The difference between viroids and viruses are as follows:
|
Viroid |
Virus |
Genome size |
246 to 467 nucleotide long |
2 kb or more in size |
RNA/ DNA |
Highly complimentary, circular single-stranded RNA |
Known to be RNA, DNA double-stranded or single-stranded |
Protein Coat |
Absent |
Present |
Host |
Plant pathogen |
Both plant and animals |
Discovery |
Potato spindle tuber viroid by Theodor Otto Diener in 1971 |
Tobacco mosaic virus by Martinus Beijerinck in 1898 |
Number known |
Around 30 |
Around 5000 described in detail but millions more known to exist. |
The phycobiont and mycobiont are the algal and fungal partners, respectively, which are in a symbiotic relationship with each other in nature. These are known as lichens and this is a mutually beneficial partnership. The algal partner, phycobiont (also called photobiont) is autotrophic and provides food to the fungi. The fungal or mycobiont partner provides shelter, nutrients and water to the algal partner.
In diatoms, the cell walls form two thin overlapping shells, which fit together closely. The walls cannot be destroyed as they are embedded with silica. The cell walls show a wide diversity in form but are usually almost bilaterally symmetrical, hence the group name – diatom.
Plants are mostly autotrophic which means that they possess the pigment chlorophyll. With the help of chlorophyll, they manufacture their own food by performing photosynthesis. However, a few members of the plant kingdom are partially heterotrophic, for example, the insectivorous plants or parasites. These plants typically grow in nitrogen and mineral deficient soils and compensate for their nutritional requirement by their partial heterotrophic nature. These insectivorous plants have evolved mechanisms to trap insects from which they derive minerals and nutrition. Bladderwort and Venus flytrap are examples of insectivorous plants.
The other partially heterotrophic plant is Cuscuta that lives like a parasite on many trees and absorbs nutrition from the host plant.
Algal bloom: The algal bloom is a phenomenon occurs when contaminants like nitrates and phosphates from untreated sewage are released into water bodies. the presence of an excessive amount of nutrients such as nitrates and phosphates overstimulate the growth of algae. This overstimulation of algal growth (algal bloom) reduces the dissolved oxygen content of the water, which in turn causes other life forms to perish. Algal bloom affects water quality and causes fish and other aquatic life mortality. Some algae can be toxic to humans too.
Algal blooms are often green but they could also be other colours like red or green depending upon the predominant species of algae causing the bloom.
Red Tides: Red or brown coloured algal blooms are called red tides. Red tides are caused by red dinoflagellates (example Gonyaulax) which accumulate due to rapid multiplication making the sea appear red. Toxins released by such large numbers of algae may kill other marine animals such as fish.
The two economic uses of heterotrophic bacteria are:
The two economic uses of archaebacteria are:
The debate whether viruses are living or non-living is an old one and till today, a consensus has not been reached.
Characteristics that classify viruses as living:
Characteristics that classify viruses as non-living:
Though it is challenging to categorise viruses, it can be said with enough confidence that viruses are some form of life. Nature is so diverse that the classification system that humans have made for their ease of use may not fit that diversity.
A complete virus particle is known as virion. It consists of nucleic acid, either DNA or RNA, surrounded by a coat of protein called capsid. These are formed from identical protein subunits called capsomeres. Proteins associated with nucleic acid are known as nucleoproteins, and the association of viral capsid proteins with viral nucleic acid is called a nucleocapsid. Viruses are classified into four types based on their structure:
Polyhedral / icosahedral Viruses: These are non-enveloped viruses in which the capsomeres are arranged in geometric shape around the genetic material which is either double or single-stranded DNA or RNA. The icosahedron shape has 20 equilateral triangle faces and 12 corners. Example: polio causing virus
Helical viruses: These viruses are non-enveloped with capsomeres arranged as a helix around the nucleic acid giving rise to elongated rods or flexible filaments. Example: tobacco mosaic virus
Complex viruses: The complex viruses have complex combinations of structures that may or may not be completely consistent between viruses of the same species. Example tailed bacteriophage, poxviruses.
Enveloped viruses: These viruses are helical and icosahedrons shapes but have an outer envelope that surrounds the virus capsomeres. The envelope is a lipid bilayer containing glycoproteins embedded in the lipid. Example: Herpes virus.
Genetic Material of Viruses:
Four viral diseases: Rabies, Influenza, Herpes, HIV
Characteristic features of Euglenoids are:
(i) Mode of nutrition
(ii) Mode of reproduction
The four major groups of protozoans are as follows:
Amoeboid protozoans:
Flagellated protozoans:
Ciliated protozoans:
Sporozoans:
The biological classification system has undergone tremendous change guided by scientific understanding and a need to define and categorize each organism precisely on the basis of its morphological, anatomical and genetic attributes. The steps given below show how the present system of classification has evolved into its form.
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