BIOD 171 module 6 Virus Composition: Size and Shape

BIOD 171 module 6 Virus Composition: Size and Shape Virus Composition: Size and Shape Viruses, although they are considered neither prokaryotic nor eukaryotic, do play a large role in microbiology. Viruses are small, non-cellular particles that cannot replicate unless inside a living host cell. As they cannot replicate on their own, viruses are therefore, by definition, 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 may be found in circular or linear arrangements. The size of viral genomes can also vary from just a few thousand nucleotide base pairs (~3,200 bp for Poliovirus) to roughly 2.5 million nucleotides from the Pandoravirus, which houses the largest known viral genome. Note: Compared to the bacterial genome of E coli (~4.6 million nucleotides) or to the human genome (~3 billion nucleotides) the viral genome is surprisingly small. While the Pandoravirus (~2.5 million), Megavirus (~1.25 million) and Mimivirus (~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