It has been duly established that the
double-stranded chromosomal DNA
type of microbe essentially inherits some typical characteristic features which remain not only constant
and absolutely specific for that microorganism, but also quite beneficial for its methodical classification
as well.
However, there are
two predominant criteria invariably employed for determining the ‘genetic
characteristics’
of microbes, namely:
(
a) DNA base composition, and
(
b) Sequence of nucleotide bases in DNA.
These
two aspects shall now be treated individually in the sections that follows:
3.2.6.1. DNA Base Composition
Importantly, one may evidently observe that the double-stranded DNA molecule is essentially
comprised of
base pairs, such as: adenine-thymine, and guanine-cytosine. However, the entire gross
aggregate of the actual nucleotide bases present in the DNA, the relevant percentage articulately constituted
by
guanine plus cytosine is known as the mole % G + C value (or more concisedly as mole %G + C). Such values usually vary from 23 to 75 for various organisms.
and thus, the (G + C)/(A + T) ratio or
G + C content i.e., the per cent of G + C in DNA, actually
reflects the
base sequence which in turn critically varies with the prevailing sequence changes as given
below:
Mole % G + C =
G +C
G +C+ A + T
× 100
Chemical methods
—are used frequently to ascertain the G + C content after due hydrolysis of
DNA and separation of its bases.
Physical methods
—are employed more often and conveniently e.g., the melting temperature
(T
m) of DNA.
3.2.6.2. Sequence of Nucleotide Bases in DNA
Based on intensive and extensive studies it has been duly revealed that the
sequence of nucleotide
bases in DNA
is not only absolutely extraordinary for each type of organism, but also designates the
most fundamental of all the characteristic features of a microorganism. As a result of this unique genetic
characteristic feature it commands an immense significance for the legitimate classification of microbes.
Besides, there are
two cardinal factors, namely : chromosomal DNA, and plasmid DNA thatmay occasionally show their very presence in the microbial cells
Plasmids
represent an altogether diverse category of extra-chromosomal genetic elements. In
fact, these are circular double-stranded DNA molecules critically present intracellularly and symbiotically
in most microorganisms. They invariably reproduce inside the bacterial cell but are not quite essential to
its viability. In addition,
plasmids are responsible for carrying out the autonomous replication within the
bacterial cells, and their presence would ably impart highly specific characteristic features upon the
cells that essentially contain them, such as:
• Capability of producing
toxins
• Render resistance to different range of
‘antibiotics’
• Make use of
‘uncommon chemical entities’ as nutrients
• Ability to produce
enzymes that specifically produce certain antibiotics
• Ability of the cell to detoxify harmful materials, and
• Production of bacteriocins*.