Understanding Cohen and Boyer's First Recombinant DNA

The question asks about the origin of the plasmid used by Stanley Cohen and Herbert Boyer when they created the first artificial recombinant DNA molecule. This was a landmark achievement in molecular biology and genetic engineering.

What is Recombinant DNA?

Recombinant DNA (rDNA) is a technology where DNA molecules from two different sources are combined into one molecule. This is typically done by inserting a piece of foreign DNA into a vector, such as a plasmid.

The Pioneering Work of Cohen and Boyer

In 1972, Stanley Cohen and Herbert Boyer successfully created the first functional recombinant DNA. Their experiment involved taking a plasmid from one bacterium and inserting a segment of DNA from another bacterium into it. This modified plasmid could then be introduced back into a host cell.

Identifying the Source of the Plasmid

Cohen and Boyer used plasmids from the bacterium Escherichia coli as their cloning vector. They inserted a gene for antibiotic resistance from a plasmid of Salmonella typhimurium into the E. coli plasmid. This created a new recombinant plasmid that conferred antibiotic resistance when put back into *E. coli* cells. This demonstrated that foreign DNA could be successfully inserted into a plasmid and expressed in a host organism.

Let's look at the options provided:

• (a) Escherichia coli: Cohen and Boyer used plasmids from *E. coli* as the backbone vector for their recombinant DNA construction.
• (b) Thermus aquaticus: This bacterium is famous for its heat-stable DNA polymerase (Taq polymerase), widely used in PCR. It was not the source of the plasmid in Cohen and Boyer's initial experiment.
• (c) Salmonella typhimurium: Cohen and Boyer used a gene (specifically, an antibiotic resistance gene) from a plasmid of *Salmonella typhimurium*, but the plasmid itself that was modified was from *Escherichia coli*. The question asks for the source of "plasmid of", implying the vector plasmid backbone.
• (d) Bacillus thuringiensis: This bacterium is known for producing Bt toxins, which are used as biological pesticides. It is not the source of the plasmid used in Cohen and Boyer's foundational experiment.

Therefore, the plasmid used as the vector backbone by Cohen and Boyer originated from Escherichia coli.

The creation of this first recombinant DNA molecule using an E. coli plasmid and a *Salmonella typhimurium* gene marked the beginning of modern genetic engineering techniques.

Summary of the Experiment

Based on this, the correct option is the one stating the plasmid was from Escherichia coli.

Revision Table: Key Facts about the First Recombinant DNA

Additional Information: Recombinant DNA Technology Basics

Recombinant DNA technology relies on several key tools and processes:

• Plasmids: These are small, circular, double-stranded DNA molecules that occur naturally in bacteria (and sometimes in eukaryotes). They are separate from the bacterial chromosome and can replicate independently. Plasmids are often used as vectors in genetic engineering to carry foreign DNA into host cells.
• Restriction Enzymes: These enzymes, also called "molecular scissors," cut DNA at specific nucleotide sequences (restriction sites). Different restriction enzymes recognize different sequences. They are essential for cutting both the vector plasmid and the foreign DNA fragment at precise locations, creating compatible ends.
• DNA Ligase: This enzyme acts as "molecular glue." It joins the cut ends of DNA fragments, covalently linking the foreign DNA insert into the plasmid vector to create the recombinant DNA molecule.
• Transformation/Transfection: This is the process of introducing the recombinant DNA molecule into a host cell (like bacteria or yeast). The host cell can then replicate the recombinant DNA and express the inserted gene.
• Gene Cloning: The process of creating many identical copies of a specific gene or DNA sequence using recombinant DNA technology. This involves inserting the gene into a vector, introducing it into a host cell, and growing the host cell culture.