GRE Subject Test: Biology : Genetic Manipulation

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

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Example Questions

Example Question #1 : Genetic Manipulation

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

Possible Answers:

Morpholino

shRNA interference

GAL4-UAS system

Crispr-Cas9

Homologous recombination

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, and the null is smaller than the hypomorph, but only by about 5%. 

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

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

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. 

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 : Genetic Manipulation

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

Possible Answers:

qPCR

FPLC

RNAi

X-ray crystallography

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 #1 : Understanding Rn Ai

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

Possible Answers:

Interfere with translation by targeting ribosomes 

Globally interfere with translation by blocking all mRNA

Interfere with translation by targeting specific tRNA molecules

Interfere with translation by blocking a target mRNA

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 #1 : Understanding Rn Ai

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

Possible Answers:

double-stranded DNA

single-stranded DNA

transcription factor mRNA

double-stranded RNA

single-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 #1 : 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

RNase III Dicer 

Endonuclease

Argonaute

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 #7 : 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:

DNA-protein complex

Double stranded RNA

Double stranded DNA

Single stranded RNA

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 #1 : 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 would be increased and more fluorescence would be apparent in the RNAi expressing fly.

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 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.

Example Question #1 : Genetic Manipulation

Which term describes DNA that has been altered by genes from a different organism, typically from a different species?

Possible Answers:

DNA-DNA hybridization

DNA fingerprinting

DNA replication

Recombinant DNA

DNA sequencing

Correct answer:

Recombinant DNA

Explanation:

Recombinant DNA are segments of DNA from one organism artifically manipulated or inserted into the DNA of another organism, using a technique known as gene splicing. This technique permits isolation and examination of properties and actions of specific genes.

DNA sequencing is the process of determining the chemical composition of a DNA molecule (in particular, the order in which the molecule's nucleic acids are arranged). DNA fingerprinting is the use of enzymes to cut DNA samples into a unique set of restriction fragments that can be used as a genetic identified for a single individual. DNA-DNA hybridization is a technique by which DNA from two species is separated into single strands and then allowed to re-form. DNA replication is the copying of the double-stranded DNA molecule, producing two identical DNA double helices.

Example Question #1 : Understanding Recombinant Dna

A scientist seeks to synthesize a rare human protein in lab. To accomplish this, she utilizes recombinant techniques to insert the DNA of a eukaryotic gene for the protein into bacterial plasmids. These plasmids are transformed into bacteria for expression. She is disappointed to discover that the gene product from the bacteria is not the correct protein. What corrective step could she take in the procedure to fix this issue?

Possible Answers:

Use archaea instead of bacteria

Ligate the gene directly into the chromosome

Use cDNA for the gene instead of DNA

Use a different genus bacteria

Insert mRNA into the plasmid instead of DNA

Correct answer:

Use cDNA for the gene instead of DNA

Explanation:

The corrective step would be to use cDNA in the plasmid rather than DNA from the gene. cDNA (complementary DNA) is the DNA copy of the mature mRNA created by the reverse transcriptase enzyme. This would allow the introns to be spliced out from the sequence of the strand. This must be done since prokaryotes lack the equipment necessary for splicing. Simply inserting the DNA gene into the plasmid would result in a translated product with introns present. cDNA avoids this problem.

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