GRE Subject Test: Biology : Lab Techniques

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

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Example Questions

Example Question #1 : Understanding Immunological Stains

4',6-diamidino-2-phenylindole, commonly known as DAPI, stains what part of the cell when performing immunohistochemistry?

Possible Answers:

A-T rich DNA

Nuclear envelope

Golgi apparatus

Nucleolus

Plasma membrane

Correct answer:

A-T rich DNA

Explanation:

The correct answer is A-T rich DNA. DAPI is a common fluorescent dye used in immunohistochemistry to stain DNA to indicate the localization of the nucleus within a cell, relative to other structures and regions. When bound to DNA, it absorbs ultraviolet light (358nm) and emits blue light (461nm). 

Example Question #22 : Lab Techniques

When performing whole-mount specimen immunohistochemistry of golgi markers, why is it necessary to porate cell membranes?

Possible Answers:

Poration promotes attachment of cells to a microscope slide

Poration allows the golgi markers to exit the cell

None of the other answers

Poration keeps the cells alive, allowing for live-cell imaging 

Poration of the membrane allows the antibody to enter the fixed cells

Correct answer:

Poration of the membrane allows the antibody to enter the fixed cells

Explanation:

When performing immunohistochemistry, antibodies are often utilized to detect proteins of interest within the cell. However, in order for antibodies to enter fixed cells, there must be holes (pores) artificially made in the cell membrane. 

Example Question #1 : Understanding Knockouts

Which of the following is not a methodology to obtain either transient or stable knockout/ knockdown of a gene?

Possible Answers:

shRNA interference

Crispr-Cas9

Morpholino

Homologous recombination

GAL4-UAS system

Correct answer:

GAL4-UAS system

Explanation:

Homologous recombination and Crispr-Cas9 create stable gene knockouts whereas shRNA and morpholino interference transiently knockdown a gene of interest. The GAL4-UAS system acutally is used to overexpress a gene of interest. By using a cell type specifc promoter, researchers are able to overexpress a specific protein in a desired cell type in a whole organism. 

Example Question #1 : Genetic Manipulation

Genetically altering an animal to reduce the expression of a gene of interest can be a labor intensive process that does not necessarily produce complete loss of a gene. A genetic null is an animal in which the gene has been completely (or nearly completely) excised and as such, no protein is produced. A genetic hypomorph is an animal in which only part of a gene has been deleted and as such, a lower amount of protein or a dysfunctional protein is produced, but it is still there. Hypomorphs can be close to null, perhaps only retaining 5-10% of normal function, or they can be close to wild-type, retaining 80-90% of gene function and having mild mutant phenotypes.

You are studying Gene H, a gene that regulates head size, and the more Gene H is expressed, the larger the organism's head is. You have one animal that is null for Gene H, and one that is a hypomorph for Gene H. You compare the head sizes of these animals. Which of the following results is least likely to be true from your experiment? You can assume that a wild-type animal will have the largest head size of the three. 

Possible Answers:

Both the null and hypomorph have smaller heads than the wild-type, but the null is the smallest by a very large factor, nearly 90% smaller than the hypomorph. 

Both the null and hypomorph have smaller heads than the wild-type, and the null is smaller than the hypomorph, but only by about 5%. 

The null and the hypomorph cause smaller head size early in development, but the hypomorph head growth accelerates later in development. 

The null has a larger head than the hypomorph by about 10%.

The null and the hypomorph have roughly the same head size, because the hypomorph is dysfunctional enough to disturb head size development. 

Correct answer:

The null has a larger head than the hypomorph by about 10%.

Explanation:

The only result that would be very unlikely is the one in which the null has a larger head size than the hypomorph. All of the other cases could be true since the nature of a hypomorph is often hard to ascertain, but given that more protein = bigger head, a true genetic null would have the least amount of protein (i.e. no protein) and represents the absolute smallest a head can get. 

Example Question #1 : Understanding Rn Ai

Which of the following techniques could help a researcher inhibit the expression of a target gene?

Possible Answers:

qPCR

RNAi

X-ray crystallography

FPLC

Correct answer:

RNAi

Explanation:

RNA interference (RNAi) takes use of the cell's internal machinery to locate a target mRNA transcript and stall its translation, or degrade it completely. It is a very powerful tool for silencing genes. Small RNA transcripts bind to mRNA, either silencing translation or labeling the mRNA for destruction.

qPCR is a technique used to measure gene expression. FPLC is used to purify proteins. X-ray crystallography is used to elucidate protein structures.

