Development of Organisms and Continuity of Life

• Anchor: “Mitösis M aintains. M eiosis M akes gametes and M inuses.” Both begin with M, so tie each one to a second M-word you can recall under pressure.
• Anchor for the seed: the radicle goes to the root and comes out first; the plumule is the plume, and a plume points upward.
• Anchor for the four roles: Grow, Repair, Replace, Reproduce — one G and three Rs.
• Anchor: the flower is built in four rings from the outside in — sepals, petals, stamens, carpel. Working from the outside inwards means you never miss one when labelling.
• Anchor: ovule → seed and ovary → fruit. Or, physically: the ovule is the small thing inside, and the seed is the small thing inside the fruit. What was inside stays inside.
• Anchor: the gonad makes the hormone that shapes the body it belongs to — testes → testosterone, ovaries → oestrogen. Both words even start with the same letters as their gland.
• One last thing. If you remember nothing else from this chapter, remember this pair of sentences: “Meiosis halves it, fertilisation restores it, mitosis keeps it the same.” and “Pollination moves pollen; fertilisation fuses nuclei; ovule becomes seed and ovary becomes fruit.” Between them they protect a remarkable proportion of the marks in Topic 16.

• State that chromosomes are made of DNA, that DNA carries the genetic information, and that a gene is a length of DNA that codes for a feature or a protein.
• Define a haploid nucleus as one containing a single set of chromosomes and a diploid nucleus as one containing two sets, arranged as pairs.
• State that a human diploid body cell contains 46 chromosomes (23 pairs) and a normal human gamete contains 23, and explain why these numbers are specific to humans.
• Define mitosis as nuclear division giving rise to genetically identical cells in which the chromosome number is maintained.
• State the roles of mitosis: growth, repair of damaged tissues, replacement of worn-out or dying cells, and asexual reproduction.
• Describe stem cells as unspecialised cells that divide by mitosis to produce daughter cells that can become specialised.
• State that cancer arises through uncontrolled cell division.
• Define meiosis as a reduction division in which the chromosome number is halved from diploid to haploid, resulting in genetically different cells, and state that it produces gametes.
• Compare mitosis and meiosis on chromosome number, genetic similarity, product and biological role.
• Define asexual reproduction as a process resulting in genetically identical offspring from one parent, and give examples.
• Define sexual reproduction as a process involving the fusion of the nuclei of two gametes to form a zygote and the production of genetically different offspring.
• Discuss the advantages and disadvantages of asexual reproduction, and of sexual reproduction, to a population of a species in the wild and to crop production.
• Identify and give the functions of the sepals, petals, anthers, filaments, stigma, style, ovary and ovules of an insect-pollinated flower.
• Define pollination as the transfer of pollen grains from the anther to the stigma.
• State that fertilisation occurs when a pollen nucleus fuses with a nucleus in an ovule, and keep the two words apart in every answer.
• Compare insect-pollinated and wind-pollinated flowers feature by feature, and explain each difference in terms of how the pollen must travel.
• Distinguish self-pollination from cross-pollination and discuss the consequences of each for genetic variation.
• Describe the growth of the pollen tube down the style to the ovule.
• State that each fertilised ovule develops into a seed and that the ovary develops into a fruit.
• Identify the testa, cotyledons, plumule and radicle of a named dicot seed and give the function of each.
• Investigate and state the conditions required for seed germination — water, suitable temperature and oxygen — and explain the role of each.
• Explain how seed and fruit dispersal by wind and by animals is an adaptation, and why dispersal is an advantage.
• Identify and give the functions of the testes, scrotum, sperm ducts, prostate gland, urethra and penis.
• Identify and give the functions of the ovaries, oviducts, uterus, cervix and vagina.
• Describe the adaptive features of sperm and egg cells and explain how each feature suits its function.
• State that fertilisation is the fusion of the nuclei from a male gamete and a female gamete, and that it normally occurs in an oviduct.
• Describe the roles of testosterone and oestrogen in the development of secondary sexual characteristics.
• Describe the roles of FSH, oestrogen, LH and progesterone in controlling the menstrual cycle, and read a hormone graph without assuming a fixed cycle length.
• Describe early development from zygote to implanted embryo, and explain that growth of the embryo is by mitosis.
• Describe the functions of the amniotic sac, amniotic fluid, placenta and umbilical cord.
• Explain how the placenta is adapted for exchange, and state that maternal and fetal blood normally remain separate while substances pass between them.
• State that some toxins and pathogens can pass across the placenta.

