1. Group I – dsDNA virus: The viruses belonging to this group follow the

simplest mechanism of transcription. They use DNA-dependent DNA

polymerase (DdDp) from the host cell to generate mRNA, which could

late translate into functional viral proteins.

Examples: Adenovirus, Herpesvirus

2. Group II – ssDNA virus: This group first needs to create a com-

plementary strand to form a dsDNA. This is achieved by using DNA

polymerase from the host to synthesize the strand which is complementary

to their ssDNA genome. After the formation of dsDNA, it makes mRNA

using the host enzyme DdDp.

Example: Parvovirus

3. Group III – dsRNA virus: This group of viruses contains an RNA poly-

merase that would transcribe the dsRNA to (+) ssRNA, which would ulti-

mately be the mRNA. This mRNA would serve two purposes. First, it would

get translated to the required viral proteins, and second, it would also serve as a

template to synthesize (−) ssRNA to form the dsRNA genome for packaging.

Examples: Reovirus, Rotavirus

4. Group IV – (+) ssRNA virus: The genome of this group of viruses is

similar to the host mRNA sequence. But the only hurdle is that the host

ribosomes do not recognize the viral RNA. Hence, the viral polymerase

moves along the (+) strand of the RNA template and elongates the (−)

stranded RNA molecule. Now this (−) ssRNA serves as a template for

polymerizing new (+) ssRNA, serving as the genome of the virus.

Examples: Picornavirus, Togavirus, Coronavirus

5. Group V – ssRNA virus: This group makes for the largest family of

viruses. They carry the RNA-dependent RNA polymerase (RdRp). This

enzyme makes two types of (+) RNA strand: 1) a short viral mRNA that

would be translated into necessary viral proteins and 2) a full-length RNA

that would be replicated to make (−) ssRNA genome for packaging into

progeny visions. Here, viral RNA is replicated separately and has a highly

organized and regulated packaging process to make sure that one of each

distinct RNA has been received by each virion.

Examples: Orthomyxovirus, Paramyxovirus

6. Group VI – (+) ssRNA-RT virus: This group of viruses contains an

enzyme called reverse transcriptase (RT). They go opposite or reverse to

the normal transcription process. The (+) ssRNA undergoes reverse tran-

scription to make a complementary DNA strand. This DNA strand acts as

a template from a dsDNA in the nucleus of the host. This dsDNA is

covalently linked to host chromosomal DNA and, therefore, replicates as a

host genome. Viruses of this group benefit from the error-prone RT en-

zyme. It provides them with their ability to evade the immune system by

minor changes to their protein capsules.

Example: Retrovirus

7. Group VII – dsDNA-RT virus: This group of viruses replicates through

RNA intermediates. The members of this group have a very different

genome because of two facts. First, one strand has a protein at the 5’ end

Introduction to basic virology

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