THE PROKARYOTIC CELL

The members of the prokaryotic world make up a vast heterogeneous group of very small unicellular organisms. This group includes eubacteria, or true bacteria, and archaebacteria. Bacteria are one of the most important groups of the microbial world.

The chief distinguishing characteristics of prokaryotic cells are:

•  Their genetic material (DNA) is not enclosed within a membrane.

•  They lack other membrane-bounded organelles

•  Their DNA is not associated with histone proteins (special chromosomal proteins found in eukaryotes).

•  Their cell walls most of the time contains the complex polysaccharide peptidoglycan.

•  They usually divide by binary fission. During this process, the DNA is copied and the cell splits into two cells.

Size, shape and arrangement of bacterial cells

Size:

Prokaryotes are among the smallest of all organisms (0.5 to 2.0 m m). Because of their small size, bacteria have a large surface-to-volume ratio. The smallest member of the genus is about 0.3µm in diameter. Even smaller cells have been reported like the nanobacteria or ultramicrobacteria appear to range from around 0.2µm to not less than 0.05µm. E. coli , a bacillusof about average size is 1.1 to 1.5 µm wide by 2.0 to 6.0 µm long. Spirochaetes occasionally reach 500 µm in length and the cyanobacterium Oscillatoria is about 7 µm in diameter. The bacterium, Epulosiscium fishelsoni , can be seen with the naked eye (600 m m long by 80 m m in diameter). Thus a few bacteria are much larger than the average eukaryotic cell (typical plant and animal cells are around 10 to 50 µm in diameter).

Shape and arrangement:

Typically bacteria have three basic shapes – spherical, rod like and spiral.  

Spherical bacteria :

Coccus (pluralcocci ) in pairs– diplococcic; in chains– Streptococci (Streptococcus); in cube like groups of eight– Sarcinae (Sarcina); in grape like structures– Staphylococci (Staphylococcus) (Fig.1)

Fig. 1. Arrangement of spherical cells