2. Systematics of Living Organisms

 

2.11 Units of Classification

Species

The species is the lowest and most basic natural taxonomic unit. It ranks below a genus and is denoted by a Latin binomial name (for example: Homo sapiens for humans). It is considered as the fundamental unit of classification in taxonomy.

A species is defined as a group of organisms that can interbreed under natural conditions to produce fertile offspring. This means that all members of the same species share common characteristics and can reproduce successfully with each other.

Earlier, species were thought to be indivisible, stable, and unchanging (static). However, modern taxonomy recognises that species can have sub-divisions such as:

• Sub-species: Naturally occurring smaller groups within a species showing slight differences.
• Varieties: Distinct forms or variations within a species (often used in plants).
• Populations: Groups of the same species living in a particular geographical area at a given time.

Thus, in modern biology, species are dynamic units that can evolve and change, making them important in the study of biodiversity and evolution.

Genus

The genus is a higher taxonomic rank than species. It is used in the biological classification of both living and fossil organisms. A genus is a group of closely related species that show similarities in their morphological characters (external form and structure).

Although species within the same genus share many similarities, they usually do not interbreed with one another. This makes genus an important category for grouping species that are alike, yet still distinct.

Examples

• Panthera: The genus that includes Tiger, Leopard, and Lion. These animals are morphologically similar but are separate species.
• Solanum: The genus to which Brinjal (eggplant) and Potato belong. Both are different species but share common features of the same genus.

Thus, the genus serves as a link between species and higher categories in classification, grouping together species that are evolutionarily and structurally related.

Family

The family is an important hierarchical taxonomic rank that comes above genus. A family represents a group of closely related genera (plural of genus). Members of the same family share several common characteristics and evolutionary relationships.

Examples

• Malvaceae Family: This includes genera such as Hibiscus, Gossypium (cotton), Sida, and Bombax. All these plants share similar floral and structural features, so they are grouped under the same family.
• Felidae Family: This family includes cats, leopards, tigers, and lions. Although they are different species, they belong to the same family due to close similarities in their body structure and behaviour.
• Canidae Family: Dogs belong to this family, which is different from Felidae. Canidae includes animals like dogs, wolves, foxes, and jackals.

Thus, the family brings together related genera into a broader category, helping biologists study their similarities and evolutionary links more effectively.

Cohort / Order

The order (sometimes also referred to as cohort) is a taxonomic rank used in the classification of organisms. It is officially recognised by nomenclature codes. An order consists of a group of closely related families that share certain definite affinities (similarities).

An order is placed above family and below class in the taxonomic hierarchy. Although families within the same order may have some differences, they still show important common features that justify their placement together.

Examples

• Order Parietales: Families such as Papaveraceae, Brassicaceae, and Capparidaceae are grouped together because they share parietal placentation (a type of arrangement of ovules in the ovary).
• Order Carnivora: This order includes the Felidae family (cats, lions, leopards, tigers) and the Canidae family (dogs, wolves, foxes). Although they are different families, they share carnivorous adaptations and are therefore grouped in the same order.

Thus, an order is a broader rank that unites related families, showing their evolutionary and structural relationships.

Class

The class is a distinct taxonomic rank in biological classification, and each class has its own unique name. It is a higher rank than order and represents a larger assemblage of organisms.

A class consists of a group of related or allied orders. This means that several orders sharing important structural and evolutionary features are grouped together into one class.

Example

• Class Mammalia: This class includes orders like Carnivora (dogs, cats, tigers) and Primates (monkeys, gorillas, gibbons). Though they belong to different orders, they share common mammalian features such as the presence of mammary glands, hair on the body, and warm-blooded nature.

Thus, the class brings together multiple related orders into a broader category, showing larger evolutionary relationships among organisms.

Division / Phylum

The division (used mainly in plant classification) or phylum (used in animal classification) is a higher taxonomic category that consists of closely related classes. It is placed above class and below kingdom in the hierarchy.

Example

• Division Angiospermae: This division includes two major classes – Dicotyledonae (dicots) and Monocotyledonae (monocots). Both classes belong to the same division because they share common features of flowering plants.
• In animals: The term phylum is used instead of division. For example, Phylum Chordata includes classes such as Mammalia, Aves (birds), Reptilia, Amphibia, and Pisces (fishes).

Thus, a division or phylum brings together multiple classes that are evolutionarily related.

Sub-kingdom

When several divisions (in plants) or phyla (in animals) show some similarities, they are grouped into a higher category called the sub-kingdom.

