GRE Subject Test: Biology : Genetic Sequences, Transcription, and Translation

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

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Example Questions

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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:

II only

I and II

I, II, and III

I 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 #1 : Understanding Introns And Exons

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

Possible Answers:

Exons . . . mRNA

Exons . . . pre-mRNA

Introns . . . pre-mRNA

Introns . . . 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 #2 : 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:

Exons have not yet been added to the transcript

The poly-A tail is still on the primary 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 : Genetic Sequences, Transcription, And Translation

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:

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

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

All of these

None of these

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 #1 : Understanding Introns And Exons

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

Possible Answers:

Intronic regions typically code for transcription factors.

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

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

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

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 #1 : Genetic Sequences, Transcription, And Translation

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

500 basepairs

1150 basepairs

950 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. 

Example Question #2 : Genetic Sequences, Transcription, And Translation

Which of the following accurately describes the promoter?

Possible Answers:

A sequence of DNA used to signal the beginning point of transcription

The protein that attaches to DNA in order to create mRNA

The binding site for DNA polymerase on DNA

The attachment point for a ribosome before translation

Correct answer:

A sequence of DNA used to signal the beginning point of transcription

Explanation:

The promoter is a specific segment of DNA that signals the starting point of transcription. RNA polymerase attaches to the promoter and proceeds to create the mRNA primary transcript.

DNA polymerase binds to the RNA primer to begin DNA replication. Ribosomes bind to the 5' cap on eukaryotic mRNA.

Example Question #2 : Genetic Sequences, Transcription, And Translation

Which conditions would result in the largest levels of lac operon transcription?

Possible Answers:

Low lactose and high glucose

High lactose and high glucose

Low lactose and low glucose

High lactose and low glucose

Correct answer:

High lactose and low glucose

Explanation:

The important thing to remember about the lac operon is that it is transcribed when glucose is absent from the cell, but lactose is present and can be utilized. As a result, the operon's transcription would be high if there are both high levels of lactose available, and very little amounts of glucose.

Example Question #1 : Genetic Sequences, Transcription, And Translation

The lac operon is typically found in prokaryotes in order to utilize lactose in the event that glucose is absent. How does the presence of lactose affect the lac operon?

Possible Answers:

It stimulates the transcription of the lac repressor gene

It attaches to the operator, blocking polymerase from attaching

It binds to the lac repressor, causing it to detach from the operator

It binds to the promoter, signaling the polymerase to attach

Correct answer:

It binds to the lac repressor, causing it to detach from the operator

Explanation:

The lac operon is set up in a way so that the lac repressor is able to be transcribed, regardless of glucose and lactose levels. The lac repressor will then attach to the operator, which inhibits transcription. If lactose is present, it will bind to the lac repressor, and make it detach from the operator.

This process allows the operon to be transcribed in the event that glucose is absent. If glucose is absent, but lactose is not present, then the repressor will remain in place and transcription will not take place.

Example Question #3 : Genetic Sequences, Transcription, And Translation

Some inherited diseases of the liver, including Wilson's Disease, are primarily or entirely genetically determined. Wilson's Disease results when a defect in a copper transporter in the small intestine occurs, leading to copper level disregulation in both the hepatocytes and the systemic circulatory system. Mutations have primarily been found in the copper transporter that helps load copper onto a transport protein, apoceruloplasmin, which normally creates serum-soluble ceruloplasmin with the addition of copper. Given this defect, serum studies of an individual with Wilson's Disease would likely show what kind of change in serum ceruloplasmin compared with a normal individual?

Possible Answers:

Decreased serum ceruloplasmin

The comparison cannot be estimated

Decreased serum apoceruloplasmin

Equivalent serum ceruloplasmin

Increased serum ceruloplasmin

Correct answer:

Decreased serum ceruloplasmin

Explanation:

The question informs us that the mutational defect in the gene involves the enzyme's ability to load copper onto apoceruloplasmin. Healthy individuals are able to load copper to apoceruloplasmin, creating serum-soluble ceruloplasmin. With this process disrupted in an individual with Wilson's Disease, we would expect that less ceruloplasmin would be produced because copper could not be transported. We would expect to see reduced serum levels of the complete protein, and high levels of copper building up in hepatocytes and circulatory serum.

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