Structure and Replication of Virus — Core Principles

Core Principles

Viruses are acellular, obligate intracellular parasites consisting of genetic material (DNA or RNA) enclosed within a protein coat called a capsid. Some viruses also have an outer lipid envelope. They lack their own metabolic machinery and must hijack a host cell's resources to replicate.

The viral life cycle involves adsorption, penetration, uncoating, biosynthesis (replication and protein synthesis), assembly, and release of new virions. Bacteriophages can undergo either a lytic cycle, leading to host cell lysis, or a lysogenic cycle, where viral DNA integrates into the host genome as a prophage.

Key viral enzymes include reverse transcriptase (in retroviruses) and RNA-dependent RNA polymerase (in some RNA viruses). Viruses are classified based on their genetic material, capsid symmetry, and presence/absence of an envelope.

Understanding these basics is crucial for comprehending viral diseases and developing antiviral strategies.

Important Differences

vs Bacteria

Aspect	This Topic	Bacteria
Cellular Nature	Acellular (not a cell)	Prokaryotic cell (unicellular)
Genetic Material	DNA or RNA (never both)	DNA (main chromosome) and sometimes plasmids
Metabolism	None (obligate intracellular parasite)	Independent (has own metabolic machinery)
Reproduction	Replication via host cell machinery	Binary fission (asexual reproduction)
Size	Extremely small (20-300 nm)	Larger (0.5-5 µm)
Cell Wall/Membrane	No cell wall, some have an envelope (host-derived)	Cell wall (peptidoglycan) and cell membrane
Ribosomes	Absent	Present (70S type)
Treatment	Antiviral drugs	Antibiotics

Viruses are acellular, obligate intracellular parasites that lack their own metabolic machinery and ribosomes, relying entirely on host cells for replication. They contain either DNA or RNA. Bacteria, in contrast, are true prokaryotic cells, possessing their own ribosomes, metabolic pathways, and a cell wall, allowing them to grow and reproduce independently. Viruses are significantly smaller than bacteria and are unaffected by antibiotics, which specifically target bacterial cellular processes. This fundamental distinction underpins their different biological roles and disease mechanisms.