Class 10 › Science › Chapter 7
How do Organisms Reproduce?
DNA copying and variation, asexual modes of reproduction, sexual reproduction in flowering plants and in human beings, and reproductive health.
1. Reproduction, DNA Copying and Variation
- Reproduction is not essential to keep an individual alive (unlike nutrition or respiration), but it creates new individuals that look like the parents because of similar body designs.
- DNA in the chromosomes (nucleus) carries the information for making proteins. A basic event in reproduction is making a copy of DNA, along with an additional cellular apparatus, so one cell divides into two.
- DNA copying is not perfectly accurate, so copies are similar but not identical. Variations that are too drastic make the cell die; the rest give subtly different survivors. This tendency for variation is the basis of evolution.
Stability
Consistent DNA copying
Maintains the body design that lets a population fill its niche, so reproduction links to the stability of species.
Variation
Survival over time
If the niche changes (e.g. heated water), a few variants (heat-resistant bacteria) may survive. Variation is useful to the species, not necessarily to the individual.
2. Modes of Asexual Reproduction
Asexual reproduction: a new generation is created from a single individual.
| Mode | How it works | Examples |
|---|---|---|
| Binary fission | Cell splits into two equal halves. In Amoeba, in any plane; in Leishmania (whip-like structure), in a definite orientation. | Amoeba, Leishmania, bacteria |
| Multiple fission | Divides into many daughter cells at once. | Plasmodium (malarial parasite) |
| Budding (unicellular) | Small buds separate and grow. | Yeast |
| Fragmentation | Simple multicellular organism breaks into pieces on maturation; each grows into a new individual. | Spirogyra |
| Regeneration | Cut pieces grow into complete organisms, using specialised cells that proliferate and form tissues in an organised sequence (development). | Hydra, Planaria |
| Budding (Hydra) | An outgrowth forms by repeated cell division at one site, grows into a tiny individual and detaches. | Hydra |
| Vegetative propagation | Root, stem or leaf develops into a new plant. | Potato (buds in notches), Bryophyllum (leaf-margin buds), money plant, sugarcane, rose, grapes |
| Spore formation | Sporangia release spores with thick protective walls; they grow when they land on a moist surface. | Rhizopus (bread mould) |
- Regeneration is not reproduction: most organisms do not depend on being cut up to reproduce.
- Why complex organisms cannot divide cell by cell: tissues and organs are organised at definite positions in the body.
- Vegetative propagation advantages: plants (banana, orange, rose, jasmine) that have lost the ability to make seeds can still multiply; plants flower and fruit earlier; offspring are genetically similar to the parent. Methods: layering, grafting.
- Tissue culture: cells from the growing tip are grown on an artificial medium into a callus, then moved to a medium with hormones, and finally into soil. Gives many disease-free plants from one parent (ornamental plants).
- Rhizopus: hyphae (thread-like) are not reproductive; the sporangia (blobs on sticks) hold the spores.
3. Why Sexual Reproduction?
- DNA copying alone makes variation slowly. Sexual reproduction combines DNA of two individuals, creating new combinations of variants and so more variation, faster.
- Problem: combining two DNA sets would double the DNA each generation. Solution: germ cells (gametes) are made by meiosis, with half the chromosomes and DNA of body cells. Fusion restores the normal number.
- In simple organisms the two gametes are similar. In complex ones they specialise: the male gamete is smaller and motile; the female gamete is large with food stores.
- Zygote: the cell formed by fusion of the male and female gametes.
4. Sexual Reproduction in Flowering Plants
Stamen
Male part
Anther makes yellowish pollen grains (carrying the male germ cell).
Pistil
Female part (centre)
Stigma (sticky top), style (middle tube), ovary (swollen base) containing ovules; each ovule has an egg cell.
Flower types
Unisexual vs bisexual
Unisexual (stamens or pistil only): papaya, watermelon. Bisexual (both): Hibiscus, mustard.
| Process | Meaning |
|---|---|
| Self-pollination | Pollen transferred from stamen to stigma of the same flower |
| Cross-pollination | Pollen transferred to another flower (by wind, water or animals) |
| Fertilisation | Fusion of male and female gametes to form the zygote |
From pollination to seed
Pollen lands on a suitable stigma and grows a pollen tube through the style to the ovary (towards the ovule).
The male gamete fuses with the egg in the ovule: fertilisation gives a zygote.
The zygote divides to form an embryo in the ovule. The ovule becomes a seed (tough coat); the ovary becomes a fruit. Petals, sepals, stamens, style and stigma may wither and fall.
The seed grows into a seedling under suitable conditions: germination.
- Pollination vs fertilisation: pollination is the transfer of pollen to the stigma; fertilisation is the fusion of gametes (after the pollen tube reaches the ovule).
- Functions of petals and sepals: as in the NCERT text they are not reproductive; they attract pollinators and protect the flower.
5. Reproduction in Human Beings: Puberty
Puberty: the period of adolescence when general body growth slows and the reproductive tissues mature (sexual maturation).
Both
Common changes
Thick hair in armpits and genital area; thinner hair on legs, arms and face; oily skin and pimples; new awareness of the body.