Example

• The divisions Angiospermae and Gymnospermae are combined to form the Sub-kingdom Phanerogams (also called Spermatophyta), which includes all seed-bearing plants.

Hence, the sub-kingdom is a broader rank that unites related divisions or phyla under one common group.

Kingdom

The kingdom is the highest taxonomic category in the biological classification system. It is composed of different sub-kingdoms, grouping together a very large number of organisms that share only a few fundamental similarities.

Examples

• Plant Kingdom (Plantae): Formed by the sub-kingdoms Phanerogams (seed-bearing plants) and Cryptogams (non-seed plants like algae, fungi, mosses, and ferns). It includes all types of plants.
• Animal Kingdom (Animalia): This kingdom includes all animals, from simple invertebrates to highly evolved vertebrates such as humans.

The taxonomic categories we studied so far (species, genus, family, order, class, phylum/division, and kingdom) are broad categories. To make classification more scientific and precise, scientists have also introduced sub-categories (like sub-species, sub-family, sub-order, etc.).

It is important to note that as we move higher up in the taxonomic hierarchy, the number of common characters decreases. For example, members of the same species have many similarities, but as we go up to genus, family, order, and finally kingdom, the similarities become fewer while the diversity increases.

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Kingdom Monera

Monera is the kingdom of organisms that are unicellular (made of only one cell) and prokaryotic (cells without a true nucleus and membrane-bound organelles). They are some of the most ancient and simplest life forms on Earth.

General Features of Monera

1. Unicellular and Prokaryotic – All members of Monera are single-celled organisms with a very simple cell structure. They lack a true nucleus, and their DNA is not enclosed within a nuclear membrane.

2. Omnipresent Nature – These organisms are found everywhere. They can live in soil, water, air, and even in extreme environments such as hot springs, salty lakes, acidic soils, or inside living organisms.

3. Nutrition –

   • Some are photoautotrophs (prepare their own food using sunlight, like cyanobacteria).

   • Some are chemoautotrophs (make food using energy from chemical reactions).

   • The majority are heterotrophs (depend on other organisms or organic material for food).

4. Genetic Material –

   • DNA is present in a simple form as a circular double-stranded molecule without a nuclear membrane. This region is called the nucleoid.

   • Apart from the main DNA, they often have smaller circular DNA pieces called plasmids, which carry extra genetic traits like resistance to antibiotics.

5. Cell Wall – The cell wall is made up of peptidoglycan (murein), a special combination of sugars and amino acids that provides strength and protection.

6. Cell Organelles – Membrane-bound organelles such as mitochondria, chloroplasts, and endoplasmic reticulum are absent. However, ribosomes are present, but they are of a smaller type (70S) compared to those in eukaryotic cells.

Reproduction in Monera

1. Asexual Reproduction – Mainly by binary fission (cell divides into two identical cells) or budding (a small part grows into a new organism).

2. Sexual Reproduction (rare) – Occurs by conjugation, where two cells temporarily join to exchange genetic material.

Shapes of Bacteria (Morphological Types)

1. Coccus – Spherical (round-shaped).

2. Bacillus – Rod-shaped.

3. Vibrio – Comma or kidney-shaped.

4. Spirillum – Spiral or spring-like shaped.

Classification Based on Evolution

1. Archaebacteria – Ancient bacteria that can survive in extreme conditions like high salt, high temperature, or acidic environments.

2. Eubacteria – Also called "true bacteria." These include the common bacteria found around us, such as those in soil, water, and inside other living beings.

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a. Archaebacteria
Archaebacteria are a special group of bacteria that differ from other bacteria due to their unique cellular features. They are often called extremophiles because they live in extreme environments where most organisms cannot survive.

1. Habitats – Found in unusual and harsh places such as volcanic craters, salty lakes, and hot springs.

2. Adaptations – Their ability to live in these conditions shows their remarkable power to survive under high stress environments.

3. Types –

   • Halophiles – Can survive in very salty conditions.

   • Thermophiles – Can live in extremely hot temperatures.

   • Methanogens – Found in the intestines of ruminant animals (like cows and buffaloes). They help produce methane gas, which is useful in biogas plants.

4. Cell Wall – Unlike other bacteria, their cell wall does not contain peptidoglycan.

5. Reproduction – They reproduce asexually by binary fission (simple cell division).

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b. Eubacteria (True Bacteria)
Eubacteria are the most common type of bacteria and are often referred to as true bacteria.

