13 
By present techniques it is possible to cut up the DNA from different 
organisms in a way that allows us to reassemble new combinations of genes — 
that is, to construct hybrid or recombinant DNA molecules in the test tube. 
The first attempts to construct hybrid DNA molecules in vitro were 
carried out at Stanford. David Jackson and Robert Symons in my lab, and 
Peter Lobban in A. Dale Kaiser's research group carried out the first such 
attempts. Our objective was to try to introduce new genes into animal 
cells in order to learn something about how gene expression is regulated 
in such higher organisms. To do this we explored the use of a virus, the 
SV40 virus, as a vector, or carrier, of the new genes to be introduced 
into these animal cells. 
We chose SV40 because it was known that this tumor virus's DNA can be 
integrated into the chromosomal DNA of the recipient cells. That is, it 
becomes a permanent part of the genetic complement of the cells it infects. 
We had available to us a preparation of DNA containing the genes 
which code for the utilization of a sugar, galactose, derived from bacteria, 
E^. coli . We also had large quantities of SV40 virus DNA. What we needed 
was a method for joining these two genetic elements together, but that had 
never really been done before. The principle, however, was already known 
to molecular biologists. It involves what has come to be known as cohesive 
or sticky ends. 
May I have the first slide, please? 
This is my simple diagram. If we look at DNA molecules, just this 
diagram of a double-stranded structure, I have tried to indicate these 
cohesive or sticky ends by little triangular lock-and-key-like structures — 
single-strand extensions at the ends of DNA molecules, with the chemical 
elements associated with them. 
It was known already that two molecules which had ends that could 
interact with each other — that is, form complementary base pairs — could in 
fact be joined to produce long chains by virtue of this interaction at their 
cohesive or sticky ends. If a molecule, in fact, is sticky at either end, 
at both ends, and these are complementary to each other, these molecules 
can be circularized and held together in this form by the interactions of 
their ends. 
We were confronted with a somewhat different problem, because the DNA 
molecules we wanted to fuse together did not have naturally occurring co- 
hesive ends. Consequently, we had to devise a procedure for generating 
artificial cohesive tails onto the ends of any DNA molecule. 
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