5.3 Virus Structure and Replication Cycles

A complete virus particle outside a host is a virion. Every virion contains a nucleic-acid genome enclosed by a protein capsid assembled from capsomeres. The genome may be DNA or RNA, single- or double-stranded, linear, circular or segmented. No virus contains both a DNA and an RNA genome as its hereditary material.

Some viruses have a lipid envelope acquired from host membranes during budding. Viral glycoproteins embedded in the envelope recognize host receptors and promote entry. The envelope is not a cell membrane supporting independent metabolism. Because lipid membranes are disrupted by detergents, drying and some environmental conditions, envelopes can improve immune evasion inside a host while reducing stability outside it.

Viruses are much smaller than most cells, often tens to a few hundred nanometres. They have no cytoplasm, ribosomes or independent ATP-generating metabolism. Some carry enzymes needed immediately after entry—such as reverse transcriptase—but they still depend on host raw materials, ribosomes and energy.

Infection begins when a viral surface protein binds a compatible receptor. Receptor presence is necessary but may not be sufficient: intracellular factors, immune defences and temperature can also restrict replication. Host range is therefore a molecular compatibility problem, not a deliberate choice by a virus.

A bacteriophage attaches to a bacterial surface and may inject its genome while the capsid remains outside. Enveloped animal viruses may fuse with the plasma membrane or enter by endocytosis before uncoating. Despite different entry routes, successful infection delivers the viral genome to a cellular compartment where it can be copied and expressed.

In a lytic pathway, viral genes redirect the host toward genome replication and protein synthesis. Capsids self-assemble, genomes are packaged, and new virions leave—often by host-cell lysis in bacteriophages. A burst releases many particles, but particle count exceeds the number that successfully infect new cells.

In a lysogenic pathway, phage DNA integrates into the bacterial chromosome as a prophage or persists in a stable state. Host-cell division copies the viral sequence without immediate virion production. Stress or other signals can induce the prophage to exit and enter a productive lytic cycle. Lysogeny is therefore a replication strategy through the host lineage, not viral inactivity in an absolute sense.

HIV is an enveloped retrovirus whose RNA enters CD4-bearing cells with reverse transcriptase and integrase. Reverse transcriptase copies viral RNA into DNA; integrase inserts that DNA into a host chromosome as a provirus. Host transcription and translation then generate viral RNAs and proteins, and virions bud through the membrane.

The terms lytic and lysogenic are most precise for bacteriophages. Animal-virus infections can be acute, persistent, latent or transforming and do not always fit a simple two-box classification. The central distinction is whether new particles are being produced immediately or the genome persists within a host cell or lineage.