DNA copying & variation
Photocopy a page many times and tiny dots and smudges slowly appear. Cells copying DNA behave in a similar way: almost perfect, never exactly perfect.
Reproduction makes new individuals. Its basic step is making a copy of DNA, the blueprint of body design, along with extra cell machinery, so one cell divides into two. Copying is accurate but not perfect, so small variations appear; they help a species survive when the surroundings change.
Equation / formula : DNA copy + new cell machinery → 2 cells · small errors → variation → survival and evolution
In the figure : Each step feeds the next: copy, divide, vary, survive.
DNA copying & variation
In the app you can move this figure with your finger.
Open in the appRule 1 : Why copying must be accurate — Accurate copying keeps the body design that suits the niche, so the population stays stable. A very drastic error may produce a cell that cannot work and dies.
- Easy : Children look like parents because DNA is copied almost exactly.
- Medium : A niche is the place or role a population fills in the ecosystem; if it stays the same, stable copying is enough.
- For exams : Reproduction is not essential for one individual to stay alive, but it is essential for the species to continue.
Rule 2 : Why variation helps — If the niche changes (heat, water level, meteorite), a population with variants has a chance because some individuals may suit the new conditions. For example, a few heat-tolerant bacteria survive when pond water warms.
- Easy : Not all bacteria in warm water die; the heat-tolerant ones live on.
- Medium : Variation helps the species, not always the individual: one variant may fail while another succeeds.
- For exams : A field of identical plants can be destroyed by one disease because all share the same weakness.
Variation is the raw material of evolution.
Saying variation always helps every individual. It helps the species over time.
Copy DNA → divide → small errors → variation → species survives change. Memory clue : "CDV": Copy, Divide, Vary.
Spot the difference
| Point | Accurate copying | Variation |
|---|---|---|
| Gives | stable features | new options |
| Helps | niche stability | survival in change |
Practice questions
- 1Why is DNA copied in reproduction?
- 2What is a niche?
- 3Why is copying never perfect?
- 4What happens to a cell with a very drastic DNA error?
- 5Why must each new cell get its own cell machinery?
- 6How do variants help bacteria when water gets hotter?
- 7Why is variation useful for the species but not always for an individual?
- 8A field of identical plants was destroyed by a disease. Explain.
Show answers (try them yourself first)
- 1To pass the body-design blueprint to the next generation.
- 2The place or role a population fills in the ecosystem.
- 3No biochemical reaction is absolutely reliable.
- 4It may fail to work and die.
- 5To carry out its own life processes.
- 6Heat-tolerant variants survive and multiply.
- 7A variant may suit a new environment (species lives on), but may fail in the present one.
- 8All plants had the same weakness, so none was resistant.
Asexual reproduction
A cut piece of money-plant in a glass of water grows roots on its own. One plant has become two without any seeds.
In asexual reproduction a single parent gives rise to new individuals without gametes. Modes: fission (binary, multiple), fragmentation, regeneration, budding, vegetative propagation and spore formation.
Equation / formula : One parent → new individual(s) · offspring almost identical to parent
In the figure : All six modes need only one parent.
Rule 1 : Single cell and simple organisms — Amoeba splits into two in any plane (binary fission); Leishmania splits in a fixed direction; Plasmodium divides into many at once (multiple fission). Yeast forms buds. Spirogyra breaks into fragments. Rhizopus makes spores in sporangia with a tough wall.
- Easy : Bread mould: hyphae are thread-like, sporangia are the small blobs on sticks.
- Medium : Spores have thick walls, survive harsh conditions and grow when they land on a moist surface.
- For exams : Complex animals cannot reproduce by cell-by-cell splitting or regeneration because their cells are organised into tissues and organs at fixed places.
Rule 2 : Regeneration, budding and vegetative propagation — Planaria and Hydra can regrow from cut pieces (regeneration, which is not the same as reproduction). Hydra also forms a bud by repeated division at one spot; it detaches when mature. In plants roots, stems or leaves grow into new plants: potato notches, Bryophyllum leaf buds, rose cuttings, layering and grafting. Tissue culture grows plants from a few cells.
