The Fundamentals of Virus Morphology and Replication

Viral Morphology

The morphology of the virion capsid is defined by its composition of multiple copies of a single protein or several different proteins, each encoded by a viral gene. A capsid, along with the nucleic acid it contains, is called a nucleocapsid. Virions exhibit two primary types of symmetry: helical and icosahedral.

Symmetry Types

Helical Symmetry

This is a helical protein structure with RNA or DNA protected inside it.

Icosahedral Symmetry

An icosahedral virus is a polyhedron composed of 20 identical, equilateral triangular faces.

In some viruses, the nucleocapsid is coated by an outer shell, or envelope, which is a fragment of the host cell's plasma membrane acquired as the virus emerges through a process called budding.

Complex Structures

Some viruses, like the T-even bacteriophages that attack Escherichia coli, have complex or binary structures. These phages have an icosahedral head that connects to a tail with helical symmetry, consisting of a contractile sheath surrounding a tubular core. The tail ends in a hexagonal base plate from which short tail pins and long tail fibers emerge. Its mission is to act as the organ for attachment to host cells.

Viral Replication Cycles

Lysogenic Pathway

This occurs when the phage DNA is integrated into the host bacterium's chromosome. It is then replicated as part of the bacterial DNA from one generation to the next. Such bacterial strains are called lysogenic, and the bacteriophages are called temperate phages. The integrated phage DNA is called a prophage, and the host cell is described as being in a state of lysogeny. Lysogeny provides the host bacterium with specific resistance to reinfection by a related virus. Furthermore, some phages and animal viruses can infect a cell and produce virions without killing it, establishing a persistent viral infection. These types of infections are suspected to be linked to some degenerative diseases.

Lytic Cycle

The lytic cycle is the more common type of viral replication and is divided into five phases.

Phase 1: Adsorption

The first phase, adsorption (or attachment), begins when the virus comes into contact with the host cell's plasma membrane. A viral protein binds to a specific receptor protein on the host cell membrane to initiate infection.

Phase 2: Penetration

The second phase is penetration. There are several ways a virus can cross the plasma membrane to enter the cytoplasm:

• Direct Penetration: The entire virus enters directly through the plasma membrane.
• Endocytosis: The virus is engulfed by the cell membrane, enclosed in a vesicle, and then released into the cytoplasm.
• Membrane Fusion: The viral envelope fuses with the host cell membrane, releasing the nucleocapsid inside. This is often combined with endocytosis, as seen in the influenza virus.

Phase 3: Eclipse Phase

During this stage, the synthesis of the viral genome and proteins occurs. The virus undergoes "uncoating," where its nucleic acid is released. The viral DNA (or RNA) directs the host cell's machinery (ribosomes, tRNAs, etc.) to translate its mRNA into viral proteins.

Phase 4: Maturation

The fourth phase is maturation (or assembly). After the synthesis of viral components is complete, they are transported to "assembly zones" within the cell. Here, the components are assembled into new nucleocapsids.

Phase 5: Release

The final stage is release (or liberation). For naked viruses in bacterial cells, this typically occurs through lysis, where the host cell breaks down, releasing all the new virions at once.