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Context: SARS-CoV-2 is the seventh coronavirus that has humans as the host. Because of its highly infectious nature, toward the end of January 2020, the WHO declared it a public health emergency of international concern. The present review is about understanding the journey of SARS-CoV-2 to its present form with an attempt to assess the genetic basis of its pandemic-causing abilities. Evidence Acquisition: The data for the present review were accessed through different publications and preprint repositories. Results: SARS-CoV-2 is a beta-coronavirus, and is approximately 60 - 140 nm in size. The appearance of its structure as a crown shape under an electron microscope led to the coining of its name ‘Coronavirus’. Comparative genome and proteome analysis exhibits similarities and differences with reference to SARS-CoV. The Open Reading Frames (ORFs) found on the SARS-CoV-2 genome, and their corresponding proteins have been discussed. Bats may act as reservoir hosts but not exclusively. The possibility of snakes as the host, as well as other intermediate hosts, before reaching humans seems plausible. This has been supported by ACE2 receptor diversity and conservation across different tissues and organisms. The role of spike glycoprotein and its interaction with the receptor through specific residues for invading host cells makes a perfect therapeutic target, but the variations therein and the resulting impact on interactions pose challenges for the same. Conclusions: Though the differences between the MERS, SARS-CoV, and SARS-CoV-2 genomes indicate amino acid changes, leading to the present pandemic situation, the fact that new variants are still emerging signifies that the journey is an ongoing one, which requires monitoring.

Background: Simple sequence repeats (SSRs) are 1 - 6 bp repeat motif sequences present across both prokaryotic and eukaryotic genomes with various clinical implications besides being tools for conservation and evolutionary studies. Objectives: Analysis of 33 Coronavirus genomes, including severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), for incidence, distribution, and complexity of SSRs patterns to understand their role in host divergence and evolution. Methods: Full-length genome sequences were extracted from National Center for Biotechnology Information (NCBI). Extraction of microsatellites was done using imperfect microsatellite extractor (IMEx) in “Advanced Mode”. Sequences were aligned with MAFFT v6.861b and the maximum likelihood tree was inferred using RAxML v8.1.20 of the GTR + GAMMA+I model with default specifications. Results: A total of 3,442 SSRs and 136 complex sequence repeats (cSSRs) were extracted from the studied 33 genomes. SSR incidence ranged from 82 (CV09) to 144 (CV60). cSSR incidence ranged from 1 (CV42, CV43, CV53) to 11 (CV32). CV61 (SARS-CoV-2) had 107 SSRs and 6 SSRs. Di-nucleotide motifs were the most prevalent followed by tri- and mono-nucleotide motifs. TG/GT was the most represented di-nucleotide motif, followed by CA/AC. In tri-nucleotide SSRs, ACA/TGT was the most represented motif followed by CAA/GTT, whereas in mono-nucleotide SSRs, T was the most observed nucleotide, followed by A. About 94% of SSRs were localized to the coding region. Twenty species, including CV61 (SARS-CoV-2), exhibit mono-nucleotide repeats exclusively in the A/T region, which were clustered in phylogenetic analysis. The sequence similarity of the genomes was assessed through heat map analysis and revealed similar sequences are expectedly placed in proximity on the phylogenetic tree. Conclusions: Mono-nucleotide exclusivity to A/T region and SSR genome signature can be a possible basis for predicting the evolution of viruses in terms of host range.

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