How do Organisms Reproduce? — NCERT Solutions, AI Tutor & Practice

Understand: Reproduction copies the DNA blueprint, but small differences (variations) creep in during DNA copying.

Analyse:

• The environment (temperature, water availability, pathogens) is not stable over time.
• If all members of a species were identical, a single unfavourable change could wipe out the entire population.
• Because variations exist, at least some individuals may possess traits suited to the new conditions.

Conclude: Variation gives the species a survival advantage over time by improving its chances of survival during drastic environmental changes, even if a particular variation does not immediately help the individual carrying it. This is the basis of the stability of a population.

Binary fission:

• The parent cell divides into two equal daughter cells.
• A single division event.
• Example: Amoeba, Leishmania (which divides in a definite plane), bacteria.

Multiple fission:

• The parent cell divides into many daughter cells almost simultaneously.
• The nucleus divides repeatedly first, then the cytoplasm divides around each nucleus.
• Example: Plasmodium (malarial parasite).

Key difference: Number of daughter cells produced in one reproductive event — two in binary fission, many in multiple fission.

Regeneration is the ability of certain fully differentiated organisms to grow back their lost body parts, or to form complete new individuals when the body is cut into pieces.

In Planaria:

• If Planaria is cut into several pieces, each piece can regenerate into a complete organism.
• Specialised cells proliferate and make large numbers of cells, which then differentiate in an organised way into various tissues and organs.

Difference from reproduction:

• Regeneration is not the normal way Planaria reproduces; it is a survival/repair response to injury.
• In true reproduction, the parent deliberately produces offspring, whereas regeneration happens accidentally when the body is broken.

Conclude: All regeneration is not reproduction, though it can result in new individuals.

Vegetative propagation is a type of asexual reproduction in plants in which new plants develop from vegetative parts such as roots, stems, or leaves (not from seeds).

Examples:

• Stem: potato (eyes/buds), ginger, sugarcane.
• Leaf: Bryophyllum (buds on leaf margins).
• Roots: sweet potato, dahlia.
• Artificial methods: cutting, layering, grafting, tissue culture.

Advantages to farmers:

1. Plants raised are genetically identical to the parent, so desirable qualities (taste, colour, size) are preserved.
2. Plants can be grown faster and can produce flowers/fruits earlier than seed-grown plants; useful for plants that produce few or no viable seeds (banana, orange, rose).

A flower typically has four whorls attached to the thalamus:

1. Calyx (sepals) – green, protects the bud. Non-reproductive.
2. Corolla (petals) – often coloured, attracts pollinators. Non-reproductive.
3. Androecium (stamens) – the male reproductive whorl. Each stamen = anther + filament. The anther produces pollen grains (male gametes).
4. Gynoecium/Pistil (carpels) – the female reproductive whorl. It has three parts:
   • Stigma – receives pollen.
   • Style – tube connecting stigma to ovary.
   • Ovary – contains ovules, each having the female gamete (egg).

Reproductive whorls: Androecium (male) and Gynoecium (female).

A flower having both is bisexual (e.g., Hibiscus); with only one, it is unisexual (e.g., papaya, watermelon).

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

Self-pollination:

• Transfer of pollen to the stigma of the same flower or another flower of the same plant.
• Does not require external agents to a great extent; occurs in bisexual flowers.
• Offspring are genetically very similar to the parent.

Cross-pollination:

• Transfer of pollen to the stigma of a flower on a different plant of the same species.
• Requires agents like wind, water, insects, birds.
• Introduces greater variation in offspring.

After pollination, the pollen grain forms a pollen tube that grows through the style to reach the ovule, where fertilisation occurs.

Site of production: Sperms are produced in the testes (a pair), located in the scrotum outside the abdominal cavity.

Path of sperm: $\text{Testes} \rightarrow \text{Vas deferens} \rightarrow \text{(joined by ducts of seminal vesicle and prostate gland)} \rightarrow \text{Urethra} \rightarrow \text{Penis}$

Roles of the testes:

1. Produce sperms (male gametes).
2. Produce the male sex hormone testosterone, which controls sperm production and secondary sexual characters.

