Question 1

Why do smaller DNA fragments move faster through an agarose gel than larger ones?

Accepted Answer

Smaller DNA fragments navigate through the porous gel matrix more easily because they encounter less resistance from the agarose network. Larger fragments have difficulty moving through the gel due to their size, causing them to migrate more slowly. This size-based separation is the fundamental principle enabling DNA gel electrophoresis to resolve fragments of different lengths.

Question 2

What role does ethidium bromide play in DNA gel electrophoresis?

Accepted Answer

Ethidium bromide is an aromatic compound that intercalates between individual base pairs of DNA, fitting into the double helix structure. When exposed to ultraviolet light, the intercalated ethidium bromide causes DNA to emit intense orange fluorescence, making DNA fragments visible as bands on the gel. However, ethidium bromide is a carcinogen, so gloves must always be worn when handling gels containing this compound.

Question 3

How does gel percentage affect DNA fragment resolution?

Accepted Answer

Lower percentage agarose gels have larger pores and better resolve larger DNA fragments, while higher percentage gels have smaller pores and make smaller fragments easier to identify. Gel percentage is expressed as weight over volume; for example, 1 gram of agarose in 100 ml of buffer creates a 1% gel. Choosing the appropriate percentage depends on the size range of DNA fragments you need to separate.

Question 4

What is the purpose of a DNA ladder in gel electrophoresis?

Accepted Answer

A DNA ladder is a series of DNA fragments of known sizes that serves as a size reference standard. By comparing the relative location of your test sample bands to the ladder fragments, you can determine the size of your DNA fragments of interest. The ladder must be loaded alongside experimental samples and should be separated to a degree that allows useful determination of sample band sizes.

Question 5

Why is loading dye added to DNA samples before gel electrophoresis?

Accepted Answer

Loading dye serves multiple functions: it helps visualize and load samples into the wells, allows samples to sink into the gel, and helps track how far samples have migrated during the run. The dye is typically prepared at a 6X concentration and provides visual confirmation that samples are properly loaded and progressing through the gel during electrophoresis.

Question 6

How can DNA fragments be recovered after gel electrophoresis?

Accepted Answer

After electrophoresis, DNA fragments of interest can be recovered through gel purification procedures. A razor blade is used to cut out the specific DNA band from the gel, and the DNA sample within that fragment is then collected and recovered. This approach enables recovery of DNA fragments after electrophoretic separation for downstream applications.

Question 7

What technique is used to separate large DNA molecules greater than 15-20 kb?

Accepted Answer

Standard DNA gel electrophoresis is not ideal for separating high molecular weight DNA like genomic DNA. Pulse field gel electrophoresis is used instead, which subjects the gel to a changing, or pulsing, electric field in different directions. This technique uses a specialized apparatus with electrode pairs arrayed in different orientations around the gel, allowing separation of large DNA samples and detection of genome size differences between organisms.

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