<< Chapter < Page Chapter >> Page >
Micrograph a shows icosahedral polioviruses arranged in a grid; micrograph b shows two Epstein-Barr viruses with icosahedral capsids encased in an oval membrane; micrograph c shows a mumps virus capsid encased in an irregular membrane; micrograph d shows rectangular tobacco mosaic virus capsids; and micrograph e shows a spherical herpesvirus envelope studded with glycoproteins.
Transmission electron micrographs of various viruses show their structures. The capsid of the (a) polio virus is naked icosahedral; (b) the Epstein-Barr virus capsid is enveloped icosahedral; (c) the mumps virus capsid is an enveloped helix; (d) the tobacco mosaic virus capsid is naked helical; and (e) the herpesvirus capsid is complex. (credit a: modification of work by Dr. Fred Murphy, Sylvia Whitfield; credit b: modification of work by Liza Gross; credit c: modification of work by Dr. F. A. Murphy, CDC; credit d: modification of work by USDA ARS; credit e: modification of work by Linda Stannard, Department of Medical Microbiology, University of Cape Town, South Africa, NASA; scale-bar data from Matt Russell)

Baltimore classification

The most commonly used system of virus classification was developed by Nobel Prize-winning biologist David Baltimore in the early 1970s. In addition to the differences in morphology and genetics mentioned above, the Baltimore classification scheme groups viruses according to how the mRNA is produced during the replicative cycle of the virus.

Group I viruses contain double-stranded DNA (dsDNA) as their genome. Their mRNA is produced by transcription in much the same way as with cellular DNA. Group II viruses have single-stranded DNA (ssDNA) as their genome. They convert their single-stranded genomes into a dsDNA intermediate before transcription to mRNA can occur. Group III viruses use dsRNA as their genome. The strands separate, and one of them is used as a template for the generation of mRNA using the RNA-dependent RNA polymerase encoded by the virus. Group IV viruses have ssRNA as their genome with a positive polarity. Positive polarity means that the genomic RNA can serve directly as mRNA. Intermediates of dsRNA, called replicative intermediates , are made in the process of copying the genomic RNA. Multiple, full-length RNA strands of negative polarity (complimentary to the positive-stranded genomic RNA) are formed from these intermediates, which may then serve as templates for the production of RNA with positive polarity, including both full-length genomic RNA and shorter viral mRNAs. Group V viruses contain ssRNA genomes with a negative polarity    , meaning that their sequence is complementary to the mRNA. As with Group IV viruses, dsRNA intermediates are used to make copies of the genome and produce mRNA. In this case, the negative-stranded genome can be converted directly to mRNA. Additionally, full-length positive RNA strands are made to serve as templates for the production of the negative-stranded genome. Group VI viruses have diploid (two copies) ssRNA genomes that must be converted, using the enzyme reverse transcriptase    , to dsDNA; the dsDNA is then transported to the nucleus of the host cell and inserted into the host genome. Then, mRNA can be produced by transcription of the viral DNA that was integrated into the host genome. Group VII viruses have partial dsDNA genomes and make ssRNA intermediates that act as mRNA, but are also converted back into dsDNA genomes by reverse transcriptase, necessary for genome replication. The characteristics of each group in the Baltimore classification are summarized in [link] with examples of each group.

Baltimore Classification
Group Characteristics Mode of mRNA Production Example
I Double-stranded DNA mRNA is transcribed directly from the DNA template Herpes simplex (herpesvirus)
II Single-stranded DNA DNA is converted to double-stranded form before RNA is transcribed Canine parvovirus (parvovirus)
III Double-stranded RNA mRNA is transcribed from the RNA genome Childhood gastroenteritis (rotavirus)
IV Single stranded RNA (+) Genome functions as mRNA Common cold (pircornavirus)
V Single stranded RNA (-) mRNA is transcribed from the RNA genome Rabies (rhabdovirus)
VI Single stranded RNA viruses with reverse transcriptase Reverse transcriptase makes DNA from the RNA genome; DNA is then incorporated in the host genome; mRNA is transcribed from the incorporated DNA Human immunodeficiency virus (HIV)
VII Double stranded DNA viruses with reverse transcriptase The viral genome is double-stranded DNA, but viral DNA is replicated through an RNA intermediate; the RNA may serve directly as mRNA or as a template to make mRNA Hepatitis B virus (hepadnavirus)

Section summary

Viruses are tiny, acellular entities that can usually only be seen with an electron microscope. Their genomes contain either DNA or RNA—never both—and they replicate using the replication proteins of a host cell. Viruses are diverse, infecting archaea, bacteria, fungi, plants, and animals. Viruses consist of a nucleic acid core surrounded by a protein capsid with or without an outer lipid envelope. The capsid shape, presence of an envelope, and core composition dictate some elements of the classification of viruses. The most commonly used classification method, the Baltimore classification, categorizes viruses based on how they produce their mRNA.

Art connections

[link] Which of the following statements about virus structure is true?

  1. All viruses are encased in a viral membrane.
  2. The capsomere is made up of small protein subunits called capsids.
  3. DNA is the genetic material in all viruses.
  4. Glycoproteins help the virus attach to the host cell.

[link] D

Got questions? Get instant answers now!

Questions & Answers

Is the "growth and maintenance phase" in a cell's life cycle when cell division is about to occur
Somto Reply
what is the common name of Basidiomycetes
Ogechukwu Reply
الاجزاء النباتية لابد من تعقيمها قبل زراعتها في القوارير
yes
tariq
whats this?
tariq
do you speak arabic?!
what are bio elements
Shahzad Reply
which are present In Body And such elements Have Great role in our Body there are 16 bio elements that maintains human Body but on The basis of amount There are 6 bio elements present in Concen. of 99% and More Valuable And Highly Concen. element is Oxygent with 65 %
Haider
how je pollution brought about
Lamina Reply
how je pollution brouhgt about
Lamina
non is pollution brouhgt about
Lamina
describe the anatomy of cell division
Ivanovic Reply
Complex traits such as height result from 
Ruben Reply
what is the difference between chloroplasts and mitochondria
Nkalubo Reply
chloroplast in plants and bacterial cell ; mitochondria in animal cells
aung
Diagram of a living cell
Eliza Reply
what is cell
Sule
A cell is the smallest basic unit of life.
John
what's biology
Ogochukwu Reply
this is da study of living and non-living thing in an eco-system
Nutty
it is the study of living and non living organism in the ecology
Akufia
I agree with you dat biology is d study of living nd nonliving features
Winner
why do plants store carbohydrates in form of starch and not glucose?
Nutty Reply
Describe the structure of starch?
Nutty
wat is diffusion
Winner
water is life!.. Discuss?
Nutty Reply
why do plants store carbohydrates in form if starch not glucose!
Nutty
study of living thing
Dennis Reply
what is beyond a liveing cell
Raymond
what is biology
Gabriel Reply
d study of living nd non living thing
Winner
what is vasectomy
Evelyn Reply
The surgical removal of d spermduct
Eniola

Get the best Biology course in your pocket!





Source:  OpenStax, Biology. OpenStax CNX. Feb 29, 2016 Download for free at http://cnx.org/content/col11448/1.10
Google Play and the Google Play logo are trademarks of Google Inc.

Notification Switch

Would you like to follow the 'Biology' conversation and receive update notifications?

Ask