GENERAL PROPERTIES OF VIRUS:
1. Acellular structure:
They are acellular particles consisting of genetic materials [DNA &RNA] enclosed in a protein coat called capsids. Some viruses also consist of lipid envelopes obtained from the host.
2. Obligate intracellular parasites:
They cannot reproduce or perform metabolic pathways independently. So, they require a host cell for replication and metabolic processes.
3. Genetic material:
- Viruses contain either DNA or RNA but not both.
- The genetic material can either be single-stranded or double-stranded.
4. Capsid:
- It helps in protecting the viral genome and helps in attachment to host cells.
- Capsids are made of protein subunits called capsomers.
5. Size:
It is usually in 20 to 300 nanometers. e.g., smallest virus bacteriophage- 27 to 32nm in size
Largest virus pithovirus sibericum- 1.5 mm found in 2014.
6. Host specificity:
Most virus only affects a specific type of host group, e.g., Bacteriophage affects E. coli.
Smallpox affects humans.
7. Reproduction:
A virus cannot replicate on its own, so it hijacks the host's replication mechanism to reproduce
e.g., lytic cycle, lysogenic cycle.
8. Metabolism:
Viruses do not contain metabolic mechanisms; they cannot generate energy or synthesize proteins.
e.g., Virion.
9. Transmission:
It can spread by air, water, direct contact, by vectors, and body fluids, etc., depending on the types of viruses, e.g., dengue, HIV, Hepatitis B virus, and Hepatitis C virus.
10. Mutation:
Viruses mutate rapidly, mainly RNA viruses, which helps to adapt to host defense mechanisms.
11. Nature:
They show characteristics of life only inside a host. Outside, they behave like inert particles.
Baltimore classification:
The common virus classification was developed by David Baltimore in the 1970s.
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 (complementary 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.
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