(a) The coding-strand DNA substitution changes GAA to GTA, which is transcribed as an mRNA change from GAA to GUA. The changed mRNA codon can specify a different amino acid, such as glutamate to valine. The amino-acid substitution can alter folding or a binding site and therefore protein function.

The coding-strand DNA substitution changes GAA to GTA, which is transcribed as an mRNA change from GAA to GUA. The changed mRNA codon can specify a different amino acid, such as glutamate to valine. The amino-acid substitution can alter folding or a binding site and therefore protein function.

(b) Reduced activator binding at the P* enhancer can reduce recruitment of transcription machinery at the promoter. Reduced transcription initiation would lower P mRNA abundance. Quantifying P mRNA in matched cells, normalised to a validated reference gene, would test the prediction.

Reduced activator binding at the P* enhancer can reduce recruitment of transcription machinery at the promoter. Reduced transcription initiation would lower P mRNA abundance. Quantifying P mRNA in matched cells, normalised to a validated reference gene, would test the prediction.

(c) Equal cell number makes mRNA amount comparable between groups. The same signal prevents signal strength from confounding the effect of allele status.

Equal cell number makes mRNA amount comparable between groups. The same signal prevents signal strength from confounding the effect of allele status.

(a) More active sites are occupied as substrate concentration increases. At high substrate concentration, most active sites are occupied so enzyme concentration limits the rate.

More active sites are occupied as substrate concentration increases. At high substrate concentration, most active sites are occupied so enzyme concentration limits the rate.

(b) Reaching the same maximum at higher substrate is consistent with competitive inhibition. Inhibitor I competes with substrate S for the active site. High substrate concentration reduces the inhibitor's effect by increasing substrate-active-site encounters.

Reaching the same maximum at higher substrate is consistent with competitive inhibition. Inhibitor I competes with substrate S for the active site. High substrate concentration reduces the inhibitor's effect by increasing substrate-active-site encounters.

(c) Expose enzyme to inhibitor I, then remove or greatly dilute free inhibitor before adding substrate. Include an otherwise identical untreated-enzyme control. Recovery of activity after inhibitor removal would support reversible binding.

Expose enzyme to inhibitor I, then remove or greatly dilute free inhibitor before adding substrate. Include an otherwise identical untreated-enzyme control. Recovery of activity after inhibitor removal would support reversible binding.

(a) An antigen-presenting cell processes virus R and displays an antigen fragment. A helper T cell with a complementary receptor binds the displayed antigen-MHC complex.

An antigen-presenting cell processes virus R and displays an antigen fragment. A helper T cell with a complementary receptor binds the displayed antigen-MHC complex.

(b) A B cell with a complementary receptor is selected after binding the antigen and receiving activation signals. The selected B cell proliferates to form a clone. Plasma cells from the clone secrete antibodies specific to the viral antigen.

A B cell with a complementary receptor is selected after binding the antigen and receiving activation signals. The selected B cell proliferates to form a clone. Plasma cells from the clone secrete antibodies specific to the viral antigen.

(c) The secondary response will be faster and usually larger than the primary response. Antigen-specific memory cells persist and rapidly undergo clonal expansion on re-exposure. Faster antibody production can clear the virus before severe disease develops.

The secondary response will be faster and usually larger than the primary response. Antigen-specific memory cells persist and rapidly undergo clonal expansion on re-exposure. Faster antibody production can clear the virus before severe disease develops.

(a) Town A cases fell by 52, from 80 to 28. Town B cases fell by 21, from 76 to 55. The larger fall in Town A is associated with, but does not alone prove, an effect of the campaign.

Town A cases fell by 52, from 80 to 28. Town B cases fell by 21, from 76 to 55. The larger fall in Town A is associated with, but does not alone prove, an effect of the campaign.

(b) Vaccinated people are less likely to become susceptible hosts or develop transmissible infection. Fewer infectious contacts interrupt transmission chains. Reduced circulation indirectly protects some susceptible people through population-level immunity.

Vaccinated people are less likely to become susceptible hosts or develop transmissible infection. Fewer infectious contacts interrupt transmission chains. Reduced circulation indirectly protects some susceptible people through population-level immunity.

(c) Comparable testing and reporting rates in both towns are needed to exclude surveillance differences. Information about other controls, movement, prior immunity or demographic differences is needed to assess confounding.

Comparable testing and reporting rates in both towns are needed to exclude surveillance differences. Information about other controls, movement, prior immunity or demographic differences is needed to assess confounding.

(a) Mutations arise independently of whether they will be useful in a later environment. The resistant variant was already present before antibiotic exposure. The antibiotic selected among existing heritable variants rather than directing a needed mutation.

