GENERAL CHARACTERISTICS OF DIFFERENT ACELLULAR ORGANISMS Dr Minakshi

GENERAL CHARACTERISTICS OF DIFFERENT ACELLULAR ORGANISMS Dr Minakshi De

Acellular organisms. Viruses They are infectious agents so small that they can be seen by magnifications of electron microscope. They are 10 -100 times smaller than bacteria, approximate size range 20 -300 nm. They can pass through pore filters which donot allow the bacteria to pass.

They are incapable of growth in artificial media, they grow only in animal, plant and microorganism cells. In these cells they reproduce by replication, a process in which many copies or replicas of each and every viral component is made. The parts are then assembled to produce the progeny virus. They don’t have both DNA and RNA. Host cells have both DNA and RNA. Another name of viruses is obligate intracellular parasite.

They lack their own metabolic machinery to generate energy and synthesise proteins and depend on the host cells to carry out these vital functions. Like the host cell they have the genetic information for replication and information in the genes for usurping the host cell’s energy generating and protein synthesising systems.

Actually in transit from one host cell to another, viruses are small packets of genes. The viral genetic material is either DNA or RNA but they don’t have both. Host cells have both DNA and RNA. Viral nucleic acid is enclosed in a a highly specialised protein coat of varying design. Outside the host cell the coat protects the viral genetic material and serves as a vehicle for entry into another specific host cell. The structurally complete mature and infectious virus is known as virion.

During reproduction in the host cell the viruses may cause disease. Example cold or flu in humans and they may cause many chronic diseases. Usually they are insensitive to antibiotics like penicillin, streptomycin, etc.

To summarise, viruses are defined as noncellular infectious entities whose genomes are a nucleic acid either DNA or RNA, they reproduce only in living cells, using the cell’s biosynthetic machinery to direct the synthesis of specialised particles (virions), they contain viral genomes and transfer them efficiently to other cells. Bacterial viruses are called bacteriophage.

Viroids They are nucleic acid entities of relatively low molecular weight and have a unique structure that cause several important diseases of cultivated plants, such as potato spindle tuber, chrysanthemum stunt, cucumber pale fruit, etc. They are the smallest known agents of infectious diseases. Unlike viruses, they donot possess a protein coat and exist as small infectious molecules of RNA. In spite of their small size, they replicate autonomously in cells of susceptible plant species. Some known viroids are single stranded, covalently closed circular RNA molecules, others may be single stranded linear molecules.

How viroids cause disease is not known, their location in the nuleus may cause interference with gene regulation in the infected host.

Prions Some progressive or gradually extending diseases which usually cause death are poorly understood and requires research. These diseases have a slow progressive course usually with a fatal outcome, incubation periods are measured in years. They are caused by transmissible agents whose properties and behaviour suggest an unconventional or atypical virus, for example unusual resistance to uv radiation and heat. These agents are known as prions, proteinaceous infectious particles because they appear to have no nucleic acids at all, protein is their only detectable component. If prions donot in fact contain nucleic acids, their ability to replicate would seem to pose a challenge to the central dogma of molecular biology.

Bacteria are typically unicellular cells, they are usually small , 0. 5 to 1 micro m in diameter and motile. Some species produce pigments, the true purple and green bacteria possess photosynthetic pigments much related to the chlorophyll of higher plants.

Endospores are formed in some species. Bacteria are free living, saprophytic, parasitic or even pathogenic. The pathogenic forms cause diseases either in plants and animals. They are unicellular, colonial or filamentous, prokaryotic organisms without nuclear membrane, nucleolus, chromatin and histone proteins.

Bacteria Nucleoid or incipient nucleus is composed of naked DNA, RNA and non-histone proteins. DNA is circular and double stranded. Cell wall is made up of peptidoglycan (amino acids+sugar) except in archaebacteria and mycoplasma (smallest known living cells that completely lack a cell wall , can survive without oxygen).

Membrane bound cell organelles are absent, ribosomes are 70 s type

Some of them are autotrophic but vast majority of them is heterotrophic. Respiratory enzymes are found associated with plasma membrane. Reproduction is asexual type. Bacteria show both autotrophic and heterotrophic nutrition. Autotrophic nutrition involves synthesis of organic materials from inorganic substances with the help of light energy (photosynthetic autotrophic) or chemical energy( chemosynthetic autotrophic). Majority of them show heterotrophic nutrition which involves obtaining readymade organic nutrients from outside sources. It is of three typessaprotrophic, symbiotic and parasitic.

Distribution and occurrence of bacteria Bacteria are the smallest, simplest and most abundant microorganisms which are primitive in nature. They were the first inhabitants on earth and still continue to flourish. They occur almost everywhere. Hundreds of them are present in a handful of soil. They also live in extreme habitats like hot springs, deserts, snow and deep oceans where very few other life forms can survive. Many of them live in or on other organisms as parasites.

