The correct answer is (B) the pathways required to synthesize virus mRNA.​

David Baltimore’s classification system, proposed in 1971, groups viruses into seven categories based on their nucleic acid type, strandedness, sense, and the specific replication strategies needed to generate mRNA from the genome. This approach highlights how viruses depend on host machinery for protein synthesis via mRNA.​

Option Analysis

Option (A): Host cell receptors used by viruses
Viruses bind specific host receptors for entry, but Baltimore classification ignores this, focusing solely on genomic replication pathways rather than attachment mechanisms.​

Option (B): The pathways required to synthesize virus mRNA
This matches the core principle: groups differ by whether genomes are DNA/RNA, single/double-stranded, positive/negative-sense, or use reverse transcription to produce mRNA (e.g., Group I transcribes dsDNA directly; Group IV uses +ssRNA as mRNA).​

Option (C): The modes of transmission of viruses
Transmission (airborne, fecal-oral) varies widely and unrelated to classification, which centers on molecular replication strategies.​

Option (D): The envelope proteins on the surface of viruses
Surface proteins aid antigenicity or entry but play no role; classification depends on genome-mRNA pathways, not structural proteins.​

David Baltimore’s classification of viruses revolutionized virology by focusing on the pathways required to synthesize virus mRNA, enabling precise grouping into seven classes for better study of replication strategies. This system, ideal for CSIR NET Life Sciences preparation, emphasizes genomic differences over morphology or hosts.​

7 Baltimore Groups

• Group I (dsDNA): mRNA transcribed directly by host RNA polymerase (e.g., Adenovirus).​

• Group II (ssDNA): Genome converts to dsDNA intermediate before transcription (e.g., Parvovirus).​

• Group III (dsRNA): Viral RdRp transcribes mRNA from one strand (e.g., Reovirus).​

• Group IV (+ssRNA): Genome acts as mRNA (e.g., Poliovirus).​

• Group V (-ssRNA): RdRp synthesizes +mRNA from negative genome (e.g., Influenza).​

• Group VI (ssRNA-RT): Reverse transcription to DNA, then mRNA (e.g., HIV).​

• Group VII (dsDNA-RT): RNA intermediate via reverse transcription (e.g., Hepatitis B).​

Why Pathways Matter

All viruses rely on host ribosomes for translation, placing mRNA synthesis central—Baltimore’s scheme maps exact routes from genome to mRNA, aiding antiviral design. Unlike ICTV taxonomy (evolutionary), it simplifies functional analysis.​

This classification proves vital for exams, distinguishing it from irrelevant factors like receptors or transmission.​