Despite being exposed to billions of viruses per day, only some viruses actually cause infection indicating that viruses affect humans in different ways. here, we will discus how the characteristics of viruses correlate with their structure.
Virion
Virus partical also known as the virion, which is the extra cellular form of a virus used to spread from one cell or organisms to another. in contrast to bacteria or fungi, viruses are complex molecular structures. however they don't count as living organisms as they dont have their own metabolism.
Viruses come in many shapes ans sizes such as bullet, icosahedral and sphere. A virion consists of a viral genome which can be either DNA or RNA, and is enclosed in a protein capsid, that provides protection. These viruses are referred to as non-enveloped viruses. This is in contrast to viruses surrounded by a biological membrane, known as an envelope which contains lipids and proteins. As its name suggests, these viruses are also called enveloped viruses.
A virion is only infectious is fully assembled. If the envelop of virus is destroyed it is no longer infectious. Viral genome don't encode the full set of proteins required for independent metabolism. however they only encode certain proteins that link the virus to the metabolic pathways of the cell they infect, such as energy metabolism or translation machinery.
There fore viruses are considered obligate intracellular parasites and the cells they infect are referred to as host cells. After entry into the cell, the viral genome is released from its protective shell and depending on whether it has a DNA or RNA genome, interferes with host transcription and translation processes.
Capsid and Envelop
Transfer of the viral genome and its release into the cell requires the viral capsid to be sufficiently stable to protect the genome while being labile enough to be released or uncoated inside the host cell. Structures with such opposite properties are called metastable.
A capsid consists of identical subunits made up of structural proteins connected to each other in a process termed self assembly. As the individual proteins assemble to form a large capsid, their surface charge and polarity is minimised while their contact region is maximised. This leads to a decrease in the energy of the capsid system and provides the driving force for capsid self assembly. At the same time, the energy stored in the order of the system increases, which can act as a driving force for viral uncoating. As capsid subunits are held togather by weak non-covalent bonds, dissociation can be initiated by exposing the capsid to thermal energy.
Viral uncoating can also be initiated by mechanical traction or a pH change. however, the trigger releasing the viral genome depends on the virus and the host cell. Contrary to what one would expact, an envelope increases viral sensitivity to physical influencing factors as biological membranes are relatively fragile structures.
Enveloped virus vs non enveloped viruses
• Consequently, enveloped virus can't survive the extreme acid of the stomach and don't usually enter through the gastrointestinal tract in the host. While non-enveloped viruses are less sensitive to extreme pH and can easily enter via gastrointestinal tract.
• Enveloped viruses are also more sensitive to heat dryness and disinfectants such as ethanol or propanol, making them an easy target of the hygienic measures. While the non enveloped viruses are less sensitive to heat, dryness and simple disinfectants.
• Advantage of viral envelop is the biological membrane that forms the envelop is drived from the host cell, originating from compartments such as the endoplasmic reticulum or golgi apparatus, or from the plasma membrane. This provides a shields protecting the virus particle more efficiently against the attack of the host immune system. although the viral envelop is embedded with viral proteins. While non-enveloped viruses are easy target for the host's immune system.
• Enveloped viruses can also exit the host cell, without disrupting the host cell membrane. In contrast, non-enveloped viruses exit the cell by lysis, which is a highly immunogenic event. enveloped viruses are usually less immunogenic than non-enveloped viruses. In other words, the envelope helps the virus to evade the host immune response.
