1. It is a circular molecule of ssRNA without a capsid
2. Discovered by T.O. Diener in 1971.
3. RNA has low molecular weight.
4. It causes citrus exocortis & potato spindle tuber disease in plants.

The word virus is derived from Latin meaning 'Poison'.

Property of self-regulation and tendency to maintain a steady state within an external environment which is liable to change is called Homeostasis. It is essential for the living organisms.

Growth is an intrinsic property of all living organisms through which they can increase cells both in number and mass.

Homoiomerous	Heteromerous
Algal cells evenly distributed in the thallus	A distinct layer of algae and fungi present

1. Can be crystallized.
2. Absence of metabolism.
3. Inactive outside the host.
4. Do not show functional autonomy.
5. Energy producing enzyme system absent.

1. Presence of nucleic acid and protein.
2. Capable of mutation.
3. Ability to multiply within living cells.
4. Ability to infect and cause diseases in living beings.
5. Show irritability.
6. Host-specific.

Generally, viruses are of three types based on shape and symmetry
1. Cuboid symmetry - Eg: Adenovirus, Herpesvirus.
2. Helical symmetry - Eg: Influenza virus, TMV.
3. Complex or Atypical symmetry - Eg: Bacteriophage, Vaccinia virus.

Human Diseases:
1. Common cold.
2. Hepatitis B.
3. Cancer.
4. SARS (Severe Acute Respiratory Syndrome).
5. AIDS (Acquired Immunodeficiency Syndrome).
6. Rabies.
7. Mumps.
8. Polio.
9. Chikungunya.
10. Smallpox.
11. Chickenpox.
12. Measles.

During crop rotations and mixed cropping. Farmers plant leguminous crops to increase the soil fertility.

S.No	Beneficial aspects	Bacteria	Role
1.	Ammonification	1. Bacillus ramosus 2. Bacillus mycoides	Convert complex proteins in the dead bodies of plants and animals into ammonia which is later converted into ammonium salt
2.	Nitrification	1. Nitrobacter 2. Nitrosomonas	Convert ammonium salts into nitrites and nitrates
3.	Nitrogen fixation	1. Azotobacter 2. Clostridium 3. Rhizobium	(i) Converting atmospheric nitrogen into organic nitrogen
			(ii) The nitrogenous compounds are also oxidized to nitrogen
			(iii) All these activities of bacteria increase soil fertility

S.No	Criteria	Monera
1.	Cell type	Prokaryotic
2.	Level of organization	Mostly unicellular, rarely Multicellular
3.	Cell wall	Present (made up of peptidoglycan and mucopeptides)
4.	Nutrition	Autotrophic (Phototrophic, Chemoautotrophic) Heterotrophic (Parasitic and Saprophytic)
5.	Motility	Motile or non-motile
6.	Organisms	Archaebacteria, Eubacteria, Cyanobacteria, Actinomycetes and Mycoplasma

Lytic Cycle :
During lytic cycle of the phage the disintegration of host bacterial cell occurs and the progeny virions are released The steps involved in the lytic cycle are as follows:
1. Adsorption:
(i) Phage (T₄) particles interact with cell wall of host (E.coli).
(ii) The phage tail makes contact between the two, and tail fibres recognize the specific receptor sites present on bacterial cell surface.
(iii) The lipopolysaccharides of tail fibres act as receptor in phages.
(iv) The process involving the recognition of phage to bacterium is called landing. Once the contact is established between tail fibres and bacterial cell, tail fibres bend to anchor the pins and base plate to the cell surface.
(v) This step is called pinning.
2. Penetration:
(i) The penetration process involves mechanical and enzymatic digestion of the cell wall of the host.
(ii) At the recognition site phage digests certain cell wall structure by viral enzyme (lysozyme).
(iii) After pinning the tail sheath contracts (by using ATP) and appears shorter and thicker.
(iv) After contraction the base plate enlarges through which DNA is injected into the cell wall without using metabolic energy.
(v) The step involving injection of DNA particle alone into the bacterial cell is called Transfection.
(vi) The empty protein coat leaving the outside of the cell is known as 'ghost'.
3. Synthesis:
(i) This step involves the degradation of bacterial chromosome, protein synthesis and DNA replication.
(ii) The phage nucleic acid takes over the host biosynthetic machinery. Host DNA gets inactivated and breaks down.
(iii) Phage DNA suppresses the synthesis of bacterial protein and directs the metabolism of the cell to synthesize the proteins of the phage particles and simultaneously replication of phage DNA also takes place.
4. Assembly and Maturation:
(i) The DNA of the phage and protein coat are synthesized separately and are assembled to form phage particles.
(ii) The process of assembling the phage particles is known as maturation.
(iii)  After 20 minutes of infection about 300 new phages are assembled.
5. Release:
The phage particles get accumulated inside the host cell and are released by the lysis of host cell wall.

Tobacco mosaic virus was discovered in 1892 by Dmitri Ivanovsky from Tobacco plant.
Structure of Tobacco mosaic virus:

1. TMV is a rod-shaped helical virus measuring about 280 x 150\(\mu\)m with a molecular weight of 39 x 10⁶ Daltons.
2. The virion is made up of two constituents, a protein coat called capsid and a core called nucleic acid.
3. The capsid is made up of approximately 2130 identical protein subunits called capsomeres.
4. The Nucleic acid consists of central single stranded RNA molecule.
5. The genetic information necessary for the formation of a complete TMV particle is contained in its RNA. It has 6,500 nucleotides.

(i) The symbiotic association between algae and fungi is called lichens.
(ii) The algal partner is called Phycobiont or Photobiont and the fungal partner is called Mycobiont.
(iii) Algae provide nutrition for fungal partner in turn fungi provide protection and also help to fix the thallus to the substratum through rhizines.
(iv) Asexual reproduction takes place through fragmentation, soredia and Isidia. Phycobionts reproduce by akinetes, hormogonia, aplanospore, etc., Mycobionts undergo sexual reproduction and produce ascocarps.
Classification:
(i) Based on the habitat lichens are classified into following types: Corticolous (on Bark) Lignicolous (on Wood) Saxicolous (on rocks) Terricolous (on ground) Marine (on siliceous rocks of sea) Fresh water (on siliceous rock of fresh water).
(ii) On the basis of morphology of the thallus they are divided into Leprose (a distinct fungal layer is absent) Crustose-crust like; Foliose-leaf like; Fruticose-branched pendulous shrub like.
(iii) The distribution of algal cells distinguishes lichens into two forms namely Homoiomerous (Algal cells evenly distributed in the thallus) and Heteromerous (a distinct layer of algae and fungi present)
(iv) If the fungal partner of lichen belongs to ascomycetes, it is called Ascolichen and if it is basidiomycetes it is called Basidiolichen.

(i) R.H. Whittaker, an American taxonomist proposed five kingdom classification in the year 1969.
(ii) The Kingdoms include Monera, Protista, Fungi, Plantae and Animalia.
(iii) The criteria adopted for the classification include cell structure, thallus organization, mode of nutrition, reproduction and phylogenetic relationship. The merits and demerits are as follows:
Merits:
(i) The classification is based on the complexity of cell structure and organization of thallus.
(ii) It is based on the mode of nutrition.
(iii) Separation of fungi from plants.
(iv) It shows the phylogeny of the organisms.
Demerits:
(i) The kingdom Monera and Protista accommodate both autotrophic and heterotrophic organisms, cell wall lacking and cell wall bearing organisms thus making these two groups more heterogeneous.
(ii) Viruses were not included in the system.