Round 1: cell division and molecular detail
At exactly which substage of meiosis does crossing over occur?
Pachytene, the third substage of prophase I. Synapsis forms the bivalent in zygotene, crossing over occurs in pachytene via the recombination nodules, and the chiasmata become visible in diplotene. 'Prophase I' scores half a mark; the substage is the question.
How many ATP molecules does complete aerobic respiration of one glucose molecule actually yield?
Around 30 to 32 by current estimates, not the 36 to 38 in older textbooks. The older figure assumed fixed proton-to-ATP ratios and ignored the cost of transporting NADH and ADP across the mitochondrial membrane. Knowing that it is an estimate with a range is itself the more advanced answer.
Which enzyme unwinds the DNA double helix during replication, and which relieves the resulting torsional strain?
Helicase unwinds the strands; topoisomerase (DNA gyrase in bacteria) relieves the supercoiling ahead of the fork. Students who name only polymerase are describing synthesis, not unwinding.
Why is the genetic code described as degenerate but unambiguous?
Degenerate because most amino acids are specified by more than one codon - 61 sense codons for 20 amino acids. Unambiguous because each individual codon specifies only one amino acid. Both halves are required; students frequently give only the first.
Which enzyme maintains the ends of linear chromosomes, and why is one needed at all?
Telomerase, a reverse transcriptase that extends telomeres. It is needed because DNA polymerase cannot replicate the extreme 3' end of a template - the end-replication problem - so chromosomes would shorten with every division without it.
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Round 2: the names designed to mislead
Which artery carries deoxygenated blood, and which vein carries oxygenated blood?
The pulmonary artery carries deoxygenated blood from the right ventricle to the lungs, and the pulmonary veins carry oxygenated blood back to the left atrium. Arteries are defined by carrying blood away from the heart, not by oxygen content - the umbilical vessels in a fetus are the other classic exception.
Photoperiodism: does a short-day plant flower because the day is short?
No - it flowers because the continuous dark period exceeds a critical length. Interrupting the night with a brief flash of light prevents flowering, even though the day length is unchanged. They are more accurately called long-night plants, and this is one of the most-tested misconceptions in plant physiology.
Which plant hormone is a gas, and what does it do?
Ethylene, which promotes fruit ripening, senescence and abscission. Because it is a gas it acts on neighbouring fruit too, which is why one ripe fruit hastens the ripening of others in the same box.
Which organelles contain their own DNA, and what does that suggest?
Mitochondria and chloroplasts. Both have circular DNA, their own ribosomes and double membranes, which supports the endosymbiotic theory - that they originated as free-living prokaryotes engulfed by an ancestral eukaryotic cell.
Which cells actually secrete antibodies?
Plasma cells, which are terminally differentiated B lymphocytes. Saying 'B cells' is imprecise: a resting B cell displays antibody as a surface receptor but does not secrete it until it differentiates.
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Round 3: mechanisms, not labels
What is the Bohr effect, and why is it useful?
A fall in pH, or a rise in carbon dioxide, shifts the oxygen dissociation curve of haemoglobin to the right, lowering its oxygen affinity. This means haemoglobin releases oxygen most readily in exactly the tissues that are metabolically active and therefore acidic - a mechanism, not just a curve shift.
What binds calcium ions to initiate muscle contraction?
Troponin. Calcium released from the sarcoplasmic reticulum binds troponin, which shifts tropomyosin off the myosin-binding sites on actin, allowing cross-bridges to form. Answering 'actin' or 'myosin' names the filaments but misses the switch.
What drives the ascent of water in a tall tree?
Transpiration pull, explained by the cohesion-tension theory: evaporation from leaf surfaces creates negative pressure, and the cohesive hydrogen bonding between water molecules plus adhesion to xylem walls transmits that tension down an unbroken column. Root pressure contributes little and cannot account for tall trees.
By what mechanism does translocation occur in phloem?
The pressure-flow or Münch hypothesis: active loading of sugars at the source lowers water potential, water enters from the xylem, and the resulting hydrostatic pressure drives bulk flow to the sink, where unloading reverses the gradient. Unlike xylem transport, this requires energy at the loading step.
Which enzyme fixes carbon dioxide in the Calvin cycle, and what is its other, wasteful activity?
RuBisCO, which carboxylates ribulose bisphosphate. It also has oxygenase activity, binding oxygen instead of carbon dioxide, which initiates photorespiration and wastes fixed carbon and energy - and is precisely what the C4 and CAM pathways exist to minimise.
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Round 4: comparisons that need precision
How does the counter-current mechanism in the kidney concentrate urine?
The loop of Henle and the vasa recta run in opposite directions, and the descending limb is permeable to water while the ascending limb actively transports salt without water. This maintains a steep osmotic gradient in the medullary interstitium, which the collecting duct then exploits to reabsorb water under the influence of ADH.
What distinguishes apoptosis from necrosis?
Apoptosis is programmed, energy-requiring and orderly - the cell shrinks, DNA is cleaved at regular intervals and fragments are cleared without inflammation. Necrosis is uncontrolled injury: the cell swells, ruptures and releases contents, provoking inflammation.
Which enzyme allows a retrovirus to integrate into a host genome, and why is it remarkable?
Reverse transcriptase, which synthesises DNA from an RNA template - the reverse of the usual flow of genetic information. Its discovery required amending the central dogma, and it is the enzyme HIV depends on.
What is a prion, and why is it unlike every other infectious agent?
A misfolded protein that propagates by inducing normal copies of the same protein to misfold. It contains no nucleic acid at all, which is why it resists treatments that destroy DNA or RNA and why it challenged the assumption that inheritance of infection requires a genome.
Where is ADH synthesised, and where is it released from?
Synthesised in the hypothalamus, transported down axons and released from the posterior pituitary. The posterior pituitary stores and releases rather than manufactures - a distinction commonly lost, since the hormone is usually just labelled 'pituitary'.
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How did you score, and what does it mean?
Look at the pattern of your misses rather than the total. Missing the substage of crossing over or the exact site of ADH synthesis means your recall is one level coarser than the exam wants - fixable by revising with precision rather than more volume. Missing the Bohr effect, the cohesion-tension theory or the pressure-flow hypothesis means you have learnt names without mechanisms, which is a different and more important gap.
Missing the pulmonary artery question or the photoperiodism question means a textbook misconception has settled in, and those are worth hunting deliberately - there are perhaps a dozen of them in the whole syllabus and they are disproportionately represented in hard questions precisely because they catch good students.
- ✓For precision gaps: revise with the page covered, writing substages and exact structures from memory.
- ✓For mechanism gaps: for each process, write one sentence on what drives it. If you cannot, you have a label rather than an understanding.
- ✓For misconception gaps: keep a running list of the counter-intuitive ones - pulmonary vessels, long-night plants, arteries defined by direction, ATP yield as a range.
- ✓Then drill under time pressure, because precision that only appears with unlimited thinking time does not survive an exam.
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