Viruses

What are the parts of virus, how can they be visible, lysogenic and lytic cycle, mechanism of formation of enveloped virus • PARTS OF A VIRUS: nucleic acid (DNA or RNA = genetic material), capsid (protein coat that protects genetic material), nucleocapsid (capsid + genome together), envelope (lipid membrane in some viruses, derived from host cell), spike proteins (surface proteins used for attachment to host cells), enzymes (sometimes present, e.g., reverse transcriptase in retroviruses) • HOW VIRUSES ARE “VISIBLE”: viruses are too small for light microscopes, so they are seen using electron microscopy; they can also be detected indirectly by observing infected cells, plaque assays (clear zones of cell death in cultures), or molecular methods like PCR • LYTIC CYCLE (active destruction cycle): virus attaches to host cell, injects genetic material, host cell machinery is hijacked to make viral DNA/RNA and viral proteins, new virus particles assemble, host cell lyses (bursts), releasing many new viruses • LYSOGENIC CYCLE (dormant/integrated cycle): virus attaches and injects genetic material, viral DNA integrates into host DNA (called a prophage in bacteria), host cell replicates normally copying viral DNA with it, can stay inactive for long periods, then may switch to lytic cycle when triggered (stress, UV, etc.) • LYTIC vs LYSOGENIC KEY DIFFERENCE: lytic = immediate replication + cell death, lysogenic = dormant integration + delayed activation • MECHANISM OF ENVELOPED VIRUS FORMATION: after replication inside host cell, viral proteins (especially spike proteins) are inserted into host cell membrane, viral nucleocapsid moves to membrane, virus buds out of the host cell taking part of the host’s lipid membrane with it, this stolen membrane becomes the viral envelope, virus leaves without immediately killing the cell (in many cases), examples include influenza and HIV