Electrophoresis Quiz
Questions: 16 · 10 minutes
1. The wells of a DNA agarose gel are accidentally positioned beside the positive electrode. What is the likely outcome when power is applied?
The DNA will remain in the wells because electrode orientation does not affect migration
The DNA will move toward the nearby positive end and may quickly run out of the gel
The DNA will separate normally but with large fragments traveling farthest
The DNA will migrate toward the negative electrode because DNA is positively charged
2. After electrophoresis, a researcher transfers DNA to a membrane and uses a labeled complementary probe to detect a particular sequence. Which technique is this?
Isoelectric focusing
Western blotting
Northern blotting
Southern blotting
3. Two linear DNA fragments, 500 and 1,500 base pairs long, are run under the same appropriate agarose-gel conditions. Which result is expected?
The 1,500-base-pair fragment travels farther because it carries more total charge
Both fragments travel the same distance because their charge-to-mass ratios are similar
The 500-base-pair fragment remains closer to the well because it binds less stain
The 500-base-pair fragment travels farther because it moves through the gel pores more readily
4. In isoelectric focusing, where does a protein stop migrating?
At the pH where its net electrical charge is zero
At the positive electrode, regardless of the protein
At the location where its molecular mass matches the gel density
At the point where the gel has the largest pores
5. An unknown DNA band is compared with a lane containing fragments of known lengths. What is the known-fragment mixture used for?
To estimate the unknown fragment's size from its migration
To determine the nucleotide sequence of the unknown fragment directly
To prevent the unknown DNA from leaving the well
To supply the electrical charge needed by the unknown DNA
6. Why is pulsed-field gel electrophoresis useful for separating very large DNA fragments?
It separates DNA by isoelectric point rather than size
It replaces the gel matrix with an antibody-coated membrane
It gives large DNA fragments a positive charge
It periodically changes the electric-field direction, helping large molecules reorient through the gel
7. What property primarily causes DNA to migrate toward the positive electrode during standard agarose gel electrophoresis?
The positive charge of its nitrogenous bases
Hydrogen bonds between complementary bases
Its negatively charged phosphate backbone
Hydrophobic interactions with the agarose matrix
8. A laboratory wants rapid, automated electrophoretic separation using very small sample volumes and efficient heat dissipation. Which format best fits these requirements?
Paper electrophoresis in an open tray
Capillary electrophoresis
A thick preparative agarose slab
A membrane transfer without prior separation
9. A gel was made with the correct running buffer, but the electrophoresis tank was accidentally filled with distilled water. What problem is most likely?
Low conductivity and poor pH control will make migration weak or unreliable
The gel pores will immediately become too small for DNA
DNA will reverse its charge and migrate toward the negative electrode
All DNA fragments will migrate at exactly the same speed
10. Why is loading dye commonly mixed with a DNA sample before it is placed in an agarose gel?
It permanently fixes DNA to the gel matrix
It digests contaminating proteins and RNA in the sample
It increases sample density for loading and provides visible tracking dyes
It gives every DNA fragment a different electrical charge
11. A DNA sample known to be intact was loaded at several times the recommended amount. Its lane appears broad and smeared, while adjacent lanes are sharp. What is the most appropriate first adjustment?
Reverse the electrode connections
Increase the sample concentration further
Load a smaller amount of the sample
Remove the molecular-size marker
12. A run has finished, and a technician needs to open the electrophoresis chamber. What should happen first?
Touch the buffer to confirm that current has stopped
Remove the gel while leaving the power supply running
Add more buffer before moving any electrical connections
Switch off and disconnect the power before opening the chamber
13. What is the principal role of SDS in SDS-PAGE?
It cuts proteins at specific amino acid sequences
It focuses proteins at their isoelectric points
It denatures proteins and gives them a broadly similar negative charge-to-mass ratio
It cross-links proteins to the polyacrylamide matrix
14. Compared with SDS-PAGE, what does native PAGE better preserve?
Noncovalent structure, complexes, and potentially biological activity
Separation based almost exclusively on polypeptide length
A uniform negative charge on every protein
Complete dissociation of proteins into individual subunits
15. Under nonreducing SDS-PAGE, a protein appears as a disulfide-linked dimer. Under reducing conditions, it appears as monomer subunits. What caused the change?
The reducing agent removed all SDS from the proteins
The reducing agent disrupted disulfide bonds between subunits
The electric field formed new peptide bonds
The polyacrylamide converted the dimer into DNA-sized fragments
16. A researcher needs better separation between DNA fragments of 200 and 250 base pairs. Which gel adjustment is generally most appropriate?
Use a lower agarose concentration to create larger pores
Use a higher agarose concentration within a suitable working range
Remove the running buffer to reduce ionic interference
Replace agarose with a sample-loading dye