Biology 5090/22 — October/November 2024

Walkthrough

The question asks to identify component F in the diagram of a virus. Viruses have a simple structure consisting of genetic material (DNA or RNA) surrounded by a protective outer layer. In the diagram, F points to the main spherical body of the virus, which is the protein coat (also called the capsid). The spike-like structures extending from the surface are glycoproteins used for attachment to host cells, but F is pointing to the coat itself.

Key Takeaways

A virus is not a cell. It has no cytoplasm, organelles, or cell membrane. Its basic structure is always genetic material (DNA or RNA) enclosed in a protein coat (capsid). Some viruses also have an outer lipid envelope and surface glycoprotein spikes.

Common Mistakes

• Writing "capsid" instead of "protein coat". While technically correct in university biology, Cambridge O Level (5090) specifically expects the term protein coat as printed in the mark scheme.
• Pointing to a spike and calling it the protein coat. The spikes are glycoproteins; the main sphere is the protein coat.

Things to Be Careful About

Always match the mark scheme's exact wording. The scheme awards the mark for protein coat; avoid vague terms like "shell" or "outer layer". If the label had pointed to the interior, the answer would be genetic material, DNA, or RNA.

Walkthrough

Fig. 2.1 shows the external structure of a virus. We have just identified the protein coat. The other essential component of a virus is its genetic material, which can be either DNA or RNA (single-stranded or double-stranded). This genetic material is located inside the protein coat and is not visible in an external diagram like Fig. 2.1.

Key Takeaways

All viruses contain two main components: a protein coat (capsid) and genetic material (DNA or RNA). The genetic material carries the instructions for making new virus particles once inside a host cell.

Common Mistakes

• Writing "nucleus" or "nucleic acid". Viruses do not have a nucleus. While genetic material is a nucleic acid, the mark scheme specifically accepts genetic material, DNA, or RNA.
• Suggesting "organelles" or "ribosomes". Viruses have no organelles; they hijack the host cell's ribosomes to make viral proteins.

Things to Be Careful About

The question asks for a component not shown in the figure. Since the figure is an external diagram, any internal component is a valid answer. Stick to the simplest, most direct answer: genetic material.

Walkthrough

Antibodies are proteins (specifically, globular proteins called immunoglobulins). To name the elements in an antibody, we need to recall the elemental composition of proteins.

All proteins are made of amino acids. Every amino acid contains:

• Carbon (C)
• Hydrogen (H)
• Oxygen (O)
• Nitrogen (N)

Many amino acids (like cysteine and methionine) also contain Sulfur (S), which allows disulfide bonds to form and stabilise the protein's 3D shape.

The question asks for four elements. The four universal elements in all proteins are C, H, O, N. Including sulfur is also correct and will score, but C, H, O, N are the essential four.

Key Takeaways

• Carbohydrates: C, H, O
• Lipids: C, H, O (some phospholipids also have P and N)
• Proteins: C, H, O, N (+ S in some)
• DNA: C, H, O, N, P

Antibodies are proteins, so they contain C, H, O, N, and often S.

Common Mistakes

• Writing "phosphorus" or "P". Phosphorus is found in DNA, RNA, ATP, and phospholipids, but not in the standard amino acids that make up antibodies.
• Writing "calcium" or "iron". These are mineral ions, not elements that make up the organic molecule itself.
• Forgetting nitrogen. Nitrogen is the key element that distinguishes proteins from carbohydrates and lipids.

Things to Be Careful About

The mark scheme awards 2 marks for all four correct, and 1 mark for three correct. You must name exactly four elements. The five acceptable elements are carbon, hydrogen, oxygen, nitrogen, sulfur. Choose the four most universal ones (C, H, O, N) to be safe.

Walkthrough

This is a 5-mark explanation question. We need to describe how an antibody's structure enables its function in the immune response. Let's break it down step-by-step:

Step 1: The target.
Pathogens (viruses, bacteria) have foreign proteins on their surface called antigens. These act as identifiers that the immune system recognises as non-self.

Step 2: Specificity.
B lymphocytes produce antibodies that are specific to a particular antigen. Each antibody is designed to target only one type of pathogen.

Step 3: Structure.
The antibody molecule has a Y-shape with an antigen-binding site at the tip of each arm. This site has a complementary shape (or structure) to the specific antigen, like a lock and key.

