Gene Expression (College Board AP® Biology)

A researcher is studying patterns of gene expression in mice. The researcher collected samples from six different tissues in a healthy mouse and measured the amount of mRNA from six genes. The data are shown in Figure 1.

mRNA EXPRESSION LEVELS

Figure 1. mRNA expression levels of six genes

Based on the data provided, identify the gene that is most likely to encode a protein that is an essential component of glycolysis. Provide reasoning to support your identification.

The researcher observed that tissues with a high level of gene H mRNA did not always have gene H protein.

Provide reasoning to explain how tissues with high gene H mRNA levels can have no gene H protein.

The small invertebrate krill species Thysanoessa inermis is adapted to cold (4^oC) 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^oC. An experimental group of krill was maintained in tanks with 4^oC seawater and then placed into tanks with 10^oC seawater for approximately three hours. The krill were then given a six-hour recovery period in the 4^oC seawater tanks. A control group of krill was moved from a tank of 4^oC seawater to another tank of 4^oC 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, Ill) 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, Ill) transcribed from hsp genes in krill heat shocked at 10^oC. Error bars represent ±

Identify the hsp mRNA that has the slowest rate of concentration increase in response to heat-shock treatment.

Describe the trend in the average concentration of mRNA I throughout the experiment.

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.

mRNAs I and II are transcribed from the same gene. Explain how a cell can produce two different mRNAs from the same gene.

Housekeeping genes encode proteins involved in universally important processes such as transcription, translation, and glycolysis. Because these genes appear to be expressed in all cells at constant levels, the expression of housekeeping genes is often used as a control when comparing how the expression of other genes varies under different conditions.

Researchers studying the effect of pesticides on declining bee populations wanted to determine whether the expression of four housekeeping genes (GAPDH, RPL32, RPS5, and TBP-AF) was in fact constant in bees across different variables. The researchers collected samples of mRNA for each of the four genes and compared how their expression varied across the developmental stage of the bee, the sex of the bee, and the cell type from which the sample was taken. The mRNA from the samples was reverse transcribed to produce DNA copies of each gene. PCR was then used to amplify the DNA, and the Cq value was determined. The Cq value is the number of PCR cycles needed to produce a specified number of DNA copies. A high Cq value for a sample indicates the gene was expressed at a low level.

To analyze whether any of the examined variables affected expression of the housekeeping genes, researchers examined the range of Cq values for each gene in response to each variable. Genes with a wide range of Cq values were determined to be affected by the variable, while genes with a narrow range of Cq values were determined to be unaffected by the variable.

Figure 1. The effect of developmental stage, sex, and cell type on the Cq value of four housekeeping genes

Based on the data in Figure 1, identify the gene that had the lowest median Cq value when bees of different developmental stages were compared.

The Cq value is inversely proportional to the amount of mRNA from that gene in the starting sample. Based on the data in Figure 1, identify the gene that has the lowest level of gene expression regardless of variable.

The scientists investigated the effect of pesticides on the expression of other genes in one cell type of a group of bees containing males and females of the same developmental stage. They hypothesized that TBP-AF would serve as the best control gene for this experiment. Use the data to evaluate their hypothesis.

Explain how expression of a gene such as GAPDH can vary from one cell type to another within the same bee.