animals, pollen nucleus in plants) and a larger, food-containing female gamete (egg or ovum). Their fusion is called fertilization, producing a single cell called the zygote, which divides repeatedly to form the embryo.

Because offspring get half their genetic material from each parent, they show variation, which is the basis of evolution. Sexual reproduction is favoured in changing environments because variation gives some individuals a better chance of surviving.

Sexual Reproduction in Flowering Plants

The flower is the reproductive organ of a flowering plant. Key parts:

• Sepals (calyx) — protect the flower in bud.
• Petals (corolla) — often colourful to attract pollinators.
• Stamen (male part) = anther (produces pollen grains, the male gamete) + filament (stalk).
• Carpel / Pistil (female part) = stigma (sticky top, catches pollen) + style (tube) + ovary (contains ovules, the female gamete).

Pollination is the transfer of pollen grains from the anther to the stigma. It is of two types:

• Self-pollination — pollen reaches the stigma of the same flower or another flower on the same plant.
• Cross-pollination — pollen reaches the stigma of a different plant of the same species, usually with the help of wind, water or animals (insects, birds). Cross-pollination produces more variation.

After pollination, the pollen grain grows a pollen tube down the style to the ovule, and the male nucleus fuses with the female nucleus inside the ovule — this is fertilization. After fertilization, the ovule becomes the seed and the ovary becomes the fruit.

Human Reproductive System

Male system: a pair of testes (which make sperm and testosterone, located outside the body in the scrotum to keep them cooler than body temperature), sperm ducts (vas deferens), glands (prostate and seminal vesicles, which add fluid to make semen) and the penis.

Female system: a pair of ovaries (which make eggs and the hormones estrogen and progesterone), fallopian tubes (oviducts — the site of fertilization; the egg travels along them to the uterus), the uterus (womb — where the fertilized egg implants and develops into a baby) and the vagina (birth canal).

Menstrual cycle: the ovary releases one egg (ovulation) about every 28 days. If the egg is fertilized, it implants in the thickened lining of the uterus and pregnancy begins. If not fertilized, the thick lining breaks down and is shed along with blood — this is menstruation (periods), which lasts about 3–5 days.

Reproductive Health

Reproductive health means having a safe, responsible and satisfying reproductive life. Important points:

• Contraception methods prevent pregnancy: mechanical (condom, diaphragm), chemical (oral pills), intra-uterine devices (Copper-T) and surgical (vasectomy in males, tubectomy in females — permanent).
• Condoms also give protection against STDs, unlike other methods.
• Sexually transmitted diseases (STDs) are infections spread through sexual contact. Examples: HIV/AIDS, syphilis, gonorrhoea, genital herpes, chlamydia, hepatitis B. AIDS is caused by HIV (Human Immunodeficiency Virus), which attacks and destroys the immune system, so the body cannot fight other infections.
• Women's health: female foeticide (killing female foetuses) is illegal in India. The Pre-Conception and Pre-Natal Diagnostic Techniques (PC & PNDT) Act bans sex determination of the foetus.

Exam Traps

1. Self-pollination vs cross-pollination: self-pollination gives less variation; cross-pollination gives more variation and is the main route for evolution of flowers.
2. Pollination vs fertilization: pollination is just the transfer of pollen to the stigma; fertilization is the fusion of male and female nuclei later.
3. Testes are outside the body (in the scrotum) because sperm need a temperature slightly lower than body temperature.
4. Vasectomy / tubectomy are permanent methods — don't confuse them with temporary methods like the pill or condom.
5. HIV (virus) causes AIDS (disease). Don't confuse the virus name with the disease name.