1 Introduction:
Biological Classification is the process of grouping living organisms based on easily observable characters like similarities and dissimilarities.
At the beginning of human civilization, classification was based on the need for food, shelter, and clothing. But not in the scientific way.
1.1 Importance of Biological Classification:
Studying every organism is impossible, but examining one or two from a group reveals its essential features.
It helps in identification of new organisms.
Classification reveals evolutionary links between organisms across time.
1.2 Earliest attempt at a scientific classification:
Aristotle was the first or earliest to attempt a more scientific basis of classification.
Based on morphology, he classified plants into trees, shrubs and herbs.
Based on the color or pigment of blood, he classified animals into red blood (Enaima) and absence of red blood (Anaima).
1.3 Two kingdom classifications:
Carolus Linnaeus is the first person to introduce a system of classification.
He introduced two kingdom classifications in the year 1735 on the basis of Gross morphology, Locomotion, Mode of Nutrition and Cell Wall. But primary criteria is Gross morphology and Locomotion.
Plantae
Animalia
Drawback:
This system did not distinguish between the eukaryotes and prokaryotes, unicellular and multicellular organisms & photosynthetic (green algae) and non-photosynthetic (fungi) organisms.
A large number of organisms did not fall into either category like bacteria and fungi.
1.3.1 Inadequacies (incomplete/insufficient) of the Two Kingdom System:
Need for Additional Characteristics: Gross morphology was insufficient; other characteristics like cell structure, nutrition, habitat, reproduction, and evolutionary relationships needed to be considered.
1.3.2 Evolution of Classification Systems:
Changes Over Time: Classification systems have undergone several changes to address the limitations and inadequacies of the two kingdom system.
Constant Kingdoms: Plant and animal kingdoms have remained constant, but the understanding of which groups/organisms belong to these kingdoms has changed.
Differing Understandings: Different scientists have understood the number and nature of other kingdoms differently over time.
1.3.3 Conclusion:
The two kingdom system was too simple and its drawbacks led to the development of new classification systems."
Extra Information
1.4 Three Kingdom classifications:
This was proposed by Ernst Haeckel in the 1860s on the basis of locomotion, Cell Wall and Unicellular or Multicellular.
Protista
Plantae
Animalia
Protista consist of protozoa, bacteria, fungi and other microorganisms.
1.5 Four Kingdom classifications:
This was proposed by Herbert F. Copeland in 1962 on the basis of locomotion, Cell Wall, unicellular or multicellular and photosynthesis.
Protista
Fungi
Plantae
Animalia
Protista still consists of protozoa, bacteria, and other microorganisms.
1.6 Five Kingdom classifications:
This was proposed by R H Whittaker in 1969.
R H Whittaker classified based on main criteria into five kingdoms as follows;
Monera
Protista
Fungi
Plantae
Animalia
The main criteria:
Cell structure (Cell type and Cell Wall),
Body organization,
Mode of nutrition,
Reproduction and
Phylogenetic relationships.
Monera contains only prokaryotes, especially bacteria in it because of the lack of nuclear membrane.
Protista contains all unicellular eukaryotes.
A fungus morphologically appears like a plant still it is placed in a separate kingdom due to heterotrophic mode of nutrition and chitinous cell wall.
Plantae have a unique feature: The cell wall is made up of cellulose.
In animal kingdom observation, the cell is not made of cell wall or lack of cell wall.
Kingdom Protista has brought together Chlamydomonas, Chlorella (earlier placed in Algae within Plants and both having cell walls) with Paramoecium and Amoeba (which were earlier placed in the animal kingdom which lack cell walls).
Changes in classification systems: Our understanding of characteristics and evolutionary relationships improves with technology, leading to changes in both current and future classification systems. These attempts establish a system that accurately reflects morphological, physiological, reproductive similarities, and evolutionary connections.
Extra Information:
1.7 Six Kingdom classifications:
This was proposed by Grey and Doolittle in 1977.