Step 4: Binding.
Because of this complementary shape, the antibody binds to the antigen, forming an antibody-antigen complex.

Step 5: Consequence (Function).
Once bound, the antibody neutralises the pathogen in several ways:

• It can directly destroy the pathogen (e.g., by disrupting its cell membrane or blocking its ability to enter cells).
• It marks the pathogen (opsonisation), making it easier for phagocytes to recognise and engulf it.
• It causes pathogens to clump together (agglutination), which also aids engulfment by phagocytes.

Any five of these linked points will score full marks.

Key Takeaways

Antibodies are Y-shaped proteins with a specific antigen-binding site. Their function relies on the lock-and-key relationship between the antibody's shape and the pathogen's antigen. Binding leads to neutralisation, clumping, or marking for phagocytosis.

Common Mistakes

• Saying "antibodies kill all pathogens". Antibodies are specific; they only bind to the antigen they were made against.
• Forgetting to mention antigens. The explanation must start with the pathogen having antigens for the antibody to bind to.
• Saying "antibodies destroy bacteria" without explaining how (e.g., by marking them for phagocytes, clumping them, or forming a complex). The mark scheme requires the mechanism (binding, marking, clumping, engulfment).
• Writing "antibodies eat the pathogen". Antibodies do not engulf pathogens; phagocytes (macrophages) do. Antibodies mark them for engulfment.

Things to Be Careful About

The mark scheme awards marks for specific connections:

• MP2 (specific) and MP3 (complementary shape) must be in connection to antigen or antibody.
• You need five distinct points. Don't just say "it binds and destroys"; break it down: binds to antigen -> forms complex -> marks/clumps -> engulfed by phagocytes.
• Use the term antibody-antigen complex if possible, as it is a precise 5090 term.

Walkthrough

The question asks why premature babies are at higher risk of infectious diseases. We need to link the length of pregnancy to the baby's immune defences.

During a normal 40-week pregnancy, the mother's immune system produces antibodies against various pathogens. These maternal antibodies can cross the placenta and enter the baby's bloodstream. This gives the baby passive immunity (immunity provided by another individual) before birth and for several months after birth.

If a baby is born prematurely (before 40 weeks), this transfer of antibodies across the placenta has not had enough time to occur. Therefore, the baby is born with fewer antibodies and lacks the passive immunity that would normally protect them in the first few weeks of life.

Key Takeaways

• Passive immunity is the transfer of antibodies from mother to baby.
• Antibodies cross the placenta during pregnancy.
• A full-term baby (40 weeks) receives a good supply of maternal antibodies; a premature baby does not.
• Passive immunity is temporary; the baby must eventually produce its own antibodies through active immunity (e.g., via vaccination or natural infection).

Common Mistakes

• Saying "the baby's immune system is not fully developed". While true, the mark scheme specifically looks for the link to antibodies and the placenta. The primary reason for increased infection risk in the weeks after birth is the lack of transferred maternal antibodies.
• Writing "the baby hasn't been vaccinated". Vaccination is an active immunity measure and is not the primary reason for increased risk specifically due to prematurity in the context of maternal transfer.
• Forgetting to mention across the placenta. The mark scheme accepts "across placenta" as a key point.

Things to Be Careful About

The mark scheme awards 2 marks for:

1. fewer antibodies (or less passive immunity)
2. across placenta (or from mother / transfer during pregnancy)

Make sure both ideas are present: the baby has fewer antibodies because they haven't been transferred across the placenta yet. Don't just write "fewer antibodies"; explain why.

Walkthrough

The question asks for one action a mother can take to protect her premature baby from infection after birth. Since the baby missed out on maternal antibodies crossing the placenta, the mother needs to provide them another way.

Breast milk (colostrum and mature milk) contains antibodies (specifically IgA) that are passed to the baby through breastfeeding. This provides the baby with passive immunity in the gut and respiratory tract, protecting against infections in the weeks after birth.

Alternatively, the mother can ensure the baby is vaccinated according to the immunisation schedule, though this takes time to build active immunity. The most direct and immediate source of antibodies for a newborn is breast milk.

