6.1: VirusesVirus Composition: Size and ShapeViruses, although they are considered neither prokaryotic nor eukaryotic, play a large role in microbiology. Viruses are small, non-cellular particles that cannot replicate unlessinside a living host cell. As they cannot replicate on their own, viruses are considered obligate intracellular parasites.With regards to composition, all viruses have two basic components: (1) genomic material comprised of either DNA or RNA and (2) a capsid, a membrane-like protective structure that contains the genetic material, similar to the nucleus of a eukaryotic cell. Unlike prokaryotic and eukaryotic genomes, which contain a double-stranded DNA genome, the viral genome is quite diverse. Viral genomes can be either double or single-stranded, and they may be found in circular or linear arrangements. The size of viral genomes can also vary from just a few thousand nucleotide base pairs (about 3,200 bp for poliovirus) to roughly 2.5 million nucleotides (like pandoravirus, which houses the largest known viral genome).Note: Compared to the bacterial genome of E. coli (about 4.6 million nucleotides) or the human genome (about 3 billion nucleotides) the viral genome is surprisingly small. While the pandoravirus (about 2.5 million), megavirus (about 1.25 million), and mimivirus (about 1.2 million) are large, they are the exception and do not conform to conventional definitions as the vast majority of viral genomes are much smaller.For many viruses, an additional membrane called the envelopesurrounds the capsid. The envelope is derived from the host cell membrane and serves as an additional barrier to the external environment. In contrast, a virus simply surrounded by the protein capsid is referred to as a nakedor non-envelopedvirus. As an important distinction, the overwhelming majority of animal viruses are enveloped whereas the majority of plant or bacteria-infecting viruses are not.It is important to note that not all viruses appear the same. The differences in virion composition can also significantly influence the shape and appearance of the virus. For instance, the smallpox virus (variola virus) is enveloped, about 200 nm long, and has a distinct dumbbell-shaped viral capsid (Figure 6.1A). In contrast, the poliovirus is non-enveloped and only about 30 nm in diameter (Figure 6.1B). These differences can be observed using an electron microscope. Although EM images are capable of resolving the differences in the shape of the virus, looks can be deceiving. Paramyxoviruses and orthomyxoviruses are both enveloped, spherical, and about 100-150 nm in diameter, butthey vary drastically in composition and function (Figure 6.1C and D). Paramyxoviruses,the causative agent of measles and mumps, contain a single-stranded linear genome and fuse with the host cell membrane to initiate entry and viral replication. In contrast,
orthomyxoviruses, the causative agent of the flu, contain eight segments of RNA and enter the host cell via endocytosis.