Question

Following steps are involved in cloning a eukaryotic gene in a bacterial plasmid vector.
I. Introduction of cloning vector into cells.
II. Insertion of eukaryotic DNA into the vector.
III. Identification of cell clones that carry the inserted eukaryotic gene.
IV. Isolation of the vector and the eukaryotic gene-source DNA.
V. Cloning of cells (and foreign DNA).
Which of the following statemet is the correct reason for the insertion of eukaryotic DNA into the plasmid vector, the sticky ends formed by digestion of both DNA types with the same restriction enzyme may join in a recombinant molecule?

• A. The eukaryotic DNA and plasmid DNA will have the same sequence
• B. The eukaryotic DNA and plasmid DNA will have complementary sequences
• C. The eukaryotic DNA can join with any plasmid DNA, regardless of sequence
• D. The plasmid DNA can join with any eukaryotic DNA, regardless of sequence
• E. The plasmid DNA and the eukaryotic DNA cannot join together due to differences in the structure of their DNA molecules

Answer 1

The correct option is B The eukaryotic DNA and plasmid DNA will have complementary sequences

When a restriction enzyme like EcoRI cuts the two DNA strands it makes a staggered cut which results in the production of sticky ends. Many enzymes apart from EcoRI make a staggered cut that leaves a single-stranded tail at both ends. These tails on the DNA fragments generated by this restriction enzyme are complementary to the DNA cuts produced on the other fragment of DNA when digested by the same enzyme. These sticky or complementary ends help in transient base-pairing of the DNA fragments. Thus, the correct answer is option B.