Q.5. Desbribe the classification and structure of Prokaryotes and Eukaryotic.
Ans. Classification of Prokaryotes-Bacteria: -
Prokaryotes include a variety of morphologicall and physiologically diverse forms exhibiting an extra-ordinary variation in their metabolic mechanisms. Unlike eukaryotes, classification of prokaryotes is largely based on their genetic relatedness expecially similarity in DNA (DNA homology). The degree DNA homology (G + C content and nucleic acid hybrization is the best indicator of genetic similarities between two organisms. As a rule of the organisms that ore 40 - 60% related for DNA homology are considered members of a single genus and those that are more than 70% related are assigned to a single form. Similarities below 30% indicate unrelated forms and if an organism is too different to be assigned to any existing genus or species it is declared a new organism and given its own name.
The recently developed molecular approaches have added to the classical approaches which are based solely on phenotypic characters. Using this approach it is now possible among bacteria to recognise a considerable number of subgroups appearing somewhat natural and many of them could be distinguished by both phenotypic and genotypic criteria. However, still it is not possible to develop a complete hierachial classification for bacteria.
The earliest recognised system of bacterial classification was devised by DH Bergey, who first published his system in 1923 in his book. Bergey;s Mannual of Determinative Bacteriology’. This is the most standard reference of descriptive bacteriology. His classification scheme is based on the available genetic data related to the sorphological and biochemical tests. In this system bacteria constitute as one of the division of the kingdom prokaryotae and are divided into 19 groups which are described in table.
The major Groups of Bacteria (Illustrated in Bergey’sMannual) -
The 9th edition of Bergey’s mannual was relased in 1983 in which prokaryotes have been spread over four divisions: (i) Graculicutes including gram negative bacteria; (ii) Firmicutes including gram positive bateria; (iii) Tenericutes including wall - less bacteria and (iv) Mendosicutes including bacteria possessing cell wall that lack peptidoglycan. Despite presenting illustrative account. Bergey’s Mannual (1974) could not accommodate cyano bacteria and archaebacteria.
Acellular, Prokaryotic an Eukaryotic Organism: The varied forms of micro-organisms for the sake of convenience can be studied under acellular, prokaryotic and eukaryotic micro - organisms.
Acellular Micro-organism :-
Micro - organisms lacking a typical cell structure such as viruses bacteriophages virion, prions, cyanophages, mycophages etc. are grouped under acellular micro - organisms. Any given virus contains with DNA or RNA but never both.
Porkaryotic Micro - organisms:-
The discovery of electron microscope has revolutionarised our knowledge to recognise a primitive and small group of organisms whose cell lacks double membraned cell organelles such as nucleus mitochandra, plastids, golgi bodies and endoplasmic reticulum. Stanier and van Niel, 1962 included such organisms in Prokaryota. In prokaryotic organisms cell does not possess any membrane around the nuclear material instead their heriditary material lies in a portion of cytoplasm called nucleoid which is well distinguished in the nuclear region. The prokaryotes are regarded as relics from the earliest time of biological evolution. There is little morphological differentiation among them and such a uniformity in shape, however, is accompained by a remarkable diversity and flexibility in metabolic mecanisms. Such a physiological versatility and flexibility, rate of growth and synthesis, the simple cell organization and simple structure of genetic material have made prokaryotes unique among plant kingdom.
In prokaryotes DNA exists as a closed orcular molecule in the cytoplasm sometimes coupled with one or more very small circular DNA molecules called plasmids. Which are, however, dispensible. Prokaryotic ribosomes are relatively small. The respiratory enzymes are present in specialized structures the mesosgmes. The photosynthetic pigments are enbedded within chromatophores which are comparable eanella of granum cell wall is made up of peptidoglycan (except archaebacteria). Reproduction takes place through asexual methods. Cilia or Hagella are single firbrillar. Prokaryotes include bacteria, Cyanobacteria and actimomycetes filamentous bacteria.
Characteristics :-
1. Aquatic bacteria that produce carbohydrates from CO2 using Photosynthetic pigments.
2. Rods that move by gliding in a layer of slime : many form complex fruiting bodies.
3. Rods surrounded by a shoath of iron or manganese oxides : some have flagella.
4. Diverse group of rods that reproduce by forming buds or appendages.
5. Slender, helically coiled bacteria moving by rotation or flexion of the cell.
6. Helically curved rods that move with a cork - screw like motion.
7. Rods and spheres that give a negative reaction of Gram staining and that require oxygen for life.
8. Rods that give a negative reaction on Gram staining and that may survive in the absence of oxygen.
9. Rods that give a negative reaction on Gram staining and that require and oxygen free environment for life.
10. Spheres that give a negative reaction on Gram staining.
11. Spheres that give a negative reaction on Gram staining and that require an oxygen - free environment for life.
12. Bacteria that use nitrogen, sulphur, and iron compunds for energy and strctural components.
13. Rods and spheres that obtain energy from carbohydrates and that form methane as an end product.
14. Spheres that live in the presence or absence of oxygen and that give a positive reaction no Gram staining.
15. Rods and spheres that form endospores during their life cycles.
16. Rods that do not form spores and that give a positive reaction on Gram staining.
17. Large group of aerobic and anaerobic rods including many soil forms.
18. Small rod - shaped bacteria that are transmitted by arthropods and that multiply only within a host cell; includes chlamydia.
19. Very small, multi - shaped bacteria lacking a true cell wall.
Differences between prokaryotes and eukaryotes: -
Eukaryotic Micro - organisms:-
Eukaryotic micro -organisms exhibit a tremendous range of behaviour in, structure and function. The structure of eukaryotic cell is generally much more complex than prokaryotic cell Eukaryotic cells are usually much larger, with a typical diameter 10 times greater than the prokaryotic cells. Eukaryotic cells also show much diversity in their size and shape. The structure of the nucleus and type of nuclear divisions present most important and fundamental differences between prokaryotes and eukaryotes. The nucleus is separated from rest of the cytoplasm by a nuclear membrane and encloses chromosomes and a nucleolus. The eukaryotic cell undergo mitosis and meiosis. Eukaryotic microbes include algae, fungi and protozoa.