Cytoplasm

Is the fluid component inside the plasma membrane. Cytoplasm is about 80% water and contains primarily proteins (enzymes), carbohydrates, lipids, inorganic ions and many low MW compounds. Major structures in the cytoplasm are DNA, ribosomes and inclusions.

Nuclear area

The striking difference between prokaryotic and eukaryotic systems is the way in which their genetic material is packaged. Prokaryotes lack a membrane-delimited nucleus. Contains a single long circular molecule of double-stranded DNA bacterial chromosomes do not include histones and are not surrounded by a nuclear envelope and located in an irregularly shaped region called nucleiod . The nuclear area can be spherical, elongated, or dumb-bell shaped. It is attached to the plasma membrane and proteins of plasma membrane are believed to be responsible for replication of the DNA. It has been discovered recently that Vibrio cholera has more than one chromosome. Electron microscope studies have shown the nucleiod in contact with either the mesosome or the plasma membrane and hence evidence that the membranes may be involved in the separation of DNA into daughter cells during division. Chemical analysis reveals that they nucleoids are composed to about 60% DNA, 30% RNA and 10% protein. E. coli, which is about 2 to 6um long, the closed DNA circle measures approximately 1400um. Hence, it is evident that the DNA is efficiently packaged to fit within the nucleoid and the DA is looped and coiled extensively (Fig. 10).

Many bacteria possess extra chromosomal double stranded, circular DNA molecules called plasmids in addition to their chromosome. They replicate independently and are associated with plasma membrane proteins. Plasmids usually contain from five to 100 genes. Plasmids may carry genes for such activities as antibiotic resistance, tolerance to toxic metals, production of toxins, and synthesis of enzymes. Plasmid DNA is used for gene manipulation in biotechnology. Because plasmids move between different bacteria, drug resistance can spread throughout a population.

 

Fig. 10. Bacterial DNA and plasmids

Ribosomes:

The cytoplasmic matrix is also packaged with ribosomes, they also may be loosely attached to the plasma membrane. They look like small, featureless particles at low magnification electron microscope. They are made up of both protein and ribonucleic acid (RNA). Ribosome's function as sites of protein synthesis; matrix ribosomes synthesize proteins destined to remain within the cell, whereas the plasma membrane ribosomes make proteins for transport to the outside.The shape of each protein is determined by its amino acids sequence and the special proteins called molecular chaperones or chaperones aid the polypeptide in folding to its proper shape. Prokaryotic ribosomes are smaller than eukaryotic ribosomes (Fig. 11). Ribosomes are composed of two subunits, each subunit being composed of protein and a type of RNA called ribosomal RNA (rRNA).They are comm. Only 70S: 30S subunit (1 molecule of rRNA) and 50S subunit (2 molecules of rRNA) and have dimensions of about 14 to 15 nm, a molecular weight of approximately 2.7 million.The S in 70S stands for Svedberg value or sedimentation coefficient. It is the sedimentation velocity in a centrifuge; the faster a particle travels when centrifuged, the greater is its Svedberg value. The Sedimentation coefficient is a function of a particle's molecular weight, volume and shape. Several antibiotics, such as streptomycin, neomycin and tetracyclines, exert their antimicrobial effects by inhibiting protein synthesis on ribosomes.

 

Fig. 11. Ribosmes in bacteria