Gene Expression · Paper 2 — Free Biology HL Practice Questions

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.

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

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.