# Comparison of Selected Characteristics of SARS-CoV-2, SARS-CoV, and HCoV-NL63
### Morphology, Structure, and Replication of SARS-CoV-2
#### 1.3.3 Replication  
The SARS-CoV-2 virus primarily attacks respiratory epithelial cells, alveolar pneumocytes, mainly type II [23], as well as intestinal tract epithelial cells. Kidney and heart tissue is secondary [24; 25]. CoVs enter cells by specifically binding to the cell receptor of the invaded cell and subsequent fusion with the membranes. _ACE2_ was specified as the binding receptor between the host cell and the spike protein through the receptor-binding domain (RBD) in SARS-CoV-2. The susceptibility of the host is dependent on the affinity of the virus to its receptor. SARS-CoV-2 recognizes ACE2 more efficiently than SARS-CoV because of its 10- to 20-fold increased binding affinity to ACE2. This binding leads to the entry of the virus into the host cell and to the activation of the S protein by a specific TMPRSS2, of the host cell. The S protein, which consists of two subunits and S1, catalyzes the fusion of the S2 subunit. Upon binding of the virion to the target cell, the protease of the cleaved TMPRSS2 protein opens the top protein of the virus and reveals the fusion peptide in the S2 subunit and the ACE2 host receptor. The virus enters the cytoplasm of the cells and fuses with the membrane of the host cell. Following the fusion, an endosome is formed around the virion. Afterward, RNA is released into the cell [8; 26]. SARS-CoV-2 viruses are positive-sense, single-stranded RNA viruses, which employ a complicated pattern of virus genome length RNA replication as well as transcription of genome length and leader-containing subgenomic RNAs. Replication of RNA viruses occurs in the cytoplasm of the host cell in several consecutive phases. Subsequently, the virus genome is exposed by the disintegration of the viral protein envelope and the transcription of the viral RNA into the RNA of the host cell. The first part of the genome to be transcribed is a viral RNA polymerase, called RNA replicase, which transcribes copies of the full-length genome in negative strands. These are used as templates to produce _messenger RNA_ (mRNA) that transcribe viral genes. Transcripts are produced by viral RNA polymerase activity. The viral RNA polymerase interacts with a transcriptional regulation sequence (TRS) located between the viral genes, which enables a link between the 5' leader sequence and the beginning of each gene. The mechanism of replication has not yet  
Figure 2: Schematic RNA genome structure SARS-CoV-2. The replicase locus of SARS-CoV-2 contains two substantial open reading frames (ORFs) and four structural proteins. The ORF1a and ORF1b locus encodes 16 nonstructural proteins. Structural proteins, the spike (S) protein is the largest structural protein encoded by the RNA genome. The envelope (E) protein is a small membrane protein that is anchored on the viral envelope and plays a role in the virion assembly and has additional effects on the infected cells of the host. The membrane (M) protein is a multi-membrane spanning protein and the most abundant in the viral envelope. It provides structural support for the virion and plays a role in the assembly and construction of virion particles. Nucleocapside (N) protein complexes with the genome RNA to form the virial nucleocapsid within the viral membrane and interact with the M in the assembly. The N protein may play a role in replicating RNA. been fully described. The genome from which the four major structural proteins arise, as well as a number of non-structural proteins (nsps) from two alternative reading frames is ORF1\(\alpha\), which generates polyprotein 1a followed by 11 nsps and ORF1b, resulting in polyprotein 1 ab, which is further processed to 16 nsps. [27]. The S protein is 150 kDa and N-glycosated. The S protein trimer forms a distinct structure on the surface of the virus [28]. The trimeric S protein is cleaved by a host furin-like protease into two domains: S1 and S2. S1 provides the binding site to the receptor, whereas S2 provides structural aid in the form of support for a functional S protein [24]. The M protein has a size of 25 to 30 kDa.