Every apple tree of a named variety is a clone, propagated vegetatively rather than grown from seed. That is deliberate: a grower who has a tree with the right fruit wants that genotype, not a genetically different seedling. The same property that makes asexual reproduction useful in a plant nursery is what makes it risky in the wild, and section I works through the trade-off.

Asexual Reproduction

A process resulting in the production of genetically identical offspring from one parent. No gametes are involved and no nuclei fuse; the offspring are produced by mitosis, which is why they carry the same genes as the parent.

Stem Cell

An unspecialised cell that divides by mitosis to produce daughter cells which can become specialised for particular functions. Stem cells are the source of the many different cell types found in a multicellular organism.

Genetic Variation

Differences in the genes carried by individuals of the same species. Sexual reproduction generates genetic variation because meiosis produces genetically different gametes and fertilisation combines them at random; asexual reproduction generates almost none, because the offspring are produced by mitosis from a single parent.

Meiosis

Reduction division in which the chromosome number is halved from diploid to haploid, resulting in cells that are genetically different from one another. Meiosis is involved in the production of gametes, and the diploid number is restored later at fertilisation.

Ovule

A structure inside the ovary of a flower that contains the female gamete nucleus. After fertilisation each ovule develops into a seed, while the ovary that contained it develops into the fruit.

Reduction Division

A nuclear division that halves the chromosome number, taking a diploid nucleus with two sets of chromosomes to haploid nuclei with one set each. Meiosis is a reduction division; mitosis is not, because mitosis maintains the chromosome number.

Mitosis

Nuclear division giving rise to genetically identical cells in which the chromosome number is maintained. Mitosis provides the cells needed for growth, for the repair of damaged tissues, for the replacement of worn-out cells, and for asexual reproduction.

Sexual Reproduction

A process involving the fusion of the nuclei of two gametes to form a zygote, and the production of offspring that are genetically different from each other. The gametes are haploid and are made by meiosis, so the zygote is diploid.

Wind-Pollinated Flower

A flower adapted to have its pollen carried by moving air. Its petals are small, dull or absent, it has no scent or nectar, its anthers dangle outside the flower, its stigma is large and feathery and exposed, and it produces very large numbers of small, light, smooth pollen grains.

Pollen Tube

A tube that grows out of a germinating pollen grain, down through the style, and into an ovule. It carries a male nucleus from the stigma to the female nucleus so that fertilisation can occur; it is the link between pollination and fertilisation.

Carpel

The female part of a flower, made up of the stigma, the style and the ovary containing ovules. The stigma receives pollen, the style supports the stigma and provides the route for the pollen tube, and the ovary contains the ovules that become seeds after fertilisation.

Seed Dispersal

The carrying of seeds or fruits away from the parent plant. Dispersal reduces competition between the seedlings and the parent and between the seedlings themselves, and allows a species to colonise new areas; it is achieved by structural adaptations to wind or to animals.

Diploid Nucleus

A nucleus containing two complete sets of chromosomes, so that every type of chromosome is present as a matching pair. Body cells are diploid; in humans a diploid nucleus contains 46 chromosomes arranged as 23 pairs.

Germination

The beginning of growth of the embryo in a seed, once conditions are suitable. Germination requires water, a suitable temperature and oxygen, and it begins with the seed absorbing water and ends with the radicle and then the plumule emerging.

Pollination

The transfer of pollen grains from the anther of a stamen to the stigma of a carpel. Pollination is a transfer only: no nuclei have fused and no seed exists at this stage, and fertilisation follows later inside the ovule.

Oviduct

A tube that carries an egg from the ovary towards the uterus. Fertilisation normally takes place in an oviduct, and the zygote then divides by mitosis as it travels down towards the uterus, where it implants.

