GRE Subject Test: Biology : Understanding Other Blots and Gels

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

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Example Questions

Example Question #14 : Lab Techniques

Which of the following techniques would be most useful to study gene expression?

Possible Answers:

Southern blot

Northern blot

Western blot

Eastern blot

Correct answer:

Northern blot

Explanation:

In order to study gene expression it would be useful to quantify the expression of a specific RNA transcript. All of the blotting techniques follow similar procedures, but differ in the macromolecule they separate or the information they obtain. Southern blotting is used for DNA fragments, western blotting is used for proteins, and eastern blotting is used to look at post-translational modifications. Northern blotting would be the most useful in this question, as it is used to look at RNA fragments.

Example Question #15 : Lab Techniques

Which of the following probes are most commonly used in southern blotting?

Possible Answers:

Antibodies

Nucleic acids

Phosphorous-32

Biotin-binding proteins

Correct answer:

Nucleic acids

Explanation:

Southern blotting is a technique used to detect a specific DNA sequence in a sample. The sample is run on an agarose gel and transferred to a membrane. A labeled nucleic acid probe is then incubated with the membrane. The probe carries a tag for identification and will only bind to the target region of DNA. If the tag is identified, then the researcher can conclude that the target gene is present in the sample.

Antibodies are most commonly used to detect specific proteins in western blotting. Biotin-binding proteins have wide uses in both detection and purification procedures. Phosphorous-32 is commonly used to label nucleotides (such as GTP) to perform enzymatic assays.

Example Question #1 : Understanding Other Blots And Gels

Which of the following blots is used to identify known RNA fragments?

Possible Answers:

Northern blotting

Southern blotting

Western blotting

Eastern blotting

Correct answer:

Northern blotting

Explanation:

A Northern blot is very similar to a Southern blot, except it is used to identify RNA fragments rather than DNA fragments. A gel is used to separate the RNA fragments based on size. A radioactive probe can then be used to identify which known RNA sequences have been combined with an RNA fragment.

A Western blot is used to identify protein subunits and fragments. Eastern blots are used to identify protein modifications, such as glycosylation.

Example Question #17 : Lab Techniques

A cell line was engineered to contain and express a DNA sequence frequently associated with pancreatic cancer, with the hope that the cell line will undergo rapid cell division much like the cancerous cells. You want to test whether or not the cell line is efficiently transcribing this DNA by measuring the quantity of mRNA produced. Which of the following methods would be best suited for testing this?

Possible Answers:

Southern blot

Western blot

Thin-layer chromatography 

Two-photon microscopy 

Northern blot

Correct answer:

Northern blot

Explanation:

Northern blots are designed to detect the quantity and identity of mRNA transcripts in cells or tissue. Electrophoresis separates RNAs by size, and then hybridization is used to detect the mRNA sequence of interest. Southern blots detect DNA sequences, but give no information about whether they are transcribed or not. Western blots detect proteins, but we are not interested in translation. Neither thin-layer chromatography nor two-photon microscopy would tell us anything about the genetic processes occurring in the cell line. 

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