Girls
Changes
Breast size increases, nipple skin darkens; menstruation begins.
Boys
Changes
Thick facial hair; voice cracks; occasional enlargement and erection of the penis.
- Changes are slow, gradual, vary between people and are triggered by hormones (testosterone in males, oestrogen in females).
- Signs of sexual maturity are visible to others, which helps in mating.
6. Male Reproductive System
| Part | Function |
|---|---|
| Testes (in the scrotum) | Produce sperms; secrete testosterone. Outside the abdomen because sperm formation needs a lower temperature. |
| Vas deferens | Carries sperms; joins a tube from the urinary bladder. |
| Seminal vesicles and prostate gland | Add secretions that form a fluid, making sperm transport easier and giving nutrition. |
| Urethra | A common passage for sperms and urine. |
| Penis | Delivers sperms into the female body. |
- Sperm: tiny; mainly genetic material with a long tail for motility.
- Testosterone regulates sperm formation and brings about the changes of puberty in boys.
7. Female Reproductive System
| Part | Function |
|---|---|
| Ovaries | Make eggs (thousands of immature eggs at birth; one egg per month matures after puberty) and some hormones. |
| Oviduct (fallopian tube) | Carries the egg to the uterus; fertilisation occurs here. |
| Uterus | Elastic bag-like organ where the embryo implants and develops. |
| Cervix and vagina | The uterus opens into the vagina through the cervix; sperms enter through the vagina. |
After fertilisation
The zygote divides into a ball of cells, the embryo, which is implanted in the uterine lining and grows into a foetus.
The placenta (disc embedded in the uterine wall) has villi on the embryo's side and maternal blood spaces around them. A large surface area allows glucose and oxygen to pass to the embryo and wastes to pass to the mother's blood.
Development takes about nine months; birth occurs through rhythmic contractions of the uterine muscles.
8. What if the Egg is Not Fertilised? Menstruation
- An unfertilised egg lives for about one day.
- The uterus prepares every month (lining thick, spongy and rich in blood). If there is no fertilisation, the lining breaks and leaves through the vagina as blood and mucus.
- This cycle is menstruation: roughly every month, lasting about 2 to 8 days.
9. Reproductive Health
- Sexual maturation does not mean the body or mind is ready for sexual acts or for bringing up children; choices should not be made under pressure.
- Sexually transmitted diseases (STDs): bacterial (gonorrhoea, syphilis) and viral (warts, HIV-AIDS). A condom helps prevent many such infections to some extent.
| Contraceptive method | How it works | Note |
|---|---|---|
| Mechanical barrier (condom, vaginal covering) | Sperm cannot reach the egg | Also reduces STD risk |
| Oral pills | Change hormonal balance so eggs are not released | Possible side effects |
| Loop / copper-T | Placed in the uterus | May irritate the uterus; does not protect against STDs |
| Surgical methods | Block the vas deferens (male) or fallopian tube (female) | Safe long term; infection risk if done improperly |
- Reasons for contraception: avoid unwanted pregnancy, protect health and readiness of the woman, and control population size.
- Female foeticide: illegal sex-selective abortion is a misuse of surgical methods; it is causing the child sex ratio to decline. Prenatal sex determination is prohibited by law.
- Population size: birth and death rates determine it; an expanding population makes it harder to improve living standards, though inequality may be the main cause of poor standards.
10. Quick Revision
| Asexual | Sexual |
|---|---|
| One parent | Two parents (two gametes) |
| Offspring almost identical; little variation (only DNA copying errors) | More variation by combining DNA of two individuals |
| Fast; no gametes | Needs meiosis (half DNA) and fertilisation |
| Unicellular | Multicellular |
|---|---|
| Whole cell divides (fission, budding) | Cannot divide cell by cell; use fragmentation, regeneration, budding, spores, vegetative propagation or sexual reproduction through a specialised cell type |
Common mistakes:
• Mixing up pollination (pollen transfer) with fertilisation (gamete fusion).
• Saying the seed develops from the ovary: the ovule becomes the seed, the ovary becomes the fruit.
• Calling regeneration the normal mode of reproduction; it is not the same as reproduction.
• Saying fertilisation happens in the uterus; it happens in the fallopian tube.
• Saying testes are inside the abdomen; they are in the scrotum because sperm formation needs a lower temperature.
• Saying the copper-T protects against STDs; only condoms help.
• Writing budding as the mode of Amoeba, Plasmodium or Leishmania; Amoeba, Leishmania undergo binary fission, Plasmodium multiple fission, yeast budding.
• Mixing up pollination (pollen transfer) with fertilisation (gamete fusion).
• Saying the seed develops from the ovary: the ovule becomes the seed, the ovary becomes the fruit.
• Calling regeneration the normal mode of reproduction; it is not the same as reproduction.
• Saying fertilisation happens in the uterus; it happens in the fallopian tube.
• Saying testes are inside the abdomen; they are in the scrotum because sperm formation needs a lower temperature.
• Saying the copper-T protects against STDs; only condoms help.
• Writing budding as the mode of Amoeba, Plasmodium or Leishmania; Amoeba, Leishmania undergo binary fission, Plasmodium multiple fission, yeast budding.