Prokaryotic Cell: 18 Main Characteristics (With Diagram)

The following points highlight the eighteen important characteristics of prokaryotic cells.

Characteristic # 1. Nuclear Material:

DNA is naked and lies variously coiled in the cytoplasm. It is often called gonophore, nuclear body or nucleoid. It is equivalent to a single naked chromosome and is, therefore, also called prochromosome. Many prokaryotes also have additional small circular DNA entities called plasmids. Plasmids carry additional specific factors like nitrogen fixation, resistance, fertility, etc.

Characteristic # 2. Nuclear Components:

Nuclear envelope, nucleoplasm, nucleolus and histone cov­ering of chromatin are absent. In eukaryote (= eukaryotic) cells, a typical nucleus is found.

Characteristic # 3. Types:

Prokaryote contains organisms like blue-green algae (BGA = cyanobacteria, e.g., Nostoc), bacteria, pleuropneumonia-like organisms or PPLO (e.g., Mycoplasma), archaebacteria, spirochaetes, rickettsiae and chlamydiae. PPLOs are the smallest free living organisms.

Characteristic # 4. Cell wall:

It is present in bacteria and cyanobacteria. A cell wall is absent in mycoplasma or PPLO.

Characteristic # 5. Flagella and Fimbriae:

Flagella are present in some bacteria only (Fig. 8.6). The bacterial fla­gella are single-stranded as compared to 11-stranded flagella of eukaryotes. In some bacteria, non-motile appendages called pili or fimbriae also occur. They take part in attachment (e.g., Neisseria gonorrhoeae) and conjugation (e.g. Escherichia coli).

Characteristic # 6. Photosynthetic Thylakoids:

Blue – green algae and some bacteria are photo-autotrophic. Their photosynthetic thylakoids lie freely in the cyto­plasm. They are not organised into chloroplasts.

Characteristic # 7. Membrane-lined Cell Organelles:

The prokaryotic (= prokaryotic) cells lack mitochon­dria, endoplasmic reticulum, Golgi apparatus, lysosomes, microtubules, microfilaments and cen­trioles.

Characteristic # 8. Vacuoles:

Typical vacuoles are doubtful. Instead complex gas vacuoles are found.

Characteristic # 9. Ribosomes:

Ribosomes are 70S as com­pared to 80S. Similar 70S ribosomes occur inside chloroplasts and mitochondria of eukaryotes.

Characteristic # 10. One-Envelope System:

In prokaryotic cells, membrane bound cell organelles are absent so that there is a single membrane that surrounds the cell. Hence, prokaryotes have a single membrane or one-envelope system. In eukaryotes many organelles are surrounded by their own covering membranes in addition to the cell membrane that covers the whole cell. Therefore, eukaryotes have a double membrane or two-envelope system of organisation.

Characteristic # 11. Cyclosis:

Cytoplasm does not show streaming movements or cyclosis.

Characteristic # 12. Spindle:

Mitotic spindle is not formed during cell division.

Characteristic # 13. Sexual Reproduction:

It is absent. Therefore, meiosis and gamete formation are unknown. They multiply very rapidly by asexual means like binary fission, sporulation etc.

Characteristic # 14. DNA Content:

It is low.

Characteristic # 15. Transcription and Translation:

Transcription and Translation occur in the cytoplasm.

Characteristic # 16. Respiratory Enzymes:

They usually lie in contact with cell membrane.

Characteristic # 17. Endocytosis and Exocytosis:

They seen to be absent in prokaryotes.

Characteristic # 18. Nitrogen Fixation:

It occurs only in some prokaryotes, bacteria and cyanobacteria.