AP Biology 6.2 DNA Replication- Exam Style questions - FRQ
Question:
Topic – Gene Expression and Regulation
(a) – 6.2 Regulation of Gene Expression
(b) – 6.2 Regulation of Gene Expression
(c) – 6.3 Transcription and RNA Processing
(d) – 6.4 Translation
The small invertebrate krill species Thysanoessa inermis is adapted to cold (4°C ) seawater. Over the past ten years, there has been a gradual increase in the water temperature of the krill’s habitat. A sustained increase in water temperature may ultimately affect the ability of the krill to survive.
One effect of higher temperatures is protein misfolding within cells. Krill have several hsp genes that code for heat-shock proteins (HSPs). These proteins help prevent protein misfolding or help to refold proteins to their normal shapes.
Scientists conducted experiments on T. inermis to detect changes in the expression of hsp genes when the krill were exposed to temperatures above 4°C. An experimental group of krill was maintained in tanks with 4°C seawater and then placed into tanks with 10°C seawater for approximately three hours. The krill were then given a six-hour recovery period in the 4°C seawater tanks. A control group of krill was moved from a tank of 4°C seawater to another tank of 4°C seawater for approximately three hours and then returned to the original tank. The scientists analyzed hsp gene expression by measuring the concentrations of three mRNAs (I, II, III) transcribed from certain hsp genes in both the heat-shocked krill (Figure 1) and the control krill. For the control krill, no transcription of the hsp genes was detected throughout the test period (data not shown).
Figure 1. Average concentration of three mRNAs (I, II, III) transcribed from hsp genes in krill heat shocked at 10°C. Error bars represent \(± 2SE_{\bar{x}}\).
(a) Identify the hsp mRNA that has the slowest rate of concentration increase in response to heat-shock treatment.
(b) Describe the trend in the average concentration of mRNA I throughout the experiment.
(c) The scientists hypothesized that the heat-shock protein (HSP) translated from mRNA I plays a greater role in refolding proteins than does the HSP translated from mRNA II. Use the data to support the hypothesis.
(d) mRNAs I and II are transcribed from the same gene. Explain how a cell can produce two different mRNAs from the same gene.
Answer/Explanation
Ans:
(a) Identify the hsp mRNA that has the slowest rate of concentration increase in response to heat-shock treatment.
• (mRNA) III
(b) Describe the trend in the average concentration of mRNA I throughout the experiment.
• (No change in concentration from 1 to 3 hours) increased concentration (slightly) between 3 and 6 hours/during the heat shock, increased concentration at a greater rate from 6 to 10 hours/for 4 hours after the heat shock, and then decreased concentration after hour 10.
(c) The scientists hypothesized that the heat-shock protein (HSP) translated from mRNA I plays a greater role in refolding proteins than does the HSP translated from mRNA II .
Use the data to support the hypothesis.
• mRNA I is still expressed at a high level after the heat-shock period, while mRNA II levels decrease after the heat shock, when proteins would need to be refolded.
(d) mRNAs I and II are transcribed from the same gene. Explain how a cell can produce two different mRNAs from the same gene.
Accept one of the following:
• The cell expresses different exons/performs alternative splicing.
• The cell uses different transcription termination sites (poly(A) sites).
• The cell uses different promoters.
(a) The hsp mRNA III has the slowest rate of concentration increase in response to heat-shock treatment, as shown on the graph.
(b) When the krill are in 40C water, there is no mRNA I. When the krill are placed in 100C tanks, the concentration of mRNA I increase from θ to 5 ng/mL. When the krill are placed back in 40C tanks for a 6 hour recovery, mRNA I concentration increase for the first 4 hours from 5 to 25 ng/ml, then decreases for the final 2 hours from 25 to 17 ng/ml.
(c) The graph shows that the mRNA I concentration increases most drastically after the krill are moved back to 40C . This suggests that after krill proteins are denatured, then mrNA II has a steep drop-off in concentration after the krill are moved back to 40C , indicating that it is not involved in refolding denatured proteins.
(d) A cell can produce two different mRNAs from the same gene through a process called alternative rNA splicing. Genes contain coding regions called exons that alternative with noncoding regions called introns. When transcription occurs, the pre-MrNA transcript has both exons and introns. The transcript then has its introns removed by a spliceosome and the exons are joined mRNAs can be produced depending on which regions are treated as exons and which are treated as introns.