BACTERIOLOGY
FOURTH LECTURE
A
geneticist approach to classification is to arrange individual organism into
groups on the basis of the homology of their DNA based sequences.
Test
of DNA homology explored the fact that double the fact that double stranded
form separated strands during controlled cooling of a heated preparation of a
DNA.
This
“annealing” process can be easily demonstrated with suitably heated homologous
DNA extracted from a single spp, but it can also occur when a mixture of DNA
from 2 related spp is used, in the latter case hybrid pairs of DNA strands are
produced.
These
hybrid pairing occur with high frequency between complimentary regions of 2 bit
of DNA and the degree of hybridization can be assessed if labelled DNA
preparation are used.
Organisms
with different G+C ratio are very unlikely to show DNA homology. However,
organisms with the same or close G+C ratios don’t necessary show DNA homology.
DNA HOMOLOGY EXPERIMENT
The
double stranded DNA molecules from organism are heated to convert them to
single strands. The single strand from one organism are then mixed with those
from other organisms and allowed to cool. If the two organisms are closely related,
heteroduplexes will be formed. In other words, a strand from one organism will
pair with a strand from the other organism as shown in the scheme below.
However,
if the two organisms are not closely related, no heteroduplexes will be formed.
This method is most useful at the specie level of classification.
Image of
the schematic illustration of the basic principle behind homology experiment.
RIBONAL DNA HOMOLOGY
EXPERIMENTS AND RIBOSOMAL DNA OLIGONUCLEOTIDE CATALOGING
It
is possible for two organisms not to be closely related as to give a high level
of DNA homology but they may still have some degree of relatedness. You may
have known that ribosome which are small granular appearing structure within
the cell synthesize protein. They are made up of proteins and RNA.
The
ribosomal RNA (rRNA), is coded for by only a small fraction of the DNA molecule
called the rRNA cistron. Studies on some bacteria have shown that the
nucleotides sequence of the RNA is highly conserved this means that during
evolution the nucleotide sequence have changed much slowly than the bulk of the
DNA molecule. The implication of this is that even if two organisms are only
distantly related and show no significant DNA Homology, there may be
considerable similarities that exist and can therefore be used as a measure of
relatedness between organism but at level beyond that of specie (at the level
of genus, family, order etc.).
RNA
homology experiment and RNA oligonucleotide cataloging are two model method
used to determine the degree of similarities between the rRNA ciston of
different organisms. These techniques are complex and they are used in
restricted to few scientific concern. Therefore genetic classification of
bacteria based on genetic relatedness allow for.
1.
Stability in classifying organism
2.
Predietability of the position of an organism in
the groups.
3.
It allows room for ancestral relationship in
bacteria i.e. phylogenetic relationship or phylogenetic classification.
4.
Genetic studies for classification purposes has
also enable the group of bacteria with two major groups which are Eubacteria
(true bacteria) and Archaeobacteria (methane oxydizers extreme halophiles and
Thermo acrdophiles).
5.
Such gametic studies with result from rRNA
oligonucleotide cataloging has put more light to demonstrate that the
Archaeobacteria are separated from the Eubacteria by a wide Margin.
BIOCHEMICAL
CLASSIFICATION
The chemical
composition of the prokaryotic cell wall is different from that of Eucaryotic
cell wall. The peptidoglycan structure is built of repeating unit of N-acetyl
glucosamine and N-acetyl muramic acid does not occur in the walls of eucaryotes
and cell wall composition is a distinguishing feature of the procaryotes added
to this is that the prokaryotic (prokaryotic) lack sterols in their cell wall
exceptions being found in the mycoplasma. With more emerging information about the
chemistry of bacteria cell wall, it is now clear that differences do exist in
the structure of Gram +VE and Gram –VE bacteria though the peptidoglycan
structure is common to all.
Secondly,
difference exist in the chemical constituent of individual and this has been of
taxonomic importance value e.g. Gram +VE bacteria contain glycerol teichoic
acid and Ribitol teichoic acid in considerable amount as well as a wide range
of monosaccharides including some that are specific for lactobacillus and clostridium
erosa streptococcus and arabinose for Corynebacterium, mycobacterium.
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BACTERIOLOGY STM311