Key Takeaways

• Breast milk contains maternal antibodies, providing passive immunity to the baby after birth.
• Colostrum (the first milk) is particularly rich in antibodies and is crucial for newborn immunity.
• Vaccination provides active immunity but takes time to develop; breast milk provides immediate passive immunity.

Common Mistakes

• Writing "keep the baby in a sterile room". While this reduces exposure, it doesn't provide biological protection (antibodies) and is not the accepted mark scheme answer.
• Writing "give the baby antibiotics". Antibiotics treat bacterial infections; they do not protect against viruses or provide immunity.
• Writing "wash hands". This is a general hygiene practice, not a direct protective action for the baby's immune system.

Things to Be Careful About

The command word is Suggest, so any reasonable action that provides immunity or reduces infection risk is acceptable. The mark scheme specifically accepts breast feeding or vaccinate child. Stick to these to guarantee the mark. Ensure the action is something the mother can take (e.g., "breast feed" not "the hospital vaccinate").

Walkthrough

The photomicrograph shows a single cell at $\times 5000$ magnification with a smooth, rounded outline and a characteristic dimpled appearance — this is the biconcave disc shape of a red blood cell (erythrocyte). No nucleus is visible, which fits: mature red blood cells lose their nucleus to make more room for haemoglobin. White blood cells would look very different — larger, often irregular or lobed, with a visible nucleus — and platelets are tiny fragments, far smaller than this cell.

Key Takeaways

• Red blood cells are biconcave discs with no nucleus, packed with haemoglobin.
• The biconcave shape gives a large surface area to volume ratio for oxygen diffusion and flexibility to squeeze through capillaries.
• Either name is accepted: red blood cell or erythrocyte.

Common Mistakes

• Calling it a white blood cell — white cells have nuclei and are less regular in shape.
• Calling it a platelet — platelets are much smaller fragments.
• Spelling 'erythrocyte' incorrectly if choosing that alternative.

Things to Be Careful About

• One mark only — give just the name, nothing else needed.
• Both 'red blood cell' and 'erythrocyte' score; pick whichever you can spell confidently.

Walkthrough

The question gives you three changes to red blood cells in uncontrolled high blood glucose (as happens in diabetes) and asks you to suggest and explain how they affect the body. The mark scheme lists seven points and asks for five, so build a logical chain:

1. Loss of specialised shape → reduced surface area. A red blood cell's biconcave disc shape exists to maximise surface area for oxygen to diffuse across. If the cell swells and loses that shape, the surface area falls, so less oxygen can be absorbed.

2. Less flexible → cannot fit through capillaries. Capillaries are extremely narrow — often barely wider than a red blood cell. Healthy red cells are flexible and fold slightly to squeeze through. Stiff, swollen cells get stuck, so oxygen delivery to those tissues fails.

3. Sticking together → clotting. Cells clumping together can trigger clot formation inside a vessel, blocking blood flow — dangerous if it happens in a small vessel supplying a vital organ.

4. Less oxygen supplied → less aerobic respiration. Haemoglobin in red blood cells carries oxygen to respiring tissues. With fewer working red cells delivering oxygen, body cells receive less oxygen, so the rate of aerobic respiration falls.

5. Less respiration → less energy. Aerobic respiration releases energy from glucose; with less of it, cells have less energy for their functions — muscle contraction, active transport, growth — so the person feels weak and tired.

Notice how the chain runs: shape → surface area → oxygen uptake → oxygen delivery → respiration → energy. Each link is a separate marking point, which is why five marks are available.

Key Takeaways

• Structure and function: the biconcave shape of red blood cells is an adaptation for maximum oxygen absorption and flexibility through capillaries.
• Oxygen is transported bound to haemoglobin in red blood cells.
• Aerobic respiration requires oxygen and releases energy from glucose — less oxygen means less energy released.
• Uncontrolled diabetes damages red blood cells, reducing oxygen delivery throughout the body.

Common Mistakes

• Stopping at 'less oxygen' without following the chain to respiration and energy — the last two marks sit in those links.
• Saying 'less respiration' without specifying aerobic respiration — anaerobic respiration does not need oxygen, so the precise word matters.
• Omitting haemoglobin — naming the molecule that carries oxygen earns its own point.
• Vague answers like 'the person gets sick' — each point must name a specific effect on a specific process.
• Giving more than five points is fine (extras are ignored), but giving fewer than five loses marks — aim for six to be safe.

