Corona Virus

Corona Virus name is derived from the Latin word "Corona" which means "Crown" or "Wreath" which is represent the characteristics of  "Virions" (the infective form of the virus). Corona Viruses are the group of viruses. These viruses infected to the Mammals and Birds.

Corona Virus History :-

1. It firstly discovered in the 1930 in chickens. Domesticated chickens having a rapid respiratory caused by Infectious Bronchitis Virus (IBV).
2. In the year of 1940 it found in the mouse caused by Mouse Hepatitis Virus (MHV) and in pigs caused diarrhea by Transmissible Gastroenteritis Virus (TGEV)
3. In 1960 Human Corona Virus were discovered.

• HCoV-229E and HCoV-OC43 in 2003
• SARS-CoV or SARS-CoV-1 - SARS Corona Virus in 2003.
• HCoV-NL63  in 2004
• Human coronavirus HKU1 (HCoV-HKU1) in 2005
• MERS-CoV or EMC/2012 or HCoV-EMC/2012) or 2012 Novel Corona Virus or 2012-nCoV discovered in 2012.
• 2019-nCoV/ COVID-19 / SARS-CoV-2 / Wuhan Virus / Wuhan Corona Virus/ Novel Corona Virus discovered in 2019.

Human Corona Virus Infected to -

Virus Name	Infected To
HCoV-229E	Human & Bat
HCoV-OC43	Human & Cattle
HCoV-NL63	Human
SARC-CoV	Humans, Bats & Palm Civets
HCoV-HKU1	Mice & Human
MERS-CoV	Humans, Bats & Camels
COVID-19	Human

Symptoms of Corona Virus : -

In humans, Corona Viruses infect the Respiratory System that can be mild or lethal.

1. Mild Symptoms - Common cold by Rhinovirus
2. Lethal Symptoms as follows according to the Corona Viruses Species : 

• Range of respiratory symptoms from the common cold to high-morbidity outcomes like - pneumonia and bronchiolitis caused by  HCoV-229E
• Flu-like and may include fever, muscle pain, lethargy, headache, cough, sore throat, dyspnea, and pneumonia and regularly decrease in the number of lymphocytes circulating in the blood by Severe acute respiratory syndrome (SARS-CoV) Virus.
• Fever, cough, rhinitis, sore throat, hoarseness, bronchitis, bronchiolitis, pneumonia, and croup caused by HCoV-NL63.infection results in an upper respiratory disease with symptoms of the common cold, but can advance to pneumonia and bronchiolitis by Human coronavirus HKU1 (HCoV-HKU1).
• Fever, cough , shortness of breath, abdominal pain, muscular pain , nausea, vomiting and diarrhea is known as  Middle East respiratory syndrome (MERS) also called camel flue caused by MERS-CoV.

• Cough, fever, tiredness, difficulty breathing (severe cases) by Severe acute respiratory syndrome Corona Virus 2 or SARS-CoV-2 or or Novel Corona Virus or COVID-19 or 2019-nCoV.

Structure of Corona Virus : -

• Corona Viruses are large pleomorphic ( ability to alter their morphology, biological functions or reproductive modes according to the environmental conditions) , spherical particles with bulbaceous surface projections. 
• The virus particles diameter is approx 120 nm (nenometer). The envelope of the virus is as a distinct pair of electron dense shells.The envelope is made up of a lipid bilayer. 
• Lipid Bilayer is anchored by the membrane (M), envelope (E) and spike (S) structural proteins.

• The envelope is having nucleocapsid. Nucleocapsid is made up of multiple copies of the nucleocapsid (N) protein which are bound to the positive-sense single-stranded RNA genome in a continuous beads-on-a-string type conformation. 
• The genome size for Corona Viruses ranges from approximately 27 to 34 kilo-bases.
• The genome organization for a coronavirus is 5′-leader-UTR-replicase/transcriptase-spike (S)-envelope (E)-membrane (M)-nucleocapsid (N)-3′UTR-poly (A) tail. 

The lipid bilayer envelope, membrane proteins, and nucleocapsid protect the virus when it is outside the host cell.

Life Cycle of  Corona Virus :- 

1. Entry - Infection begins when the viral spike (S) glycoprotein attaches to its complementary host cell receptor. After attachment, a protease of the host cell cleaves and activates the receptor-attached spike protein by endocytosis or direct fusion of the viral envelop with the host membrane.On entry into the host cell, the virus particle is uncoated, and its genome enters the cell cytoplasm. The coronavirus RNA genome has a 5′ methylated cap and a 3′ polyadenylated tail, which allows the RNA to attach to the host cell's ribosome for translation. The host ribosome translates the initial overlapping open reading frame of the virus genome and forms a long polyprotein. The polyprotein has its own proteases which cleave the polyprotein into multiple nonstructural proteins.
2. Replication : - A number of the nonstructural proteins coalesce to form a multi-protein replicase-transcriptase complex (RTC). The main replicase-transcriptase protein is the RNA-dependent RNA polymerase (RdRp). It is directly involved in the replication and transcription of RNA from an RNA strand. The other nonstructural proteins in the complex assist in the replication and transcription process. The exoribonuclease nonstructural protein, for instance, provides extra fidelity to replication by providing a proofreading function which the RNA-dependent RNA polymerase lacks.One of the main functions of the complex is to replicate the viral genome. RdRp directly mediates the synthesis of negative-sense genomic RNA from the positive-sense genomic RNA. This is followed by the replication of positive-sense genomic RNA from the negative-sense genomic RNA.The other important function of the complex is to transcribe the viral genome. RdRp directly mediates the synthesis of negative-sense subgenomic RNA molecules from the positive-sense genomic RNA. This is followed by the transcription of these negative-sense subgenomic RNA molecules to their corresponding positive-sense mRNAs.
3. Release : - The replicated positive-sense genomic RNA becomes the genome of the progeny viruses. The mRNAs are gene transcripts of the last third of the virus genome after the initial overlapping reading frame. These mRNAs are translated by the host's ribosomes into the structural proteins and a number of accessory proteins. RNA translation occurs inside the endoplasmic reticulum. The viral structural proteins S, E, and M move along the secretory pathway into the Golgi intermediate compartment. There, the M proteins direct most protein-protein interactions required for assembly of viruses following its binding to the nucleocapsid. Progeny viruses are then released from the host cell by exocytosis through secretory vesicles.

Transmission : - 

The interaction of the coronavirus spike protein with its complement host cell receptor is central in determining the tissue tropism, infectivity.

Ex.. - The coronavirus infects human cells by attaching to the angiotensin-converting enzyme 2 (ACE2) receptor.

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