Mutations arise independently of whether they will be useful in a later environment. The resistant variant was already present before antibiotic exposure. The antibiotic selected among existing heritable variants rather than directing a needed mutation.

(b) Antibiotic Z kills or inhibits susceptible bacteria more strongly. Resistant bacteria survive and reproduce at a higher rate under treatment. They pass the resistance allele to descendants, increasing its frequency over generations.

Antibiotic Z kills or inhibits susceptible bacteria more strongly. Resistant bacteria survive and reproduce at a higher rate under treatment. They pass the resistance allele to descendants, increasing its frequency over generations.

(c) Selection favouring resistance would weaken when the antibiotic is absent. If resistance carries a fitness cost, susceptible bacteria may increase relative to resistant bacteria.

Selection favouring resistance would weaken when the antibiotic is absent. If resistance carries a fitness cost, susceptible bacteria may increase relative to resistant bacteria.

(a) Species J and K are inferred to be the closest pair. Their sequences differ at only 4 positions, fewer than either comparison with L.

Species J and K are inferred to be the closest pair. Their sequences differ at only 4 positions, fewer than either comparison with L.

(b) Sequence similarity provides evidence about relative relatedness. The fossil provides evidence of morphology and occurrence in the past. Radiometric age with uncertainty constrains the timing of the lineage but is not an exact divergence date.

Sequence similarity provides evidence about relative relatedness. The fossil provides evidence of morphology and occurrence in the past. Radiometric age with uncertainty constrains the timing of the lineage but is not an exact divergence date.

(c) The inference uses one gene and may not represent the whole evolutionary history. Fossil assignment and dating have uncertainty. Additional genes or fossils could support or change the proposed relationship.

The inference uses one gene and may not represent the whole evolutionary history. Fossil assignment and dating have uncertainty. Additional genes or fossils could support or change the proposed relationship.

(a) A guide RNA base-pairs with a complementary target sequence. Cas9 cuts DNA at or near the targeted site. Cellular DNA repair can introduce a sequence change that disrupts the regulatory site.

A guide RNA base-pairs with a complementary target sequence. Cas9 cuts DNA at or near the targeted site. Cellular DNA repair can introduce a sequence change that disrupts the regulatory site.

(b) Disrupting the regulatory sequence is associated with reduced transcription or mRNA abundance. Less mRNA provides fewer templates for translation. The protein reduction is consistent with, but does not prove only, reduced mRNA availability.

Disrupting the regulatory sequence is associated with reduced transcription or mRNA abundance. Less mRNA provides fewer templates for translation. The protein reduction is consistent with, but does not prove only, reduced mRNA availability.

(c) Sequence the target region and assess likely off-target sites to verify edit specificity. Use independent edited lines or a rescue treatment to distinguish the intended edit from clone-specific effects.

Sequence the target region and assess likely off-target sites to verify edit specificity. Use independent edited lines or a rescue treatment to distinguish the intended edit from clone-specific effects.

(a) Increasing light intensity will increase the mean rate of oxygen production by the algae. The prediction applies while other relevant conditions are controlled.

Increasing light intensity will increase the mean rate of oxygen production by the algae. The prediction applies while other relevant conditions are controlled.

(b) The independent variable is assigned light intensity. The dependent variable is change in dissolved oxygen per unit time.

The independent variable is assigned light intensity. The dependent variable is change in dissolved oxygen per unit time.

(c) Keep water and electrical equipment separated or use a low-voltage sealed light source. Keep algal amount, temperature, solution volume or carbon dioxide availability constant.

Keep water and electrical equipment separated or use a low-voltage sealed light source. Keep algal amount, temperature, solution volume or carbon dioxide availability constant.

(d) Replicates reveal variability and permit a more reliable treatment mean. A dark control estimates oxygen change not caused by light-dependent photosynthesis.

Replicates reveal variability and permit a more reliable treatment mean. A dark control estimates oxygen change not caused by light-dependent photosynthesis.

(a) Mean rate increases from pH 4 to pH 7 and decreases at higher pH. The highest tested mean is 6.1 mL per minute at pH 7.

Mean rate increases from pH 4 to pH 7 and decreases at higher pH. The highest tested mean is 6.1 mL per minute at pH 7.

(b) Extreme pH can alter charges and interactions maintaining enzyme structure. A changed active-site shape or chemistry reduces productive substrate binding.

Extreme pH can alter charges and interactions maintaining enzyme structure. A changed active-site shape or chemistry reduces productive substrate binding.

(c) Without spread or uncertainty, the reliability of differences between means cannot be judged. Record individual trials and present a suitable spread or uncertainty measure with additional pH values near 7.

Without spread or uncertainty, the reliability of differences between means cannot be judged. Record individual trials and present a suitable spread or uncertainty measure with additional pH values near 7.