- Easy : A money-plant piece with a leaf node grows roots; a piece with no node does not.
- Medium : Vegetative propagation gives early flowering and keeps the parent's features; banana and jasmine, which lost the ability to make seeds, are grown this way.
- For exams : Tissue culture: cells from a growing tip are grown in artificial medium into a callus, then moved to a hormone medium to form plantlets, which give disease-free plants.
Asexual offspring are almost identical to the parent, so there is little variation.
Mixing up regeneration with reproduction. Regeneration repairs or regrows from damage; reproduction does not need damage.
Fission, budding, fragmentation, regeneration, spores, vegetative: one parent each. Memory clue : "FBF-RSV": Fission, Budding, Fragmentation, Regeneration, Spores, Vegetative.
Spot the difference
| Point | Binary fission | Multiple fission |
|---|---|---|
| Daughter cells | two | many at once |
| Example | Amoeba | Plasmodium |
Practice questions
- 1Name one organism each for binary fission and multiple fission.
- 2How does Hydra reproduce by budding?
- 3Name the reproductive parts of Rhizopus.
- 4What is regeneration? Is it the same as reproduction?
- 5Why is vegetative propagation used for banana?
- 6How do spores help an organism survive?
- 7Why can humans not reproduce by regeneration?
- 8Describe the steps of tissue culture.
Show answers (try them yourself first)
- 1Amoeba (binary); Plasmodium (multiple).
- 2A bud grows by repeated division at one spot, matures and detaches.
- 3Sporangia (with spores).
- 4Growing a new individual or part from a piece; no, reproduction does not need injury.
- 5It has lost the ability to make seeds.
- 6Thick walls protect them until they find moisture.
- 7Their cells are organised into tissues and organs, which cannot be rebuilt from pieces.
- 8Cells → callus in medium → hormone medium → plantlets → soil.
Sexual reproduction & flowering plants
A papaya or a pumpkin flower must receive pollen from another flower before a fruit forms. Seeds need two parents' cells to meet.
Sexual reproduction joins germ cells (gametes) of two individuals to form a zygote; it creates more variation. Gametes have half the chromosomes (made by meiosis), so the number is restored after fusion. In a flower the stamen (male) makes pollen and the pistil (female: stigma, style, ovary) holds the ovules.
Equation / formula : Pollination → pollen tube → fertilisation → zygote → embryo → seed + fruit
In the figure : Pollination happens first; fertilisation comes only after the pollen tube reaches the ovule.
Sexual reproduction & flowering plants
In the app you can move this figure with your finger.
Open in the appRule 1 : Gametes and the flower — The male gamete is small and motile (in the pollen tube); the female gamete is large with stored food (egg in the ovule). Flowers can be unisexual (papaya, watermelon) or bisexual (hibiscus, mustard). The anther makes pollen; the ovary holds ovules.
- Easy : Papaya has separate male and female flowers; hibiscus has both parts in one flower.
- Medium : Why does the female gamete store food? The zygote needs energy to grow into an embryo.
- For exams : Meiosis halves the chromosomes in gametes so that fusion restores the normal number instead of doubling it.
Rule 2 : Pollination, fertilisation and seed — Pollen moving from anther to stigma is pollination: self (same flower) or cross (another flower, carried by wind, water or animals). A pollen tube grows down the style to the ovule, where the male gamete fuses with the egg (fertilisation). The zygote becomes an embryo, the ovule a seed, the ovary a fruit; the petals and other parts wither. A seed germinates into a seedling under suitable conditions.
- Easy : Insects visit bright, scented flowers and carry pollen; wind-pollinated flowers make light pollen in large amounts.
- Medium : Pollination is not fertilisation: pollination is pollen reaching the stigma; fertilisation is fusion of the gametes.
- For exams : If the stigma or style is removed, the pollen cannot reach the ovule, fertilisation fails and no fruit forms.