Example Question #23 : Lab Techniques

Which of the following statements best describes the function of RNAi?

Possible Answers:

Globally interfere with translation by blocking all mRNA

Interfere with translation by blocking a target mRNA

Interfere with translation by targeting specific tRNA molecules

Interfere with translation by targeting ribosomes 

Correct answer:

Interfere with translation by blocking a target mRNA

Explanation:

RNAi is a process that utilizes small molecules of RNA (miRNA or siRNA) to target specific molecules of mRNA, repressing their translation or cleaving them into non-functional units. This essentially prevents the expression of a particular protein by neutralizing its mRNA transcript. Ribosomes may fail to bond with the region of double-stranded RNA, created from the mRNA and RNAi dimer, or it may attract nucleases.

RNAi is not involved in globally halting translation and is not used to target tRNA or ribosomes.

Example Question #21 : Lab Techniques

Dicer is an endonuclease that cleaves __________ during the process of RNA-interference.

Possible Answers:

transcription factor mRNA

single-stranded RNA

single-stranded DNA

double-stranded DNA

double-stranded RNA

Correct answer:

double-stranded RNA

Explanation:

Double-stranded RNA molecules are dicer's substrate, and their presence initiates the RNAi process. The other molecules listed are not cleaved by dicer proteins.

Example Question #4 : Understanding Rn Ai

Which protein, associated with the RNA-inducing silencing complex (RISC), activates and cleaves mRNA in RNAi? 

Possible Answers:

RNA helicase 

None of these

Endonuclease

Argonaute

RNase III Dicer 

Correct answer:

Argonaute

Explanation:

The correct answer is argonaute. Argonaute is part of the RISC and binds small non-coding RNAs. These RNAs guide argonaute to their specific targets via complementary base pairing, leading to mRNA cleavage and subsequent inhibition of translation. Dicer generates double-stranded RNA fragments, and endonucleases and RNA helicases are not specific to RNAi. 

Example Question #1 : Genetic Manipulation

Many strains of the model organism Drosophila have been engineered to express RNAi transgenes; that is, these transgenes express a moiety that is capable of targeting specific mRNAs to be degraded. This effectively results in downregulation of certain genes, permitting scientists to study how loss of that gene effects the organism. 

What is the structure of the RNAi moiety that targets and binds to specific mRNAs?

Possible Answers:

Double stranded RNA

Single stranded RNA

Double stranded DNA

DNA-protein complex

Single sheet polypeptide

Correct answer:

Double stranded RNA

Explanation:

RNAi molecules are double stranded RNAs. Their sequences are complementary to the sequence of the mRNA in which they are designed to destruct, thus allowing them to bind and trigger the degradation process. 

Example Question #4 : Understanding Rn Ai

Many strains of the model organism Drosophila have been engineered to express RNAi transgenes; that is, these transgenes express a moiety that is capable of targeting specific mRNAs to be degraded. This effectively results in downregulation of certain genes, permitting scientists to study how loss of that gene effects the organism. 

Consider the following hypothetical situation. Gene A codes for the protein Enzyme A; that is, Gene A is expressed and is translated to make Enzyme A. You obtain a fly expressing an RNAi that targets Gene A's mRNA. If you express this RNAi in a fly (Drosophila), which of the following is the most likely result you'd expect if you were able to monitor Enzyme A levels with a fluorescent antibody and compared a wild-type fly to one expressing the RNAi?

Possible Answers:

Enzyme A levels would be reduced in the RNAi expressing fly, resulting in less fluorescence in the RNAi expressing fly.

Enzyme A levels would be decreased in both the wild-type and RNAi expressing flies.

Enzyme A levels, and therefore fluorescence, would be decreased only in the nucleus of the RNAi expressing fly because this is where translation occurs.

Enzyme A levels would be increased and more fluorescence would be apparent in the RNAi expressing fly.

Enzyme A levels would be unchanged because RNAi targets mRNA, thus no differences in fluorescence would be apparent between wild-type and RNAi expressing flies. 

Correct answer:

Enzyme A levels would be reduced in the RNAi expressing fly, resulting in less fluorescence in the RNAi expressing fly.

Explanation:

Since we can assume the RNAi correctly targets and degrades the proper mRNA, we can expect that the levels of Enzyme A will be decreased in the fly expressing the RNAi against Enzyme A. Because Gene A's mRNA will be degraded, little or no protein will be produced from Gene A, and thus the fluorescent antibody against Enzyme A will have nothing to bind to, which will result in less fluorescence in the fly expressing the RNAi versus the fly with undisturbed translation of the mRNA.

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