A peak into the world of virus: How they invade cells and multiply

Various school syllabi carry a small portion on necessary information regarding multiple microbes that affect living organisms. However, the school texts never get beyond their typical characteristics and common diseases they cause. And now, the outbreak of COVID-19 has now stunned the world with the rate of replication and spread of the novel coronavirus that causes the disease. Brief research by Onmanorama streamlines an array of curious and basic facts about how the virus infects a cell.

Viruses are the smallest obligate intracellular infective agents containing only one type of nucleic acid (DNA or RNA) as their genome. The size of viruses ranges from 20-300 nm (nanometre). The virus consists of a nucleic acid core (genome) surrounded by a protein coat, and it is lipoprotein in nature.

The whole material is known as nucleocapsid. Generally, there are six important virus family identified as DNA viruses (Poxiviridae, Herpesvirdae, Adenoviridae, Papovaviridae, Hepadnaviridae, Parvoviridae) and 14 RNA viruses (Orthomyxoviridae, Paramyxoviridae, Picornaviridae, Rhabdoviridae, Filoviridae, Caliciviridae, Reoviridae, Togaviridae, Flaviviridae, Bunyaviridae, Coronaviridae, Arenaviridae, Astroviridae, Retroviridae).

Virus invasion cycle

Replication of viruses is through six critical stages (1) Adsorption or attachment (2) Penetration (3) Uncoating (4) Biosynthesis (5) Maturation (6) Release.

In the first stage, the virus comes in contact with the cells on the host cell surface. Then the virus starts to penetrate using its spikes, a process known as viropexis. Alternatively, in case of the enveloped (lipid coat) viruses, the envelope may fuse with the plasma membrane of the host cell releasing the nucleocapsid into the host cytoplasm. Further, the uncoating - the process of striping the virus of its outer layers and capsid releases the nucleic acid into the host cell. Then the biosynthesis occurs for the protein synthesis to reciprocate its generation. A messenger RNA initiates the protein synthesis, which is essential to reproduce its next generation.

The first step for that is a transcription of messenger RNA (mRNA) from the viral nucleic acid, this transcription of the viral nucleic acid into mRNA is the usual step for all viruses except those RNA viruses. The second step is the translation of the mRNA into non-structural proteins. There are enzymes which initiate and maintain synthesis of virus components. They may also induce shutdown of host protein and nucleic acid synthesis. Then happens replication of viral nucleic acid, in which the nucleic acid replicates inside the nucleus of the host cell. Finally, synthesis of structural proteins which constitutes daughter virus capsids or lipid coat of daughter virus.

Similarly, the viral genome is also derived from the synthesis. The next step is maturation, where the viral nucleic acid and lipid portions assemble to form a complete daughter virus. This assembly takes place either in the nucleus or cytoplasm of the host cell. Further, the completed viruses are released by process of budding from the cell membrane over a period. Viruses are generally released in a burst-like fashion from the cells as the lyse.

In these all stages, most of the virus attacks, the host cell may not have affected, but in the case of poliovirus, the host tissue gets damaged. In the case or COVID 19, the immune system works against the virus wherein, the human body recognizes the virus as an antigen (foreign particle). The body, then, excites its antibodies through the white blood cells which consist of antibodies which recognise the viruses and shoot off to the infected spot to fight away the attackers (viruses). The antibodies and related cells will get damaged and die during the fight against the virus. The damaged white blood cells accumulate in the infected portions, especially in the lungs. The mass accumulation of antibodies and related cells will cause a respiratory block in the lungs. Usually, the infected human will have flu with high temperature. All these situations indicate that the human immune system is fighting against viruses. Persons who have a sound immune system will overcome the attack; however, the immunocompromised patients will die.

What makes the novel coronavirus different is that the RNA-based SARS-CoV-2 binds fast with the receptor on the human cell, angiotensin-converting enzyme ACE-2. Understanding the RNA-based virus is far more complicated than the DNA-based virus and hence the delay in developing a vaccine or medicine for infection caused by these viruses.

It was late in 2019 that novel coronavirus infection COVID-19 was reported in Wuhan in the Hubei province of China. Soon it started spreading at an exponential rate and the now over 2 lakh persons have been infected and over 37,000 deaths reported around the world as of now with Italy and Spain being the most affected countries.

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