Continuity and Change — Free Biology HL Practice Questions

Alternative splicing is a key post-transcriptional regulatory mechanism in eukaryotes. The Drosophila gene dsx (doublesex) is alternatively spliced to produce two different transcription factors that control sex determination. In females, the splicing pattern includes exon 4, producing the $\text{Dsx}^\text{F}$ protein, which represses male-specific genes. In males, exon 4 is skipped, producing $\text{Dsx}^\text{M}$ protein, which represses female-specific genes. The splicing decision is regulated by the female-specific protein Tra (transformer), which binds to a splicing enhancer sequence in the pre-mRNA. Researchers created a transgenic Drosophila line in which the splicing enhancer sequence in exon 4 was mutated so that Tra protein cannot bind. The table below summarises exon 4 inclusion data. Genotype — Exon 4 inclusion (%) Wild-type females — 95 Wild-type males — 5 Mutant females — 15 Mutant males — 4

A strain of E. coli is cultured in a medium containing only lactose as a carbon source, in the absence of glucose. After $30$ minutes, glucose is added to the medium. The graph below shows the relative concentration of $\beta$-galactosidase over time.

The lac operon in Escherichia coli is a classic model for transcriptional regulation. The operon contains three structural genes (lacZ, lacY, lacA) controlled by a promoter and operator region. A regulatory gene, lacI, encodes the Lac repressor. When lactose is present, allolactose (an isomer of lactose) binds to the Lac repressor, causing a conformational change that prevents operator binding, allowing transcription. Additionally, when glucose is absent, elevated intracellular cAMP binds to CAP (catabolite activator protein); the cAMP–CAP complex binds to the CAP site upstream of the promoter, enhancing RNA polymerase binding and increasing transcription rate. A research team studied a mutant strain of E. coli (strain M1) carrying a point mutation in lacI that alters the allolactose-binding site of the repressor. The mutant repressor retains full operator-binding ability but cannot bind allolactose. $\beta$-galactosidase activity (encoded by lacZ) was measured under three conditions: - Condition A: $2\%$ glucose, no lactose - Condition B: $2\%$ lactose, no glucose - Condition C: $1\%$ glucose and $1\%$ lactose Wild-type $\beta$-galactosidase activity was high only in Condition B. Strain M1 showed negligible $\beta$-galactosidase activity in all three conditions.

A marathon runner consumes a high-carbohydrate meal 2 hours before a race. During the race, her blood glucose concentration is monitored. Time (min) — Blood glucose $(\text{mmol}\,\text{L}^{-1})$ 0 — 5.2 30 — 4.8 60 — 4.5 90 — 4.3 120 — 4.0

A patient with Type 2 diabetes mellitus has a fasting blood glucose concentration of $9.8\,\text{mmol}\,\text{dm}^{-3}$ (normal range: $3.9\text{–}5.6\,\text{mmol}\,\text{dm}^{-3}$). The patient's pancreatic beta cells still produce insulin, but body cells show reduced responsiveness to the hormone. A doctor prescribes Metformin, a drug that increases the sensitivity of liver and muscle cells to insulin.

In a species of sea urchin (Arbacia punctulata), external fertilization occurs in seawater. Researchers observed that the acrosomal reaction in sperm is triggered by a specific glycoprotein on the surface of the egg's jelly coat.

A student investigated sporophyte production in Funaria hygrometrica over two years. The table shows the number of sporophytes counted in sample plots. Year — Rainfall classification — Sporophytes counted Year 1 — Wet — $84$ Year 2 — Dry — $12$ The life cycle of Funaria hygrometrica involves a dominant haploid gametophyte generation. The gametophyte produces gametes by mitosis, and fertilization requires water for sperm to swim to the egg.

In Drosophila melanogaster the diploid chromosome number is $2n = 8$. A geneticist studies a temperature-sensitive mutation that causes complete failure of spindle fibre formation at the restrictive temperature ($30^\circ\text{C}$). Cells are assumed to begin G1 of the cell cycle before entering division.

In a laboratory, a culture of human skin cells is treated with a chemical that prevents the formation of spindle fibres during mitosis.