Archaebacteria
Eubacteria
Protista
Fungi
Plantae
Animalia
1.8 Seven Kingdom classifications:
This was proposed by Thomas Cavalier-Smith in 1995.
Archaebacteria
Eubacteria
Protista
Fungi
Plantae
Animalia
Chromista (algae group)
Note: SAVINAY KUMAR JC
In eight kingdom classification Archezoa is a kingdom containing archamoeba. It has flagella for locomotion and lack of mitochondria.
This kingdom was proposed by Thomas Cavalier-Smith in 1993.
1.9 Three Domain systems:
This was proposed by Carl Woese et.al in 1990
Archaea
Bacteria
Eukarya
Three domains of life include 6 kingdoms as Archaebacteria, Eubacteria, Protista, Fungi, Plantae, Animalia
1.10 Difference between Prokaryotic and Eukaryotic cell:
S* = Svedberg’s unit SAVINAY KUMAR J C
Note:
The criteria for classification changes. Justify the statement by taking Paramecium and fungi as examples.
Paramecium was included under Animalia due to the lack of Cell Wall. But, later it was placed in Protista due to its unicellularity.
Fungi were included under Plantae due to the presence of Cell Wall. But, later it was placed in Mycota (Fungi) due to heterotrophic mode of nutrition and chitinous cell wall.
2 General Characteristics of Kingdom Monera: (Gr: Monera = single or solitary)
Bacteria are the sole members of the kingdom monera. They are the most abundant microorganisms.
These are the most primitive microscopic unicellular and prokaryotic organisms.
Its cell wall is made up of Peptidoglycan (polysaccharide and amino acid).
In this cell organelles are absent except the ribosome.
They are found in soil, water, air, in extreme habitats like the Deep Ocean, hot deserts, hot springs, inside and outside of other living organisms.
Mode of nutrition: Autotrophic and Heterotrophic.
Reproduction in bacteria:
Primarily asexual by binary fission and spore formation at the time of unfavorable condition.
Pseudo-sexually by conjugation and transformation, through this transfer DNA from one bacterium to the other bacterium.
Locomotory organelle: Flagella and in a few cases locomotory organelle is absent.
Ex: Archaebacteria, Mycoplasma, Cyanobacteria, Chemosynthetic bacteria, Heterotrophic bacteria.
Note:
Hundreds of bacteria are present in a handful of soil.
Bacteria is a plural form and bacterium is a singular form.
2.1 Shape of Bacteria:
Bacteria are grouped under four categories based on their shape as follows;
Spherical Coccus (pl: Cocci)
Rod Shaped Bacillus (pl: Bacilli)
Comma shaped Vibrium (pl: Vibrio) and
Spiral Spirillum (pl: Spirilla)
2.2 Bacterial Complexity:
In this understanding the complexity of bacterial behavior and metabolism.
Simple Structure, Complex Behavior: Bacteria have a simple cellular structure, but exhibit complex behaviors.
Metabolic Diversity:
Bacteria display the most extensive metabolic diversity among all organisms.
Ability to thrive in various environments and utilize different energy sources
Autotrophic Bacteria: Synthesize their own food from inorganic substrates.
Heterotrophic Bacteria: Depend on other organisms or dead organic matter for food.
2.2.3 Types of Bacteria:
Majorly bacteria are divided into two types based on cell wall composition as follows;
Archaebacteria
Eubacteria.
2.3.1 Archaebacteria: (Gr: Archaea = ancient / very old)
They live in harsh habitats such as extreme salty areas (Halophiles), Hot springs (Thermoacidophiles) and Marshy areas (Methanogens).
Many Archaebacteria have cell walls made up of polysaccharide pseudomurein and this feature is responsible for their survival in extreme conditions.
Methanogens are present in the gut of several ruminant animals such as cows and buffaloes.SAVINAY KUMAR JC
Methanogens are responsible for the production of methane (biogas) in absence of oxygen from the dung of ruminant animals by the reduction process.
