Bacteriophage and Eukaryotic Virus Replication: Stages and Structures

Bacteriophage Replication

The duplication of a bacteriophage is known as the lytic cycle and can be grouped into different stages:

1. Attachment
2. Entry
3. Replication
4. Assembly
5. Lysis and Release

The time required for viral multiplication is 30 to 35 minutes.

Retroviruses

Retroviruses are a type of virus present only in eukaryotic cells. Retroviruses have RNA as genetic material and within their capsid, they also bear a molecule of the reverse transcriptase enzyme that is able to catalyze the transformation of their messenger RNA molecules into DNA. In a later stage, the viral genome, DNA copied and called provirus, is integrated into the cell's DNA and can be transmitted to the descendants of the same when it divides. The next stage, which can occur after a long time, is that proviral genes are transcribed by the host's RNA polymerase. In this process, it produces a large amount of RNA molecules identical to the infective RNA. These will then be translated by the host's enzymatic machinery to produce all the proteins needed by the mature viral particle.

Eukaryotic Virus Replication

Eukaryotic viruses are those that infect eukaryotic cells. The viral nucleic acid contains the genetic information necessary to create all the specific viral macromolecules in a highly organized manner. One of the characteristics of viral multiplication is that, shortly after its interaction with a host cell, the infecting virus disintegrates and loses infectivity. This cell cycle phase is called the eclipse period. This period is of intense synthetic activity, since it redirects the infected cell's metabolic machinery in function of the needs of the virus, i.e., the synthesis of new viral particles. There are metabolic processes of the host cell that are not altered significantly, but it is forced to synthesize viral proteins and nucleic acids.

Attachment

The first stage of viral infection is the interaction of the virus with a specific receptor site on the cell surface of the host cell. The receptor molecules differ for different viruses: in some cases, they are proteins and in others, oligosaccharides. The presence or absence of receptors plays a crucial role in viral pathogenesis.

Penetration

After attachment, the viral particle enters the cell. In some systems, it is by endocytosis-mediated loss of receptors. The viral envelope occurs during or shortly after penetration of the same. As already commented, at this moment the original virus loses infectivity.

Synthesis

Soon after the viral genome is discovered, the synthetic phase of the virus replication cycle is initiated. The essential aspect of viral replication is that they have to transcribe the specific mRNA of the viral nucleic acid to be successfully expressed and duplicated while achieving genetic information. At this stage, viruses rely on the components of the cell to translate mRNA. The envelope proteins assemble to build the capsid, which encapsulates and stabilizes the viral nucleic acid against the extracellular environment. The result may be dozens or hundreds of progeny viral particles from a virus that infects the host cell.

Viruses

Viruses are acellular structures and are not considered living organisms. Their dimensions are between 20 and 300 nm. Due to their small size, they could not be observed until the mid-twentieth century. Viruses were photographed for the first time in 1942. Viruses have no metabolism and get their own reproduction as obligate intracellular parasites. They can parasitize bacteria and eukaryotic cells.

Virus Structures

Their structure is very simple:

• An outer shell of proteins, called a capsid. Sometimes they also have an outer shell that comes from the biofilm of the infected cell lipid; in it are, however, virus-specific proteins.
• Inside, they contain a nucleic acid, which can be DNA or RNA. The DNA may be linear or closed. Both DNA and RNA can be formed by a single strand or a double strand.

The capsid is formed by protein subunits called capsomeres. The union of capsomeres and their distribution in space determine the shape of the virus. Three basic models are distinguished:

1. Polyhedral Viruses: The capsid is formed by numerous capsomeres that form a polyhedron. This polyhedron can have many faces. This is the case of the chickenpox virus.
2. Helical Viruses: The capsomeres are wound in a spiral, forming a helical cylinder. This is the tobacco mosaic virus.
3. Complex Viruses: A region of the capsid, named the head, is polyhedral-shaped, and an area called the tail has a helical pattern. An example is the T2 virus that infects bacteria.