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Daily Current Affairs – 2020

Topic: For Prelims and Mains

RNA Virus & DNA Virus

3rd June, 2020

What is virus?

Viruses are small obligate intracellular parasites, which by definition contain either a RNA or DNA genome surrounded by a protective, virus-coded protein coat. Viruses may be viewed as mobile genetic elements, most probably of cellular origin and characterized by a long co-evolution of virus and host.

For propagation viruses depend on specialized host cells supplying the complex metabolic and biosynthetic machinery of eukaryotic or prokaryotic cells. A complete virus particle is called a virion.

The main function of the virion is to deliver its DNA or RNA genome into the host cell so that the genome can be expressed (transcribed and translated) by the host cell.

The viral genome, often with associated basic proteins, is packaged inside a symmetric protein capsid.

The nucleic acid-associated protein, called nucleoprotein, together with the genome, forms the nucleocapsid.

In enveloped viruses, the nucleocapsid is surrounded by a lipid bilayer derived from the modified host cell membrane and studded with an outer layer of virus envelope glycoproteins.


Classification of Viruses

Morphology: Viruses are grouped on the basis of size and shape, chemical composition and structure of the genome, and mode of replication. Helical morphology is seen in nucleocapsids of many filamentous and pleomorphic viruses.

Helical nucleocapsids consist of a helical array of capsid proteins (protomers) wrapped around a helical filament of nucleic acid.

Icosahedral morphology is characteristic of the nucleocapsids of many “spherical” viruses. The number and arrangement of the capsomeres (morphologic subunits of the icosahedron) are useful in identification and classification. Many viruses also have an outer envelope.

Chemical Composition and Mode of Replication: The genome of a virus may consist of DNA or RNA, which may be single stranded (ss) or double stranded (ds), linear or circular.

The entire genome may occupy either one nucleic acid molecule (monopartite genome) or several nucleic acid segments (multipartite genome). The different types of genome necessitate different replication strategies.


RNA Virus Genomes:

  • RNA viruses, comprising 70% of all viruses, vary remarkably in genome structure. Because of the error rate of the enzymes involved in RNA replication, these viruses usually show much higher mutation rates than do the DNA viruses.
  • Mutation rates of 10-4lead to the continuous generation of virus variants which show great adaptability to new hosts.
  • The viral RNA may be single-stranded (ss) or double-stranded (ds), and the genome may occupy a single RNA segment or be distributed on two or more separate segments (segmented genomes).

In addition, the RNA strand of a single-stranded genome may be either a sense strand (plus strand), which can function as messenger RNA (mRNA), or an antisense strand (minus strand), which is complementary to the sense strand and cannot function as mRNA protein translation .

Sense viral RNA alone can replicate if injected into cells, since it can function as mRNA and initiate translation of virus-encoded proteins. Antisense RNA, on the other hand, has no translational function and cannot per se produce viral components.


DNA Virus Genomes

  • Most DNA viruses contain a single genome of linear dsDNA. The papovaviruses, comprising the polyoma- and papillomaviruses, however, have circular DNA genomes, about 5.1 and 7.8 kb pairs in size.
  • DsDNA serves as a template both for mRNA and for self-transcription. Three or 2 structural proteins make up the papovavirus capsid: in addition, 5-6 nonstructural proteins are encoded that are functional in virus transcription, DNA replication and cell transformation.

Facts for Prelims:

ACE-2 enzymes:

According to a study, ACE-2 enzymes are responsible for why men seem to be more vulnerable to Covid-19 than women.


  • Angiotensin Converting Enzyme 2(ACE2) is an enzyme molecule that connects the inside of our cells to the outside via the cell membrane.
  • The enzyme is found in our lungs, heart, intestines, blood vessels and muscles.
  • The enzyme essentially acts as a port of entry that allows the coronavirus to invade our cells and replicate.
  • Since men have higher concentrations of ACE2 compared to women, they are more vulnerable to coronavirus.


GI tag for Thanjavur Netti & Arumbavur Wood Carving:

Thanjavur Netti:

  • Thanjavur Netti Works (Thanjavur Pith Work) is made from Netti (pith) from a marshy plant called as Aeschynomene Aspera.
  • This is a traditional art form and has been transferred from one generation to the next.
  • The pith work models are delicate to handle and are usually preserved inside glass boxes.
  • The notable works include Brihadeeswara Temple, Hindu Idols, door hangings and decorative pieces.The government has also recognised the pith work industry as one of the major handicraft symbols.

Arumbavur Wood Carving:

  • Arumbavur wood carvings are sculptures primarily made out of the wooden logs of Mango, Lingam tree, Indian Ash tree, Rosewood, Neem tree.
  • The wood supply comes from the Pachamalai Hills along the Trichy – Perambalur boundary and the Thanjavur – Kumbakonam zone.
  • It’s work revolves around idols and deities, temple chariots and temple cars, door panels of houses, pooja rooms and temples, decorative figures and pooja mandapam.
  • Uniqueness:The entire design carved is made out of a single block of wood given that a single error could damage the entire piece.


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