Biogas is a renewable energy source that can be used for cooking, heating, and electricity generation.
2.3.2 Eubacteria:
These bacteria are called true bacteria and they are abundant in nature.
They are characterized by the presence of a rigid cell wall and if Motile bacteria possess a flagellum.
They have a rigid cell wall called Peptidoglycan made up of true murein.
2.3.2 1 Types of Eubacteria:
Eubacteria further divided into three types based on mode of nutrition;
i) Cyanobacteria (Holophytic):(Gr: Cyano = blue)
It is also known as blue green algae.
It has ‘chlorophyll a’ similar to green plants due to this, they are photosynthetic autotrophs.
Forms: unicellular, colonial or filamentous.
The colonies or filaments are generally surrounded by gelatinous sheath.
Habitat: freshwater/marine or terrestrial algae.
They often form blooms in polluted water bodies.
In colonial or in filamentous Cyanobacteria, few cells are specialized for fixing atmospheric nitrogen called Heterocysts (Gr: Hetero = different; cysts = sac).
Cyanophycin granules help in storage of nitrogen.
They lack flagellated cells.
Ex: Nostoc, Anabaena and Oscillatoria (not consisting of Heterocysts).
ii) Chemosynthetic autotrophic bacteria:
It uses chemical reactions to produce food.
They are oxidizing various inorganic substances such as nitrates (NO3-), nitrites (NO2-) and ammonia.
After oxidizing, they get ATP as an energy molecule.
They play a great role in recycling nutrients like nitrogen, phosphorus, iron and sulfur.
Ex: Ferrobacillus, Nitrobacter, Nitrifying Bacteria.
iii) Heterotrophic bacteria:
These bacteria do not synthesize their organic food. They get their food from external sources.
They are most abundant in nature.
In these the majority are decomposers (saprophytes), few are parasitic and symbiotic bacteria.
Parasitic Bacteria: They obtain their organic food from living cells. They are harmful to human beings, crops, farm animals and pets.
Ex: Cholera, Tetanus, Citrus Canker (plant disease) and Typhoid.
Saprophytic (Gr: Sapro = rotten/decomposed; phytic = plants) Bacteria: These bacteria obtain their nutritional requirements from dead and decaying matter.
Ex: Clostridium
Symbiotic Bacteria: This bacteria engage in a relationship with another organism where both parties benefit. This mutually advantageous interaction is known as symbiosis.
Ex: Rhizobium in legumes plants (beans family)
iv) Mycoplasma: (Gr: Myco = fungus; plasma = fluid)
It is also called pleuropneumonia like organisms (PPLO).
They are obligate parasites for animals and plants.
They lack a cell wall and the smallest living cell in nature.
They can survive without oxygen.
2.4 Use (friends) of bacteria to mankind:
Lactobacillus = Curd, Cheese from milk.
Streptomyces = produce streptomycin antibiotics.SAVINAY KUMAR JC
Rhizobium = fixing nitrogen in legume roots.
Acetobacter aceti = Acetic acid used in Vinegar production.
Clostridium = production of butyric acid.
2.5 Harmful (foes) bacteria to mankind:
Causes disease like
Vibrio Cholerae= Cholera
Salmonella typhi= Typhoid
Clostridium tetani = Tetanus
Xanthomonas citri= Citrus canker
3 General characteristic Kingdom Protista: (Gr: Prot = very first; ista = member)
All single celled eukaryotes are placed under Protista, but undefined in terms of mode of Nutrition.
They are eukaryotic organisms that cannot be classified as plants, animals, or fungi (heterogeneous group and behavior), but they link with each of these kingdoms.
Habitat: Primarily aquatic and few terrestrial.
Mode of nutrition: Autotrophic like holophytic and heterotrophic like saprophytic, holozoic and a few are parasitic.
Kelp, or 'seaweed,' is a large Multicellular Protist that provides food, shelter, and oxygen for numerous underwater ecosystems.
Locomotory organelles: Flagella and Cilia
Reproduction: Sexually and asexually reproduces.
