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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

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Questions & Answers

What is cytoplasm
Nitta Reply
Parasitic adaptation of tapeworm
Angela Reply
Describe osmoregulator or osmoconformers and how these tools allow animals to adapt to different environment.
Alick Reply
describe the secondary function of a leaf to a plant.
Twizera Reply
hi I'm asking a question about HIV infection, can HIV infection transmitted from Mother to unbron child? please help me I So confused.
Khushboy Reply
no it can't
Ahmad
yes it can but their is an injection that can be used to prevent it.
Gift
Ahmad how no?
Khushboy
Hi im new
innocent
no, unless when giving birth
Bright
YES
Kaole
it is can be transmitted but there is an injection that the mother is injected to prevent the disease
Nia
OK
Kaole
I agree with nia
peter
but I think HIV infection can be transmitted through blood so the unborn baby can be affected by HIV when a slight mistake occurred that what i think
Salim
yes, but there is a treatment however, using a retroviral therapy and injection to prevent the unborn child unscathed from the infection
Gula
what is the difference between primary and secondary active transport in detail? I didn't understand the steps in the textbook specifically
Fathima Reply
you are a doctor?
Mohammed
what is the meaning of connective tissue?
Mohammed Reply
what are the characteristics of living things
Owolo Reply
hi
Mohammed
what's the meaning of connective tissue?
Mohammed
Reproduction, adaptation, interaction, movement, growth, respiration, made of cells, responsive to environment (homeostasis), metabolic action (consumption of food converted into energy)
garret
Movement, reproduction, nutrition, irritability, growth, excretion, respiration, death, adaptation, competition
peter
state two most important factors that favour exponential growth of population of a gazelle in a pack
Eliza Reply
growth
Ben
what are the two types of electron microscope
Sharlom Reply
light microscope and early microscope
Sama
Enzymes are biological catalyst which alter any reaction and protein in nature
Nkoue Reply
Thanks
Gaudi
Your welcome sir
Nkoue
guyz you enjoying
Royd
What is translation and transcription
Nkoue
Transcription is making RNA from DNA. Translation is going from RNA to proteins.
Eric
full meaning of RNA and DNA
Charity
Nkoue what homeostits means?
Mohammed
Mohammed khalfan, In biology, homeostasis is the state of steady internal, physical, and chemical conditions maintained by living systems.[1] This dynamic state of equilibrium is the condition of optimal functioning for the organism and includes many variables, such as body temperature and fluid bal
Khushboy
what is the definition of enzymes
Royd Reply
enzymes are biological catalyst that speed up chemical reaction.
Alale
What are enzymes?
Gaudi
Enzymes are made of proteins and lower the energy of activation. In other words, they bring things together which helps to lower the amount of energy for a reaction to go forward.
Eric
they are catalyses that speeds up chemical reaction.... e.g they break down the food we consume.
Azeez
These are catalyst that speeds up the chemical reaction.
Micheal
What is connective tissue?
Mohammed
what homeostits means ?
Mohammed
what is a spirogyra
Talabi Reply
Spirogyra is a filamentous chlorophyte green algae of the order Zygnematales. It is named for the helical or spiral arrangement of the chloroplasts. That is characteristic of the genus. It is commonly found in freshwater habitats. And there are more than 400 species of Spirogyra in the world.
Eric
what is the mean of biology
Bello Reply
what is cell
Bello
A cell is the smallest living unit.
Eric
Hi I'm new in this group can someone please help with the list features shared by plants and charopytes that are not shared with most other eukaryotes
Wendy
iz a basic units of a living thing?
simongc
what is eutrophication
Chinaza Reply
hi.. I'm asking a question about HIV infection.... Can HIV infection transmitted from Mother to unbron child?
Khushboy
Eutrophication is an enrichment of water by nutrient salts that causes structural changes to the ecosystem such as: increased production of algae and aquatic plants, depletion of fish species, general deterioration of water quality and other effects that reduce and preclude use.
Khushboy

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Source:  OpenStax, Biology. OpenStax CNX. Feb 29, 2016 Download for free at http://cnx.org/content/col11448/1.10
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