Lesson 5: Biological Classification- Virus and Lichens

Characteristics  of Kingdom Plantae

1) It includes all eukaryotic chlorophyl containing organism commonly called as plants.

2) The plant cells have eukaryotic structure with prominent chloroplasts and cells are mainly made  up of cellulose.

3) The mode of nutrition is autotrophic but few members are partially heterotrophic such as insectivorous plants or parasites.

4)Kingdom Plantae includes all the plants except some algae and other members of Monera, Protista and Fungi.

5)The plants cells have unique rigid, cellulose cell wall that prevents contraction and make these organisms distinct from rest of other

6)Cell wall is a non permeable outer covering that insulate the plant cell from external environment.

7)All the plant cells have large central vacuole that store water and nutrients, thus helping plants in osmo-regulation.

8)They also lack locomotion.

9)Green plants synthesize food by the process pf photosynthesis using water, mineral and CO₂ with help of chlorophyll and solar energy.

10)Plants complete their life in two phases: gametophyte and saprophyte. These two phases alter with each other and this phenomenon is called “alternation of generation”.

11)Plants reproduce both sexually and asexually.

12)They occupy land as well as water habitat (terrestrial and aquatic plants respectively).

Virus :

They are non-cellular, non-cytoplasmic infectious agents. They are ususally smaller than bacteria. They contain only a single type of nucleic acid , either DNA or RNA. It was discovered by Iwanowiski in 1892.

General structure:

The viruses vary in shapes and size, namely rod shape ( tobacco mosaic  virus) tadpole like (bacteriphage), helical (influenza virus)  or polyhedral (adenovirus)

Chemical structure and composition :

Viruses have a simple structure . It is made up of nucleic acid core and protein  coat.

Nucleic Acid:  A virion contains either DNA or RNA. It may appear as single stranded or double stranded forms.  Some virus like  AIDS, mumps, poliomylitis, etc have  only RNA, while in chicken pox, hepatitis B have  double stranded DNA. The infectious propery of the virion is due to its nucleic acid.

Capsid or Protein coat: The protein coat of virus is called capsid., gives a specific shape to the virus. The protein coat is made up of capsomeres. It is present when the virus is outside the host cell.  In some e.g. Influenza, mumps the capsid has a covering of lipoprotein around it. This is called the envelope. This envelope is a  fragment of plasma membrane aquired from the previous cell invaded by the virus.

On the basis of the classification the viruses are classified as:

a) Deoxyviruses: 

(i)contains double stranded DNA. e.g. pox virus, cauliflower mosaic virus.

(ii) contains single  stranded DNA  e.g.  Cyanophages

b) Riboviruses:

(i)contains double stranded RNA e.g. Mycophages reovirus, Wound tumor virus

(ii)contains single stranded RNA e.g. TMV, HIV, influenza virus.

Structure of some viruses:

(i)TMV is a simple rod-shaped helical virus. It was discovered by D. Ivanowski in 1892 and crystallized by W.M Stanley (1935). It consists of centrally located  single stranded  RNA  enveloped by a protein coat. The rod is considered to be 3,000 A^o in length and about 180 A^o in diameter. The protein coat is  called 'capsid'. R. Franklin estimated 2,130 sub-units, namely, capsomeres in a complete helical rod and 49 capsomeres on every three turns of the helix; thus there would be about 130 turns per rod of TMV.   The diameter of RNA helix is about 80 A^o and the RNA molecule lies about 50 A^o inward from the outer-most surface of the rod.The ssRNA is little more in length (about 3300 A^o ) slightly protruding from one end of the rod. The RNA molecule consists of about 7300 nucleotides; the molecular weight of the RNA molecule being about 25,000 dalton.

  (ii) Pox virus/ variola : They vary in their shape depending upon the species but are generally shaped like a brick or as an oval form similar to a rounded brick. They are amongst the largest of animal viruses. The virion is exceptionally large, its size is around 200  nm in diameter and 300 nm in length and carries its  genome  in a single, linear, double-stranded segment of DNA. Edward Jenner discovered vaccine against Small pox.

(iii) AIDS virus: It consists of  single stranded RNA.There are 2 copies of ssRNA.  The outer most covering is of glycoprotein , followed by lipid bilayer, . The innermost layer comprises if two protein layers.HIV has just nine genes  Three  genes, called gag, pol and env, contain information needed to make structural proteins for new virus particles. The other six genes, known as tat, rev, nef, vif, vpr and vpu, code for proteins that control the ability of HIV to infect a cell, produce new copies of virus, or cause disease.

(iv) T4 bactriophage:    These are the viruses that grow as intrcellular parasites of bacteria .  They appear in various shapes and sizes. Independently discovered by Frederick W. Twort (1915) and Felix de Herelle (1917). They are approximately 200nm long and 80-100nm wide. The head may be icosahederal or filamentous. The head is composed of proteins. The nucleic acid is found within the head.The head as a protection for the nucleic acid. Some phages have tails attached to the phage head. During the infection the nucleic acid passes through this tail.At the end of the tail the more complex phages like T4 have a base plate and one or more tail fibers attached to it. The base plate and tail fibers are involved in the binding of the phage to the bacterial cell.

(v)influenja virus:  They are spherical in shape measuring 800-1200A in diameter.The protein capsid encloses a single stranded RNA. The nucleocapsid is enclosed in an envelope of lipoproteins. The envelope of influenja virus has a projection called spikes. It can withstand low temperature and is killed at 650 C.

Reproduction in viruses

The reproduction is of two types: Phagic and Pinocytic as in T4 bacteriphage.

