Interpreting and evaluating experimental results

Growth factor signaling and the cell cycle

Unit 4
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

Many animal cells divide only when they receive a signal from a growth factor. In a certain type of cell, growth factor GF binds to a receptor in the plasma membrane. This activates Kinase A, which phosphorylates and activates Kinase B. Active Kinase B leads to increased production of a cyclin, which allows the cell to pass the G₁ checkpoint and enter S phase.

Experiment

Researchers grew identical cultures of these cells in a medium without growth factor for 24 hours. They then gave each culture one of four treatments: no GF; GF; GF plus inhibitor X, a molecule that blocks the activity of Kinase A; or inhibitor X alone.

After 24 more hours, the researchers measured the relative amount of phosphorylated Kinase B and the percent of cells in S phase. Each treatment had six replicate cultures.

Source: Hypothetical study

Table 1. Phosphorylated Kinase B and cells in S phase, mean ± 2SE

Treatment	Relative amount of phosphorylated Kinase B	Cells in S phase (%)
No GF	1.0 ± 0.2	6 ± 2
GF	4.8 ± 0.5	38 ± 4
GF + inhibitor X	1.3 ± 0.3	9 ± 3
Inhibitor X alone	0.9 ± 0.2	5 ± 2

Source: Hypothetical data

Suggested time: 25 minutes

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Part (a)

1 point

Describe what happens to a cell's DNA during S phase of the cell cycle.

Your answer to part (a)

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Part (b)

3 points

(i) Identify the treatment that serves as the negative control for the effect of GF. (ii) Justify the researchers' inclusion of the inhibitor X alone treatment. (iii) Based on Table 1, describe the effect of GF on the relative amount of phosphorylated Kinase B.

Your answer to part (b)

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Part (c)

3 points

(i) Calculate the percent increase in cells in S phase in the GF treatment compared with the no-GF treatment. (ii) Based on Table 1, determine whether the percent of cells in S phase in the GF + inhibitor X treatment is significantly different from the no-GF treatment. Support your answer with the data. (iii) Based on Table 1, describe the relationship between the relative amount of phosphorylated Kinase B and the percent of cells in S phase across the four treatments.

Your answer to part (c)

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Part (d)

2 points

Researchers study a line of these cells with a mutation that makes Kinase B active at all times, even when it is not phosphorylated by Kinase A. (i) Predict the percent of mutant cells in S phase in the no-GF treatment compared with the percent of normal cells in S phase in the no-GF treatment. (ii) Justify your prediction.

Your answer to part (d)

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