Ch. 17 - Transcription, RNA Processing, and Translation

Freeman - Biological Science 8th Edition

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Freeman 8th Edition

Ch. 17 - Transcription, RNA Processing, and Translation

Problem 13

Chapter 17, Problem 13

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Eating even a single death cap mushroom (Amanita phalloides) can be fatal due to a compound called α-amanitin, a toxin that inhibits transcription.
Toxins like α-amanitin are used for research in much the same way as null mutants (Chapter 16)—to disrupt a process and see what happens when it no longer works. Researchers examined the ability of α-amanitin to inhibit different RNA polymerases. They purified RNA polymerases I, II, and III from rat liver, incubated the enzymes with different concentrations of α-amanitin, and then tested their activity. The results of this experiment are shown here. These findings suggest that cells treated with α-amanitin will have a reduced level of:
a. tRNAs
b. rRNAs
c. snRNAs
d. mRNAs

Verified step by step guidance

Step 1: Begin by understanding the role of RNA polymerases I, II, and III in the cell. RNA polymerase I is responsible for synthesizing rRNA (ribosomal RNA), RNA polymerase II synthesizes mRNA (messenger RNA) and snRNA (small nuclear RNA), and RNA polymerase III synthesizes tRNA (transfer RNA) and some small RNAs.

Step 2: Analyze the experimental setup described in the problem. Researchers tested the activity of RNA polymerases I, II, and III after incubation with different concentrations of αα-amanitin, a toxin known to inhibit RNA polymerase activity.

Step 3: Examine the results of the experiment. Determine which RNA polymerase's activity was most inhibited by αα-amanitin. Typically, αα-amanitin strongly inhibits RNA polymerase II, while RNA polymerases I and III are less affected.

Step 4: Connect the inhibition of RNA polymerase II to the type of RNA it synthesizes. Since RNA polymerase II is responsible for producing mRNA and snRNA, its inhibition would lead to reduced levels of these RNA types in the cell.

Step 5: Based on the findings, conclude that cells treated with αα-amanitin will have a reduced level of mRNAs (messenger RNAs), as RNA polymerase II is the primary target of the toxin.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

RNA Polymerases

RNA polymerases are enzymes responsible for synthesizing RNA from a DNA template. In eukaryotic cells, there are three main types: RNA polymerase I synthesizes rRNA (ribosomal RNA), RNA polymerase II synthesizes mRNA (messenger RNA) and some snRNA (small nuclear RNA), and RNA polymerase III synthesizes tRNA (transfer RNA) and other small RNAs. Understanding the specific functions of these polymerases is crucial for interpreting the effects of inhibitors like αα-amanitin.

αα-amanitin

αα-amanitin is a cyclic peptide toxin derived from the Amanita phalloides mushroom, known for its ability to selectively inhibit RNA polymerase II. This inhibition disrupts the transcription of mRNA, which is essential for protein synthesis. By understanding how αα-amanitin affects different RNA polymerases, researchers can deduce the consequences of its application on cellular processes and RNA production.

Transcription and RNA Types

Transcription is the process by which RNA is synthesized from a DNA template, leading to the production of various RNA types, including mRNA, rRNA, tRNA, and snRNA. Each type of RNA plays a distinct role in the cell: mRNA carries genetic information for protein synthesis, rRNA forms the core of ribosomes, tRNA transports amino acids during translation, and snRNA is involved in RNA splicing. The inhibition of transcription by αα-amanitin will primarily affect the levels of these RNA types, particularly mRNA.

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Introduction to Types of RNA

Related Practice

Textbook Question

Controlling the rates of transcription and translation is important in bacteria to avoid collisions between ribosomes and RNA polymerases. Calculate what the maximum rate of translation by a ribosome in a bacterial cell would have to be, in units of amino acids per second, so as not to overtake an RNA polymerase that is synthesizing mRNA at a rate of 60 nucleotides per second. How long would it take for this bacterial cell to translate an mRNA containing 1800 codons?

Textbook Question

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Eating even a single death cap mushroom (Amanita phalloides) can be fatal due to a compound called α-amanitin, a toxin that inhibits transcription.

What would you predict to be the immediate outcome of adding α-amanitin to a cell?

a. Reduced DNA synthesis

b. Reduced production of one or more types of RNA

c. Reduced binding of tRNAs to anticodons

d. Reduced rate of translocation of ribosomes translating mRNA

Textbook Question

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Eating even a single death cap mushroom (Amanita phalloides) can be fatal due to a compound called α-amanitin, a toxin that inhibits transcription.

α-Amanitin inhibits transcription by binding inside an RNA polymerase to a region other than the active site that catalyzes addition of a nucleotide to the RNA chain. Based on the model of RNA polymerase shown in Figure 17.3, predict how the toxin might function to inhibit transcription.

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Textbook Question

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Eating even a single death cap mushroom (Amanita phalloides) can be fatal due to a compound called α-amanitin, a toxin that inhibits transcription.

If you wanted to use α-amanitin to shut down 95 percent of transcription by RNA polymerase II, roughly what concentration of α-amanitin would you use? Note that the scale on the x-axis of the graph in Question 13 is logarithmic rather than linear, so that each tick mark shows a tenfold higher concentration.

Textbook Question

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Eating even a single death cap mushroom (Amanita phalloides) can be fatal due to a compound called α-amanitin, a toxin that inhibits transcription.

Biologists have investigated how fast pre-mRNA splicing occurs by treating cells with a toxin that blocks the production of new pre-mRNAs, then following the rate of splicing of the pre-mRNAs that were transcribed before adding the toxin. Why is addition of a toxin important in this study?

Textbook Question

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Eating even a single death cap mushroom (Amanita phalloides) can be fatal due to a compound called α-amanitin, a toxin that inhibits transcription.

The primary cause of death from α-amanitin poisoning is liver failure. Suppose a physician informs you that liver cells die because their rate of protein production falls below a level needed to maintain active metabolism. Given that α-amanitin is an inhibitor of transcription, you wonder if this information is correct. Propose an experiment to determine whether the toxin also has an effect on protein synthesis.