HSC Biology 2023

(a) [1 mark]. A phosphate group, a five-carbon sugar (deoxyribose in DNA) and a nitrogenous base (adenine, thymine, cytosine or guanine).

(b) [2 marks]. Transcription pairs each DNA base with its complement, and in RNA uracil (U) replaces thymine. For the template read from CAGT:

The key point is that the base pairing with adenine is uracil, not thymine.

Marker's note. Name all three nucleotide components (sugar, phosphate, base), and remember that uracil replaces thymine when building the complementary mRNA strand.

(a) [2 marks]. Phagocytes recognise a pathogen as non-self by its surface antigens, then engulf (enclose) it by phagocytosis to form a vesicle inside the cell. Enzymes in lysosomes then break the pathogen down, destroying it.

(b) [4 marks]. A pathogen entering the body carries antigens (protein markers) recognised as non-self. An antigen-presenting cell (e.g. a macrophage) displays the antigen and activates a matching helper T cell, which releases cytokines. These activate the specific B cell whose receptor fits the antigen. That B cell divides and differentiates into plasma cells, which secrete large numbers of antibodies specific to that antigen. The antibodies bind the pathogen, neutralising it and tagging it for destruction. Memory B cells also form for a faster future response.

Marker's note. In (a) use precise terms such as "engulf" or "enclose". In (b) give the steps in sequence (antigen recognition, helper T cell, plasma B cells, antibodies) and keep the antibody-mediated response distinct from the cell-mediated response.

[4 marks].

(Any two hormones from the graph are acceptable if the function and peak trimester match.)

Marker's note. Match each hormone to a genuine function using correct terminology, and read the graph carefully to identify the trimester in which each named hormone peaks.

(a) [3 marks]. In myopia (short-sightedness) the eyeball is too long, or the cornea and lens refract light too strongly. Light from distant objects is therefore focused at a point in front of the retina rather than on it. Because the image does not form on the light-sensitive retina, distant objects appear blurred while near objects are seen clearly. So the altered structure (an over-long eye) directly changes the function (focusing) and causes the disorder.

(b) [3 marks]. LASIK laser surgery assists with refractive disorders such as myopia. A thin flap is cut and lifted from the surface of the cornea, then a laser precisely removes (reshapes) corneal tissue to flatten the cornea so light is refracted less. The flap is laid back into place. This shifts the focal point back onto the retina, so distant objects are focused sharply and the patient no longer needs corrective lenses.

Marker's note. Name a specific disorder and identify the parts of the eye involved (e.g. the cornea or lens), then link the change in structure to the change in function. In (b) match the technology to the named disorder and state its effect on vision.

(a) [3 marks]. Individual H is affected, and because Huntington's is dominant she carries at least one H allele. She has unaffected children (J and L), each of whom must have received a recessive normal allele (h) from her. So H must also carry an h allele. Therefore H is heterozygous (Hh), not homozygous HH.

The square shows that an Hh parent can have hh (unaffected) children only if she carries the h allele, which is why J and L being unaffected proves H is Hh.

(b) [1 mark]

Glutamine (the codon CAG codes for glutamine).

(c) [2 marks]

There is an inverse relationship: as the number of CAG repeats increases, the age of onset of Huntington's disease decreases. For example, a person with about 60 repeats develops symptoms at around 20 years of age, whereas fewer repeats give a later onset. More repeats produce a more severely altered Huntingtin protein, so the disease manifests earlier.

(d) [3 marks]

Individual S is predicted to be not affected: their gel band sits at about 15 CAG repeats, which is within the normal range (10 to 26) and matches their unaffected father P. Individual U is predicted to develop Huntington's disease at about 45 years of age: U's band is at about 38 repeats, in the affected range (37 to 80) and matching their mother Q, whose recorded age of onset is around 45.

Marker's note. In (a) reason from the children's genotypes (unaffected children force an h allele in H). In (b) read the codon chart from the centre without re-transcribing. In (c) name the variables correctly (age of onset versus number of repeats) and quote data. In (d) use both diagrams and tie the number of repeats to the predicted age of onset.

(a) [2 marks].

• Dependent variable: the number of times mosquitoes landed on the clothing in an hour.
• Controlled variable: the number of mosquitoes in each container (20), or the size of the container, or the observation time (one hour). Any one is acceptable.

(b) [3 marks]. Wearing clean clothing reduces the transmission of malaria. The infected mosquitoes landed far more often on the worn clothing (Container A, averaging about 17 landings) than on the clean clothing (Container C, averaging about 11), and the same pattern holds for the uninfected containers. Because an infected mosquito can only pass on malaria when it lands on and bites a person, fewer landings on clean clothing means fewer chances to transmit the disease. The trend is consistent across all five repeats, supporting the conclusion.

Marker's note. Read the dependent variable from the results table (landings, not "transmission of malaria"), and give a single controlled variable. In (b) quote quantitative data (averages or totals), and link the conclusion back to the aim, that clean clothing reduces transmission.

