Phage have also been great toolkits for molecular biology.
First, various enzymes were purified, many still in use today.
Later, whole phage machinery were borrowed to move DNA segments
around. Two of the best studied phage are T7 and lambda. Both
have a lot of great history, and both have recently undergone
very interesting makeovers. T7 is a lytic phage; after infection
it simply starts multiplying and soon lyses (breaks open) its
host. T7 provided an interesting early computational conumdrum,
one which I believe is still unsolved. Tom Schneider has an
elegant theory about information and molecular biology, which
can be summarized as locational codes contain only as much
information as they need to be located uniquely in a genome, no
more, no less. Testing on a number of promoters suggested the
theory valid. However, a sore thumb stuck out: T7 promoters
contain far more information than the theory called for, and a
clever early artificial evolution approach showed that this
information really wasn't needed