Classification of viruses
There are a number of classifications, but the most important which is accepted internationally is the Baltimore Classification.
Baltimore classification of viruses
The classification is based on the replication strategies.
DNA virus
A. Replication
1. The double stranded DNA separate
2. Each strand acts as a template to form double stranded DNA viral genome.
XXXXXXXXXX XXXXXXXXXX
YYYYYYYYYY
XXXXXXXXXX
YYYYYYYYYY
YYYYYYYYYY XXXXXXXXXX
YYYYYYYYYY
Almost all DNA viruses replicate in the Nuclease.
Hepatitis B virus is an exception (It uses reverse trancriptase to synthesis new circular DNA genome).
B. Translation
One of the strand acts as a template for synthesizing RNA that acts as a mRNA
Nuclease
XXXXXXXXXX DNA
YYYYYYYYYY
Nuclease XXXXXXXXXX DNA-RNA complex
YYYYYYYYYY
xxxxxxxxxx
Cytoplasma xxxxxxxxxx mRNA
RNA viruses
Viruses that have RNA as their genome can have single stranded or double stranded RNA segment. Viruses with single stranded RNA that can directly translated to produce structural and functional protein molecules are considered to have a Positive RNA segment whereas those RNA strand that have to form complementary RNA strand that act as mRNA are considered to have Negative stranded RNA segment.
I. Positive stranded genome has the same polarity as mRNA.
A. Replication
- Synthesis of Negative stranded RNA from Positive Stranded RNA
- The new negative stranded RNA segment acts as a template to synthesis the genomic RNA
xxxxxxxxxxxxxxx
xxxxxxxxxxxxxxx
yyyyyyyyyyyyyyy
yyyyyyyyyyyyyyy Template
xxxxxxxxxxxxxxx xxxxxxxxxxxxxxx xxxxxxxxxxxxxxx
- Translation
The Positive stranded RNA acts as mRNA to synthesis structural and functional proteins.
xxxxxxxxxxxxxxx mRNA
II. Negative stranded genome has the opposite polarity to mRNA.
A. Replication
1. Formation of positive strand from the negative stranded RNA genome.
2. The new positive stranded will act as a template to synthesis Negative stranded viral genome. Retroviruses are the exceptions as they use reveres transcriptase to synthesis complementary DNA from RNA.
yyyyyyyyyyyyyyy
yyyyyyyyyyyyyyy
xxxxxxxxxxxxxxx
xxxxxxxxxxxxxxx Template
yyyyyyyyyyyyyyy yyyyyyyyyyyyyyy yyyyyyyyyyyyyyy
Most RNA virus replicate in the cytoplasma.
B. Translation
1. Formation of Positive Stranded RNA from Negative stranded RNA genome
2. The new positive stranded RNA segment acts as mRNA
III. Ambisens genome contains characteristics of both positive and negative stranded RNA.
Replication and translation
The genome can act as mRNA in some regions and in other regions the complementary genetic sequence may act as mRNA
Replications site
DNA viruses replicates in the nucleus except Poxviruses
RNA virus replicates in the cytoplasma except influenza virus
Classification of medically important viruses
Other methods of classification are based on
1. The nature of the nucleic acid (DNA or RNA)
2. The symmetry of capsid (helical, icosahedral or complex)
3. The presence or absence of envelope
4. The size of the virus and capside
DNA virus
1. Complex ( capsid symmetry is neither helical nor icosahedral)
In this group there is only one family member ( Poxiviradiae).
All viruses in this group have envelope.
Their size ranges between 170-200 X300-450 nm.
This group of viruses is the only DNA viruses that have virion polymerase (other than Hepadnaviradia).
It is also the only DNA virus that replicates in the cytoplasm.
2. Icosahedral capsid
a. Enveloped virus that include Herpes and Hepadna viruses.
b. Naked (non-enveloped) viruses. These are Adenovirus, Papovaviruses and Parvovirus.
Parvovirus is the only single strand DNA virus.