Bacterial cells that reproduce and divide in a normal manner may be induced to grow in filaments by changing the conditions of the media. Variations in the Mg 2+ content of the media may exert a marked effect on cell division of some bacteria. In a Mg deficient medium, gram-positive rods grow in the form of long filaments. They revert to the normal forms when transferred to the same medium supplemented with suitable concentrations of Mg. Inhibition of cell division occurs also in media supplemented with an excess of Mg.

Shape of bacteria Bacteria exhibit mainly four kinds of shape. 1. The spherical Coccus 2. The rod-shaped Bacillus 3. The comma shaped Vibrium 4. The spiral Spirillum.

All bacteria exhibit pleomorphism, occurrence of different morphological stages during the life of an organism. But a bacterial specie is generally associated with a definite cell form when grown on a standard medium under controlled conditions.

The spherical bacteria divide in one, two or three planes producing pairs or chains , clusters or packets of cells. Some are apparently perfect spheres , others are slightly elongated or ellipsoidal in shape. The streptococci divide in only one plane. They grow normally in pairs or chains. The staphylococci divide in two planes , producing pairs, tetrads or clusters of bacteria that resemble bunches of grapes. The rod shaped forms are usually cylinder with the ends more or less round. Rods may show marked variation in their length/width ratio. Some of them are very long in comparison to their width, others are so short that they may be compared with the spherical forms.

Young cells are in general larger than the old organisms of the same species. As a culture ages, the cells become progressively large until a maximum is reached. After which the reverse effect occurs

The bacteria can be kept in a conical flask containing a suitable medium , which has a carbon source (glucose, sucrose, lactose, etc), nitrogen source(protein, amino acids, ammonium salts, nitrates), mineral source (organic salts in micro amounts). Some bacteria require vitamin as their growth factors. The p. H of the medium has to be adjusted to 7. 2 -7. 6, i. e. neutrality. Size of the bacterial cells is so small that when they are suspended in water they behave like colloids.

Capsule formation Under strain, the bacterial cells produce some gel materials by which the cell wall is covered. This material which covers the cell wall is known as capsule. The capsule is always an extracellular material. It is never intracellular. The function of capsule is to protect the cell from poison or from adverse conditions.

Endospore formation It is the resting system of living system. Sometimes the protoplasm gets dehydrated and forms spherical or elliptical body. This spherical or elliptical body formation by dehydration is known as sporulation. The endospore is more resistant to outermost environmental conditions

Reproduction -Bacteria reproduce asexually mainly by fission. Sometimes under unfavourable conditions they produce spores. The bacteria exhibit a primitive form of sexual reproduction which differs from eukaryotic sexual reproduction because there is no gamete formation and fusion. However the essential feature of sexual reproduction i. e. exchange of genetic material does take place from one bacterium to another.

Here genetic recombination occurs without gamete formation. So it is called parasexuality. It occurs through three methods. 1. Transformation- Absorption of DNA segment from the surrounding medium by a living bacterium. 2. Conjugation- Transfer of genes from donor to recipient by conjugation tube. 3. Transduction- Transfer of genes by viruses.

Bacteria has the following economic importance. Beneficial effects 1. Role in agriculture-scavenging role, saprophytic bacteria obtain food from organic remains such as animal excreta, fallen leaves, meat, etc. They decompose these substance with the help of digestive enzymes aerobically or anaerobically, by this they clean nature. e. g. Pseudomonas. 2. Nitrification- Rhizobium bacteria living in the root nodules of leguminous plant symbiotically helps in fixing atmospheric nitrogen.

Bacteria has the following economic importance. Beneficial effects 3. Production of organic manure-Saprophytic bacteria help in breaking complex organic substances into simpler forms. Thus in this process, they help to convert farm refuse, dung and other wastes to manure. 4. Fermentation activity of bacteria-By this process, lactic acid is produced. 5. Disposal of sewage- By decomposing sewage bacteria helps in the disposal of sewage. 6. Role in industry- Streptococcus lactis bacteria convert milk sugar lactose into lactic acid that coagulates milk protein, casein. Then milk is converted to curd, cheese, etc needed for the industry. 7. Production of antibiotics, vitamins, etc.

Harmful effects of bacteria 1. Food spoilage-Saprophytic bacteria besides decomposing dead matters, also cause rottening of vegetables, fruits, meat, etc. 2. Food poisoning- May cause food poisoning, specially Staphylococcus aureus and cause diarrhoea and vomiting. 3. Damaging domestic articles-Some bacteria may cause deterioration of cotton, leather and wooden articles.

Harmful effects of bacteria 3. Denitrification. Denitrifying bacteria convert nitrate of the soil to gaseous nitrogen and hampers plants. 4. Desulphurication. Bacteria like Desulfovibrio convert soil sulphates into hydrogen sulphide. 5. Causing diseases-Over 90% of human diseases and 10% of plant diseases are caused by bacteria.