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genomes in the way of synthesizing virus or even getting the whole genome 
into a prokaryotic system is virtually impossible. So we think of the last 
category as clearly less risky than the others, if that is the right word. 
Now, a significant, important class of viruses are the so-called retro- 
viruses. They are of particular interest — well, first, the word "retro" 
comes from the word "reverse transcriptase" — the enzyme that is carried in 
the virus particle that translates the RNA of the virus into DNA, following 
which the DNA is inserted into the cell DNA. These are important because 
they are present in normal chromosomes of many species, not yet known for 
humans, but clearly present in mice and chickens and many other laboratory 
and domestic animals. The virus genome is present as normal gene in the 
animals. One of the early concerns in relation to recombinant DNA work was 
that, in shotgun cloning from cells, one could pick up the genomes of these 
viruses inadvertently. 
The other cause for at least thinking of these in a special category is 
that the leukemia viruses, the viruses that cause leukemia and mammary tu- 
( mors in mice and in ch ickens--not many mammary tumors in chickens, but plen- 
ty of leukemias — they belong in this class. 
This illustrates their life cycle in kind of a sloppy way. The virus 
contains reverse transcriptase; it infects a cell; the RNA is released, is 
i transcribed into DNA; the DNA is integrated into the cell, and then tran- 
scribes RNA, which synthesizes the viral protein, and RNA puts it together 
again to make a virus which is an enveloped virus. In addition, the genome 
sequences in the chromosome of the cell can be induced to start synthesizing 
virus if it is a whole, complete viral genome. There are many copies of 
partial pieces, just small pieces of the viral genome in the species that 
carry these. Some of them are complete genomes; most are defective short 
segments. But they are very complex viruses, and they fall in the category 
that I think could not possibly be synthesized by a bacterium. The nucleic 
acid could. 
Another word that we have to define is "baculoviruses . " These are 
viruses that occur only in insects. They don't replicate as far as is 
known in any plant or animal. Several baculoviruses are extremely narrow 
in their host range. Most of them only grow in certain species and don't 
cross even species lines, much less genera or class. And they are so viru- 
lent for the larval stage of certain insects that they have been developed 
for pest control purposes, and are very widely disseminated for control of 
Douglas-fir tussock moth, and the cotton bollworm. These have been tested 
more extensively than any other virus for host range and safety in a wide 
variety of species. Dozens and dozens of fish, Crustacea, mammals, birds, 
everything has been tested to get the EPA certification that these baculo- 
viruses are safe for distribution. They provide some very interesting 
features for recombinant DNA research. 
That is the last slide, I think, if I could have the lights. 
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