Questions and Answers
What is the genome of coronaviruses made of?
What is the name of the structural protein responsible for receptor binding and membrane fusion between the virus and host cell?
What is the name of the multi-protein complex that mediates replication and transcription of RNA?
How many genera of coronaviruses are officially recognized as of 2020?
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What is the name of the protease that cleaves the polyproteins at specific sites to yield nonstructural proteins?
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How do coronaviruses enter the host cell?
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What are the three structural proteins that direct protein-protein interactions required for virus assembly?
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How are coronaviruses transmitted?
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Study Notes
Coronaviruses: Structure, Genome, and Replication Cycle
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Coronaviruses are enveloped viruses with a positive-sense single-stranded RNA genome and a nucleocapsid of helical symmetry.
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They have characteristic club-shaped spikes that project from their surface, which in electron micrographs create an image reminiscent of the stellar corona, from which their name derives.
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Coronaviruses cause respiratory tract infections in humans and birds, ranging from mild to lethal, and can also cause diseases in other animals such as cows, pigs, and mice.
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The name "coronavirus" was coined by June Almeida and David Tyrrell in 1968, referring to the characteristic appearance of virions by electron microscopy.
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As of 2020, 45 species of coronaviruses are officially recognized and classified into four genera: Alphacoronavirus, Betacoronavirus, Deltacoronavirus, and Gammacoronavirus.
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The viral envelope is made up of a lipid bilayer in which the membrane (M), envelope (E), and spike (S) structural proteins are anchored.
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The S protein is composed of an S1 and S2 subunit, and is responsible for receptor binding and membrane fusion between the virus and host cell.
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The genome size of coronaviruses ranges from approximately 26 to 32 kilobases, one of the largest among RNA viruses, and contains a 5′ methylated cap and a 3′ polyadenylated tail.
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The replicase polyprotein self-cleaves to form 16 nonstructural proteins (nsp1–nsp16), and the later reading frames encode the four major structural proteins: spike, envelope, membrane, and nucleocapsid.
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Infection begins when the viral spike protein attaches to its complementary host cell receptor, followed by cleavage and activation by a host cell protease.
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Depending on the host cell protease available, the virus can enter the host cell by endocytosis or direct fusion of the viral envelope with the host membrane.
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The host ribosomes translate the initial overlapping open reading frames ORF1a and ORF1b of the virus genome into two large overlapping polyproteins, pp1a and pp1ab, which undergo further processing to produce the viral proteins.Functions and Mechanisms of the Coronavirus Replicase-Transcriptase Complex
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Polyproteins have their own proteases that cleave the polyproteins at specific sites to yield nonstructural proteins.
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Nonstructural proteins include replication proteins such as RNA-dependent RNA polymerase, RNA helicase, and exoribonuclease.
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Nonstructural proteins coalesce to form a multi-protein replicase-transcriptase complex (RTC) which mediates replication and transcription of RNA.
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RTC's RNA-dependent RNA polymerase mediates synthesis of negative-sense genomic RNA from positive-sense genomic RNA and replication of positive-sense genomic RNA from negative-sense genomic RNA.
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RTC's RNA-dependent RNA polymerase mediates synthesis of negative-sense subgenomic RNA molecules from positive-sense genomic RNA, followed by transcription of these molecules to their corresponding positive-sense mRNAs.
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RTC is capable of genetic recombination through template switching during genome replication, driving genetic variability and emergence of novel coronaviruses.
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Replicated positive-sense genomic RNA becomes the genome of progeny viruses, while mRNAs are translated into structural and accessory proteins.
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Structural proteins S, E, and M direct protein-protein interactions required for virus assembly following M protein's binding to the nucleocapsid.
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Progeny viruses are released from host cells by exocytosis through secretory vesicles, and can infect other host cells.
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Coronavirus spike protein's interaction with its complementary cell receptor determines tissue tropism, infectivity, and species range of the released virus.
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Coronaviruses are transmitted by either aerosol, fomite, or fecal-oral route, depending on the species.
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Human coronaviruses infect respiratory tract epithelial cells, while animal coronaviruses infect digestive tract epithelial cells.
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Description
Test your knowledge on coronaviruses with this informative quiz! Learn about the structure, genome, and replication cycle of these enveloped viruses with positive-sense single-stranded RNA. Discover the characteristic club-shaped spikes that give them their name, and the four genera they belong to. Dive into the functions and mechanisms of the coronavirus replicase-transcriptase complex, and understand how these viruses infect host cells and cause respiratory tract infections. Take this quiz to become an expert on coronaviruses!