1. Cell Wall – Made of peptidoglycan, which gives strength and protection.

2. Nutrition – They can be autotrophs (make their own food) or heterotrophs (depend on others for food).

   • Autotrophic forms:

     • Photosynthetic bacteria such as Chlorobium (green sulphur bacteria) and Chromatium.

     • Chemosynthetic bacteria such as sulphur bacteria that obtain energy from chemical reactions.

   • Heterotrophic forms:

     • Most abundant group of eubacteria.

     • Function as decomposers, breaking down complex substances into simple molecules or minerals.

3. Multicellular Filamentous Forms – Some are long thread-like organisms found in freshwater. Their body is covered with a mucilaginous sheath for protection.

4. Genetic Material – Prokaryotic type (not enclosed by nuclear membrane).

5. Pigments – Possess chlorophyll-a, chlorophyll-b, carotene, and xanthophylls for photosynthesis.

6. 
   Heterocyst – Special thick-walled cells in some filamentous bacteria that help in nitrogen fixation (conversion of atmospheric nitrogen into usable form).

1. Economic and Biological Importance –

   • Anaerobic bacteria like Lactobacilli help in milk curdling.

   • Azotobacter helps in nitrogen fixation in soil.

   • Streptomyces produces antibiotics.

   • Some bacteria assist in composting and degrading oil spills.

   • On the negative side, many eubacteria are pathogens (disease-causing), responsible for illnesses like typhoid, cholera, tuberculosis, and tetanus.

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Mycoplasma
Mycoplasmas are the smallest living organisms known.

1. Cell Wall – They do not have a cell wall, which makes them flexible and shapeless.

2. Pathogenic Nature – Many types of Mycoplasma are harmful and cause diseases in plants and animals.

3. Antibiotic Resistance – They are resistant to many common antibiotics because most antibiotics target the bacterial cell wall, which Mycoplasma lacks.

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2. Kingdom Protista

Protista is the kingdom that includes all unicellular but eukaryotic organisms. Unlike Monera, their cells have a true nucleus enclosed by a nuclear membrane and contain membrane-bound organelles.

1. Basic Characteristic – Protists are single-celled organisms, but their cell is complex and organised like that of higher eukaryotes.

2. Evolutionary Link – They act as a connecting link between other eukaryotic kingdoms such as Plantae, Fungi, and Animalia because they share some features with each of them.

   • Some Protists resemble plants (they can photosynthesise).

   • Some Protists resemble fungi (they can absorb nutrients from decaying matter).

   • Some Protists resemble animals (they can move and capture food).

3. Diversity – This kingdom shows a huge variety in structure, nutrition, and mode of living. Some are autotrophic, some are heterotrophic, and some show both types of nutrition depending on conditions.

4. Examples – Common members include amoeba (animal-like), paramecium (animal-like), euglena (plant + animal-like), and unicellular algae (plant-like).

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a. Plant-like Protists (Chrysophytes)

Plant-like protists are also called Chrysophytes and are commonly known as phytoplanktons.

1. Microscopic Nature – They are extremely small and can only be seen under a microscope.

2. Photosynthetic Ability – Most of them prepare their own food by photosynthesis and act as major producers in oceans, forming the base of the aquatic food chain.

3. Diatoms – A large number of these protists are known as diatoms.

   
   
   

   • Their body wall is made up of two overlapping parts that fit together like a soap-box.

   • These parts are impregnated with silica, which makes them strong and glass-like.

4. Diatomaceous Earth – When diatoms die, their silica shells accumulate on the ocean floor for millions of years. This deposit is called diatomaceous earth.

• It is granular in texture.

• It has many uses, especially in polishing surfaces and filtration processes (like filtering oils and beverages).

Dinoflagellates

Dinoflagellates are a group of protists that are mostly found in aquatic habitats, especially in marine waters.

1. Nature – They are mainly photosynthetic and contribute to primary production in oceans.

2. Cell Wall – The cell wall is composed of cellulosic stiff plates, which provide rigidity and protection.

3. Flagella – They have a pair of flagella (whip-like structures) that help in movement.

4. Photosynthetic Pigments – Dinoflagellates contain a variety of pigments such as yellow, green, brown, blue, and red, giving them different colours.

5. Red Tide Phenomenon – A well-known dinoflagellate is Gonyaulax, which is responsible for the phenomenon called red tide. During red tide, their population increases rapidly, and the water appears red in colour. In such cases, even the sea looks red due to their abundance.

