GRE Subject Test: Biology : Genetics, DNA, and Molecular Biology

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

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Example Questions

Example Question #5 : 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 primary RNA transcript contains both intronic and exonic regions.

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.

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:

1150 basepairs

500 basepairs

1000 basepairs

950 basepairs

250 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 #1 : Genetic Sequences, Transcription, And Translation

Which of the following accurately describes the promoter?

Possible Answers:

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

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

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 #1 : Genetic Sequences, Transcription, And Translation

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

Possible Answers:

Low lactose and low glucose

High lactose and high glucose

Low lactose and high 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 : Understanding Operons And Promoters

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

It binds to the promoter, signaling the polymerase to attach

It attaches to the operator, blocking polymerase from attaching

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 #4 : 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

Increased serum ceruloplasmin

Decreased serum apoceruloplasmin

Equivalent serum ceruloplasmin

The comparison cannot be estimated

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.

Example Question #1 : Understanding Genetic Patterns

An error occurs during DNA replication, resulting in the insertion of a base pair. Replication of the original 3' DNA strand (template) produces a mutant 3' strand (mutant), as diagrammed below:

Template

3' - AUGGCCATTTTTATA - 5'

Mutant

3'- AUGCGCCATTTTTATA - 5'

Of the answers below, which best describes the type of mutation depicted above?

Possible Answers:

Frameshift mutation

Missense mutation

Duplication mutation

Repeat expansion

Nonsense mutation

Correct answer:

Frameshift mutation

Explanation:

The addition of a number of nucleotides that is not a multiple of three shifts the reading frame of the codons in the gene. One base pair was inserted early in the strand, thus shifting the codon reading frame +1 to the right.

Missense and nonsense mutations imply base pair substitutions, which did not occur in the diagram. Similarly, nothing was duplicated, and repeat expansion would require multiple repetitions of a short DNA sequence.

Example Question #41 : Genetics, Dna, And Molecular Biology

Consider the following scenario:

Gene-Z is a transcription factor required for transcribing the genes that contribute to populating the motor cortex with the proper number of neurons. Mutant mice for Gene-Z have 50% fewer motor neurons than controls (non-mutants). You genetically engineer a transgenic DNA construct that acts as a dominant negative to Gene-Z.

You transfect an embryonic mouse brain with this dominant negative and measure that all motor neurons get and expressed this construct. What would you expect the number of neurons in the transfected brain to be if the dominant negative works with 100% efficacy, and a normal motor cortex has 100,000 cells in the region you are examining?

Possible Answers:

Correct answer:

Explanation:

This question requires the knowledge that a dominant negative is expected to work in the same direction as a genetic mutant; it is a loss-of-function of the gene, by expressing a version of the gene that outcompetes the actual gene but cannot perform the proper biological process. Because we are assuming the dominant negative works perfectly, it should act just like the mutant and reduce cell number by 50%, giving us 50,000 cells. 

Example Question #42 : Genetics, Dna, And Molecular Biology

Which of the following statements about the function of transcription factors is not true?

Possible Answers:

Transcription factors can be activated by intracellular signaling cascades to modulate DNA expression in a context-dependent manner. 

Transcription factors can only be activated by phosphatases.

Transcription factors can stabilize or destabilize the binding of RNA polymerases to DNA prior to transcription.

Transcription factors can bind to enhancer and promoter regions to upregulate DNA expression.

Transcription factors can bind to enhancer and promoter regions to downregulate DNA expression.

Correct answer:

Transcription factors can only be activated by phosphatases.

Explanation:

Transcription factors can be activated or deactivated by any number of processes occurring within the cell and nucleus, and this it not limited to phosphatases (which remove phosphate groups from proteins). All of the other answers accurately describe possible activity and function of transcription factors.

Example Question #43 : Genetics, Dna, And Molecular Biology

Which of the following is true?

I. Transcription factors typically bind directly to the genes that they regulate.

II. Transcription factors can bind enhancer and promoter regions upstream of the genes they regulate. 

III. In eukaryotes, transcription factors frequently associate with coactivators.

IV. Transcription factors can either upregulate or downregulate transcription of a gene. 

Possible Answers:

I, III, and IV

II, III, and IV

II and IV

II and III

I and III

Correct answer:

II, III, and IV

Explanation:

Only the first statement in this question is false. Transcription factors typically (in fact, almost always) bind upstream of the gene to enhancer or promoter regions, and are rarely found interacting with the gene's coding region itself. 

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