A research team investigated the use of microalgae (Chlorella vulgaris) as a renewable biofuel feedstock. The algae were grown in open raceway ponds ($500\,\text{m}^2$ each) under natural sunlight over a 30-day growth period. Data collected: - Initial algal biomass: $0.25\,\text{g\,L}^{-1}$ (dry mass) - Final algal biomass: $1.80\,\text{g\,L}^{-1}$ (dry mass) - Pond volume: $100{,}000\,\text{L}$ - Lipid content of dried algae: $32\%$ by mass - Energy content of algal lipids: $37.8\,\text{MJ\,kg}^{-1}$ - Energy required for harvesting and lipid extraction: $8.2\,\text{MJ}$ per kg of dry algae processed - $\text{CO}_2$ absorbed during growth: $1.83\,\text{kg CO}_2$ per kg dry algae - $\text{CO}_2$ emitted during biodiesel combustion: $2.68\,\text{kg CO}_2$ per kg biodiesel - $\text{CO}_2$ emitted from processing energy: $0.94\,\text{kg CO}_2$ per kg dry algae Useful relationships: $$\text{Net energy} = \text{Energy output} - \text{Energy input}$$ $$\text{Mass} = \text{concentration} \times \text{volume}$$

The island nation of Palau has committed to protecting $80\%$ of its marine territory as a 'no-take' marine reserve. A team of ecologists is evaluating the sustainability of this conservation strategy by comparing fish communities inside the reserve with those in adjacent fishing zones. Data from a 2023 survey: - Simpson's diversity index ($D$) inside the reserve: $0.92$ - Simpson's diversity index ($D$) in the fishing zone: $0.68$ - Total fish biomass inside the reserve: $4500\,\text{kg km}^{-2}$ - Total fish biomass in the fishing zone: $2100\,\text{kg km}^{-2}$ - Annual fish catch in the fishing zone: $380\,\text{kg km}^{-2}\,\text{year}^{-1}$ - Estimated annual fish recruitment in the fishing zone: $420\,\text{kg km}^{-2}\,\text{year}^{-1}$ - Reserve area: $250\,\text{km}^2$; fishing zone area: $800\,\text{km}^2$

A genetic counsellor analyses a family pedigree for a rare autosomal recessive disorder controlled by a single gene (alleles $D$ and $d$). The disorder causes severe metabolic dysfunction and is often fatal in early childhood. An affected individual (genotype $dd$) has two unaffected parents. The affected individual marries an unaffected woman with no known family history of the disorder.

In a species of fruit fly (Drosophila melanogaster), grey body colour ($G$) is dominant over black body colour ($g$). A female fly heterozygous for body colour is crossed with a male fly that has a black body. In a large experiment, 800 offspring are produced: 560 have grey bodies and 240 have black bodies.

A student investigates osmosis using cylinders of potato (Solanum tuberosum) tissue. She cuts five cylinders of equal size and mass, then places each in a different concentration of sucrose solution. After 30 minutes, she removes the cylinders, blots them dry, and measures their mass again. Her results are shown below. Sucrose concentration $(\text{mol dm}^{-3})$ — $0.0$ — $0.2$ — $0.4$ — $0.6$ — $0.8$ Percentage change in mass $(\%)$ — $+12$ — $+5$ — $-2$ — $-8$ — $-15$

A researcher studies water uptake in the freshwater plant Elodea canadensis. She places a leaf in a $10\%$ sucrose solution and observes the cells under a microscope. Initially, the chloroplasts are distributed throughout each cell. After 10 minutes, the chloroplasts are clumped in the centre of each cell, and the cell membrane has pulled away from the cell wall.

A research team investigates the production of a therapeutic enzyme, human deoxyribonuclease I (DNase I), in E. coli bacteria. The wild-type human DNASE1 gene is inserted into a bacterial expression plasmid. The protein produced is enzymatically inactive despite being successfully translated. Analysis reveals that the recombinant DNase I lacks removal of an N-terminal signal peptide that targets the protein for secretion in human cells. The team engineers a modified DNASE1 gene coding directly for the mature form of the protein (without the signal peptide). This modified gene is expressed in E. coli. The resulting protein is active but is degraded by intracellular proteases within 30 minutes, whereas human wild-type DNase I is stable for over 24 hours in the bloodstream.