Testis

The male gonad. The testes produce sperm by meiosis and also produce the hormone testosterone; they are held in the scrotum outside the main body cavity, where the temperature is slightly lower than core body temperature and suitable for sperm production.

Sperm Cell

The male gamete in humans. It is small and produced in very large numbers, has a haploid nucleus, is motile by means of a flagellum, contains many mitochondria that release energy for movement, and carries an acrosome of enzymes that help it penetrate the coverings of the egg.

Zygote

The single diploid cell formed when a sperm nucleus fuses with an egg nucleus at fertilisation. In humans a zygote contains 46 chromosomes, 23 from each gamete, and it begins to divide by mitosis to form an embryo.

Follicle Stimulating Hormone

A hormone produced by the pituitary gland, usually written FSH. It stimulates an egg to develop inside a follicle in the ovary, and stimulates the ovary to secrete oestrogen. FSH does not cause ovulation; LH does.

Testosterone

A hormone produced by the testes. Testosterone controls the development of male secondary sexual characteristics at puberty and is needed for the production of sperm.

Amniotic Fluid

The fluid held inside the amniotic sac around a developing fetus. It cushions the fetus against mechanical shock, allows the fetus to move, and provides a stable surrounding environment during development.

Implantation

The embedding of an early embryo, a ball of cells, into the thickened lining of the uterus. Implantation happens several days after fertilisation, once the embryo has travelled down the oviduct, and it is the point from which the placenta begins to develop.

Placenta

The organ that forms in the uterus during pregnancy and acts as the exchange surface between the maternal and fetal circulations. It has a large surface area and a thin barrier, so oxygen and dissolved nutrients diffuse towards fetal blood while carbon dioxide and urea diffuse towards maternal blood; the two circulations remain separate.

