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Nucleic Acids · Paper 2 — Free Biology HL Practice Questions

1MasterySAQ-SGene expression regulation7 marksPaper 2~11 min

The bacterium E. coli can use either glucose or lactose as a carbon source. When both sugars are present, the lac operon is repressed until glucose is depleted. The graph below shows the relative intracellular lactose concentration over time after both sugars are added simultaneously at

t = 0

(a)

State the role of the lac repressor protein regulating the lac operon when lactose is absent. [1 mark]

(b)

Explain why intracellular lactose concentration remains low during the first 20 minutes, even though lactose is present in the growth medium. [2 marks]

(c)

Calculate the average rate of increase of intracellular lactose concentration between

t = 20\,\text{min}

and

t = 40\,\text{min}

. Give your answer in arbitrary units per minute. [2 marks]

(d)

The plateau at

9.0

arbitrary units is reached at

t = 40\,\text{min}

and maintained thereafter. Deduce what this plateau indicates about the balance between lactose uptake and lactose metabolism in E. coli after

t = 40\,\text{min}

. [2 marks]

Solutions

2MasterySAQ-SDNA replication and transcription8 marksPaper 2~12 min

A researcher studies transcription of a gene in a eukaryotic cell. The gene is

1200

base pairs long. RNA polymerase II moves along the template strand at a rate of

50

nucleotides per second. - The gene contains

4

introns, each

150

base pairs long - Average mass of one ribonucleotide in mRNA:

5.0 \times 10^{-22}\,\text{g}

(a)

State the role of RNA polymerase during transcription. [1 mark]

(b)

Calculate the time, in seconds, taken for RNA polymerase II to transcribe the entire gene. [2 marks]

(c)

Calculate the mass, in grams, of the mature mRNA molecule after splicing. [2 marks]

(d)

Explain how RNA splicing produces a functional mRNA molecule from the pre-mRNA transcript, and suggest one advantage this process provides to the cell. [3 marks]

Solutions

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