A group of students studies the effects of a novel drug, Modifin, on post-translational modification of the tumour suppressor protein p53 in human cell cultures. p53 is activated by phosphorylation at serine residues in response to DNA damage. Two sets of cells are exposed to UV radiation. Set A receives no Modifin. Set B receives Modifin immediately after UV exposure. Phosphorylated p53 (detected at serine 15) and total p53 are measured over 6 hours. - Set A (control): phosphorylated p53 peaks at 2 hours ($100\,\text{units}$), declines to baseline by 6 hours; total p53 remains constant at $100\,\text{units}$ throughout. - Set B (Modifin-treated): phosphorylated p53 peaks at 2 hours ($100\,\text{units}$), remains at $80\,\text{units}$ at 6 hours; total p53 increases by $50\%$ over 6 hours. - Modifin specifically inhibits phosphatase PP2A, which removes phosphate groups from proteins. - Rate of p53 translation is constant and identical in both sets.

A research team studies DNA replication in thermophilic archaeon Pyrococcus furiosus, which lives at $100^\circ\text{C}$. The DNA polymerase incorporates nucleotides at $500$ nucleotides per second at $100^\circ\text{C}$. Replication is bidirectional from a single origin of replication.

In 1958, Meselson and Stahl investigated the mechanism of DNA replication using E. coli. Bacteria were grown for many generations in a medium containing heavy nitrogen ($^{15}$N), ensuring all DNA was fully labelled. They were then transferred to a medium containing light nitrogen ($^{14}$N) and allowed to replicate. After each round of replication, DNA was extracted and its density analysed by caesium chloride density gradient centrifugation.

The CFTR gene encodes the CFTR chloride ion channel protein. Mutations in this gene cause cystic fibrosis. The wild-type CFTR protein is $1480$ amino acids long. A patient with severe cystic fibrosis has a single cytosine (C) nucleotide deleted from exon 10 of the CFTR gene. The wild-type mRNA sequence around this region is: $$5'\text{-} \ldots \text{AAG AAC AUA UCC UUC GAU GAG}\ldots\text{-}3'$$ corresponding to the amino acid sequence: Lys – Asn – Ile – Ser – Phe – Asp – Glu. The deletion removes the first nucleotide (U) of the UCC codon (Ser) from the mRNA.

A geneticist studies a population of Drosophila melanogaster examining mutations in a gene responsible for eye colour. The wild-type allele produces red eyes. Three mutant strains are isolated: - Strain 1: A single base substitution in the coding region changes codon GAG (Glu) to GUG (Val). - Strain 2: A deletion of a single adenine nucleotide in the coding region. - Strain 3: A chromosomal translocation moves a segment of chromosome 2 containing the eye colour gene to chromosome 3. All three strains show white eyes instead of red.

International climate agreements such as the Paris Agreement rely on Nationally Determined Contributions (NDCs), which are each country's self-defined targets for reducing greenhouse gas emissions. A key biological mitigation strategy promoted in many NDCs is the adoption of agricultural practices that increase soil carbon sequestration, including no-till farming, cover cropping, and agroforestry.

Coral reefs are highly sensitive to ocean warming. In 2016, a marine heatwave caused mass coral bleaching on the Great Barrier Reef. Scientists monitored two coral species: the branching coral Acropora and the massive coral Porites. Before the heatwave, Acropora covered $40\%$ of the reef area and Porites covered $30\%$. After the heatwave, Acropora cover fell to $8\%$ and Porites cover fell to $21\%$.

In a population of marine iguanas (Amblyrhynchus cristatus) on the Galápagos island of San Cristóbal, researchers measured the body length of 500 adult iguanas. Iguanas with a medium body length ($60$–$70\,\text{cm}$) were best able to dive and feed on algae in the intertidal zone. Very small iguanas (under $50\,\text{cm}$) were easily swept away by strong currents, while very large iguanas (over $80\,\text{cm}$) had difficulty manoeuvring in rocky crevices where algae grow. Over a 10-year period, researchers recorded lifetime reproductive output: Size class — Body length — Mean offspring per lifetime Small — $<50\,\text{cm}$ — $2$ Medium — $60$–$70\,\text{cm}$ — $8$ Large — $>80\,\text{cm}$ — $3$

A population of Darwin's finches (Geospiza fortis) on the island of Daphne Major in the Galápagos experiences a severe drought in 1977. The drought reduces the availability of small, soft seeds, forcing finches to eat large, hard seeds that require a deep beak to crack open. Researchers measured beak depth (a heritable trait) before and after the drought. Before drought (1976) — $9.2\,\text{mm}$ — $0.8\,\text{mm}$ After drought (1978) — $10.1\,\text{mm}$ — $0.6\,\text{mm}$