DESCRIPTION OF LIFE CYCLE PICTURE: A coronavirus virion lands on the cell surface at upper right and binds to cell surface receptors using its projecting spikes. It then enters the cell by membrane fusion with the plasma membrane or by receptor mediated endocytosis (a clathrin coated vesicle is seen floating in the region). For more details on the different types of viral entry, please see our viral entry graphic. The viral genome (which is +ve ss RNA, drawn as a yellow string) enters the cytoplasm. The genomic RNA creates a -ve version of itself and so forms a replication complex attached to membrane (the replication complexes are drawn as orange stringy structures attached to bubble like vesicles at mid right). These replication complexes then produce new genomic RNAs and (subgenomic) mRNAs (1, 5, 7 ). mRNAs code for new viral proteins, including N, M & S. The N protein joins the new genomic RNA to form new RNPs (ribonucleoproteins). These RNPs attach to the membrane where S (Spike) proteins and M proteins have previously assembled. The RNP buds into the lumen of the vesicle (lower right) and finally the membrane bound RNP and its radiating spikes detaches and comes to lie free in the lumen as an immature virion. These particles progress up the periphery of the Golgi apparatus, maturing as they do so into a denser and more icosahedral form (3, 4, 6). The new virus particles collect in large vesicles and are finally released onto the cell surface to start the cycle again.
*Although the replication cycle is said to occur in the cytoplasm, the N protein might also target the nucleolus where it could enhance the life cycle in various ways (2, 8). The nucleus is shown in the lower right hand corner of the image and the N protein presumably enters through the nuclear pores (the flower-like structures decorating the surface of the nucleus).
CORONAVIRUSES AND SARS (Severe Acute Respiratory Syndrome): A new type of coronavirus has been identified as the cause of the emergent disease called Severe Acute Respiratory Syndrome or SARS. The "SARS Virus" was officially declared the causative agent on 16th April 2003 by the World Health Organisation. The genome for this novel virus was sequenced by various groups just prior to this announcement.
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