Kingdom Monera (Including Virus and Viroids)
Two kingdom classification includes Plantae and Animalia − proposed by Linnaeus
Advanced and most popular system of classification - five kingdom classification proposed by R. H. Whittaker (1969)
Criteria for five kingdom classification − cell structure, thallus organization, mode of nutrition, reproduction, and phylogenetic relationships
Earlier classification system
includes bacteria, cyanobacteria, fungi, mosses, ferns, gymnosperms, and angiosperms under ‘Plants’ based on the presence of cell wall
brought together groups with different characteristics under one roof
did not differentiate between fungi which are heterotrophs and autotrophic green plants though they also have different cell wall composition
Kingdom Monera
Bacteria
Sole members of this kingdom
Most abundant microorganisms
Occur almost everywhere including extreme environments such as hot springs, deep oceans, snow, and deserts
Many live in or on other organisms as parasites
Can be classified in four groups based on shape
Cell structure is very simple but complex in behaviour
Show the most extensive metabolic diversity as shown below
Archaebacteria
Unique since they live in the most harsh habitats
Bacteria based on habitat can be named as
Different from other bacteria in having a different cell wall structure which provides capacity to survive in these extreme conditions
Methanogens present in the guts of ruminants are responsible for methane (biogas) production from their dung.
Eubacteria (True Bacteria)
Characteristic feature − presence of rigid cell wall and if motile, a flagellum
Cyanobacteria (Blue green Algae)
Have chlorophyll a similar to green plants; hence, they are photosynthetic autotrophs
Unicellular, colonial or filamentous, marine or terrestrial algae
Colonies surrounded by gelatinous sheath
Some can fix atmospheric nitrogen in specialized cells called heterocysts. Example − Nostoc and Anaebaena
Chemosynthetic Autotrophs
Oxidize various inorganic substances such as nitrites, nitrates, ammonia, etc.
Play key role in recycling nutrients such as nitrogen, sulphur, iron, etc.
Heterotrophic Bacteria
Most abundant in nature
Important decomposers
Helpful in making curd from milk, production of antibiotics, fixing nitrogen in legumes
Some are pathogens, causing damage to humans, crops, cattle, and pets.
In humans, they cause diseases such as cholera, typhoid while they cause citrus canker in crop plants and anthrax in cattle.
Mycoplasma
Smallest known living creature
Devoid of cell wall
Can survive without oxygen
Many are pathogens of humans, animals, and plants.
Reproduction in Bacteria
Reproduce asexually mainly by fission
Under unfavourable conditions − reproduction through spores
Sexual reproduction may also take place in a special way by DNA transfer from one bacterium to another.
Viruses and Viroids
Viruses
Being acellular, they are not placed anywhere in Whittaker’s five kingdom classification.
Viruses are not truly ‘living’ as they are living only inside the host body, else inert.
They are non-cellular organisms characterized by inert crystalline structure outside the living cells.
Obligate (compulsory) parasites
Once inside a living cell, they take over the host cell machinery to replicate themselves, killing the host.
Meaning of virus - venom or poisonous fluid
Term coined by Pasteur
D. J. Ivanowsky (1892) led the foundation of discovery of virus. He identified causative agent of Mosaic disease, which are smaller than bacteria as they can pass through bacteria-proof filters.
M. W. Beijerinek (1898) found that the extract of infected plant causes disease in healthy plants. He called the fluid as Contagium vivum fluidum (infectious living fluid).
W. M. Stanley (1935) crystallized viruses and demonstrated that crystals contain largely proteins, inert outside the host cell.
Structure of Viruses
In addition to proteins, viruses also contain genetic material, either DNA or RNA, never both.
It is a nucleoprotein and the genetic material is infectious.
The protein coat is called capsid. It protects the nucleic acid and is made up of small subunits called capsomeres, which are arranged in helical or polyhedral geometric forms.
Viruses cause diseases such as mumps, small pox, herpes influenza, and AIDS in humans.
In plants, they cause mosaic, leaf rolling and curling, yellowing, vein clearing, dwarfing and stunted growth.
Viroids
Discovered by T.O. Diener in 1971
Smaller than viruses and responsible for spindle fibre disease in potato
Unlike viruses, they lack protein coat and exist as free RNA, hence named viroids.
Viroid RNA is of low molecular weight.