2.1 Types of virus

You have now considered a great variety of viruses, which differ in size, shape and life-cycle, some of which are shown in Figure 4. In this course you will focus on Influenza-A, an orthomyxovirus, and SARS-CoV2, which is a coronavirus.

Figure 4 The morphology and approximate relative sizes of different families of virus

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The icosahedral viruses are from left to right: adenovirus, reovirus, papovavirus, picornavirus, parvovirus.

Figure 4 The morphology and approximate relative sizes of different families of virus

In addition to their obvious differences in size and shape, viruses are classified according to their genetic material and how it is replicated:

• DNA or RNA
• single-stranded (ss) or double stranded (ds)
• positive sense or negative sense; this relates to whether the genome directly encodes protein (positive) or whether it must be replicated before it can direct protein synthesis (negative).

This is referred to as the Baltimore system of classification. The details of virus replication are beyond the scope of this course. It is sufficient to know that Influenza-A has a genome with eight segments of ssRNA, which is negative-sense, whereas SARS-CoV2 has a single segment of ssRNA, which is positive sense.

One important point to note is that replication of the ssRNA genomes of these viruses involves an intermediate of dsRNA; double-stranded RNA is not a standard component of uninfected host cells.