INTRODUCTION OF VIRUS

2. BALTIMORE CLASSIFICATION
OF VIRUS
USMAN AHMAD

3. INTRODUCTION:
The Baltimore classification groups viruses into families depending on their type of
genome.
Genome:
The complete genetic information (either DNA or, in some viruses, RNA) of
an organism, typically expressed in the number of base pairs.

4. CONTINUE…..
Virus classification is the process of naming viruses and placing
them into a taxonomic system. Much like the classification systems
used for cellular organisms, virus classification is the subject of
ongoing debate and proposals. This is mainly due to the
pseudo-living nature of viruses, which is to say they are non-
living particles with some chemical characteristics similar to
those of life. As such, they do not fit neatly into the
established biological classification system in place for
cellular organisms.

5. Baltimore classification (first defined in 1971) is a classification
system that places viruses into one of seven groups depending on
a combination of their nucleic acid (DNA or RNA), strandedness
(single-stranded or double-stranded), Sense, and method of
replication. Named after David Baltimore, a Nobel Prize-winning
biologist, these groups are designated by Roman numerals and
discriminate viruses depending on their mode of replication and
genome type.

6. Other classifications are determined by the disease caused by the
virus or its morphology, neither of which are satisfactory due to
different viruses either causing the same disease or looking very
similar. In addition, viral structures are often difficult to determine
under the microscope.

8. Classifying viruses according to their genome means that those in a given category will
all behave in a similar fashion, offering some indication of how to proceed with further
research. Viruses can be placed in one of the seven following groups:
I: dsDNA viruses (e.g. Adenoviruses, Herpesviruses, Poxviruses)
II: ssDNA viruses (+)sense DNA (e.g. Parvoviruses)
III: dsRNA viruses (e.g. Rotaviruses)
IV: (+)ssRNA viruses (+)sense RNA (e.g. picornaviruses, togaviruses)
V: (−)ssRNA viruses (−)sense RNA (e.g. Orthomyxoviruses, Rhabdoviruses)
VI: ssRNA viruses (+)sense RNA with DNA intermediate in life-cycle (e.g. Retroviruses)
VII: dsDNA-RT viruses (e.g. Hepadnaviruses)

9. 1. Double stranded DNA viruses
Most double-stranded DNA viruses replicate within the host cell nucleus,
including polyomaviruses, adenoviruses, and herpesviruses—poxviruses, however,
replicate in the cytoplasm. Adenoviruses and herpes viruses encode their own replication
factors.
Adenoviruse Herpesviruse

10. 2. Single stranded DNA viruses (+) sense DNA
The ssDNA viruses constitute the widespread, diverse and important group of viruses
affecting all three domains of life. Metagenomic mining and characterization of Rep like
sequences have led to explosion of CRESS DNA virus data which reveal the ubiquitous
presence of these viruses in diverse habitat

11. 3. Double stranded RNA viruses :
They replicate in core capsid in cytoplasm.
They do not have to use host replication polymerase.
Replication is monocistronic and include individual segmented genome.
Double-stranded RNA viruses include the rotaviruses, known globally as a common
cause of gastroenteritis in young children, and bluetongue virus, an economically
significant pathogen of cattle and sheep. The family Reoviridae is the largest and most
diverse dsRNA virus family in terms of host range.

12. 4. Single stranded RNA viruses (+)sense RNA:
Viruses can be differentiated based on how they store their genomic
information, such as by DNA or double-stranded RNA. Positive-sense
single-stranded RNA (+ssRNA) viruses are one such way and it is a key
aspect of the infectious cycle of the virus.
Virus with polycistronic mRNA :
Genome RNA form mRNA which is translated into polyproteins then
cleaved into proteins.

13. Virus with complex transcription :
Subgenomic mRNA ribosomal frameshifting and proteolytic processing of
polyproteins used.
Example: Families Astrovirdae
CONTINUE……

14. 5. Single stranded RNA viruses (−)sense RNA:
Negative-strand RNA virus: Also known as an antisense-strand RNA virus, a virus
whose genetic information consists of a single strand of RNA that is the negative or
antisense strand which does not encode mRNA (messenger RNA).
Virus with non-segmented genome:
Negative stranded genome is transcribed into monocistronic mRNA
Replication occurs in cytoplasm.

15. Virus with segmented genome:
Each genome segment produce monocistronic mRNA.
Replication occurs in nucleus.
Example: Arenaviridae and Orthomyxoviridae.

16. 6. Single stranded RNA viruses (+)sense:
Positive-sense single-stranded RNA (+ssRNA) viruses are one such way and
it is a key aspect of the infectious cycle of the virus. Two important examples
of +ssRNA viruses are severe acute respiratory syndrome coronavirus 2
(SARS-CoV-2) and Hepatovirus A, which cause coronavirus and hepatitis A,
respectively.
Replicate through DNA intermediates.
They use reverse transcriptase to convert positive sense RNA to DNA.

17. The DNA are spliced into hosts’ genome using integrase and then replication
commences.
Example: Retroviruses.

18. 7. Double stranded DNA viruses:
They replicate through single-stranded RNA intermediates.
DNA is filled in to form covalently closed circles these serve as
template for production of viral mRNA and sub genomic RNA.
The pregenome RNA serves as a template for viral reverse
transcriptase which produce DNAgenome
Example: Hepadnaviridae.