Things to Be Careful About

• The command word is 'suggest and explain', so every effect needs its reason — an observation alone ('less oxygen') scores nothing without the consequence ('so less aerobic respiration').
• 'Suggest' signals you apply known biology to an unfamiliar context: use what you know about red blood cell structure and respiration, not memorised diabetes facts.
• Any five of the seven listed points score, so choose the clearest links and write them as complete cause-and-effect statements.

Walkthrough

The graph in Fig. 4.1 plots the curvature of the lens against time. The y-axis shows that a higher position means 'most curved' and a lower position means 'least curved'.

1. Start of graph (most curved): The lens is most curved (thickest) to focus on a near object. Action P is reading a book, which is a near object. So P is first.
2. At time H (drop to least curved): The lens becomes least curved (flattest) to focus on a very distant object. Action R is looking at a bird high in the sky, which is very far away (infinity). So R is second. This matches the drop to the bottom of the graph.
3. End of graph (rise to intermediate level): The lens curvature increases to a level between 'most' and 'least'. This is for an object at a medium distance. Action Q is reading a wall clock. A clock is further away than a book but closer than a bird in the sky. So the lens is less curved than for the book but more curved than for the bird. Thus Q is last.

Correct order: P → R → Q.

Key Takeaways

• Near vision (accommodation): Ciliary muscles contract, suspensory ligaments relax, lens becomes more curved (thicker).
• Distant vision: Ciliary muscles relax, suspensory ligaments tighten, lens becomes less curved (flatter).
• The degree of curvature corresponds to the distance of the object: nearest = most curved, furthest = least curved.

Common Mistakes

• Assuming 'most curved' means looking at a distant object. (It's the opposite: a thicker lens bends light more for near objects).
• Confusing the order of Q and R. A wall clock is closer than a bird in the sky, so the lens for the clock is more curved than for the bird (intermediate vs least curved).
• Forgetting to include the arrows in the sequence.

Things to Be Careful About

• Read the y-axis carefully: 'most curved' is at the top, 'least curved' is at the bottom.
• Ensure the sequence matches the time axis (left to right).
• The mark scheme specifically asks for the letters P, R, Q in order.

Walkthrough

At time H, the graph shows the lens shape changing from 'most curved' to 'least curved'. This represents the eye adjusting from viewing a near object (the book, P) to a distant object (the bird, R).

1. Ciliary Muscles: To flatten the lens for distant vision, the ciliary muscles must relax. (Contraction makes the lens rounder for near vision).
2. Suspensory Ligaments: When the ciliary muscles relax, the ring they form gets larger (or rather, the tension on the ligaments changes). The suspensory ligaments (also called zonules of Zinn) become taut (tight / under tension).
3. Lens Shape: The taut ligaments pull on the equator of the lens, pulling it flat. The lens becomes less curved (flatter).

Mark scheme points:

• Name: ciliary muscles.
• Action: relax.
• Name: suspensory ligaments.
• Action: tighten / become taut / under tension.

Key Takeaways

• Near object: Ciliary muscles contract → ligaments relax → lens thickens/more curved.
• Distant object: Ciliary muscles relax → ligaments tighten → lens thins/less curved.

Common Mistakes

• Saying 'ciliary muscles contract' when the lens is flattening. (Contraction makes it rounder).
• Forgetting to name the structures (ciliary muscles, suspensory ligaments). Just saying 'muscles relax' is not enough.
• Saying the ligaments 'relax' when the muscles relax. (They are linked: muscle relax = ligament tight).
• Using the term 'ciliary body' instead of 'ciliary muscles' (though often accepted, stick to the specific muscle name).

Things to Be Careful About

• The question asks to 'Outline, with reference to named structures'. You must name the ciliary muscles and suspensory ligaments.
• The change is at H, which is a drop in curvature (flattening). Ensure your explanation matches this direction (relax/tighten), not the reverse.

Walkthrough

Fig. 4.2 shows a front view of the eye.

• The top label line points to the coloured ring of muscle surrounding the central opening. This is the iris.
• The bottom label line points to the black circular opening in the centre. This is the pupil.

In dim light, the pupil is dilated (large) to let in more light.