Sexual reproduction is slower and costlier than asexual, but gives variation, which helps the species survive.
Saying the ovule becomes the fruit. The ovule becomes the seed; the ovary becomes the fruit.
Pollination first, then fertilisation; ovule → seed, ovary → fruit. Memory clue : "POSF": Pollination, Ovule, Seed, Fruit (ovary).
Spot the difference
| Point | Pollination | Fertilisation |
|---|---|---|
| Meaning | pollen reaches stigma | male and female gametes fuse |
| Where | stigma | ovule |
Practice questions
- 1Name the male and female reproductive parts of a flower.
- 2What are the three parts of the pistil?
- 3Differentiate unisexual and bisexual flowers.
- 4What is cross-pollination?
- 5How is the pollen tube formed and what does it do?
- 6What do the ovule and ovary become after fertilisation?
- 7Why is meiosis needed to form gametes?
- 8Give two advantages of sexual reproduction over asexual.
Show answers (try them yourself first)
- 1Stamen (male) and pistil (female).
- 2Stigma, style, ovary.
- 3Unisexual: only stamens or only pistil. Bisexual: both.
- 4Pollen moves from one flower to another.
- 5It grows from the pollen grain through the style to the ovule carrying the male gamete.
- 6Ovule → seed; ovary → fruit.
- 7To halve the chromosomes so that fusion restores the normal number.
- 8More variation; better chance of survival in changing conditions.
Human reproductive systems & puberty
Around early teenage years voices crack, hair grows in new places and periods begin. These are signs that the reproductive system is maturing.
Puberty is the stage when general growth slows and the reproductive tissues mature; it is driven by testosterone in boys and oestrogen in girls. Males: testes (sperms), vas deferens, seminal vesicles, prostate, urethra. Females: ovaries (eggs), oviducts, uterus, vagina.
Equation / formula : Testes → sperms → vas deferens → urethra · Ovary → egg → oviduct → uterus
In the figure : Testes sit outside the body in the scrotum because sperm formation needs a lower temperature.
Rule 1 : Changes at puberty — Both: thick body hair, oily skin, pimples. Girls: breasts enlarge, periods begin. Boys: facial hair, voice cracks. The changes are slow, begin at different ages and are different for each person.
- Easy : A boy's voice cracking is a sign of sexual maturation, not a disease.
- Medium : Reproductive tissues mature only when general growth slows, because the body's resources first go to growth.
- For exams : Maturing of the body does not mean body and mind are ready for sex or for bringing up children.
Rule 2 : Male and female systems — Sperms are tiny, mostly genetic material with a tail. The urethra is a common passage for sperms and urine. The prostate and seminal vesicles add fluid that helps sperm move and gives nutrition. One egg is released from an ovary every month, travels in the oviduct to the uterus; the uterus opens into the vagina through the cervix.
- Easy : Testes make sperms; ovaries make eggs.
- Medium : Born with thousands of immature eggs, a girl's eggs start maturing at puberty.
- For exams : Both testes and ovaries make gametes and also hormones (testosterone, oestrogen).
Males: testes → vas deferens → urethra. Females: ovary → oviduct → uterus → cervix → vagina.
Saying the vas deferens is in the female system, or that the ovary stores sperm. Vas deferens is male; the ovary makes eggs.
Testes: sperm + testosterone. Ovary: egg + oestrogen. Uterus: baby grows. Memory clue : "T-S, O-E": Testes–Sperm, Ovary–Egg.
Spot the difference
| Point | Testes | Ovaries |
|---|---|---|
| Gamete | sperm | egg |
| Hormone | testosterone | oestrogen |
Practice questions
- 1What is puberty?
- 2Name two changes common to boys and girls at puberty.
- 3Why are testes outside the abdominal cavity?
- 4Name the common passage for sperm and urine.
- 5What is the role of the prostate gland and seminal vesicles?
- 6Trace the path of an egg from the ovary to the vagina.
- 7Which hormone brings puberty changes in girls and boys?
