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Physical Description And Classification

Viruses are distinguished from free-living microbes, such as bacteria and fungi, by their small size and relatively simple structures. Diminutive viruses such as parvovirus may have a diameter of only 25 nanometers (nm, 10-9 meters). Poxviruses, the largest known viruses, are about 300 nanometers across, just at the detection limits of the light microscope. Typical bacteria have diameters of 1,000 nanometers or more. Information on the structure of viruses has been obtained with several techniques, including electron

Table 1. Classification of selected viruses by nucleic acid replication strategy (Baltimore Scheme).

Nucleic Acid Polarity Family Examples Host Diseases/pathologies
ss DNA + Parvoviridae parvovirus B19 humans erythema infectiosum (fifth disease)
ds DNA +/- Myoviridae Bacteriophage T4 E. coli bacterial lysis
Papillomaviridae HPV types 2, 16, 18, 33 humans warts, cervical and other cancers
Herpesviridae herpes zoster virus humans chicken pox, shingles
Poxviridae variola virus humans smallpox
ss RNA non-seg. + Picornaviridae poliovirus types 1-3 humans poliomyelitis
rhinovirus (100+ serotypes) humans common cold
Togaviridae equine encephalitis virus insects/horses CNS disease in horse and humans
ss RNA non-seg. - Rhabdoviridae rabies virus mammals rabies
Paramyxoviridae measles virus humans measles
ssRNAt segmented - Orthomyxoviruses influenza virus mammals, birds influenza
ssRNA segmented -and/or ambi Bunyaviridae Sin Nombre virus rodents hanta fever
Arenaviridae Lassa fever virus primates hemorrhagic fever
ds RNA +/- Reoviridae Rice dwarf virus plants stunting
ssRNA DNA rep. int. + Retroviridae HIV types 1, 2 humans AIDS
HTLV type I humans adult T-cell leukemia
ds DNA +/-RNA rep. int. +/- Hepadnaviridae hepatitis B virus humans hepatitis, hepatocellular carcinoma
ss=single-stranded;ds=doublestranded; non-seg.=non-segmented; ambi = ambisense;rep. int = replicativeintermediate; HPV= human papillomavirus;CNS = centralnervous system.

microscopy (EM). The limit of resolution of traditional EM is about 5 nm. With advanced EM techniques, such as cryogenic EM (cryoEM, in which the sample is rapidly frozen instead of exposed to chemical fixatives), coupled with computer image processing, smaller structures (1-2 nm) can be resolved. However, X-ray crystallography is the only method that allows for atomic-level resolution. Small viruses that produce uniform particles can be crystallized. The first atomic-level structure of a virus, tomato bushy stunt virus, was solved in 1978.

There is great diversity among viruses, but a limited number of basic designs. Capsids are structures that contain the viral genomes; many have icosahedral symmetry. An icosahedron is a three-dimensional, closed shape composed of twenty equilateral triangles. Viral proteins, in complexes termed "capsomers," form the surface of the icosahedron.

Other viruses, such as the virus that causes rabies, are helical (rod shaped). The length of helical viruses can depend on the length of the genome, the DNA or RNA within, since there are often regular structural interactions between the nucleic acids of the genome and the proteins that cover it.

A lipid-containing envelope is a common feature of animal viruses, but uncommon in plant viruses. Embedded in the envelope are surface proteins, usually glycoproteins that help the virus interact with the surface of the cell it is infecting. A matrix layer of proteins often forms a bridge between the surface glycoproteins and the capsid. Some viruses, such as the picornaviruses, are not enveloped, nor do they have a matrix layer. In these viruses, cell-surface interactions are mediated by the capsid proteins.

Some viruses have compound structures. The head of the T4 bacterial virus (bacteriophage) is icosahedral and is attached via a collar to a contractile

Table 2. The polarity, or sense, of a strand is an indication of how its sequence relates to messenger RNA (mRNA).

Molecule Sequence Polarity or Sense
Complementary RNA A U U G G G C U C negative
Coding strand DNA T A A C C C G A G positive
Complementary DNA A T T G G G C T C negative
mRNA U A A C C C G A G positive

tail with helical symmetry. Large viruses, such as the herpesviruses and poxviruses, can have higher-ordered and more complex structures.

Classification of viruses considers the genome characteristics, virion shape and macromolecular composition, and other properties, such as antigenicity and host range. A scheme for classification of viruses based on the type of nucleic acid (DNA or RNA) present in the virus particle and the method of genome replication was devised by David Baltimore, co-discoverer of reverse transcriptase (see Table 1). Reverse transcriptase is an enzyme that converts retroviral genomic single-stranded (ss) RNA into doubled-stranded (ds) DNA.

Viral genomes can be RNA or DNA, positive or negative in polarity, ss or ds, and one continuous (sometimes circular) molecule or divided into segments. By convention, messenger RNA (mRNA) that can be directly translated to protein is considered positive sense (or positive in polarity). DNA with a corresponding sequence (that is, the coding strand of double-stranded DNA) is also a positive-sense strand. An RNA or DNA molecule with the reverse complementary sequence to mRNA is a negative-sense strand. A few viruses have been identified that contain one or more "ambisense" genomic RNA segments that are positive sense in one part of the molecule (this part can be translated directly into protein) and negative sense (reverse complement of coding sequence) in the rest of the molecule.

Additional topics

Medicine EncyclopediaGenetics in Medicine - Part 4Virus - Physical Description And Classification, Infection Outcomes, Viral Cancers, Vaccines - Virus Replication Cycle