overview_of_eukaryotic_gene

Question 1 of 7

Animation Assessment

Question

What effect might you expect in a eukaryotic organism in which an enzyme essential for adding the 5’ cap to RNA polymerase II transcripts was nonfunctional?

The organism would be unable to produce stable mRNA transcripts.

The organism would be unable to efficiently splice RNA transcripts.

The organism’s ribosomes would not recognize any mRNA transcripts produced.

The organism would be unable to survive.

All of the above will result from nonfunctional mRNA capping.

Correct. The failure to cap an RNA polymerase II transcript will interfere with splicing, stability, and translation in eukaryotic cells. This would be a condition incompatible with life.

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Incorrect. The failure to cap an RNA polymerase II transcript will interfere with splicing, stability, and translation in eukaryotic cells. This would be a condition incompatible with life.

Question

If the 3’ consensus sequence was mutated in a particular gene, what effect would you expect on the RNA transcript?

If the 3’ consensus sequence were mutated, transcription would not initiate.

If the 3’ consensus sequence were mutated, splicing would not occur.

If the 3’ consensus sequence were mutated, the RNA transcript would not be capped.

If the 3’ consensus sequence were mutated, the transcript will not be polyadenylated and will be unstable.

All of the above would result if the 3’consensus sequence were mutated.

Correct. If the 3’ consensus is mutated, the RNA transcript will not cleave properly and no polyadenylation will occur. This will not affect either capping or splicing, but without polyadenylation, the transcript will most likely be unstable.

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Incorrect. If the 3’ consensus is mutated, the RNA transcript will not cleave properly and no polyadenylation will occur. This will not affect either capping or splicing, but without polyadenylation, the transcript will most likely be unstable.

Question

If a eukaryotic gene underwent a single base pair substitution in the middle of an exon, what effect would you expect on the transcript of that gene?

A single base pair substitution in an exon would be unlikely to affect the transcript unless it created a new splice consensus site.

A single base pair substitution in an exon will almost always disrupt mRNA splicing.

A single base pair substitution in an exon will prevent mRNA capping.

A single base pair substitution in an exon will destabilize the mRNA transcript.

All of the above will result from a single base pair substitution within an exon.

Correct. With the exception of the branch point consensus, the internal sequence of the exon does not influence intron splicing. Therefore, in most cases a single nucleotide substitution in the middle of an exon would not affect the transcript. If, however, the base substitution created a new splice consensus site, it could lead to aberrant splicing. A base substitution in the middle of an exon would not prevent mRNA capping and would not be expected to destabilize the transcript.

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Incorrect. With the exception of the branch point consensus, the internal sequence of the exon does not influence intron splicing. Therefore, in most cases a single nucleotide substitution in the middle of an exon would not affect the transcript. If, however, the base substitution created a new splice consensus site, it could lead to aberrant splicing. A base substitution in the middle of an exon would not prevent mRNA capping and would not be expected to destabilize the transcript.

Question

What effect on the transcript would you expect if a eukaryotic gene underwent a two base pair deletion at the 5’ end of an intron?

A two base pair deletion at the 5’ end of an intron would not be expected to have any effect on the resulting transcript.

A two base pair deletion at the 5’ end of an intron would most likely prevent the normal spicing of this intron.

A two base pair deletion at the 5’ end of an intron would prevent cleavage and polyadenylation at the 3’ end.

A two base pair deletion at the 5’ end of an intron would destabilize the mRNA.

A two base pair deletion at the 5’ end of an intron would prematurely end transcription of the gene.

Correct. The two nucleotides at the 5’end of the intron are in the heart of the 5’ splice consensus. Their deletion would prevent splicing at that site. The disruption of splicing should not prevent cleavage at the 3’ end nor should it result in premature transcription termination. Disrupted splicing may indirectly lead to transcript destabilization or problems in transcript export, but answer B, is clearly the best choice.

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Incorrect. The two nucleotides at the 5’end of the intron are in the heart of the 5’ splice consensus. Their deletion would prevent splicing at that site. The disruption of splicing should not prevent cleavage at the 3’ end nor should it result in premature transcription termination. Disrupted splicing may indirectly lead to transcript destabilization or problems in transcript export, but answer B, is clearly the best choice.

Question

Which of the following steps in mRNA processing occurs first?

Fifty to 250 adenine nucleotides are added to the 3’ end of the RNA transcript.

The introns are removed from the primary transcript.

The mRNA transcript is exported to the cytoplasm.

The 5’ end of the primary transcript is capped.

The primary transcript is cleaved at the 3’ end.

Correct. Capping of the 5’ end takes place shortly after transcription initiation.

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Incorrect. Capping of the 5’ end takes place shortly after transcription initiation.

Question

Which of the following best describes the first step of intron removal during mRNA processing?

The 3’ end of the intron is cleaved from the following exon and simultaneously attached to the branch point within the intron.

The intron is cleaved at the 3’ end with simultaneous joining of the exons flanking the intron.

The 5’ end of the intron is cleaved.

The 5’ end of the intron is cleaved from the preceding exon and simultaneously joined to the branch point within the intron.

The 5’ and 3’ ends of the intron are simultaneously cleaved as the exons flanking the intron are joined.

Correct. The first step of intron removal is cleavage at the 5’ end of the intron with a simultaneous transesterification reaction joining of the 5’ phosphate of the intron to the 2’ hydroxyl group on the ribose sugar of the adenosine nucleotide at the branch point.

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Incorrect. The first step of intron removal is cleavage at the 5’ end of the intron with a simultaneous transesterification reaction joining of the 5’ phosphate of the intron to the 2’ hydroxyl group on the ribose sugar of the adenosine nucleotide at the branch point.

Question

Which of the following best describes the consensus sites required for splicing introns during mRNA processing?

consensus sequences at the 5’ and 3’ splice sites as well as a branch point consensus sequence within the intron

consensus sequences only at the 5’ splice site and 3’ splice site of the intron

consensus sequences at the 5’ and 3’ splice sites as well as a branch point consensus sequence within the following exon

consensus sequences at the 5’ and 3’ splice sites as well as a branch point consensus sequence within the preceding exon

consensus sequences at the 5’ and 3’ ends of the mRNA

Correct. There are three different consensus sequences that play important roles during mRNA intron splicing. One of these encompasses the 5’ end of the intron and includes a nearly invariant GU dinucleotide at the intron 5’ end. Another consensus encompasses the 3’ end of the intron and includes a nearly invariant AG dinucleotide at the intron 3’ end. The third consensus encompasses the branch point that is typically 18-40 nucleotides upstream of the 3’ splice site of the intron and invariably contains the branch point adenosine nucleotide.

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Incorrect. There are three different consensus sequences that play important roles during mRNA intron splicing. One of these encompasses the 5’ end of the intron and includes a nearly invariant GU dinucleotide at the intron 5’ end. Another consensus encompasses the 3’ end of the intron and includes a nearly invariant AG dinucleotide at the intron 3’ end. The third consensus encompasses the branch point that is typically 18-40 nucleotides upstream of the 3’ splice site of the intron and invariably contains the branch point adenosine nucleotide.

Chapter 14.2. Animation Activity: Overview of Eukaryotic Gene Expression

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