GRE Subject Test: Biology : Understanding PAGE and SDS-PAGE

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

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Example Questions

Example Question #5 : Electrophoresis And Blots

A researcher is working with a protein that contains four subunits of differing molecular weights. If the researcher performs SDS-PAGE, how many distinct bands should he see on the gel?

Possible Answers:

Three

Four

One

Two

Correct answer:

Four

Explanation:

SDS-PAGE requires that proteins are denatured before they are run through the gel, typically by the addition of detergents and then heating the sample. Since the protein has four subunits that are all different molecular weights, we would see four distinct bands that represent the four subunits. If the subunits had the same molecular weight, we would only see one band.

Example Question #6 : Electrophoresis And Blots

Which of the following is a primary factor that dictates how far a protein will migrate during SDS-PAGE?

Possible Answers:

Size

Degree of secondary structure

Number of subunits

Degree of tertiary structure

Correct answer:

Size

Explanation:

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 #7 : Electrophoresis And Blots

Which of the following is true about SDS-PAGE?

Possible Answers:


It is used to anaylze DNA fragments

The main ingredient in the gel is agarose

Staining with ethidium bromide allows visualization of results

It separates proteins by charge

It requires a protein-denaturing gel

Correct answer:

It requires a protein-denaturing gel

Explanation:

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?

Possible Answers:

None of these

Running multiple lanes in one gel

Resolution down to DNA strands with single base length differences

Running multiple DNA strands in a single gel lane

Staining the DNA for visualization

Correct answer:

Resolution down to DNA strands with single base length differences

Explanation:

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).

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