Viruses do not have a metabolism, are not capable of replicating themselves outside of living cells. So viral replication can only be done in living cells
Viruses do not have a metabolism, are not capable of replicating themselves outside of living cells. Therefore, viral replication can only be carried out in living cells by means of host cell metabolism. This suggests that virus parasitism in living cells is imperative.
Viral replication is a complex process in which viral nucleic acids play a key role in transmitting their genetic information to host cells. The virus directs the metabolic processes of the host cell to the synthesis of new viral particles.
In general, viral replication in cells can be divided into five stages:
Adsorption => Infiltration => Synthesis of structural components => Assembly => Release.
Virus adsorption to the cell surface
Adsorption occurs when specific structures on the surface of the virus particle bind to virus-specific receptors located on the surface of the cell. Due to the above specificity, each virus species can only adsorb and infect a certain type of cell called their receptor cells. For example, the flu virus only infects the epithelial cells of the upper respiratory tract, the HIV virus only invades white blood cells called CD4 lymphocytes.
Virus entry into cells
Animal viruses, after attaching to specific receptors on the surface of the receptor cells, will enter the cell by the cellular mechanism. Once inside the cell, the viral capsid will be broken down by the cell's decapsidase enzyme, releasing the viral nucleic acid. This is the "undressing" phase.
For phage, after adsorbing on the cell surface, the tail capsule shrinks, the inner core punctures the cell membrane and injects nucleic acids into the cell, while the casid stays outside.
Synthesis of viral structural components
As soon as the viral nucleic acid is released, the virus loses its ability to infect and enters the latent phase, during which the virus is no longer visible in the cell. This is the stage when the virus transmits its genetic information to the host cell and forces the host cell to redirect its activity to the synthesis of viral components. First, viral nucleic acids are multiplied, then viral proteins are synthesized. The viral nucleic acids determine the specificity of the protein. Thus, the antigenic structure of the virus is not determined by the host cell but by the viral nucleic acids. The replication mechanism of viral DNA and RNA is different. Here are examples of three viruses with three different types of nucleic acids:
In viruses containing double-stranded DNA: the viral genetic information is first copied from DNA to messenger RNA by the DNA-dependent RNA polymerase. Viral messenger RNAs act as messengers to generate viral DNA and proteins.
In viruses containing positive single-stranded RNA: the viral genetic information encoded in the RNA molecule is copied to a complementary RNA by the RNA-dependent RNA polymerase, and from there they are templated to generate new RNAs. viral RNA. At the same time, viral RNA also plays the role of messenger RNA to synthesize viral proteins.
In reverse transcriptase RNA viruses: the genetic information encoded in the viral RNA is reverse transcribed to produce a DNA intermediate by reverse transcriptase (RNA-dependent DNA polymerase). .From the intermediate DNA the genetic information of the virus will be copied to messenger RNA, from which they continue to be copied to synthesize viral RNA and viral proteins.
Assembly of virus components
After the basic components of the virus have been synthesized and have been abundantly accumulated in the host cell, assembly begins. It appears that assembly of the components of the virions occurs spontaneously because of the specific molecular interaction of the capsid macromolecules with the viral nucleic acids to form virions.
Correct assembly produces complete viruses (virions) and incorrect assembly produces incomplete viruses (DIP particles) or pseudo viruses (Pseudo virions).
Release of viral particles from the cell
Viruses exit host cells in different ways depending on the virus species.
Many viruses are released in a manner that disrupts the cell membrane causing cell destruction and the viruses are released simultaneously. Either released by exocytosis or through specific channels without damaging the host cell.
The enveloped viruses are released by budding through specific sites of the host cell membrane and the virus receives a portion of the host cell membrane.
The replication time of viruses is often much shorter than that of bacteria. For example, from the original virus, a cell infected with an influenza virus can produce thousands of new viruses after about 5-6 hours.