GRE Subject Test: Biology : Understanding Introns and Exons

Study concepts, example questions & explanations for GRE Subject Test: Biology

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Example Questions

Example Question #1 : Understanding Introns And Exons

In a eukaryotic cell, a molecule of pre-mRNA is found to have four exons and three introns. Which of the following are possible combinations of the exons, if the order in which they are written is the order in which they will be translated?

I. Exon 1, Exon 2, Exon 3, Exon 4

II. Exon 1, Exon 3, Exon 4

III. Exon 4, Exon 1, Exon 2, Exon 3

Possible Answers:

I only

I, II, and III

I and II

II only

Correct answer:

I and II

Explanation:

This question is asking about alternative splicing. Alternative splicing is a means by which several different proteins can arise from the same pre-mRNA due to the order in which the exons are organized. This typically takes the form of exon skipping. Therefore, both 1 and 2 are potential mature mRNAs that could arise from this pre-mRNA. Option 3 is not an acceptable transcript, however, because alternative splicing maintains the integrity of the genomic order of the exons (i.e. exon 4 will not come before exon 1, 2, or 3).

Example Question #2 : Understanding Introns And Exons

__________ are parts of __________ molecules that do not contain information about a protein's primary structure.

Possible Answers:

Introns . . . mRNA

Introns . . . pre-mRNA

Exons . . . mRNA

Exons . . . pre-mRNA

Correct answer:

Introns . . . pre-mRNA

Explanation:

After transcription, the resulting RNA molecule must undergo post-transcriptional modification before it becomes mature mRNA. Before these modifications, it is known as heteronuclear RNA (htRNA) or pre-mRNA.

Introns are portions of pre-mRNA molecules that are spliced prior to translation. Unlike exons, introns do not contain information about the structure of the protein. Only after intron splicing is the molecule considered mRNA.

Example Question #3 : Understanding Introns And Exons

The primary transcript is much longer than the mRNA that will eventually be translated. This can be explained by which of the following?

Possible Answers:

The poly-A tail is still on the primary transcript

Exons have not yet been added to the transcript

Introns have not yet been removed from the transcript

The 5' cap has not yet been added to the transcript

Correct answer:

Introns have not yet been removed from the transcript

Explanation:

Immediately following transcription, the primary transcript will undergo a variety of changes before being translated. One of the largest changes is that a spliceosome complex will remove introns from the primary transcript. Introns are not involved in protein creation, and their removal makes the transcript much shorter. The final mRNA transcript consists of a string of exons, a 5' cap, and a 3' poly-A tail.

Example Question #4 : Understanding Introns And Exons

In most cases, introns are spliced out of mature messenger RNA (mRNA) and are not a part of the final translated protein product of a gene. Even though they are not included in the final protein, why are introns important?

Possible Answers:

None of these

Introns are involved in some special regulatory functions like mRNA export and non-sense mediated decay

All of these

Introns can generate non coding RNAs that influence gene expression

Introns allow for alternative splicing of exons to create multiple proteins from one gene sequence

Correct answer:

All of these

Explanation:

These are all reasons that introns are important, despite the fact that they are not included in final proteins. Introns can allow for alternative splicing of exons, in which exons are placed in different orders to create different proteins from one gene. In the gene Dscam in Drosophila, alternative splicing allows for around 38,000 different proteins from one gene sequence. Some introns become non-coding RNAs that control expression of genes. Lastly, it has recently been shown that introns are involved in some special functions like mRNA export - in which mRNA's are moved between the nucleus and other cellular compartments.

Example Question #5 : Understanding Introns And Exons

In eukaryotes, which of the following is true about introns and exons?

Possible Answers:

The mature mRNA transcript only contains the introns because the exons have been spliced out.

Exons are repeating sequences that are typically found at the distal ends of a gene.

Intronic regions typically code for transcription factors.

The mature mRNA transcript contains a mix of introns and exons.

The primary RNA transcript contains both intronic and exonic regions.

Correct answer:

The primary RNA transcript contains both intronic and exonic regions.

Explanation:

The primary RNA contains introns and exons because it has not been processed yet, and therefore the introns have not been spliced out. Mature mRNA contains only exons, which are the coding sequences that ultimately get translated. Intron regions are non-coding and are not included in mature transcripts. Note that post-translational modifications such as splicing only occurs in eukaryotes.

Example Question #6 : Understanding Introns And Exons

If a gene produces a pre-RNA that is 1200 basepairs long and has the following intron-exon structure:

Exon 1 - 200 bp

Intron 1 - 100 bp

Exon 2 - 50 bp

Intron 2 - 150 bp

Exon 3 - 700 bp

How many basepairs long would we expect the mRNA to be?

Possible Answers:

250 basepairs

950 basepairs

500 basepairs

1150 basepairs

1000 basepairs

Correct answer:

950 basepairs

Explanation:

This question requires you to know that preRNA contains both intronic and exonic regions, but the introns get spliced out to produce the mRNA. Therefore, you had to subtract the total intron basepairs (250) from the total length of the preRNA (1200), which gives an mRNA length of 950 basepairs. 

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