Key Takeaways

• Iris: Coloured part, contains muscles that control pupil size.
• Pupil: The hole in the centre that lets light in.

Common Mistakes

• Confusing the iris and the cornea. The cornea is the clear outer layer, not the coloured ring.
• Calling the pupil the 'black spot' instead of the pupil.
• Swapping the labels.

Things to Be Careful About

• Ensure labels match the lines. Top line = Iris, Bottom line = Pupil.

Walkthrough

Fig. 4.3 is the outline of the eye in bright light.

• In bright light, the pupil reflex causes the pupil to constrict (get smaller) to reduce the amount of light entering the eye.
• The candidate must draw the iris (the coloured part) and the pupil.
• The pupil should be drawn as a small circle (small diameter) in the centre.
• The iris fills the rest of the space inside the eyelids.

Key Takeaways

• Bright light: Pupil constricts (gets smaller).
• Dim light: Pupil dilates (gets larger).

Common Mistakes

• Drawing the pupil larger (that's for dim light).
• Forgetting to draw the iris (just drawing a small black circle is often not enough; the iris needs to be shown as the structure controlling it, or at least the space filled). The mark scheme says 'pupil drawn smaller diameter'. Usually, drawing the iris ring is implied or required.
• Drawing the pupil in the wrong position (it should be central).

Things to Be Careful About

• The drawing must show a clear reduction in pupil size compared to Fig. 4.2.
• Use a sharp pencil, continuous lines, no shading (standard biological drawing rules).

Walkthrough

The change in pupil size in response to light is an automatic, involuntary response. It does not involve conscious thought. This is called a reflex (or reflex action).

Key Takeaways

• Reflexes are rapid, involuntary responses to stimuli.
• The pupil reflex is a classic example.

Common Mistakes

• Saying 'automatic' (though true, the biological term is reflex).
• Saying 'conscious' (it is involuntary).
• Saying 'hormonal' (it is nervous).

Things to Be Careful About

• Use the precise term 'reflex'.

Walkthrough

Why do pupils constrict in bright light?

1. Protection: The retina contains light-sensitive cells (rods and cones). Too much light can damage these cells. The pupil restricts the amount of light entering.
2. Prevent damage/saturation: 'Too much light' or 'high light intensity' can overwhelm the receptors or cause damage.
3. Rapid response: The reflex is rapid, allowing immediate adjustment.

Mark scheme points:

• Protect.
• Rapidly (optional but good).
• Retina / rod cells / cone cells / light receptors.
• From too much light / high light intensity.

Key Takeaways

• The pupil regulates light intensity reaching the retina.
• Bright light -> small pupil -> less light -> protection.

Common Mistakes

• Saying 'to see better' (vague).
• Saying 'protect the eye' (too vague, specify retina/receptors).
• Saying 'to stop the light hurting' (not scientific enough).

Things to Be Careful About

• Must mention 'protect' and 'retina/receptors' and 'too much light'.

Walkthrough

First describe what the bars show. Read the first bar (~33 000 tonnes in 2012) and the last bar (~2 000 tonnes in 2020): the overall trend is a decrease, even though 2014 rises above 2013 — say 'overall' to cover that fluctuation. A data quote comparing two years earns a mark.

Then suggest why the government kept cutting the quota. The most likely reason is overfishing: catches under the early high quotas exceeded the rate at which herring replaced themselves, so the stock fell. Each year the government lowered the quota to compensate for the shrinking stock and let it recover. Another biological reason is that fishing removes fish before they have bred, so recruitment of young fish falls. Finally, natural factors — increased predation, disease, competition, or a fall in prey — could also reduce the population independently of fishing.

Note the mark scheme caps explanations at three marks, so description (2 marks) + best three explanations = 4 marks total.

Key Takeaways

• Describe trends with an overall statement plus supporting data quotes.
• Quotas are conservation tools: they are cut when stocks decline because previous removal rates were unsustainable.
• Population size depends on both human harvesting and natural factors (predation, disease, competition).

Common Mistakes

• Describing every year individually instead of giving the overall trend.
• Forgetting to quote data — the comparative quote is a separate mark.
• Giving more than three explanations when only three score.
• Saying 'fish are dying out' vaguely instead of naming overfishing or a specific cause.

