Bacteriophage and Virus Replication: Lytic Cycle, Retroviruses and Eukaryotic Viruses

Bacteriophage Replication

Bacteriophage replication: The lytic cycle can be divided into different stages schematically: 1. Attachment, 2. Penetration, 3. Replication, 4. Assembly, 5. Lysis, and entry and liberation. The time necessary for viral multiplication is approximately 30 to 35 minutes.

Retroviruses

Retroviruses are: a type of virus found only in cells, present in eukaryotes. Retroviruses are characterized by having genetic material that is enclosed inside their capsid. They have an envelope, a single-stranded RNA molecule, and reverse transcriptase, an enzyme capable of catalyzing its transformation from an RNA molecule into DNA.

In the first stage, the genome of the virus is copied into DNA to form a provirus, which integrates into the cell's DNA and may be transmitted to the descendants of the cell when it divides. In the next stage, which can occur after a long period of time, the provirus genes are transcribed by RNA polymerase into RNA. In this process, the first production lot is identical to the original RNA and is infectious.

These RNA molecules are then translated by the enzymatic machinery of the cell to produce all the proteins needed for the mature viral particle.

Replication of Eukaryotic Viruses

Replication of eukaryotic viruses: Viruses that infect eukaryotic cells contain nucleic acid. The viral genetic information is necessary for specific macromolecules of all viruses, and they have a highly organized structure.

One of the characteristics of viral multiplication is that, after a short period of time following interaction with the cell, the infecting virus disintegrates and loses its infectivity. This cell cycle period is called the eclipse period. During this period there is intense synthetic activity, since the infected cell redirects its metabolic machinery to the synthesis of viral components. New particles appear where the metabolic processes of the cell are not significantly altered, although it is forced to synthesize viral proteins and nucleic acids.

Attachment

The first step of the viral infection process is the interaction of the virus with a specific receptor site on the cell surface. The molecules that serve as receptors differ between viruses: some are proteins, and others are oligosaccharides. The presence or absence of receptors plays a determining role in viral pathogenesis.

Penetration

After attachment, the viral particle enters the cell. In some viruses, this occurs through endocytosis mediated by receptors. The viral coat is lost during or shortly after penetration, and the original virus loses its infectivity at this moment.

Synthesis

After the viral genome is uncoated, it initiates the synthetic replication phase of the viral cycle. The essential aspect is viral replication, in which specific mRNAs are transcribed from the nucleic acid and viral duplication occurs. The genetic information is successfully transferred, and the viral components return to the cell's machinery to be translated.

The proteins that make up the capsid are assembled, which encapsulates the viral nucleic acid and stabilizes it outside the host. The result can be the production of hundreds of new viral particles from a single infecting virus.

Viruses

Viruses: Viruses are acellular structures and are not considered alive. Their size ranges from 20 to 300 nm. Due to their small size, they could not be observed with a microscope until the twentieth century, when they were photographed for the first time in 1942.

Viruses do not have their own metabolism and cannot reproduce on their own. They are obligate intracellular parasites of living organisms. They can infect bacterial and eukaryotic cells.

Structure of Viruses

Structure of viruses: Their structure is very simple, consisting of an outer protein covering called the capsid. Sometimes there is also an external wrapper, which is a lipid bilayer derived from the membrane of the infected cell; these, however, are specific viral proteins.

Inside, they contain a nucleic acid, which may be DNA or RNA. The DNA can be linear or circular. Both DNA and RNA can be single-stranded or double-stranded.

The protein subunits that form the capsid are called capsomeres. The arrangement and distribution of capsomeres in the virus determine its three basic models:

1. Polyhedral viruses: The capsid is formed by numerous capsomeres arranged in a polyhedral shape with many faces. In this case, the virus is a varicella virus.
2. Helical viruses: The capsomeres are arranged in a spiral form to create a cylinder. Helical viruses include the mosaic virus and the tobacco mosaic virus.
3. Complex viruses: One area of the capsid, called the head, has a polyhedral shape, while another area, called the tail, has a helical shape. An example is the T2 virus, which infects bacteria.

Replication Cycle

The replication cycle has two distinct periods: an inactive extracellular period and an active intracellular infectious period. During the extracellular period, viral particles do not show any kind of metabolic or reproductive activity and are completely inert. They can be easily destroyed by physical agents such as high temperatures or chemicals.

The second period is divided into several stages:

• Entry: Viruses enter the cells through various mechanisms. The virus either is injected or penetrates with its genetic material. The action of enzymes dissolves the protein capsid, and the nucleic acid of the virus comes into direct contact with the cytoplasm of the cell.
• Protein and nucleic acid synthesis and replication: In this stage, the virus uses the metabolic machinery of the cell to obtain copies of itself. Some viral genes encode the proteins of the capsid, and some viruses carry genetic information for enzymes that synthesize the nucleic acid that will be replicated. Viruses with larger genomes carry their own enzymes for replicating their genetic material.
• Assembly and release: Hundreds of copies of the virus are produced inside the cell, capable of infecting new cells. Viral particles can leave the cell by budding, which includes fragments of the infected cell's membrane, or by lysis, in which the cell breaks open and releases all the viral particles at the same time.