Interpreting and evaluating experimental results

A small RNA that silences a gene

Unit 6
9 points
About 25 minutes

You can use a calculator on this question, just like on exam day.

A long question built around a real-world experiment and its data in a table or graph. You describe the biology behind it, pick out the experiment's variables and controls, analyze the data or do a calculation, and make and justify a prediction. On the exam: Question 1 of 6 in Section II (90 minutes, 50% of the exam score); about 25 minutes suggested.

The question and its sources

Some small RNA molecules made by cells regulate gene expression after a gene has been transcribed. Researchers studied whether a small RNA called miR-R regulates Gene K in cultured human cells. Gene K codes for protein K. Near its 3′ end, the mRNA of Gene K has a 22-nucleotide sequence that is complementary to miR-R. This sequence is not part of the protein-coding region.

Experiment

The researchers grew identical dishes of cells and gave them one of three treatments, with six dishes per treatment. Every dish received the same delivery reagent, a chemical that carries small RNA molecules into cells.

Treatment 1: delivery reagent only, with no RNA. Treatment 2: a scrambled RNA, with the same length and the same nucleotides as miR-R but in a random order that is not complementary to any mRNA in these cells. Treatment 3: miR-R.

After 48 hours, the researchers measured the number of Gene K mRNA molecules per cell and the amount of protein K in each dish.

Source: Hypothetical study

Table 1. Gene K mRNA and protein K after 48 hours, mean ± 2SE

Treatment	Gene K mRNA (copies per cell)	Protein K (ng per mg of total protein)
Reagent only	152 ± 12	48 ± 4
Scrambled RNA	148 ± 14	47 ± 5
miR-R	63 ± 9	11 ± 3

Source: Hypothetical data

Suggested time: 25 minutes

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Part (a)

1 point

Describe ONE way that a small regulatory RNA such as miR-R can reduce the expression of a gene.

Your answer to part (a)

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Part (b)

3 points

(i) State the null hypothesis for the effect of miR-R on the amount of protein K. (ii) Justify the researchers' inclusion of the scrambled RNA treatment. (iii) Based on Table 1, describe the effect of miR-R on the amount of protein K compared with the scrambled RNA.

Your answer to part (b)

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Part (c)

3 points

(i) Calculate the percent decrease in protein K in cells given miR-R compared with cells given the scrambled RNA. (ii) Based on Table 1, determine whether the scrambled RNA had a significant effect on the number of Gene K mRNA molecules compared with the reagent-only treatment. Support your answer with the data. (iii) Gene K mRNA decreased by about 57 percent with miR-R, compared with the scrambled RNA, while protein K decreased by a larger percent. Explain how miR-R could cause protein K to decrease by a larger percent than Gene K mRNA.

Your answer to part (c)

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Part (d)

2 points

The researchers make a line of cells in which several nucleotides in the 22-nucleotide sequence of Gene K are changed, so the mRNA sequence is no longer complementary to miR-R. The protein-coding region is not changed. (i) Predict the amount of protein K in these mutant cells given miR-R, compared with the same mutant cells given the scrambled RNA. (ii) Justify your prediction.

Your answer to part (d)

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