• “Genes contain chromosomes.” Why wrong It inverts the containment chain. The gene is the smallest unit named here, not the largest. Say instead Chromosomes carry genes; a gene is a length of the DNA that makes up a chromosome.
• “Each chromosome carries one gene.” Why wrong A human chromosome carries many hundreds of genes along its length. Say instead A chromosome carries many genes, each at its own position along the DNA.
• “Every cell in the body has a different genetic code.” Why wrong All the body cells of one organism come from one zygote by mitosis, so they carry the same genes. Cells differ because different genes are used in different cells. Say instead Body cells are genetically identical; they differ in which genes are active.
• “The pollen grain is a sperm cell.” Say instead The pollen grain contains the male nucleus. The grain is not itself the gamete, and it stays on the stigma while the nucleus travels down the tube.
• “The style turns into the pollen tube.” Say instead The style is part of the flower and stays where it is. The pollen tube is a new structure, grown by the pollen grain, that travels through the style.
• “Fertilisation happens on the stigma.” Say instead Fertilisation happens inside the ovule, at the far end of the journey. Only pollination happens on the stigma.
• “Pollination and fertilisation are the same event.” Say instead Pollination is the transfer of pollen; fertilisation is the fusion of nuclei. A flower can be pollinated without ever being fertilised.
• “Any pollen will do.” Say instead The pollen must be compatible — from the same species. Incompatible pollen may land on the stigma but will not produce a tube that reaches an ovule.
• “Genes contain chromosomes.” Why wrong It reverses the containment chain. The gene is the smallest unit in the sequence, not the largest. Say instead Chromosomes are made of DNA, and a gene is a length of that DNA. Cell → nucleus → chromosome → DNA → gene.
• “Haploid always means 23 chromosomes.” Why wrong 23 is the human number. Chromosome number is species-specific: a pea has 7 in a haploid nucleus, a dog has 39. Say instead Haploid means one set of chromosomes. In humans that set happens to be 23.
• “Mitosis halves the chromosome number.” Why wrong Mitosis maintains it — that is half the definition. Halving is what meiosis does. Say instead Mitosis: 46 → 46. Meiosis: 46 → 23.
• “Meiosis produces genetically identical cells.” Why wrong It contradicts the definition and removes the whole point of meiosis, which is variation. Say instead Meiosis produces genetically different cells; mitosis produces genetically identical ones.
• “Fertilisation produces a haploid zygote.” Why wrong Two haploid nuclei fusing gives two sets of chromosomes, not one. Say instead Fertilisation produces a diploid zygote: 23 + 23 = 46 in a human.
• “The embryo grows by meiosis.” Why wrong If it did, the chromosome number would halve at every division and the embryo would soon have none. Say instead All growth after fertilisation is by mitosis. Meiosis happened earlier, to make the gametes.
• “Asexual reproduction produces variation.” Why wrong The offspring come from one parent by mitosis, so they carry the same genes. Any variation you can see is environmental, not genetic. Say instead Asexual reproduction produces genetically identical offspring; there is very little genetic variation between them.
• “Sexual reproduction guarantees that the offspring survive environmental change.” Why wrong Variation is not insurance. Most offspring may still die; what changes is the chance that some are suited to the new conditions. Say instead Variation increases the probability that some individuals survive a change, so the population is more likely to persist.
• “Self-pollination is asexual reproduction.” Why wrong Gametes are still made by meiosis and two haploid nuclei still fuse. That is sexual reproduction, regardless of where the pollen came from. Say instead Self-pollination is sexual reproduction with less genetic variation than cross-pollination.
• “Pollination is the fusion of nuclei.” Why wrong That is the definition of fertilisation. At pollination nothing has fused and no offspring exists. Say instead Pollination is the transfer of pollen grains from an anther to a stigma.
• “A pollen grain is a sperm cell.” Why wrong The grain contains the male nucleus. The grain itself stays on the stigma; only the nucleus travels down the tube. Say instead The pollen grain contains a male nucleus, which passes down the pollen tube to the ovule.
• “Fertilisation happens on the stigma.” Why wrong The female nucleus is inside an ovule, at the far end of the style. Nothing can fuse with it on the stigma. Say instead Fertilisation happens inside the ovule, after the pollen tube has grown down the style and entered it.
• “The style becomes the pollen tube.” Why wrong The style is a permanent part of the flower. The tube is a new structure grown by the pollen grain. Say instead The pollen tube grows through the style.
• “The ovule becomes the fruit.” Why wrong The ovule is the small structure inside; it cannot become the large structure around it. Say instead Each fertilised ovule becomes a seed.
• “The ovary becomes a seed.” Why wrong The same reversal, the other way round. The ovary is the container. Say instead The ovary becomes the fruit, which contains the seeds.
• “The cotyledon is the embryonic root.” Why wrong A cotyledon is a seed leaf and a food store. The embryonic root is the radicle. Say instead Radicle = embryonic root; plumule = embryonic shoot; cotyledons = food store.
• “Light is required for germination.” Why wrong Seeds are usually buried and germinate in the dark. Light is needed for photosynthesis, which comes after germination. Say instead Germination requires water, a suitable temperature and oxygen. If a supplied experiment shows a light effect for one species, report that result without generalising it.
• “A seed that has not germinated is dead.” Why wrong It may simply be slower, or one of the three conditions may be missing. A dry seed can stay viable for years. Say instead The seeds did not germinate within the observation period. Transferring them to suitable conditions would test whether they are viable.
• “Fertilisation normally occurs in the uterus.” Why wrong It normally occurs in an oviduct. The uterus is where implantation happens, days later. Say instead Fertilisation: oviduct. Implantation: uterus.
• “Sperm mitochondria give the sperm energy.” Why wrong Mitochondria are the site of aerobic respiration; they release energy rather than supplying it from nowhere. Say instead The many mitochondria release energy in respiration, which is used for the movement of the flagellum.
• “FSH triggers ovulation.” Why wrong FSH stimulates the follicle and the egg to develop. Ovulation is triggered by a surge of LH. Say instead FSH develops the follicle; LH releases the egg. The name of FSH states its job.
• “Progesterone causes the follicle to develop.” Why wrong Progesterone is secreted after ovulation. It cannot cause something that has already happened. Say instead Progesterone maintains the thickened uterus lining after ovulation, and inhibits FSH and LH.
• “The menstrual cycle is 28 days long.” Why wrong 28 is a convenient average. Real cycles differ between people and between months. Say instead Cycle length varies. Take the day of ovulation from the graph or table you are given, not from memory.
• “Maternal and fetal blood mix in the placenta.” Why wrong The two circulations are brought close together but remain separate; only dissolved substances cross the barrier. Say instead Substances diffuse across the placenta; blood does not cross it. The two circulations stay separate.
• “The placenta blocks every harmful substance.” Why wrong It is not a perfect barrier, and the syllabus expects you to know that. Say instead Some toxins and pathogens can cross the placenta, including some viruses, alcohol, nicotine, carbon monoxide and certain drugs.
• “Amniotic fluid supplies the fetus with nutrients.” Why wrong That is the placenta's job. The fluid's roles are protection and movement. Say instead Amniotic fluid cushions the fetus against mechanical shock and allows it to move; nutrients cross the placenta.