[7 marks]. The study has the basic form of a cohort epidemiological study (defined groups monitored over time), but as a method for establishing a link between air pollution and neurological symptoms it is weak and not valid, so any causal conclusion would be unreliable.

Strengths: the sample sizes are reasonable (500 per city, 1500 total), the groups are comparable in age and include both sexes, and the same 12-month monitoring is applied to each city.

Weaknesses that undermine validity:

• No measure of the exposure. Air pollution levels in each city, and each person's actual exposure, are not recorded, so there is no way to show that higher pollution corresponds to more symptoms (no dose-response).
• Confounding variables are not controlled. Occupation, smoking, locality within the city (near industry or not), socioeconomic status and ethnicity are not accounted for, and any of these could produce similar symptoms.
• Time period may be too short. Chronic neurological effects of pollution can take many years to appear, so 12 months may miss them.
• Symptoms are not quantified by severity, only as present or absent, which is a blunt measure.

Judgement: because exposure is not measured and confounders are uncontrolled, the study cannot establish that air pollution causes the symptoms. A valid study would measure pollution exposure for each subject, control for occupation and other risk factors, use a much larger and more varied population, and run for longer.

Marker's note. Engage directly with specific features of the study from the stimulus, use several of them to support the analysis, and finish with a clear judgement about validity. Avoid simply describing the table.

(a) [2 marks]. Size is a clear distinguishing feature: a bacterium is a cellular prokaryote about 1 to 10 micrometres across, whereas a virus is non-cellular (just nucleic acid in a protein coat) and far smaller, about 0.05 to 0.1 micrometres. So a bacterium is much larger than a virus and is cellular, while a virus is not.

(b) [2 marks]. Attempt to culture a water sample on nutrient agar. Bacteria will grow into visible colonies on the agar because they can reproduce independently; a virus will not grow, because it needs living host cells to replicate. If colonies form, the pathogen is bacterial; if none form (and disease is still confirmed), it is likely viral. Examining a sample under a high-power light microscope is a useful check, since bacteria are visible but viruses are too small to see.

Marker's note. Compare one distinguishing feature (size, cellular versus non-cellular, or how each replicates) rather than just listing features. In (b) outline a workable method (e.g. agar culture) and state the expected result for each pathogen type.

(a) [2 marks]. Vasoconstriction is a physiological adaptation in endotherms. When the body cools, the blood vessels near the skin surface narrow, reducing blood flow to the skin. Less warm blood reaches the surface, so less heat is lost to the environment and core body temperature is maintained. (Vasodilation in heat, shivering, or sweating are also acceptable.)

(b) [4 marks].

• Thick waxy cuticle: a waterproof waxy layer covers the leaf surface and acts as a barrier to evaporation, so water cannot easily escape from the cells beneath. This reduces water loss and helps keep the plant's water balance.
• Sunken stomata: the stomata sit in pits in the epidermis, often with hairs. This traps a pocket of humid air directly over each stoma, so the water-vapour gradient between the leaf and the air is smaller, which lowers the rate of transpiration and conserves water.

Marker's note. In (a) choose a genuinely physiological adaptation (not a behavioural or structural one) and link it to the internal environment. In (b) for each plant adaptation give what it is, how it works, and why it conserves water, not just a name.

[5 marks]. Vaccinations and the recommended schedule are highly valuable for controlling tetanus. A vaccine introduces a harmless form of the antigen (e.g. inactivated tetanus toxin) so the immune system mounts a primary response and forms memory cells, without causing the disease. This gives immunity should the real pathogen be encountered.

The graph supports the schedule: each successive dose raises the tetanus antibody level higher and the level stays above the "immune" threshold for longer, so each booster extends the period of protection. The drop in case numbers after vaccines were introduced in 1953 shows the program works. The fact that most remaining cases occur in people over 65 fits the data: those people were vaccinated long ago and their antibody levels have since fallen below the protective level because they have not had a recent booster.

Judgement: the schedule of repeated boosters is well justified, since it is needed to keep antibody levels above the immune threshold across a lifetime, and it would be improved by ensuring older adults receive boosters.

Marker's note. Make a clear judgement (not just a description of the graph), show correct understanding of what a vaccine does in the body, and use all the stimulus, the graph and the information about over-65s, to support the assessment.

[4 marks]. Recombinant DNA technology is used to manufacture human insulin for treating diabetes. A restriction enzyme cuts the human insulin gene from human DNA, leaving sticky ends. The same restriction enzyme cuts open a bacterial plasmid (e.g. from E. coli), giving complementary sticky ends. DNA ligase joins the insulin gene into the plasmid to form recombinant DNA. The plasmid is taken up by the bacterium, which is then cultured; as the bacteria multiply they transcribe and translate the human gene, producing large amounts of human insulin. The insulin is harvested, purified and injected by people with diabetes to manage their blood glucose.