The Protista is classified into five phylum’s Chrysophytes, Dinoflagellates, Euglenoids, Slime Moulds and Protozoans.
Note:
All single-celled eukaryotes are placed under Protista, but the boundaries of this kingdom are not well defined. What may be ‘a photosynthetic protistan’ to one biologist may be ‘a plant’ to another. Because One biologist might call them protists, while another might consider them plants.
The micro-organisms that float passively on the surface of water are known as Planktons.
Planktons are divided into two types,
Phytoplankton and
Zooplankton (weak swimming animals).
3.1 Chrysophytes: (Gr: Chryso = Gloden; Phyta = plant)SAVINAY KUMAR JC
This group consists of diatoms and golden algae. So, this group is called desmids.
They are found in freshwater and marine.
The cell wall is made up of silica known as frustules. These walls are indestructible.
Diatoms have special cell walls that are made up of two thin layers. These layers overlap and fit together just like the two parts of a soap box (epitheca and hypotheca).
After the death of diatoms, its cell wall deposits in their habitats over a billion for years has been known as diatomaceous earth.
Diatomaceous earth is used in polishing, filtration of oils and syrups. Because of its gritty nature or texture (rough or harsh).
They contain pigment diatomin (fucoxanthin), with the help of these, it takes place photosynthesis.
Diatoms are the ’chief producers or primary producers’ in the oceans (Plankton).
Its locomotory organelle is flagella and in few cases pseudopodia.
3.2 Dinoflagellates: (Gr: Dino = 2 unequal or terrible or fearful; flagella = whip)
They are marine and photosynthetic organisms.
Their cell wall is made up of ‘stiff cellulose plates’ on the outer surface.
They appear in different forms like yellow, green, red, blue or brown depending on the main pigments present in their cells.
It has a locomotory organelle that is two flagella; one lies longitudinally and the other transversely in a furrow (depression) between the wall plates.
Very often, red dinoflagellates undergo rapid cell division, which leads to the sea appearing red (red tides) and releasing toxins. It kills marine habitats like fishes.
Ex: Gonyaulax (red tides causes).
3.3 Euglenoids: (Gr: Eu = true; glene = pupil of the eyes; oid = one)
Habitat: They were found in mostly freshwater and stagnant water but rarely in near marine habitats.
Instead of a cell wall, euglenoids have a protein-rich layer called a pellicle, which makes their bodies flexible.
It contains chlorophyll a and b pigment as in the plants.
In the presence of sunlight it shows autotrophic and absence of sunlight it shows heterotrophic behavior. Due to this, it is called mixotroph.
They possess two flagella: one short and one long.
Most green euglenoids store their carbohydrates as paramylon/paramylum.
Ex: Euglena
Note:
Euglena is called a connecting link as it shows the features like photosynthesis and locomotion as in plants and animals respectively. SAVINAY KUMAR JC
3.4 Slime Moulds: (Eng: Shaping the fluid or matrix)
Slime Moulds are saprophytic Protists.
They engulf organic material as they move along decaying twigs and leaves.
Life Cycle:
Under favorable conditions slime molds form an aggregation called a plasmodium.
After turning into a plasmodium structure, it aggregates, spreads and grows into several feet. SAVINAY KUMAR JC
In unfavorable conditions plasmodium differentiates and forms fruiting bodies bearing spores at their tips.
Characteristics of Spores:
The spores have true walls.
They are extremely resistant and can survive for many years, even under adverse conditions.
Spores are dispersed by air currents.
Ex: Dictyostelium, Physarum, Acrasia
3.5 Protozoans: (Gr: Proto = primitive and zoan = animal)
All protozoans are heterotrophic and free living as predators or parasites.
They are considered as primitive relatives of animals.
They Have four major classes as follows;
Amoeboid protozoans
Flagellated protozoans
Ciliated protozoans
Sporozoans
3.5.1 Amoeboid protozoans:(Lat: amoeba = no definite shape)
These organisms live in freshwater, Marein (sea or ocean), moist soil and parasitic.
