Viral Replication

Ever wondered why when the seasons change and it becomes colder you tend to get sick? Well, it's not due to you catching a 'chill' but rather because the mucus that lines your mucous membranes dries up due to the colder, drier weather. Without this protective layer, you become vulnerable to attack from different types of microbes - mainly viruses. A viral infection or in other words, viral replication, refers to the formation of viruses after they have infected healthy host cells. They will inject their viral genetic material into the host cell and ‘hijack’ the protein synthesis components to make new viruses which are then released.

We often refer to viruses as microorganisms; however, they are non-living. Viruses cannot survive without a host; therefore, we should refer to them as microbes instead.

The structure of viruses

Viruses come in all shapes and sizes and may have different features from one another. The features will depend on the target host cell. Main viral features include:

• Nucleic acids such as DNA or RNA - the genetic material needed to generate new viruses.

• Nucleocapsid (capsid in short) - a protein coat enclosing the nucleic acid.

• Lipid envelope - a lipid membrane surrounding the capsid. Not all viruses have an envelope.

• Viral tegument - a group of clustered proteins that line up between the envelope and nucleocapsid.

• Attachment (envelope) proteins - proteins that allow the virus to identify and attach to the correct host cell.

Fig. 1 - The main components of a virus

Bacteriophages

Some viruses are specialised for certain groups of living organisms. Bacteriophages, also known as phages, infect bacterial cells and do not infect humans (phew)! Instead, they are primarily used in research. It is thought that they can be used, in combination with or instead of antibiotics, in the future to treat bacterial infections.

Fig. 2 -

Viral replication steps

Viral replication is broken down into four main stages:

• Attachment of virus to host cell

• Viral entry into the host cell

• Viral replication and assembly

• Release of virions

Attachment of virus to host cell

The attachment proteins present on the viral capsid or lipid envelope complement some receptors on the host cell. The ability of a virus to infect a specific host cell is called viral tropism. The proteins on the viral agent’s surface determine whether or not the virus can infect a host cell.

Fig. 3 -

Viral entry into the host cell

The entry of the virus into the host cell differs for enveloped and non-enveloped viruses. For enveloped viruses, their membrane will fuse with the host cell membrane and the capsid is then internalised into the cell.

Non-enveloped viruses and some enveloped viruses will enter via endocytosis, whereby the host cell will take up the virus by engulfing it into an intracellular vesicle.

In either case, the viral capsid will be degraded by enzymes, allowing the viral nucleic acid to be released into the cytoplasm, ready to use the host cell’s protein synthesis components.

Viral replication and assembly

Following the degradation of the viral capsid, viral transcription and translation (using the host cell’s machinery) can commence. Some viruses can also integrate their genetic material into the host cell’s DNA and wait for a better moment to start making copies of themselves.

Here is a little refresher on transcription and translation:

• Transcription - the viral DNA strand is copied into messenger RNA (mRNA), which carries the information to the ribosomes (sites for protein synthesis).

• Translation - the sequence carried by mRNA is translated to a sequence of amino acids in the ribosomes.

Newly synthesised viral proteins are packaged into virions. A virion is a newly made virus particle that contains the structure of the “parent” virus before it had entered the host cell. Assembly is also referred to as viral maturation.

Release of virions

There are three main ways in which the release of the newly made virions occurs:

• Budding - non-enveloped virions will “borrow” the host cell’s membrane and create their envelope (HIV and SARS-COV-2).

• Cell apoptosis - the infected host cell will undergo programmed cell death when under viral attack. Viruses that cause the host cell’s death are cytolytic (herpes simplex virus and poliovirus).

• Exocytosis - vesicles containing virions will fuse with the host membrane and release the virions. The host cell remains intact (varicella-zoster virus).

Some viral proteins end up being presented on a structure called the major histocompatibility complex (MHC), specifically MHC-I, on the infected cell’s surface. This phenomenon acts as a marker, indicating to T lymphocytes that a viral infection is happening, allowing T lymphocytes to initiate an anti-viral response.

Lytic vs lysogenic viral replication of bacteriophages

There are two types of bacteriophages: lytic bacteriophages and lysogenic bacteriophages.

During the lytic viral cycle (virulent infection), the bacteriophage will take over and destroy the cell. It will synthesise viral proteins that will break down the host cell’s DNA. The bacteriophage can then control the cell; because it integrates its viral nucleic acid into the host genome, it will make the host synthesise viral particles to make new bacteriophages that are then released.

The host cell does not die during the lysogenic cycle (non-virulent infection). The phage will integrate itself into the host cell’s genome and passively replicate with the host. The host is relatively unharmed. If the cell is stressed, the bacteriophage can enter the lytic cycle.

Fig. 4 - An overview of the lytic cycle of bacteriophages

Viruses do not undergo binary fission

Viruses are non-living, so they cannot replicate by binary fission, like prokaryotic microorganisms.

Binary fission is an asexual reproduction whereby two new genetically identical daughter cells are created from one parent cell.

Differences between bacteria and viruses

Viruses and bacteria can sometimes be confused because they can cause similar symptoms like cough and fever. But they differ significantly. Listed below are just some of their differences:

• Bacteria are living microorganisms that can live inside another organism or in external environments. Viruses are non-living microbes that need a host cell to survive.

• Bacteria divide and replicate by binary fission. Viruses replicate by hijacking the host cell’s protein synthesis organelles.

• Antibiotics can treat bacterial infections. Viral infections are treated with anti-viral drugs and vaccines, but often the immune system clears the virus.