All GRE Subject Test: Biology Resources
Example Question #1 : Electrophoresis And Blots
Which of the following techniques would be most useful to study gene expression?
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 #2 : Electrophoresis And Blots
Which of the following probes are most commonly used in southern blotting?
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?
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 #2 : 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?
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.