Things to Be Careful About

• Use the word 'overall' since 2014 breaks the pattern.
• Quote actual values with units (tonnes) and years.
• Explanations must be biological reasons, not restatements of the graph.

Walkthrough

Quotas are only one tool. The syllabus expects you to know several others, and each must be paired with a correct explanation:

• Education — if people understand why stocks must be conserved, they voluntarily comply with regulations.
• Closed seasons — banning fishing during spawning lets adults breed and young fish grow, rebuilding the stock.
• Protected areas (marine reserves) — areas where no fishing occurs act as reservoirs of breeding fish that spread into fished waters.
• Net mesh size — larger holes let small, immature fish escape, so they survive to breed later.
• Monitoring and enforcement — patrols and catch records ensure quotas and rules are actually followed.

Each pair (method + matched explanation) earns 2 marks; you need two pairs for all four marks.

Key Takeaways

• Conservation methods work either by reducing the number caught, protecting breeding fish, or ensuring rules are followed.
• Always link the method to how it helps the stock — a bare name scores only half.

Common Mistakes

• Naming a method but leaving the explanation blank or generic ('to save fish').
• Repeating the quota idea — quotas are already given in the stem.
• Mismatching, e.g. saying mesh size stops overfishing by limiting total tonnage rather than letting young fish escape.

Things to Be Careful About

• Give exactly two methods, each with its own matched explanation.
• Make the mechanism explicit — the explanation mark depends on the biological effect on the fish stock.

Walkthrough

The table gives milk production per single cow in four different years. The question asks about a fixed herd of 250 cows, so there are two steps.

Step 1 — find how much more one cow produces in 2010 than in 1970: subtract the 1970 value from the 2010 value, $9400 - 4500 = 4900$ kg per year per cow.

Step 2 — scale this up to the whole herd: $4900 \times 250 = 1\,225\,000$ kg per year.

Step 3 — express in standard form. Standard form means $A \times 10^n$ where $A$ is between 1 and 10. Move the decimal point six places left: $1.225 \times 10^{6}$ kg per year.

Note that the mark scheme awards 2 marks for the unconverted figure of 1 225 000, so even if you forget standard form you still score most of the credit — but the third mark requires the conversion.

Key Takeaways

• Read both years' values carefully before subtracting; using the wrong pair of years loses everything downstream.
• Standard form places one non-zero digit before the decimal point; count the places moved to get the power of ten.
• Always attach the unit given on the answer line (kg per year).

Common Mistakes

• Subtracting in the wrong order or using 1950 instead of 1970 as the starting year.
• Forgetting to multiply by 250 and giving only 4900 — the mark scheme allows 1 mark for showing $\times 250$ or 4900, but not full credit.
• Writing $12.25 \times 10^{5}$ — that is not standard form because 12.25 is greater than 10.
• Omitting the unit 'kg per year'.

Things to Be Careful About

• The mark scheme says the response on the answer line takes precedence, so make sure your final written answer is the converted standard-form value, not the intermediate 1 225 000.
• Keep the multiplication sign in $\times 10^{6}$; writing '1225e6' or '1.225E6' may not be credited.
• Round nothing here — 1.225 is exact.

Walkthrough

Selective breeding (artificial selection) is when humans choose which organisms breed, based on a desired characteristic, rather than leaving it to natural selection. The mark scheme credits any three of four points, so structure your answer around the cycle:

1. Selection of parents — choose cows that already produce the most milk. The characteristic must be named in the context: increased milk production, not just 'the best cows'.
2. Breeding — cross/reproduce the selected parents so their genes are passed on.
3. Selection of offspring — from the offspring, pick those showing the greatest milk production and breed only these.
4. Repetition — repeat the cycle over many generations (or many years). Each generation shifts the average yield upward, which is exactly the pattern in Table 6.1: 4500 kg in 1970 rising to 9400 kg in 2010.

Notice the '+' joins in the mark scheme: 'selection + of offspring with increased milk production' means naming the trait AND saying it is the offspring being selected — either half alone does not score that mark.

Key Takeaways

• Selective breeding = choose parents with desired trait → breed → select best offspring → repeat for generations.
• Always name the specific trait being selected for in the given context.
• The repetition across generations is what produces the gradual improvement seen in data tables like Table 6.1.

