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.
Often confused with
Side-by-side differences the NEET paper likes to test.
| Aspect | Structure and Replication of Virus | 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.
Why it is tested: NEET relevance: Understanding the fundamental differences between viruses and bacteria is crucial for correctly identifying their characteristics, modes of infection, and appropriate treatment strategies. Questions often test these distinctions, particularly regarding their cellular nature, genetic material, and susceptibility to antibiotics. This comparison helps clarify why viruses are not considered 'living organisms' in the same sense as bacteria.