Marker's note. Name a specific genetic technology and use precise terminology (restriction enzyme, sticky ends, ligase, plasmid), then describe its steps and match it to a genuinely relevant medical application.

[7 marks]

Bushfires and the introduction of males had opposite effects, the fires harming the population and gene pool while the introductions rescued both, so overall the management intervention was beneficial.

Bushfires (harmful)

the Mt Buller population was already small (about 90 in 1996), so allele diversity in the gene pool was low. Successive fires killed individuals and pushed the population down, removing alleles from the gene pool. With so few survivors, allele frequencies shifted at random and rare alleles were lost, a population bottleneck and genetic drift. This left the population small (around eight individuals by 2007) and genetically depleted, reducing its capacity to adapt.

Introduction of males (beneficial)

in 2007 and 2012, six males from the isolated Mt Bogong population were introduced. Because the two populations had been separated, these males carried different alleles, so breeding them in caused gene flow into Mt Buller. This raised genetic diversity, restored variation and increased the chance that some individuals carried adaptive alleles. The graph shows the population recovering strongly, from about eight in 2007 to roughly 150 by 2016.

Judgement: the bushfires reduced both population size and genetic diversity through a bottleneck and drift, while the genetic-rescue introductions reversed this through gene flow, so the intervention markedly improved the long-term health of the Mt Buller gene pool and population.

Marker's note. Use the correct terms (genetic drift / bottleneck effect for the fires, gene flow for the introductions), explain the effect on both population size and gene pool, quote data from the graph, and finish with an informed judgement.

[4 marks].

Similarities:

• Both use meiosis in reproduction: in humans meiosis produces gametes (sperm and eggs), and in fungi meiosis produces spores.
• Both involve a fusion of genetic material that forms a diploid (2n) zygote during sexual reproduction.

Differences:

• In humans all reproduction is sexual, whereas fungi reproduce both sexually and asexually (asexual spores by mitosis).
• In humans fertilisation of two haploid gametes immediately produces a diploid zygote, whereas in fungi the cytoplasm of two mycelia fuses first, giving a stage with two separate nuclei (n + n) before the nuclei finally fuse to form the zygote.

Marker's note. Translate the stimulus into more than one genuine similarity and difference, and link each process to an outcome (e.g. "meiosis produces gametes in humans and spores in fungi"), rather than just listing common features.

[5 marks]. The biotechnologies in the table have a mixed effect on cattle biodiversity, but the overall effect is to decrease it.

• Selective breeding decreases biodiversity: only cows with the desired trait (high milk yield) are bred, so over generations the gene pool narrows to the favoured alleles and the dairy breeds become genetically uniform.
• Artificial insemination decreases biodiversity sharply: as the example shows, one record-holding bull sired cattle in 50 countries, so the genes of a few elite bulls dominate the population and many other bulls contribute nothing.
• Whole organism cloning decreases biodiversity, since clones are genetically identical to the donor, but its effect is small because only 30 to 40 clones exist in Australia and they are not used commercially.
• Transgenic organisms could increase biodiversity by introducing genes not originally present, but as the table notes their use is not widespread, so the current impact is minimal.
• Hybridisation increases biodiversity by combining the genes of Bos taurus and Bos indicus into new gene combinations, though it can reduce it if hybrids are then bred in preference to the pure breeds.

Judgement: because the widely used technologies (selective breeding and artificial insemination) strongly narrow the gene pool, while the diversity-increasing ones (transgenics, hybridisation) are limited in use, the net effect of these biotechnologies is to reduce the biodiversity of cattle.

Marker's note. Use the specific examples in the table, judge whether each biotechnology increases, maintains or decreases biodiversity, link the technology to its effect on the gene pool (not just the individual organism), and reach an overall judgement concisely.

(a) [1 mark]. A point (substitution) mutation - one base pair is replaced by another.

(b) [4 marks]. During protein synthesis the DNA base sequence is transcribed into mRNA codons, which are translated into a specific order of amino acids. The substitution changes one base, which changes one codon, and the effect depends on the new codon:

• No effect (silent): because the genetic code is degenerate, the new codon may still code for the same amino acid, so the protein is unchanged.
• A different amino acid (missense): the new codon may code for a different amino acid, changing the primary structure. This can alter the way the polypeptide folds, changing the three-dimensional shape of the protein and so its function, possibly making it non-functional.
• A premature stop (nonsense): the new codon may become a stop codon, ending translation early so a short, incomplete and usually non-functional protein is made.

So a single substitution can range from no change to a completely non-functional protein, depending on how the codon and therefore the amino acid sequence is altered.

Marker's note. Sequence the steps of protein synthesis correctly, then link a change in the base sequence to a change in the amino acid sequence, the protein's shape and finally its function. Cover several possible outcomes (silent, missense, nonsense) and engage with the stimulus.