Euglenoids

Euglenoids are a unique group of protists that share features of both plants and animals.

1. Cell Wall – They do not have a true cell wall. Instead, their body is covered by a strong but flexible layer called a pellicle, which is made of protein.

2. Movement – They have two flagella for locomotion: one is long and the other is short.

3. Nutrition –

   • In the presence of sunlight, they act like plants and perform photosynthesis, as they contain pigments similar to higher plants (chlorophylls).

   • In the absence of sunlight, they behave like animals and feed as heterotrophs, absorbing nutrients from the environment.

4. Dual Nature – Because they can switch between autotrophic and heterotrophic modes, euglenoids are considered mixotrophs.

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b. Animal-like Protists (Protozoans)

Animal-like protists are called protozoans. They resemble primitive animals because they lack a cell wall and are heterotrophic in nature, depending on other organisms for food.

1. Basic Features – Protozoans are unicellular, lack cell walls, and move with the help of special structures. They are considered primitive animal forms.

2. Amoeboid Protozoans –

   • Use pseudopodia (temporary finger-like projections) for movement and capturing food.

   • Example: Amoeba, which is free-living.

   • Example: Entamoeba, which is an endoparasite (lives inside the body of host) and causes amoebic dysentery in humans.

3. Flagellated Protozoans –

   • Move with the help of flagella.

   • Example: Trypanosoma, a parasite that causes sleeping sickness.

4. Ciliated Protozoans –

   • Move using numerous tiny hair-like structures called cilia.

   • Example: Paramecium, which also has a gullet (cavity) opening on its surface to direct food inside.

5. Sporozoans –

   • These do not have locomotory organs but form spores during one stage of their life cycle.

   • Example: Plasmodium, the parasite responsible for malaria.

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c.Fungi-like Protists:

1. Group – 

         ○Fungi-like protists are generally included under the group Myxomycetes.

2. Nature –

          ○They are saprophytic organisms, meaning they feed on dead and decaying organic matter.

3. Habitat – 

           ○These organisms are commonly found on decaying leaves where there is plenty of organic material for.                them to decompose.

4. Cell Aggregation –

               ○Their individual cells come together and aggregate to form a large, visible mass of protoplasm called a plasmodium.

5. Plasmodium –  

              ○This plasmodium is not related to the malaria parasite, but it.                                 resembles a slimy, multinucleated structure that can move slowly like an amoeba.

6. Reproduction – 

               ○The plasmodium produces spores.

7. Spore Resistance –

                ○The spores are very tough and resistant, enabling them to survive harsh environmental conditions like heat, drought, or lack of nutrients.

8. Example – 

            ○A common example of fungi-like protists is Stemonitis.

■ Kingdom Plantae:

1. Dominance of Autotrophs –

               ●The kingdom mainly consists of autotrophic organisms, which prepare their own food through photosynthesis.

2. Semi-autotrophic Members – 

             ●Some plants show partial                                   heterotrophy. 

            ●Examples include insectivorous.                        plants such as the Venus fly trap,                      pitcher plant, and bladderwort.

           ●These plants are autotrophic but also                trap and digest insects to.                                    supplement their nutrition.

3. Parasitic Members – Along with autotrophs, the kingdom also has heterotrophic parasitic plants like Cuscuta (dodder plant), which depends on host plants for food.

4. Cellular Nature – All members are multicellular and made up of eukaryotic cells that contain chlorophyll for photosynthesis.

5. Cell Wall – The cells have a rigid cell wall that is mostly composed of cellulose, providing support and structure.

6. Alternation of Generations – Plants show a unique feature known as alternation of generations, meaning their life cycle alternates between two distinct phases:

                ●The gametophytic phase (haploid) that produces gametes.

               ●The sporophytic phase (diploid) that produces spores.

7. Major Divisions – Based on reproductive structures, the kingdom is broadly divided into two groups:

       ●Cryptogamae (Cryptogams) – Non-flowering, seedless plants like algae,                bryophytes, and pteridophytes.

        ●Phanerogamae (Phanerogams) –Flowering, seed-producing plants such.            as gymnosperms and angiosperms.

■Kingdom Fungi:

1. Basic Nature – 

●Fungi are eukaryotic heterotrophs that perform extracellular digestion, meaning they secrete enzymes outside their body to break down food and then absorb it.

2. Habitat – 

●They thrive mostly in warm and humid environments, which favor their growth and reproduction.