Common Mistakes

• Describing genetic modification instead of selective breeding — they are different processes; no genes are transferred between species here.
• Saying 'select the biggest cows' without naming milk production as the trait.
• Stopping after one round of breeding without mentioning repetition over generations — that loses the fourth listed point.
• Confusing natural selection with artificial selection: here humans do the choosing.

Things to Be Careful About

• Give exactly three points since the question asks for the main stages worth 3 marks; the scheme lists four options ('any three from'), so pick the clearest three.
• Use the word 'offspring' — selecting the original parents twice does not show you understand generational improvement.

Walkthrough

Genetic modification moves a useful gene from one organism into another. The crop plants here have been engineered to carry a gene giving extra vitamins, and the cows eat these crops. The standard sequence the syllabus expects:

1. Select the gene — identify the DNA that codes for vitamin production. Naming the trait matters: 'select gene/DNA for production of vitamins' is the first marking point.
2. Cut out the gene — enzymes cut the required section of DNA out of the donor organism's chromosome.
3. Insert into a plasmid — a plasmid is a small ring of DNA found in bacteria. The gene is inserted into it. Note the '+' join: insertion AND plasmid together earn that mark.
4. Use the plasmid as a vector — the plasmid carries (transfers) the gene into the cells of the crop plant. A vector is anything used to transfer DNA into another cell. The modified cells are then grown into whole crop plants.

Each step is a separate mark, so all four sentences are needed for all four marks.

Key Takeaways

• Gene transfer sequence: select gene → cut out → insert into plasmid → plasmid acts as vector to deliver the gene to the host organism.
• A plasmid is bacterial DNA used as a carrier (vector) of the new gene.
• The new gene must be described by its function (vitamin production), not just called 'a gene'.

Common Mistakes

• Omitting the plasmid entirely and saying the gene is 'put directly into the plant' — the vector step carries its own marks.
• Saying 'vector' without saying what it is or does — the scheme wants plasmid + as vector/to introduce the gene.
• Vague verbs like 'put the gene in' without mentioning cutting the gene out first.
• Not linking the gene to vitamins — an unnamed gene scores only partially.

Things to Be Careful About

• Two of the four marks use '+': 'insert gene + plasmid' and 'plasmid + as vector'. Both halves of each must appear.
• Keep the steps in logical order; examiners read outlines sequentially.
• Do not describe selective breeding here — this part is specifically about genetic modification.

Walkthrough

These crops contain a chemical making them resistant to insect pests — insects feeding on the crops die. The question asks for potential risks, meaning reasoned possibilities, not proven facts. Any two of the following score:

1. Harm to non-target insects — the chemical cannot distinguish pests from useful insects, so pollinators (bees) or other harmless insects may also be killed, reducing insect populations.
2. Disruption of food chains and ecosystems — fewer insects means less food for insect-eating birds, frogs and other carnivores, so effects spread through the food web.
3. Gene transfer — pollen carrying the resistance gene could fertilise wild relatives or neighbouring non-GM crops, spreading the gene into other populations where its effects are uncontrolled.
4. Unknown long-term effects — the technology is relatively new, so unforeseen consequences on the environment or on people eating the crops cannot be ruled out.

This is a 'suggest'-style application task: take general principles (food webs, interdependence, gene flow) and apply them to the specific case of insect-resistant GM crops.

Key Takeaways

• Risks of GM crops fall into categories: harm to non-target organisms, disruption of food chains/ecosystems, gene escape to wild populations, unknown long-term effects.
• 'Potential risk' invites reasoned possibility — link each risk to a mechanism, not just a worry.

Common Mistakes

• Giving vague answers like 'it might be dangerous' with no mechanism — no mark without a reason.
• Listing advantages instead of risks, or repeating the same risk twice in different words (e.g. 'kills insects' and 'kills pests' — same point).
• Saying the crops will definitely cause harm — the question asks for potential risks, so hedge appropriately.
• Confusing this with risks of selective breeding — the risks asked about are specific to genetically modified insect-resistant crops.

Things to Be Careful About

• Exactly two risks are needed for 2 marks; give two distinct ones, not variations of one idea.
• Each risk should name what is affected (insects, food chain, other plant populations, long-term environment) — specificity earns the mark.