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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 chro-mosomes.

It has also been revealed that the ‘bacterial nucleus’ is not duly enclosed in a well-defined mem-branous structure, but at the same time comprises of the genetic material of the bacterial cell.

Interest-* Not connected to the chromosomes.

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 deter-mined and recorded in Table 2.9 below :

Table 2.9. The Genome Size of Certain Bacteria S.No. Microorganisms (Bacteria) Genome size (Daltons × 10⁹)

1 Bacillus subtilis 2.500

2 Escherichia coli 25.000 (± 0.5)

3 Micrococcus salivarius 3.300

4 Mycoplasma pneumoniae 0.480

5 Peptococcus aerogenes 0.816

6 Peptococcus saccharolydicus 1.250

7 Staphylococcus aureus 1.458

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 × 10³ kilo base pairs

Molecular weight : 2.5 × 10⁹ 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 in its appearance.

2.6.9. Ribosomes

Ribosome refers to a cell organelle made up of ribosomal RNA and protein. Ribosomes may exist singly, in clusters called polyribosomes, or on the surface of rough endoplasmic reticulum. In protein synthesis, they are the most favoured site of messenger RNA attachment and amino acid assem-bly in the sequence ordered b the genetic code carried by mRNA.

In other words, the specific cytoplasmic area which is strategically located in the cell material bound by the cytoplasmic membrane having granular appearance and invariably rich in the macromolecular RNA-protein bodies is termed as ribosome.

Characteristic Features : Following are some of the cardinal characteristic features of the

‘ribosomes’, namely:

(1) Contrary to the animal or plant cells, there exists no endoplasmic reticulum to which ribosomes are bound intimately.

(2) Interestingly, there are certain ribosomes that are found to be virtually ‘free’ in the cyto-plasm ; whereas, there are some, particularly those critically involved in the synthesis of proteins require to be transported out of the cell, get closely linked to the inner surface of the cytoplasmic membrane.

(3) The number of ‘ribosomes’ varies as per the ensuing ‘rate of protein synthesis’, and may reach even upto 15,000 per cell. In fact, greater the rate of proteins synthesis, the greater is the rate of prevailing ribosomes.

(4) Ribosomes represent ribonucleoprotein particles (comprising of 60 RNA ; 40 Protein) hav-ing a diameter of 200 Å, and are usually characterised by their respective sedimentation physical properties as depicted in Fig. 2.16.

(5) Prokaryotic Ribosome. In the event when the ribosomes of the prokaryotes undergo ‘sedi-mentation’ in an ultra-centrifuge, they normally exhibit a sedimentation coefficient of 70 S (S = Svedberg Units), and are essentially composed of two subunits i.e., a 50 S and a 30 S subunit (almost fused as shown in Figure 2.16). Consequently, these two subunits get dis-tinctly separated into a 50 S and a 30 S units*. As a result the 50 S unit further gets segre-gated into a RNA comprised of two daughter subunits of 5 S and 23 S each together with thirty two (32) altogether different proteins [derived from 50 – (5 + 23) = 22 sub-units].

Likewise, the 30 S gets fragmented into two segments i.e., first, a RNA comprised of only one subunit having 16 S plus twenty one (21) precisely different proteins [derived from 30 – 16 = 14 sub-units], (see Fig. : 2.16).

(6) Eukaryotic Ribosome : This is absolutely in contrast to the ribosomes of the corresponding prokaryotic organisms, that do possess a sedimentation coefficient of 80 S, and are essen-tially comprised of two subunits each of 60 S and 40 S, respectively.

* The ‘mother ribosomes’ when placed in a low concentration of Mg²⁺ ions get dissociated into two smaller

‘daughter ribosomes’.

30S 50S

50S 30S

5S

23S RNA

RNA 16S 70S

32 Different proteins 21 Different proteins Fig. 2.16. The Prokaryotic Ribosome and its Components

(7) Polysomes. In a situation when these ‘ribosomes’ are specifically associated with the mRNA in the course of active protein synthesis, the resulting product is termed as ‘polysomes’.

It is, however, pertinent to mention here that there are a plethora of ‘antibiotics’ viz., chloramphenicol, erythromycin, gentamycin, and streptomycin, which exert their predomi-nant action by causing the inhibition of ‘protein synthesis’ in ribosomes.