3. Body Structure –

●The fungal body may be unicellular (single-celled) or multicellular.

●In unicellular fungi, the protoplast often contains multiple nuclei. Example: Saccharomyces (Yeast).

●Large fungi such as mushrooms show a compact mass of cells.

●In filamentous fungi, the body is known as a mycelium, which is composed of thread-like filaments called hyphae.

4. Types of Hyphae –

●Septate Hyphae – Have cross-walls (septa) dividing them into compartments.

●Non-septate Hyphae – Lack septa and are multinucleate. Such multinucleate hyphae are called coenocytic hyphae.

5. Cell Wall – The fungal cell wall is made up of chitin (a polysaccharide) or sometimes fungal cellulose, which provides rigidity and protection.

6.Mode of Nutrition -

●Mostly saprophytic (feeding on dead and decaying matter).

●Some are parasitic, deriving nutrition from living hosts.

●Few are predatory fungi, capturing and digesting small organisms.

7. Symbiotic Relationships –

●Some fungi live in symbiosis with algae, forming lichens.

●Others form associations with the roots of higher plants, called mycorrhiza.

8. Reproduction –

●Asexual Reproduction occurs by fragmentation, fission, and budding.

●Sexual Reproduction also occurs in many fungi, often involving spore formation.

9. Economic Importance –

■Useful Fungi:

●Mushrooms are edible and consumed as food.

●Yeast (Saccharomyces) is used in bakery and brewing industries for fermentation.

●Penicillium produces penicillin, a widely used antibiotic.

■Harmful Fungi:

●Some fungi cause diseases in plants and animals. 

○Example: Puccinia, responsible for rust disease in wheat.

The fungi are further classified on the basis of their structure, mode of spore formationand fruiting bodies as follows:

A. Phycomycetes:

1. Common Name – 

●Members of this group are popularly called algal fungi because of their simple structure and resemblance to algae.

2. Mycelium Structure –

●The mycelium is composed of aseptate coenocytic hyphae, meaning the hyphae are not divided by septa and contain multiple nuclei in a continuous cytoplasm.

3. Habitat –

●They commonly occur in moist and damp places.

●They are frequently found growing on decaying organic matter.

●Some members are present in aquatic habitats.

●Certain species live as parasites on plants, deriving nutrition from them.

4. Examples –

●Mucor – A common saprophytic fungus.

●Rhizopus (bread mould) – Often grows on stale bread and other food materials.

●Albugo – A parasitic fungus that infects mustard and related plants.

B. Ascomycetes:

1. Common Name – 

●Ascomycetes are popularly known as sac-fungi because their spores are produced inside a sac-like structure called an ascus.

2. Body Nature –

●Most members are multicellular.

●Rarely, some are unicellular, such as yeast (Saccharomyces).

3. Hyphal Structure – 

●The hyphae are branched and septate, meaning they are divided into compartments by cross-walls.

4. Nutritional Modes – These fungi exhibit diverse modes of nutrition:

●Decomposers – Break down dead organic matter.

●Parasites – Live on and harm host organisms.

●Coprophilous fungi – Grow on dung and help in decomposition.

5. Economic and Edible Importance –

●Morels and truffles are edible forms of sac-fungi that are considered delicacies in food.

●Neurospora is extensively used in genetic and biochemical experiments, earning the name “Drosophila of the Plant Kingdom.”

6. Examples – Some well-known members include:

●Aspergillus – Commonly used in industrial fermentation.

●Penicillium – Source of the antibiotic penicillin.

●Claviceps – Known for producing toxic alkaloids that contaminate cereals like rye.

●Neurospora – Widely studied in genetics.

●Saccharomyces (Yeast) – Used in baking and brewing industries.

C. Basidiomycetes:

1. Common Name – Basidiomycetes are popularly called club fungi because their spores are produced on club-shaped structures known as basidia.

2. Hyphal Structure – They possess branched and septate hyphae, which are divided into compartments by cross-walls.

3. Reproductive Features –

●Sexual reproduction occurs by the formation of basidiospores on the basidia.

●Asexual reproduction is generally absent or rare in this group.

4. Diversity of Members –

●Agaricus (mushrooms) – Edible fungi widely used as food.

●Ganoderma (bracket fungi) – Grows on trees, forming shelf-like structures.

●Ustilago (smuts) – Plant pathogens that cause smut diseases in crops like maize.