Why outside the body: The scrotum keeps the testes at a temperature about 1–3 °C lower than body temperature, which is necessary for the formation of healthy sperms.

Accessory glands: Seminal vesicles and prostate gland add secretions that nourish the sperms and provide a fluid medium for easy transport. Sperms + these secretions = semen.

If fertilisation occurs:

• The fertilised egg (zygote) travels down the fallopian tube to the uterus.
• The uterine lining becomes thick and spongy, richly supplied with blood, to nourish the developing embryo.
• The embryo gets nutrition from the mother's blood through a special tissue called the placenta, which allows exchange of glucose, oxygen and waste substances between mother and embryo.
• The child develops over about 9 months and is delivered by rhythmic contractions of the uterus muscles.

If fertilisation does NOT occur:

• The thickened uterine lining is no longer needed.
• It breaks down and is shed slowly through the vagina as blood and mucus.
• This is called menstruation and lasts about 2–8 days, occurring roughly every 28 days.

Contraceptive methods can be grouped as follows:

1. Barrier methods:

• Prevent sperm and egg from meeting physically.
• Example: condoms, diaphragm.
• Advantage: Condoms also prevent transmission of STDs (e.g., HIV/AIDS).

2. Chemical/Hormonal methods:

• Change the hormonal balance so that eggs are not released.
• Example: oral contraceptive pills.
• Advantage: Highly effective when taken correctly.

3. Intra-Uterine Devices (IUDs):

• A device like the copper-T placed in the uterus prevents pregnancy.
• Advantage: Long-term protection.

4. Surgical methods:

• Vasectomy (cutting/tying vas deferens in males) and tubectomy (cutting/tying fallopian tubes in females).
• Advantage: Permanent and reliable.

Note: Contraception also helps in maintaining the health of the mother and controlling population growth.

Asexual reproduction:

• Involves a single parent.
• The offspring is produced by mitosis and is essentially a copy of the parent.
• Variations arise only from small errors during DNA copying — hence variation is limited.

Sexual reproduction:

• Involves two parents, each contributing gametes.
• Gametes are formed by a special division (meiosis) that halves the chromosome number, so each parent has half the usual DNA. During fertilisation: $\text{Gamete}(n) + \text{Gamete}(n) \rightarrow \text{Zygote}(2n)$
• The offspring receives a combination of DNA from both parents, producing new combinations of characters.
• Reshuffling of genes and combining two different DNA sets creates much greater variation.

Conclude: Because sexual reproduction mixes genetic material from two individuals, it generates far more variation, which is the raw material for evolution.

Sexually Transmitted Diseases (STDs) are infections that spread mainly through sexual contact from an infected person to a healthy person.

Bacterial STDs:

• Gonorrhoea
• Syphilis

Viral STDs:

• AIDS (caused by HIV)
• Genital warts / Hepatitis-B

Prevention:

1. Use of condoms during sexual intercourse acts as a barrier and reduces transmission.
2. Avoiding sexual contact with infected persons and maintaining awareness/hygiene.
3. Bacterial STDs can often be treated with antibiotics if detected early; viral STDs like AIDS have no complete cure, so prevention is most important.

Fragmentation:

• The body of a simple multicellular organism breaks up into two or more fragments on maturation.
• Each fragment grows into a new individual.
• Example: Spirogyra (a filamentous alga) simply breaks into smaller pieces which grow into new organisms.

Budding:

• A small outgrowth called a bud develops on the body of the parent due to repeated cell division at one specific site.
• The bud grows, develops into a tiny individual, and detaches from the parent to live independently.
• Example: Hydra, yeast.

Difference: | Fragmentation | Budding | |---|---| | Parent body breaks into many pieces | A single bud grows out from the parent | | No specific site | Bud arises at a specific point |

Both are asexual, producing offspring genetically identical to the parent.