Thursday, 21 February 2013

DNA is the genetic material of bacteria,viruses & animals


The idea that genetic material is nucleic acid had its roots in the
discovery of transformation in 1928. The bacterium Pneumococcus
kills mice by causing pneumonia. The virulence of the bacterium is
determined by its capsular polysaccharide. This is a component of the
surface that allows the bacterium to escape destruction by the host. Several
types (I, II, III) of Pneumococcus have different capsular
polysaccharides. They have a smooth (S) appearance.
Each of the smooth Pneumococcal types can give rise to
variants that fail to produce the capsular polysaccharide. These
bacteria have a rough (R) surface (consisting of the material
that was beneath the capsular polysaccharide). They are avirulent.
They do not kill the mice, because the absence of the polysaccharide
allows the animal to destroy the bacteria.
When smooth bacteria are killed by heat treatment, they lose
their ability to harm the animal. But inactive heat-killed S bacteria
and the ineffectual variant R bacteria together have a quite
different effect from either bacterium by itself. the
mouse dies as the result of a Pneumococcal infection. Virulent
S bacteria can be recovered from the mouse postmortem.
In this experiment, the dead S bacteria were of type III. The live R
bacteria had been derived from type II. The virulent bacteria recovered
from the mixed infection had the smooth coat of type III. So some property
of the dead type III S bacteria can transform the live R bacteria so
that they make the type III capsular polysaccharide, and as a result become
Figure 1.4 shows the identification of the component of the dead
bacteria responsible for transformation. This was called the transforming
principle. It was purified by developing a cell-free system, in which
extracts of the dead S bacteria could be added to the live R bacteria before
injection into the animal. Purification of the transforming principle
in 1944 showed that it is deoxyribonucleic acid (DNA).


Having shown that DNA is the genetic material of bacteria, the
next step was to demonstrate that DNA provides the genetic material
in a quite different system. Phage T2 is a virus that infects the bacterium E. coli. When phage particles are added to bacteria, they adsorb
to the outside surface, some material enters the bacterium, and
then -20 minutes later each bacterium bursts open (lyses) to release a
large number of progeny phage.
Figure 1.5 illustrates the results of an experiment in 1952 in which
bacteria were infected with T2 phages that had been radioactively labeled
either in their DNA component (with 32P) or in their protein component
(with 35S). The infected bacteria were agitated in a blender, and
two fractions were separated by centrifugation. One contained the
empty phage coats that were released from the surface of the bacteria.
The other fraction consisted of the infected bacteria themselves.
Most of the 32P label was present in the infected bacteria. The
progeny phage particles produced by the infection contained ~30% of
the original 32P label. The progeny received very little—less than
1%—of the protein contained in the original phage population. The
phage coats consist of protein and therefore carried the 35S radioactive
label. This experiment therefore showed directly that only the
DNA of the parent phages enters the bacteria and then becomes part
of the progeny phages, exactly the pattern of inheritance expected of
genetic material.


When DNA is added to populations of single eukaryotic cells
growing in culture, the nucleic acid enters the cells, and in some
of them results in the production of new proteins. When a purified DNA
is used, its incorporation leads to the production of a particular protein.
Although for historical reasons these experiments are described as
transfection when performed with eukaryotic cells, they are a direct
counterpart to bacterial transformation. The DNA that is introduced
into the recipient cell becomes part of its genetic material, and is inherited
in the same way as any other part. Its expression confers a new trait
upon the cells (synthesis of thymidine kinase in the example of the figure).
At first, these experiments were successful only with individual
cells adapted to grow in a culture medium. Since then, however, DNA
has been introduced into mouse eggs by microinjection; and it may become
a stable part of the genetic material of the mouse
Such experiments show directly not only that DNA is the genetic
material in eukaryotes, but also that it can be transferred between different
species and yet remain functional.
The genetic material of all known organisms and many viruses is
DNA. However, some viruses use an alternative type of nucleic acid, ribonucleic acid (RNA), as the genetic material. The general principle
of the nature of the genetic material, then, is that it is always nucleic
acid; in fact, it is DNA except in the RNA viruses.

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