They move and capture their prey by putting out pseudopodia (false feet).
Marine forms have silica on their surface.
Ex: Amoeba (fresh water), Lobosa (marine), Arcella (Moist soil) and Entamoeba (Parasite)
3.5.2 Flagellated protozoans:
They are free-living or parasitic. They have flagella.
The parasitic forms cause disease such as sleeping sickness.
Ex: Trypanosoma
3.5.3 Ciliated protozoans: (Gr: cilia = eyelashes)
These are aquatic, actively moving organisms because of the presence of thousands of cilia.
They have a cavity (gullet) that opens to the outside of the cell surface.
During movement, the cilia directs the water carrying food particles into the cavity.
Ex: Paramecium.
3.5.4 Sporozoans: (Gr: Sporo = Seed)
They are Parasite Protozoans.
When they infect any cell it produces spores inside the host cell (infectious spore-like stage in their life cycle).
Ex: Plasmodium (causes malaria), a disease which has a staggering effect on the human population.
Note: Notorious parasite is plasmodium and Notorious Protozoan is sporozoans.
4 General characteristics of Kingdom Fungi: (Gr: Fungus = Mushroom)
This kingdom is also known as Kingdom Mycota.
Diversity in morphology: Fungi exhibit a wide range of shapes and sizes. They are eukaryotic, rarely unicellular (yeast) and most of them are multicellular (mushroom) in nature.
Habitat: Fungi are cosmopolitan* and occur in air, water, soil and on animals and plants. They prefer to grow in warm and humid places. Even it grows on moist bread and rotten fruits.
Mode of Nutrition: Fungi are ‘achlorophyllous’. So it is a heterotrophic creature like sporophytic, coprophiles, parasitic and symbiotic.
Cell wall is made up of chitin or fungal cellulose (polysaccharides).
It stores food in the form of Glycogen and oils.
Body of the fungus is called mycelium. It consists of a long, slender thread-like structure or filamentous structure or network-like structure of fungi called hyphae.
Hyphae are coenocytic (aseptate) or septate (cross walls), branched or unbranched, monokaryotic or dikaryotic.
They reproduce vegetative, asexually and sexually.
They all are heterotrophic in nature like saprophytic, coprophiles, parasitic or symbiotic.
Ex: Yeast, Penicillium.
Note:
*Cosmopolitan means they are found in a wide variety of environments across the world.
Coenocytic hyphae are continuous tubes filled with multinucleated cytoplasm.
Dikaryon means two nucleuses in a cell.
4.1 The sexual cycle of Fungi:
It involves the following three steps;
Fusion of protoplasms between two motile or non motile gametes is called plasmogamy.
Fusion of two nuclei is called karyogamy.
Meiosis in zygote resulting in haploid spores. SAVINAY KUMAR JC
4.2 Pre fertilized events in Fungi:
When a fungus reproduces sexually:
Two compatible haploid hyphae fuse.
In some fungi, fusion creates diploid cells (2n) immediately.
In ascomycetes and basidiomycetes, a dikaryotic stage occurs:
Each cell has two nuclei (dikaryophase).
Nuclei later fuse, forming diploid cells
Fruiting bodies are then formed, where reduction division takes place to produce haploid spores.
4.3 Reproductive Structures in Fungi:
Reproductive structures are also known as fruiting bodies in fungi. The various spores are
produced in distinct structures called fruiting bodies.
Vegetative means: fragmentation, binary fission and budding.
Asexual structures: Conidia, Sporangiospores and zoospores.
Sexual structure: oospores or zygospores, ascospores and basidiospores.
4.4 Types of Sexual Reproduction in Fungi:
Isogamous: Fusion of similar gametes in size and structure is known as isogamous. Ex: Mucor
Anisogamous: Fusion of dissimilar gametes in size and structure is known as anisogamous. Ex: Penicillium
Oogamous: Fusion between one large non motile female gamete and a smaller motile male gamete is termed as oogamous. Ex: Albugo
4.5 Classification of Fungi:
The morphology of the mycelium, mode of spore formation (reproduction) and fruiting bodies based on these character fungi is divided into four classes.
