65. A DNA fragment digested with HindIII and EcoRI was ligated with a vector digested with HindIII and EcoRI
sites present in the polylinker of the vector. Upon screening of transformants by digestion with HindIII and
EcoRI, it was found that all the transformants contained only the self-ligated vector and there was no
recombinant clone (containing insert cloned in the vector). This is possibly due to:
1. Only one of the restriction enzymes digested the vector
2. Both the restriction enzymes digested the vector
3. Only one of the restriction enzymes digested the insert
4. Both the restriction enzymes digested the insert
Introduction
Cloning a DNA fragment into a vector typically requires precise cutting and ligation processes involving restriction enzymes. In this scenario, a DNA fragment is digested with HindIII and EcoRI and then ligated into a vector that has also been cut with these same enzymes. However, when screening the transformants, it was found that all the clones were self-ligated vector and no recombinant clones (with the insert) were observed. This suggests that something went wrong during the cloning procedure.
Possible Cause: Restriction Enzyme Digestion
The most probable reason for this outcome is related to the digestion of either the vector or the insert. Let’s consider the options:
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Only one of the restriction enzymes digested the vector:
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If only one of the enzymes (HindIII or EcoRI) effectively digested the vector, the vector may still have uncut sites that allow it to ligate back onto itself without incorporating the insert. This would result in self-ligation of the vector and no recombinant clones.
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Both the restriction enzymes digested the vector:
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If both HindIII and EcoRI effectively digested the vector, it should have created compatible ends for the insert to ligate. However, without proper digestion of the insert, self-ligation of the vector could still dominate if the insert is not available to ligate.
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Only one of the restriction enzymes digested the insert:
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If only one enzyme successfully digested the insert, the ends may not be compatible with the digested vector. This could prevent the successful ligation of the insert into the vector, leaving only self-ligated vector molecules. This is a very likely cause of the issue.
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Both the restriction enzymes digested the insert:
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If both the HindIII and EcoRI sites were present and properly digested in the insert, the resulting sticky ends should have allowed for ligation with the vector. This would be less likely to result in a failure to obtain recombinant clones, as long as the insert was intact and digested correctly.
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Conclusion
The most likely reason for obtaining only self-ligated vector clones is that only one of the restriction enzymes digested the insert. This would leave incompatible ends on the insert, preventing it from ligating with the vector. To troubleshoot, it is essential to confirm the complete digestion of both the vector and the insert and ensure that both fragments have compatible sticky ends for ligation.
Final Answer:
3. Only one of the restriction enzymes digested the insert