●Puccinia (rust fungi) – Parasitic fungi responsible for rust diseases in wheat and other plants.

5. Ecological Role – 

●Many basidiomycetes act as decomposers, breaking down wood and organic matter, thus playing an essential role in nutrient recycling.

d. Deuteromycetes:

1. Common Name – 

●Deuteromycetes are also known as imperfect fungi because their sexual stage of reproduction is either absent or not yet discovered.

2. Reproduction – 

●They reproduce only by asexual spores (conidia), which help in rapid dispersal.

3. Habitat and Role – 

●Many members are saprophytic, decomposing dead organic matter, while some are parasitic and cause plant diseases.

4. Examples –

●Alternaria – A plant pathogen responsible for leaf spots and blights.

●Colletotrichum – Causes anthracnose disease in many plants, leading to crop losses.

5. Economic Importance – 

●While several are harmful, some species are later reclassified into Ascomycetes or Basidiomycetes once their sexual stages are discovered, making this group a temporary, artificial classification.

■Acellular Organisms:

A. Viruses:

1. Origin of Name –

The term virus was first used by Louis Pasteur, who associated it with the meaning “venom” or “poison.”

2. Naming by Beijernck – 

Later, M. J. Beijernck observed that viruses could migrate through an agar gel and termed them “contagium vivum fluidum” (infectious living fluid), highlighting their infectious nature.

3. Stanley’s Contribution – 

Scientist W. M. Stanley demonstrated that viruses are inert outside the host cell and can even be crystallized, proving that they are not truly living organisms in isolation.

4. Acellular Nature – 

Viruses are not made up of cells. Instead, they consist of:

●A protein coat (capsid).

●A nucleic acid strand (DNA or RNA).This simple structure makes them acellular organisms.

5. Dependence on Host –

●Viruses lack their own cell machinery (ribosomes, enzymes for metabolism).

●They are obligate parasites, meaning they can reproduce only inside a specific host cell.

6. Inactivity Outside Host – 

Viruses remain inactive and inert when outside a host, but once they enter, they take over the cellular machinery and direct it to produce multiple copies of themselves.

7. Definition – 

Due to their structure, viruses are best described as infectious nucleoprotein particles, existing at the border between living and non-living.

Types of Viruses

1. Classification Based on Genetic Material – Viruses are grouped depending on the type of nucleic acid present in them:

●DNA Viruses – Contain DNA as genetic material. Most are double-stranded DNA, though rarely single-stranded DNA forms exist.

●RNA Viruses – Contain RNA as genetic material. Most are single-stranded RNA, though rarely double-stranded RNA is found.

2. Capsid Structure –

●The genetic material of viruses is surrounded by a protein coat called capsid.

●The capsid is composed of smaller protein subunits known as capsomeres.

●These capsomeres are arranged in polyhedral (many-sided) or helical (spiral-shaped) patterns.

●The main function of the capsid is to protect the genetic material from damage.

3. Special Type – Bacteriophages –

●Viruses that infect bacterial cells are known as bacteriophages.

●They usually possess double-stranded DNA as their genetic material.

●Bacteriophages are important tools in molecular biology and genetic research.

4. Diseases in Plants – Viral infections cause severe damage in plants, leading to:

●Leaf curling.

●Yellowing of leaves.

●Mosaic formation (patchy discoloration).

5. Diseases in Animals – Some viral diseases affecting animals include:

●Foot and mouth disease.

●Swine flu.

6. Diseases in Humans – Viruses are responsible for a wide range of disorders such as:

●Smallpox.

●Mumps.

●Herpes.

●Common cold.

●AIDS (Acquired Immunodeficiency Syndrome) caused by HIV.

b. Viroids:

1. Discovery – 

The existence of viroids was first reported by T. O. Diener in 1971 while studying plant diseases.

2. Causative Agent – He discovered that the potato spindle tuber disease was caused not by a virus but by a much simpler infectious agent.

3. Structure –

●Viroids are composed of single-stranded RNA (ssRNA).

●They are not enclosed in a protein coat (capsid), unlike viruses.

●This makes them structurally even simpler and smaller than viruses.

4. Nature of RNA – 

The RNA present in viroids is of low molecular weight and significantly smaller in size compared to viral RNA.

5. Infectious Property – 

Despite their simplicity, viroids are infectious RNA strands that can multiply within host plant cells and cause diseases.

6. Example – 

The best-known example is the Potato spindle tuber disease, which leads to abnormal growth and reduced yield in potato crops.