Biology Reference
In-Depth Information
68
This process consists of packaging a piece of
E. coli
DNA into the phage
head accidentally. Thus, it occurs only rarely about one out of a thousand
times. This hybrid phage is known as the transducing particle. If it infects
the recipient
E. coli
, the small piece of DNA from the donor can thus be
introduced into the recipient. The size of the donor DNA has a length of
about two minutes, or roughly 1 X 10
5
base-pairs (bp), i.e. the original size
of the P1-phage DNA. This length is ideal to complement the above-
mentioned experimental location of genetic markers by conjugation.
A typical experiment involved the infection of the donor strain having the
good gene, e.g.
thr
+
, with the P1-phage. The progeny phages together with
the transducing particles were separated from the bacteria by centrifugation,
and used to infect the recipient strain carrying
thr
-
, for example. Similar to
the conjugation experiment, the streptomycin marker could be used to
eliminate any contaminating donor strain, although not absolutely required.
Transductants were then isolated on selecting plates and counted. Due to
the much small size of the donor DNA fragment, the frequency of obtaining
transductants would be much smaller than that of recombinants from
conjugation experiments. Fortunately, it was usually much larger than the
reversion rate.
If the distance between two genetic markers is less than two minutes, they
can be transduced into the recipient simultaneously by one transducing
particle. Let the markers in the donor be denoted by A+ and B+, and the
corresponding ones in the recipient by A- and B-. In one experiment, let
the experimentally measure number of transductants with A+ marker be
[A+], and that of transductants with both A+ and B+ markers be [A+B+].
Then the co-transduction frequency is defined as:
c.f. = [A+B+] / [A+].
Since transduction is a rare event, if the separation of the genetic markers A
and B is larger than two minutes, c.f. = 0. Also, if genetic markers A and B
are the same, c.f. = 1. The question is how to convert other values of c.f.
into the physical distance between markers A and B. One simple-minded
suggestion is to draw a straight line between the above two points (Fig. 3-2).
However, there is no biological reasoning for this approach. Furthermore,
its validity can be tested by considering three consecutive genetic markers,
A, B and C. Experimentally, one can measure the co-transduction
