Based upon various intensive and extensive investigations carried out on the
bacterial cell, one
may observe that the major cytoplasmic contents of it essentially include not only the nucleus but also
ribosomes, proteins, water-soluble components, and reserve material. It has also been observed that a
plethora of bacteria do contain
extrachromosomal DNA i.e., DNA that are not connected to the chromosomes.
It has also been revealed that the
‘bacterial nucleus’ is not duly enclosed in a well-defined membranousstructure, but at the same time comprises of the genetic material of the bacterial cell. Interest
ingly, several altogether sophisticated meticulous and methodical investigations pertaining to the actual
status/content(s) of the bacterial nucleus reveal amply that :
(
a) Electron microscopy : Electron micrographs of the bacterial nucleus under investigation
evidently depict it as a region very tightly and intimately packed with
fibrillar DNA i.e., consisting of
very small filamentous structure.
(
b) Cytological, biochemical, physical, and genetic investigations : Such investigations with
respect to a large cross-section of bacterial species revealed that the
‘bacterial nucleus’ essentially
contains a distinct singular molecule of definite circular shape, and having a double-stranded DNA.
The
genome size of DNA i.e., the complete set of chromosomes, and thus the entire genetic
information present in a cell, obtained painstakingly from a variety of bacterial species has been determined
and recorded
Specifications of
E. coli: The size of DNA in E. coli together with certain other specifications
are as given below :
Average length : Approx. 1000
μm
Base pairs : 5 × 10
3 kilo base pairs
Molecular weight : 2.5 × 10
9 Daltons (± 0.5 × 109)
• The ensuing DNA happens to be a highly charged molecule found to be dissociated with any
basic proteins as could be observed in higher organisms.
• Neutralization of charge is duly caused either by
polyamines e.g., spermine, spermidine, or
by
bivalent cations e.g., Mg2+, Ca2+.
Plasmid DNA :
Besides, the apparent and distinct presence of the bacterial ‘nuclear DNA’, they
invariably contain
extrachromosomal* DNA termed as plasmid DNA that replicates autonomously. It
has been duly observed these
plasmid DNAs exhibit different specific features, such as :
• confer on the bacterial cell,
• drug resistance,
• ability to generate
bacteriocins i.e., proteinaceous toxins.• ability to catabolize uncommon organic chemical entities (viz., in Pseudomonas).
Nevertheless, the actual size of
plasmid DNA usually found in these specific structures may be
nearly 1/10th or even less in comparison to that invariably found in the
bacterial nucleus ; however, the
exact number of copies may change from one to several. Besides, these structures are not enclosed in a
membrane structure. Importantly, the
plasmid DNA is mostly circular in shape and double stranded inits appearance.