A student added a 5′ exonuclease enzyme instead of a restriction enzyme to digest his purifiedplasmid
DNA sample. What is he likely to observe when he runs his plasmid digest on anagarose gel?
1. free nucleotides from the 5′ end only
2. free nucleotides from both ends
3. no digestion of plasmid DNA
4. plasmid DNA will be digested similar to the restriction enzyme

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Detailed Explanation:

Correct Answer: 1. Free nucleotides from the 5′ end only

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What is Exonuclease and What Does It Do?

An exonuclease is an enzyme that catalyzes the removal of nucleotide bases from the ends of a DNA molecule. Specifically, a 5′ exonuclease removes nucleotides from the 5′ end of a DNA strand. The exonuclease cleaves single-stranded DNA, processing the ends of the DNA by progressively removing nucleotides from the 5′ end, without affecting the rest of the DNA molecule.

In contrast, a restriction enzyme is a type of endonuclease that typically cuts DNA at specific, short sequences, known as restriction sites, and it does not remove nucleotides from the ends of the DNA molecules.

What Happens When 5′ Exonuclease is Used Instead of a Restriction Enzyme?

If a student mistakenly uses a 5′ exonuclease enzyme to digest their plasmid DNA instead of a restriction enzyme, the following will likely occur:

• The 5′ exonuclease will begin to remove nucleotides from the 5′ end of the plasmid DNA. It will not cut at specific sites as a restriction enzyme would. Instead, it will simply degrade the 5′ ends of the plasmid.

• The exonuclease activity will progressively trim the plasmid DNA from the 5′ ends, resulting in the loss of the 5′ nucleotide sequence.

• As the exonuclease works only on the 5′ ends, the plasmid DNA will not be cleaved into defined fragments (as would happen with a restriction enzyme) but will be progressively degraded.

Expected Results on an Agarose Gel:

When the student runs the plasmid DNA on an agarose gel, they are likely to observe:

• Free nucleotides at the 5′ end of the plasmid DNA, which are the result of the exonuclease activity.

• The plasmid will appear as a smear or a few short fragments, since the 5′ exonuclease will degrade the 5′ end of the plasmid without generating specific, predictable fragments like a restriction enzyme would.

Thus, the most likely observation on the gel would be free nucleotides from the 5′ end only, which corresponds to Option 1.

Why the Other Options are Incorrect:

• Option 2: Free nucleotides from both ends:

  • Exonucleases only degrade from one end of the DNA molecule (in this case, the 5′ end). Therefore, you would not see free nucleotides from both ends of the plasmid.

• Option 3: No digestion of plasmid DNA:

  • The exonuclease will still act on the DNA, removing nucleotides from the 5′ end, so the plasmid DNA will not remain completely intact.

• Option 4: Plasmid DNA will be digested similar to the restriction enzyme:

  • Restriction enzymes cut at specific sites, while exonucleases degrade the ends of the DNA molecule. Therefore, the digestion pattern will be very different, with exonuclease producing a smear or fragments instead of distinct pieces.

Conclusion:

Using a 5′ exonuclease enzyme instead of a restriction enzyme will result in the removal of nucleotides from the 5′ end of the plasmid DNA, causing the DNA to appear as a smear or shorter fragments when analyzed on an agarose gel. This explains why the student would observe free nucleotides from the 5′ end only on the gel.