Question

How can the genome of a (+) ssRNA virus also act as mRNA?

Answer 1

Viruses are tiny infectious agents which can replicate only when they are inside the living organism. They can infect all living organisms. When they are not inside the living organism they behave like independent particles. They are consist of genetic material which can be either DNA or RNA, a protein coat (capsid) which protect the genetic material. The genetic material contains the genes which encodes the protein through which virus act when they come in contact with living organism. Viruses which have DNA as genetic material called DNA virus and those having RNA as their genetic material are called as RNA virus. Generally tRNA viruses have single stranded RNA as genetic material but may be have double stranded RNA (Rotavirus). Single stranded RNA (ssRNA) viruses can be further classified according to polarity (sense) of their RNA into positive-sense, negative-sense or ambisense RNA viruses.

Positive-sense single stranded RNA virus (+)ss RNA virus: Virus which uses positive sense RNA as genetic material.

Negative-sense single stranded RNA virus (-)ss RNA virus: Virus which uses negative sense single stranded RNA as genetic material.

(+)ssRNA viral genome can directly serves as mRNA and directly synthesize protein in host cell because their genome have 5' cap or protein which behave similar to 5' cap and often contain poly(A) tail sequence at 3' end which is characteristic feature of mRNA. Therefore, genome of (+)ss RNA virus can act directly as mRNA. Whereas in case of (-)ss RNA virus, genetic material RNA can not be used directly for protein synthesis because it is complementary to the positive strand. So, it has to be converted into positive strand before translation.