The phagic reproduction is of two types:

a) lytic cycle : occurs in virulent phages.The DNA in these type of viruses reproduces through the lytic cycle. When these viruses reproduce, they break open, or lyse, their host cells, resulting in the destruction of the host.

b) Lysogenic Cycle:  In this the phage's DNA recombines with the bacterial chromosome. Once it has inserted itself, it is known as a prophage. A host cell that carries a prophage has the potential to lyse, thus it is called a lysogenic cell.

Pinocytic reproduction: It occurs in plant and animal viruses. e.g. Tobacco Mosaic virus. These viruses act chemically. Attachment and Penetration are their main methods of action.  The genome replicates by RNA-RNA viruses and retroviruses.

Diseases caused by virus in Man

Cryptogram of virus:A descriptive code summarising the main properties of a virus.The viruses have been named  by a system of nomenclature as designated by International Committee of Virus Nomenclature. It consists of two parts. The first part is the common name of the virus  and the second part consists of a coded information of it.  This is termed as Cryptogram.e.g:

1. Cryptogram of TMV :           R/1:2/5;E/E:S/A

2. Cryptogram of Pox virus: D/2,160/5-7.5, X/*,V/O

3. Cryptogram of polio virus: R/1, 2.5/30,S/S.V/O

4.Cryptogram of T4 bacteriophage: D/2, 130/40, X/X, B/O

The explanation of a cryptogram is s follows:

(i) The first pair represents the type of nucleic acid or the number of strands in the nucleic acid.

(ii) The second pair represents the molecular weight of nucleic acid or the amount of nucleic acid as expressed in percentage.

(iii) The third pair denotes the shape of the virus or the shape of the nucleoprotein.

(iv) The fourth pair denotes the host or carrier used in the transmission of virus.

Viroids : They are group  of pathogens discovered in 1967 by Diener and Raymer. Viroides are the smallest known agents of infectious disesase. They are uncapsulated and are capable of replicating autonomously in susceptible cells. In plants viroids consists of single stranded RNA. It is believed that in animals they might have DNA. It causes Potato Spindle Tuber  Disease(PSTD)

Prions:  A disease-causing agent  discovered by Prusiner etal (1982) that is neither bacterial nor fungal nor viral and contains no genetic material.Prions have been held responsible for a number of degenerative brain diseases, including  scrapie (a fatal disease of sheep and goats), mad cow disease, Creutzfeldt-Jacob disease, fatal familial insomnia, Kuru etc.

Lichens

Lichens are small, non-vascular plants.  Lichen was discovered bt Tulsane but the term 'lichen' was coined by Theophrastus.It is a thallus of dual organisms i.e. fungus and algae growing together in a tissue.The two organisms live in close proximity and appear as a single plant. Some lichens are extremely tough and grow in very inhospitable habitats. The symbiotic relationship enables lichens to exist in  a vast spectrum of habitats and climates all over the world including extreme environments - from the polar regions to the equator and inter-tidal zones to mountain peaks. Depending on their habitat  they are termed as terricolous ( growing in sil) , saxicolous ( on stones/ rocks) , corticolous ( on bark) and lignicolous ( on wood).

Characteristic feature of lichens:Lichens are divided into two groups: ascolichens (in which fungal component is an Ascomycete),  e.g. Graphis, Cladonia, Usnea etc and basidiolichens (in which the fungal component is a Basidiomycete) e.g. Chlorella,  Cora..Lichens  grow slowly i.e 1-2 mm/year and live for about 1000 years. They lack protective, conductive and assimilatory tissues. They can maintain their water balence. The lichens reproduce sexually as well as asexually. The sexual reproduction involves a  fungal partner. Soredia are most efficient means of asexual reproduction. The soredia are composed of algal cells clasped and surrounded by fungal hyphae.

The fungus partner is known as mycobiont and the algal partner is called phycobiont.  The mycobiont is dominant forming about 90-95% of the thallus .

Lichens are of three main types:

(a) crustose lichens (flat thalli, without any lobe, growing on stone rocks, bark or any hard substrata, and appears like crust, for example Haemmatomma puniceum),

(b) foliose lichens (thalli are flat, much lobbed and leaf-like appearing as twisted leaves, have distinct lower and upper surface, attached to substrate with rhizoid-like structure called rhizinae for example Physcia)

(c) frutiose lichens (thalli are most conspicuous, most complex, and slender and freely branched, and branches are cylindrical, flattened and form thread like tuft, thalli not differentiated into upper and lower surfaces, for example Cladonia).

Special Structures in the thallus of lichen:

(i) Cyphellae: Help in exchange of gases. Present in lower cortex.

(ii) Cephalodia: Help to retain moisture and its algal partner fix nitrogen also

(iii) Breathing pores: for aeration, present in upper cortex of thallus

Economic importance of Lichens

(i)Some lichen compounds are used in pharmaceuticals and cosmetics. Some species of Evernia and Ramalina yield essntial oils which are used in manufacture of soap.

(ii)Lichens are sensitive to air pollutants, primarily sulfur dioxide and heavy metals.

(iii)Pioneers of vegetation: They initiate weathering of  rocks into soil particles

(iv) As food for fodder: Reindeer moss ( Cladonia rangiferina) of the arctic region is eaten by reindeer and cattle. Iceland moss (Cetraria islandica) is used as food by man in Iceland. Species of Parmelia are used as curry powder in India.

(v) Dye and Litmua are obtained from Lichens.

(vi) Lobaria pulmonaria is used for lung troubles, Usnea barbaria for strengthening hair and for uterine ailments. Xanthria parietina is used in jaundice and Parmelia saxatilis has been used for epilepsy. Peltigera canina is used  against hydrophobia.

(vii) They are indicators of air pollution. They are sensitive to SO₂ and die at higher levels of SO₂.