Phycomycetes or Zygomycetes
Ascomycetes
Basidiomycetes and
Deuteromycetes
4.5.1 Phycomycetes: (Gr: Phyco = Algae or seaweed; Mycota = Fungi)
These members are also known as Zygomycetes. Commonly called bread molds.
They are found in aquatic habitats, on decaying wood in moist and damp places, few are obligate parasites on plants.
Mycelium is multicellular, coenocytic and unbranched / branched.
Asexual reproduction takes place by zoospores (motile) and aplanospores (non-motile). These spores are endogenously produced in sporangium.
Sexual reproduction by fusion of two gametes. These spores are called zygospores. Gametes may be Isogamous, or anisogamous or oogamous.
Ex: Mucor, Rhizopus and Albugo (Plant parasite on Mustard)
4.5.2 Ascomycetes: (Gr: Asco = Sac / half cup)
They are commonly called sac fungi.
Its mycelium is unicellular (few) or multicellular, branched and septate.
Mode of nutrition: saprophytic, coprophilous (growing on dung), symbiotic and parasitic
Asexual Spores are conidia and bearing conidiophores. These spores were produced exogenously.
Sexual spores are ascospores. These spores are produced inside (endogenously) the ascii (singular: ascus) and have an aggregated structure called ascocarp (fruiting body).
Ex: Yeast (Unicellular), Penicillium, Neurospora, Claviceps, Morels, Truffles and Aspergillus (Multicellular).
4.5.3 Basidiomycetes: (lat: Basidio = club)
It is commonly called Mushrooms or club fungi.
They grow in soil, on logs and tree stumps and few are parasites.
Its mycelium is Multicellular, unbranched or branched and septate.
Vegetative reproduction is common by fragmentation.
Asexual Spores are absent.
Sexual Reproduction:
Sex organs are absent, but sexual reproduction takes place through the dikaryon method.
Basidiospores are formed outside the basidium (exogenously) and plurally called basidia.
Its fruiting bodies are called basidiocarp.
Ex: Agaricus (Agaricus bisporus is an edible mushroom), Ustilago (smut), Puccinia (wheat rust), Bracket fungi, Puffballs and Toadstools.
Life Cycle of Dikaryon method:
Plasmogamy: Fusion of two vegetative or somatic cells of different strains or genotypes.
Dikaryotic Structure: Resultant structure that eventually forms the basidium.
Karyogamy and Meiosis: Occur in the basidium, producing four basidiospores.
4.5.4 Deuteromycetes: (Gr: Deutro = Imperfect)
In these members most are saprophytic and few are parasitic.
Its mycelium is Multicellular, septate and branched.
They reproduce only by asexual spores known as conidia.
Its sexual reproduction is still not reported due to this, it is called imperfect fungi. If a sexual cycle is discovered, it would be reclassified under ascomycetes or basidiomycetes.
They help in mineral cycling through Saprophytic (decompose litters).
Ex: Alternaria, Colletotrichum and Trichoderma.
4.6 Ecological Uses of Fungi:
Majority of decomposers are fungi that help in recycling of minerals. Ex: Mucor
They present in the roots of some crop plants helps in bringing good yield by absorbing minerals. Ex: Mycorrhiza
It helps in increasing soil fertility.
Some fungi are capable of degrading pollutants and contaminants from the soil called bioremediation.
4.7 Economical Uses of Fungi:
They are used in production antibiotics like Penicillin etc. Ex: Penicillium
Mushrooms, Morels and truffles are edible and rich in minerals and vitamins.
Yeast is used in the preparation bread, fermented alcohol and preparation food like dosa and idli.
Neurospora is used extensively in biochemical and genetic work.
Alternaria is used in the preparation of citric acid.
