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Viruses can have genetic material in the form of SS RNA,DSRNA,SS DNA or DS DNA. Covid 19 is caused by corona virus which belongs to the family of Nidovirales. The reason why it has ssRNA may be due to the fact that it is highly conservable, for ribosomal shifting and 3′ nested sub-genomic mRNAs (for the expression of downstream genes). Also, the ss rna can function both as a genome and a messenger rna and can be directly translated into protein. If it were DNA it would have to go through transcription first in order to ofm a mRNA hence delaying the process of protein formation.