- 8Why does sexual maturity not mean readiness to have a child?
Show answers (try them yourself first)
- 1The stage when growth slows and reproductive tissues mature.
- 2New hair in armpits/genital area; oily skin and pimples.
- 3Sperm formation needs a temperature lower than the body's.
- 4Urethra.
- 5They add fluid that helps sperm move and nourishes them.
- 6Ovary → oviduct → uterus → cervix → vagina.
- 7Testosterone (boys) and oestrogen (girls).
- 8The body matures gradually; body and mind may not yet be ready for the responsibility.
Fertilisation, menstruation & reproductive health
A baby grows for about nine months inside the mother, fed through a special tissue. If no baby begins, the body cleans the prepared lining each month.
After fertilisation in the oviduct the zygote divides into an embryo, which gets implanted in the uterus lining and grows into a foetus, nourished by the placenta. If no egg is fertilised, the uterus lining breaks down and leaves as menstruation (about every month, 2 to 8 days).
Equation / formula : Sperm + egg → zygote → embryo → implantation → foetus → birth (≈ 9 months)
In the figure : The placenta has villi on the embryo side and blood spaces on the mother side for exchange of food, oxygen and wastes.
Rule 1 : Placenta and menstruation — The embryo gets glucose and oxygen from the mother's blood through the placenta and passes its wastes back. The uterus lining thickens every month in case of pregnancy; if there is no fertilisation, the lining breaks and leaves through the vagina as blood and mucus. The child is born by rhythmic contractions of the uterine muscles.
- Easy : An unfertilised egg lives for about a day.
- Medium : The uterus lining prepares every month because an egg is released every month and might be fertilised.
- For exams : Menstruation occurs because the egg was not fertilised, so the prepared lining is not needed.
Rule 2 : Reproductive health and contraception — Sexual contact can spread infections (gonorrhoea, syphilis, warts, HIV-AIDS); a condom helps reduce this. To avoid pregnancy: barrier (condom), oral pills (change hormones, may have side effects), copper-T or loop in the uterus (does not stop infections), and surgical blocking of the vas deferens or oviduct. Sex-selective abortion is illegal and harms the sex ratio. Population size depends on birth and death rates.
- Easy : Condom: barrier method and reduces many infections.
- Medium : Pills work by changing hormone balance so eggs are not released; copper-T prevents pregnancy but gives no protection from infections.
- For exams : Reasons for contraception: avoid pregnancy for which the woman is not ready, plan family size, and (for condoms) reduce infections.
Fertilisation takes place in the oviduct; the embryo is implanted in the uterus.
Saying copper-T protects from sexually transmitted infections. It only prevents pregnancy.
Egg fertilised → baby; egg not fertilised → menstruation. Condom is the only one here that also reduces infections. Memory clue : "ZEFB": Zygote, Embryo, Foetus, Baby; placenta = the feeding link.
Spot the difference
| Point | Pollination | Menstruation |
|---|---|---|
| Seen in | flowering plants | human females |
| Meaning | pollen reaches stigma | uterine lining sheds when no fertilisation |
Practice questions
- 1Where does fertilisation occur in humans?
- 2What is an embryo? Where does it get implanted?
- 3What is the function of the placenta?
- 4Why does menstruation occur?
- 5Name two sexually transmitted infections.
- 6List four methods of contraception.
- 7Will a copper-T protect against sexually transmitted infections? Explain.
- 8Why is sex-selective abortion illegal and harmful?
Show answers (try them yourself first)
- 1In the oviduct (fallopian tube).
- 2The ball of cells formed from the zygote; implants in the uterus lining.
- 3Nourishes the embryo with glucose and oxygen and removes its wastes.
- 4The egg was not fertilised, so the prepared lining is not needed.
- 5Gonorrhoea and HIV-AIDS (or syphilis, warts).
- 6Condom, oral pills, copper-T/loop, surgical blocking.
- 7No; it only prevents pregnancy and is not a barrier to infections.
- 8It reduces the number of girls and upsets the sex ratio of society.