Note:
Fungi causing wheat rust = Puccinia
Parasitic fungi on mustard = Albugo (causes white spots)
Symbiotic fungi = Lichens and Mycorrhiza
Bread Mould = Rhizopus
Poisonous fungi or inedible fungi = Toadstools
Smut (black powder) = Ustilago
5 Viruses, Viroids, Prions and Lichens:
Viruses, Viroids, Prions and lichens not included in the five kingdom classification of R.H.Whittaker.
Viruses, Viroids, and Prions are acellular or non cellular organisms.
Lichens are symbiotic associations between algae and fungi.
5.1 Viruses: (Lat: virus = venom or poisons fluid)
Viruses are acellular organisms, crystallizable, non-living infectious agents but made up of proteins and nucleic acid.
Viruses have not been included in biological classification because they are not regarded as truly 'living' organisms.
2.5.1.1 History of Viruses:
The virus term was 1st used by Louis Pasteur (1880).
It was first discovered by D.J. Ivanowsky in a tobacco infected plant (1892).
In 1898, M.W. Beijerinck demonstrated that an extract from infected tobacco plants could transmit disease to healthy plants. He referred to this infectious fluid as “Contagium Vivum Fluidum” (contagious living poison fluid) and formally coined the term “virus.”
W.M. Stanley crystallized the virus first (1935). Crystals consist largely of proteins.
5.1.2 General Characteristics:
They are obligate parasites.
Viruses are ultramicroscopic and acellular organisms.
It is made up of protein coat and nucleic acid (Nucleoprotein) but it contains either DNA or RNA but not both.
Genetic material may be ssRNA, dsRNA or dsDNA.
The protein coat is called capsid and its small subunit called capsomeres. It protects viral genetic material.
Capsomeres in the form of helical or polyhedral geometrical form.
In viruses genetic material is infectious.
Outside the host cell, viruses don’t perform respiration and metabolism.
They are insensitive to antibiotics
Ex: HIV, H1N1 or influenza, Herpes (oral and genital rashes), Mumps (infected salivary gland), Smallpox, TMV, Bacteriophage (Infects bacteria).
5.1.3 Types of Viruses:
Based on type of organisms it is classified into following types;
Animal Viruses: May contain either ssRNA or dsRNA or dsDNA.
Bacteriophages: Viruses that infect bacteria usually have dsDNA.
Plant Viruses: Typically contain ssRNA.
Symptoms of Viruses in Plants:
Mosaic Formation
Leaf rolling and Curling
Yellowing and Vein Clearing
Dwarfing and Stunted growth.
5.2 Viroids:SAVINAY KUMAR JC
It was discovered by T.O. Diener in 1971.
It causes diseases only in plants.
It is made up of only circular ssRNA as genetic material and lacks a protein coat.
The RNA of the viroid was low molecular weight.
It is smaller than viruses and it causes disease like Potato Spindle tuber disease.
5.3 Prions:
It was discovered by Stanley B. Prusiner.
The Prions were similar in size to viruses.
It is an infectious abnormally folded protein that converts normal protein into abnormal protein.
Most of the time it causes disease in the brain. Due to this, it is called neurological infectious disease.
Ex: Mad cow disease in cattle’s, Cr-Jakob Disease (CJD) in humans (Creutzfeldt).
Mad cow disease is also called ‘Bovine Spongiform Encephalopathy’ (BSE).
Transmits: Contact with saliva, Sharing food, towel, brush of infected individuals and also due to inheritance.
5.4 Lichens:
Lichens are symbiotic associations (Mutually useful associations) between algae and fungi.
The algal component is known as ‘Phycobiont’ and the fungal component is known as ‘mycobiont’.
Algae (autotrophic) provide food for fungi and fungi (heterotrophic) provide shelter, minerals and water for algae.
Lichens are very good pollution indicators. They do not grow in polluted areas. Especially when air is polluted (air pollution) with sulfur dioxide and nitrogen.
SAVINAY KUMAR JC
