replication and transcription model of the replicase-transcriptase complex of a coronavirus. rdrp for replication (red), exon for proofreading (dark blue), exon cofactor (yellow), rbps to avoid secondary structure (light blue), rna sliding clamp for processivity and primase domain for priming (green/orange), and a helicase to unwind rna (downstream). a number of the nonstructural replication 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 protein nsp14 is a 3'-5' exoribonuclease which provides extra fidelity to the replication process. the exoribonuclease provides a proofreading function to the complex which the rna-dependent rna polymerase lacks. similarly, proteins nsp7 and nsp8 form a hexadecameric sliding clamp as part of the complex which greatly increases the processivity of the rna-dependent rna polymerase. the coronaviruses require the increased fidelity and processivity during rna synthesis because of the relatively large genome size in comparison to other rna viruses. one of the main functions of the replicase-transcriptase 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. the other important function of the replicase-transcriptase 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 replicated positive-sense genomic rna becomes the genome of the progeny viruses. the various smaller mrnas are transcripts from the last third of the virus genome which follows the reading frames orf1a and orf1b. these mrnas are translated into the four structural proteins (s, e, m, and n) that will become part of the progeny virus particles and also eight other accessory proteins (orf3 to orf9b) which assist the virus. recombination when two sars-cov genomes are present in a host cell, they may interact with each other to form recombinant genomes that can be transmitted to progeny viruses. recombination likely occurs during genome replication when the rna polymerase switches from one template to another (copy choice recombination). human sars-cov appears to have had a complex history of recombination between ancestral coronaviruses that were hosted in several different animal groups. assembly and release 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 released from the host cell by exocytosis through secretory vesicles.