All GRE Subject Test: Biology Resources
Example Questions
Example Question #11 : Lab Techniques
Which of the following is a primary factor that dictates how far a protein will migrate during SDS-PAGE?
Number of subunits
Degree of secondary structure
Size
Degree of tertiary structure
Size
The primary factor dictating how far a protein will migrate during SDS-PAGE is the size of the protein. One of the key features of SDS-PAGE is that the protein sample is denatured and covered in a detergent prior to being run through the gel. This essentially eliminates any complications from the degree of folding or the number of subunits. In fact, subunits will migrate according to their own molecular weights.
Example Question #1 : Understanding Page And Sds Page
Which of the following is true about SDS-PAGE?
It is used to anaylze DNA fragments
Staining with ethidium bromide allows visualization of results
The main ingredient in the gel is agarose
It requires a protein-denaturing gel
It separates proteins by charge
It requires a protein-denaturing gel
SDS-PAGE requires a denaturing protein gel that separates proteins based on size. The primary ingredients are polyacrylamide and sodium dodecyl sulfate—SDS-PAGE refers to sodium dodecyl sulfate-polyacrylamide gel electrophoresis. In order to visualize the results, proteins separated via SDS-PAGE are transferred to a nitrocellulose membrane, where they are probed with antibodies for a specific protein of interest.
Example Question #1 : Understanding Page And Sds Page
As opposed to electrophoresis with a more standard agar gel, what does polyacrylamide gel electrophoresis (PAGE) allow for when working with DNA?
Running multiple lanes in one gel
Staining the DNA for visualization
Resolution down to DNA strands with single base length differences
None of these
Running multiple DNA strands in a single gel lane
Resolution down to DNA strands with single base length differences
Polyacrylamide gels allow for resolution of DNA strands down to single base pair differences. All these other options are applicable to any kind of gel electrophoresis. This property of polyacrylamide allowed for some of the earliest forms of Sanger sequencing, in which DNA sequences were read by their respective chain lengths across a 4 column gel (with one column each for adenine, cytosine, guanine, and thymine).
Example Question #12 : Lab Techniques
Which of the following techniques would be most useful to study gene expression?
Western blot
Northern blot
Southern blot
Eastern blot
Northern blot
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 #13 : Lab Techniques
Which of the following probes are most commonly used in southern blotting?
Biotin-binding proteins
Antibodies
Nucleic acids
Phosphorous-32
Nucleic acids
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 #14 : Lab Techniques
Which of the following blots is used to identify known RNA fragments?
Northern blotting
Western blotting
Southern blotting
Eastern blotting
Northern blotting
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 #1 : Understanding Other Blots And Gels
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?
Western blot
Thin-layer chromatography
Northern blot
Southern blot
Two-photon microscopy
Northern blot
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.
Example Question #15 : Lab Techniques
Which of the following techniques could help improve the purity of a protein sample while maintaining the protein's native state?
SDS-PAGE
Southern blotting
Column chromatography
All of these answers
Column chromatography
The only choice that would actually be useful in improving the purity of a protein sample would be column chromatography. It is possible to attach a specific ligand to your protein of interest to help separate it from a mixture via this method.
SDS-PAGE results in a separated, but denatured, protein sample. Detergents used in this process damage the protein structure, making it impossible to retain the native state. Southern blotting is a technique used to identify specific sequences of DNA in a sample and has nothing to do with increasing the purity of a protein sample.
Example Question #16 : Lab Techniques
A researcher is trying to identify if caffeine is present in an unknown sample. He chooses to use thin layer chromatography and compare it to a known sample of caffeine. After running the test, he discovers that his unknown sample has the exact same Rf value as the standard caffeine. Which of the following is true about his result?
The unknown substance is definitely not caffeine
It is likely that the unknown substance is caffeine
The unknown substance is definitely caffeine
It is unlikely that the unknown substance is caffeine
It is likely that the unknown substance is caffeine
The fact that the two substances have the same Rf value is strong evidence that they are the same substance; however, it is not conclusive evidence. The researcher would have to verify these results with another test (such as NMR) to conclusively state that caffeine is present in his sample. Rf value is determined by polar and hydrophobic properties that may not be unique to a given compound. Many different compounds are capable of having the same or similar Rf values; therefore, his result does not conclusively show that caffeine is in his sample.
Example Question #17 : Lab Techniques
A certain immunostaining technique initially separates antigens to be identified by electrophoresis, then transfers them to a solid membrane. This process was used as the standard for diagnosis of the presence of human immunodeficiency virus. What process is being described?
Immuno-electron microscopy
Western blot
ELISA (enzyme-linked immunosorbent assay)
Chemiluminescence
Flow cytometry
Western blot
The Western blot test identifies antigens by separating them using electrophoresis on a gel, then transferring to a solid membrane by blotting. The process allows clinicians to establish is a specific protein or set of proteins is present in a sample.
Enzyme-linked immunosorbent assay (ELISA) methodology involves identifying the presence of antigens or antibodies in blood by binding them to enzymes that would result in a color change. In flow cytometry, cells are tagged with an antibody carrying a fluorescent label and passed through a detctor. In immuno-electron microscopy, antibodies are labelled with heavy metals and viewed with an electron microscope. Chemiluminescence is the emission of light as the result of a chemical reaction. Luminol is an example of chemiluminescence.
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