2. Introduction:
Bacteria have four important
advantages for "traditional types of
genetic experiments":
They are haploid (no masking).
New generation is produced every 20
minutes.
Easy to grow in ENORMOUS
NUMBERS.
Individual members of these large
populations are GENETICALLY
IDENTICAL (or very nearly so).
5. 3 MAJOR TYPES of genetic
transfer found in bacteria:
Transformation - this is what Avery
characterised remember? Somehow
or another, the bacteria can take up
externally provided DNA.
Conjugation - DNA is transferred
from one bacteria cell to another, via
"sex pilli".
Transduction - this is where DNA is
introduced into bacteria by injection
from a bacteriophage.
6. Bacterial Transformation
"Transformation" is simply the
process where bacteria manage to
"uptake" or bring in a piece of
external DNA (somehow or
another). Usually, this process is
used in the laboratory to introduce a
small piece of PLASMID DNA into a
bacterial cell.
7.
8. DNA is the genetic material -
The First demonstration of
bacterial transformation.
Experiments done by
Frederick Griffith (in London)
in 1928 found there were two
different types of the
bacterium Streptococcus
pneumoniae:
9. An "S" or SMOOTH coat strain,
which is lethal to the mice.
An "R" or rough strain, which will
not hurt the mouse.
11. However, if he were to take a mixture of the
heat-inactivated S strain,
mixed with the R strain, the bacteria would
die. Thus there was some
Material in the heat-killed S strain that was
responsible for "transforming" the R strain
into a lethal form.
12. Griffith (and a lab co-worker) was
killed in their laboratory in 1940
from a German bomb. However,
their work continued on in the U.S.,
and in 1944, Oswald Avery, C.M.
MacLeod, and M. McCarty carefully
demonstrated that the ONLY
material that was responsible for
the transformation was DNA - thus,
DNA was the "Genetic material" -
however, many scientists were still
not sure that it was REALLY DNA
(and not proteins) that was the
genetic material. responsible for
"transforming" the R strain into a
lethal form.
13. McCarty,M., The Transforming
Principle - Discovering that
Genes are made of DNA, (New
York: W.W.Norton & company,
1985) - Although this book was
written about 40 years after the
experiments took place (1985),
it is an excellent history of the
research that was going on in
the early 1940's
14. The genetic transfer of streptomycin resistance (strr) to the
streptomycin sensitive (strs) cells of E.coli.
The recovery of strs cells depends on the concentration of
the strr DNA.
15. Co-transformation is simply the
simultaneous transformation of two
different DNA fragments.
First, you must obtain your DNA - you
can do this by isolating DNA from a nice
bacteria that has some DNA you want to
use....
First, you must obtain
you DNA - you can do
this by isolating DNA
from a nice bacteria that
has some DNA you
want to use....
16. Now you do the transformation and
have a look at the products - if you're
transforming into a strain that lacks the
gene of interest (which you usually are),
then the process is quite easy - just look
for colonies that carries the trait of
interest:
17.
18. Bacterial Conjugation
Bacterial conjugation is the
pocess in which DNA is
transferred from a bacterial
donor cell to a recipient cell by
cell-to-cell contact. It has been
observed in many bacterial
species and is best understood
in E.coli, in which it was
discovered by Joshua
Lederberg in 1951.
19.
20. "SEX in a Blender"
By some clever use of timing
experiments, it is possible to generate
a genetic map of E.coli!
