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 4

Chapter 17, Problem 4

Compared with mRNAs that have a cap and tail, predict what will be observed if a eukaryotic mRNA lacked a cap and poly(A) tail.
a. The primary transcript would not be processed properly.
b. Translation would occur inefficiently.
c. Enzymes on the ribosome would add a cap and poly(A) tail.
d. tRNAs would become more resistant to degradation.

Verified step by step guidance

Understand the role of the cap and poly(A) tail in eukaryotic mRNA. The 5' cap is added to the beginning of the mRNA during processing and helps protect the mRNA from degradation, assists in ribosome binding during translation, and facilitates nuclear export. The poly(A) tail, added to the 3' end, also protects the mRNA from degradation and aids in translation efficiency.

Analyze the options provided in the question. Option (a) refers to improper processing of the primary transcript, which is not directly related to the absence of a cap and tail since these modifications occur after transcription. Option (b) suggests inefficient translation, which is plausible because the cap and tail are critical for ribosome binding and stability. Option (c) implies that enzymes on the ribosome would add the cap and tail, which is incorrect because these modifications occur in the nucleus, not on the ribosome. Option (d) mentions tRNAs, which are unrelated to mRNA processing and translation.

Focus on the biological consequences of lacking a cap and poly(A) tail. Without these modifications, the mRNA is more susceptible to degradation by exonucleases, and the ribosome may struggle to efficiently bind and initiate translation.

Consider the process of translation initiation in eukaryotes. The 5' cap is recognized by initiation factors that recruit the ribosome, and the poly(A) tail interacts with proteins that stabilize the mRNA and enhance translation. Without these features, translation efficiency would be significantly reduced.

Conclude that the most likely observation is inefficient translation (option b), as the absence of the cap and poly(A) tail compromises mRNA stability and ribosome binding, which are essential for effective protein synthesis.

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Key Concepts

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

mRNA Capping and Polyadenylation

In eukaryotic cells, mRNA undergoes capping and polyadenylation, which are essential modifications. The 5' cap protects the mRNA from degradation and assists in ribosome binding during translation. The poly(A) tail at the 3' end also stabilizes the mRNA and facilitates its export from the nucleus. Without these modifications, mRNA is more susceptible to degradation and less efficient in translation.

Translation Efficiency

Translation efficiency refers to how effectively ribosomes synthesize proteins from mRNA. A properly capped and polyadenylated mRNA enhances ribosome recognition and binding, leading to more efficient translation. If an mRNA lacks these features, it may not be recognized by the ribosome, resulting in slower or incomplete translation processes.

mRNA Degradation Mechanisms

Eukaryotic mRNAs are subject to degradation by cellular enzymes, particularly when they lack protective features like the 5' cap and poly(A) tail. These modifications help prevent exonucleases from degrading the mRNA. Without them, the mRNA is more vulnerable to degradation, which can lead to reduced protein synthesis and cellular function.

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Related Practice

Textbook Question

What does a bacterial RNA polymerase produce when it transcribes a protein-coding gene?

a. rRNA

b. tRNA

c. mRNA

d. snRNA

Textbook Question

Where is the start codon located?

a. At the start (5′ end) of the mRNA

b. In the DNA just upstream of where transcription starts

c. At the downstream end of the 5′ untranslated region (UTR)

d. At the upstream end of the 3′ untranslated region (UTR)

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Textbook Question

Splicing begins:

a. As transcription occurs.

b. After transcription is complete.

c. As translation occurs.

d. After translation is complete.

Textbook Question

RNases and proteases are enzymes that destroy RNAs and proteins, respectively. Which of the following enzymes, if added to a spliceosome, would be predicted to prevent recognition of pre-mRNA regions critical for splicing?

a. An RNase specific for tRNAs

b. An RNase specific for snRNAs

c. A protease specific for initiation factors

d. A protease specific for a release factor

Textbook Question

For each of these statements about the genetic code, select True or False.

a. T/F Wobble pairing accounts for the redundancy of the genetic code.

b. T/F There are 64 different tRNAs that read the 64 possible codons.

c. T/F All possible codons are used, but not all codons specify an amino acid.

d. T/F Some codons are recognized by proteins, not by tRNAs.

Textbook Question

In a particular bacterial species, temperature-sensitive conditional mutations cause expression of a wild-type phenotype at one growth temperature and a mutant phenotype at another—typically higher—temperature. Imagine that when a bacterial cell carrying such a mutation is shifted from low to high growth temperatures, RNA polymerases in the process of elongation complete transcription normally, but no new transcripts can be started. The mutation in this strain most likely affects:

a. The terminator sequence

b. The start codon

c. Sigma

d. One of the polypeptides of the core RNA polymerase