• The single most valuable habit in this topic. Before writing any answer that mentions pollination, fertilisation, meiosis or mitosis, say the definition silently to yourself first. Most lost marks in Topic 16 are not gaps in knowledge — they are the right idea attached to the wrong word.
• Detailed stages are not required. The 5090 syllabus does not ask for prophase, metaphase, anaphase or telophase, and naming them earns nothing. What it asks for is the definition and the roles. Spend your time on those.
• What meiosis does not do. Meiosis is not fertilisation. It produces gametes; it does not join them. Writing “meiosis is when the sperm and egg join” loses every mark in the question, because it names the wrong process entirely. Meiosis halves; fertilisation restores.
• Answering a “discuss” question. Give a point, its cost, and then tie the judgement to the context in the question. If the context is a wild population facing a changing environment, variation wins. If the context is a grower who wants a uniform crop of a known variety, uniformity wins. The same two facts support opposite conclusions in different contexts, and saying so is what a “discuss” mark scheme is looking for.
• Turn description into explanation. A question worth four marks rarely wants four descriptions. Pair each feature with its reason: “the stigma is large and feathery, which gives a large surface area to catch pollen grains carried in the air”. That single sentence usually scores both the description mark and the explanation mark.
• “The sperm and egg join” is not the definition. The mark is for nuclei fusing. Two cells touching, meeting or joining describes the geography, not the event. Write: “the nucleus of the sperm fuses with the nucleus of the egg”.
• On the umbilical vessels. Note that here the naming looks backwards compared with the rest of the body: the umbilical vein carries oxygenated blood, because vessels are named by direction of travel relative to the heart, not by what they carry. If a question asks you to label the vessels, label them by direction and say which is oxygenated.
• Two circulations, one wall. If asked why the two bloods are kept separate, a safe syllabus-level answer is that the mother's and the fetus's blood may differ — for example in blood pressure, which is much higher in the mother and could damage the delicate fetal vessels. Keep the emphasis on the fact itself: they remain separate, and substances pass across.

• Topic 16 is unusually predictable. It is heavy on definitions, labelled diagrams and ordered sequences, and light on calculation. That shapes how you should revise it: precision of wording earns more here than breadth of reading.
• Expect single items on chromosome numbers, on which division halves the number, on identifying a flower or seed part from a diagram, and on which hormone does what. The wrong options are usually the confusable partner — LH offered against FSH, ovule against ovary — so eliminating by definition works.
• Label a diagram, then explain a function. Very common: label four parts of a flower or the male or female reproductive system for four marks, then explain one structure’s adaptation for two or three.
• “Describe what happens from the moment a pollen grain lands on a stigma until a seed is formed” is a classic four-to-six mark item. So is a matched comparison table of insect and wind pollination, or of mitosis and meiosis.
• A menstrual-cycle hormone graph to read, or germination percentages from an investigation to compare. You will be asked to use the values printed, identify a control, or explain an anomalous result.
• Germination is the standard practical here: identify the independent variable, name two variables to control, state the germination criterion, and explain why replicates are needed.