For the individual: A variation that produces a drastically incorrect DNA copy means the new cell cannot work with its cellular apparatus and will die. So the variation is harmful to that particular individual cell/organism.

For the species: When environmental conditions change suddenly (e.g., a disease outbreak, temperature rise, new predator), most individuals in a population may not survive. However, if a small number of individuals happen to carry a variation that makes them resistant or better adapted to the new condition, those individuals survive and reproduce. The species as a whole therefore continues to exist.

Key principle: Variation provides the raw material for natural selection. Without variation, an entire species could be wiped out by a single environmental change. With variation, at least some individuals have a chance of survival — ensuring the species persists over time. This is the basis of Darwin’s theory of evolution by natural selection.

Amoeba: Binary fission can occur in any plane — there is no fixed orientation. This is because Amoeba has a relatively simple, irregular body shape with no specialised structures at a fixed end. The cell simply splits anywhere.

Leishmania: Binary fission occurs in a specific, fixed orientation — along the length of the cell. This is because Leishmania has a whip-like flagellum at one end, giving it a definite body organisation. For the daughter cells to inherit this structure properly, division must happen in relation to the position of the flagellum.

Reason for the difference: The degree of body organisation determines the method of fission. The more organised and specialised the cell’s structures, the more precisely controlled the division must be to ensure each daughter cell inherits the correct body design and can function independently.

Planaria (simple organism): Has a relatively simple body organisation. It contains specialised regenerative (undifferentiated) cells throughout its body. When cut, these cells can rapidly proliferate, divide, and differentiate into all the required cell types and tissues to rebuild a complete organism.

Cow (complex organism): Has a highly complex body with specialised organs and tissues (heart, liver, nervous system, etc.). The cells are so highly differentiated that they have lost the ability to convert back into other cell types. There are not enough undifferentiated stem cells spread throughout the body to regenerate missing organs. Additionally, the intricate spatial arrangement and interconnection of organs cannot simply be rebuilt from a fragment.

Conclusion: Regeneration is only possible when the organism has a simple enough body plan and sufficient undifferentiated cells throughout its body. Increased specialisation and complexity makes regeneration practically impossible.

Three advantages of vegetative propagation:
1. Faster flowering and fruiting: Plants grown vegetatively bear flowers and fruits earlier than those grown from seeds, because they begin with a more mature plant body.
2. Genetic uniformity: All offspring are genetically identical to the parent, preserving desirable characteristics (taste, size, disease resistance) with certainty.
3. Propagation of seedless varieties: Plants that have lost the ability to produce viable seeds (like banana, orange, some roses) can only be reproduced vegetatively.

Banana and vegetative propagation: Cultivated banana plants produce fruits without seeds (through selective breeding). Since they produce no seeds, the only way to grow new banana plants is through vegetative methods — typically using suckers (shoots that grow from the base of the parent plant) or tissue culture. This also ensures the new plants are identical to the high-quality parent variety.

Asexual reproduction: Creates clones from a single parent — variations only arise from occasional DNA copying errors, making variation a slow, rare process. The offspring are nearly identical to the parent.

Sexual reproduction: Combines DNA from two different individuals. Each individual already carries its own set of accumulated variations from previous generations. When gametes fuse, entirely new combinations of genetic material are created — producing novel variation in every generation. Additionally, meiosis (which creates gametes) also shuffles genetic material, adding further variation.

Importance for species: Greater variation means the population has a wider range of traits. If the environment changes, more individuals are likely to possess at least some advantageous traits that allow survival. Sexual reproduction therefore accelerates adaptation and is a powerful driver of evolution — it is the reason most complex organisms reproduce sexually despite the energy cost.

A sperm cell (gamete) contains 23 chromosomes — exactly half the number found in a normal body cell.

Why half? Gametes are produced by meiosis (reductive cell division). During sexual reproduction, one sperm fuses with one egg (23 + 23 = 46). The resulting zygote therefore has the full, correct chromosome number (46). If gametes were not reduced to half (23) by meiosis, the zygote would have 92 chromosomes — and each subsequent generation would double the chromosome number, making the DNA unmanageable for cellular processes. Meiosis is therefore the essential mechanism that maintains the correct chromosome number across generations during sexual reproduction.

1. Pollen grain produced in the anther of a stamen (male reproductive part).
2. Pollination: Pollen grain is carried by wind, water, insects, or other animals from the anther to the sticky stigma of a flower (same or different flower of same species).
3. Pollen germination: Once on a suitable stigma, the pollen grain germinates and a pollen tube grows out of it.
4. Pollen tube travels: The tube grows down through the style — the middle elongated section of the pistil — toward the ovary.
5. Reaching the ovule: The pollen tube enters the ovary and reaches an ovule, which contains the female egg cell.
6. Fertilisation: The male germ-cell travels through the pollen tube and fuses with the female egg cell inside the ovule, forming a zygote.
7. Development begins: The zygote divides repeatedly to form an embryo. The ovule develops a tough coat and becomes a seed; the ovary ripens into a fruit.

Reason for external location: The production of sperm (spermatogenesis) requires a temperature slightly lower than the normal human body temperature (~37°C). The scrotum, located outside the abdominal cavity, maintains the testes at approximately 35°C — about 2°C below core body temperature. This cooler environment is essential for normal sperm development.

If testes were inside the body: The elevated internal body temperature (~37°C) would interfere with the enzymatic reactions required for sperm formation. Sperm production would be significantly reduced or completely stopped, resulting in infertility. This is in fact what happens in a condition called cryptorchidism (undescended testes), where the testes fail to descend into the scrotum — untreated, it can cause fertility problems.

Additional role of testes: They also secrete the hormone testosterone, which drives the development of male secondary sexual characteristics at puberty and regulates sperm production.

Why it occurs: Every month, one ovary releases a mature egg (ovulation). In anticipation of a possible fertilised egg, the lining of the uterus (endometrium) thickens and becomes richly supplied with blood vessels to nourish a potential embryo.

If the egg is not fertilised, it survives for only about one day. Since no embryo will arrive, the thickened uterine lining is no longer needed. Hormonal signals cause the lining to break down. It exits the body through the vagina as blood and mucus over approximately 2–8 days — this is menstruation.

After menstruation, the cycle resets: the uterine lining begins to rebuild, and the next egg starts maturing. The full cycle takes approximately 28 days.

Why it is healthy: Menstruation is a sign that the female reproductive system is functioning normally. It demonstrates that ovulation is occurring and the uterus is capable of preparing for pregnancy. Absence of menstruation (other than during pregnancy) can indicate hormonal imbalance or health issues.

Answer: No, the copper-T will NOT protect against STIs.

How copper-T works: It is an intra-uterine device placed inside the uterus. It prevents pregnancy by creating an environment in the uterus that is hostile to sperm movement and prevents implantation of any fertilised egg. It acts as a physical/chemical barrier within the uterus.

Why it does not prevent STIs: STIs (such as HIV-AIDS, gonorrhoea, syphilis, warts) are transmitted through intimate contact with infected body fluids or skin-to-skin contact during the sexual act itself — before any sperm even enters the uterus. The copper-T offers no protection at this stage since it is inside the uterus, not at the point of contact between partners.

What does protect against STIs: Condoms (male or female) provide the best protection against STIs among contraceptive methods, because they act as a physical barrier during the sexual act, preventing the exchange of body fluids and direct skin contact in the genital area.