Viral replication is when viruses make copies of themselves after entering healthy host cells. The virus injects its genetic material into the host cell and takes over its protein-making parts to create new viruses. Even though we call them microorganisms, viruses aren't alive. They need a host to survive, so it's better to call them microbes. Remember, these tiny microbes can cause big problems if they're not kept in check!

The structure of viruses

Viruses come in many different shapes and sizes, and they can have unique features depending on the host cell they're targeting. Some of the main features of viruses include nucleic acids like DNA or RNA, which are essential for creating new viruses. There's also a protein coat called the nucleocapsid (or capsid for short) that surrounds the nucleic acid. Some viruses also have a lipid envelope, which is a membrane made of lipids that covers the capsid. However, not all viruses have this envelope. The viral tegument is another feature, which is a group of proteins that sits between the envelope and nucleocapsid. Finally, there are attachment proteins that help the virus identify and attach to the right host cell. Knowing these features can help us understand how viruses work and how to fight them.

Bacteriophages

Some viruses can specialise 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.

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, allowing the virus to enter the host cell and replicate. This 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. For example, SARS-CoV-2 contains attachment proteins (spikes) complementary to the ACE2 receptors on a host cell. This allows the virus to enter the host cell and replicate.

Viral entry into the host cell

The process of viral entry into the host cell can differ depending on whether the virus is enveloped or non-enveloped. Enveloped viruses have a membrane that can fuse with the host cell membrane, allowing the capsid to enter the cell. Examples of viruses that enter host cells by membrane fusion include HIV and Ebola virus. On the other hand, non-enveloped viruses and some enveloped viruses enter the host cell via endocytosis, where the host cell engulfs the virus in an intracellular vesicle. Regardless of the entry method, the viral capsid will eventually be degraded by enzymes, releasing the viral nucleic acid into the cytoplasm. This nucleic acid can then use the host cell's protein components to replicate and produce new viruses. Some viruses, such as bacteriophages, have an alternative way of introducing their genetic material into host cells: they adhere to the outside of the cell and inject the genetic material directly into the cytoplasm.

Viral replication and assembly

After the viral capsid is degraded, the viral genome can be transcribed and translated using the host cell's machinery. Some viruses can even integrate their genetic material into the host cell's DNA and remain dormant until a later time.

Transcription is the process of copying the viral DNA into messenger RNA (mRNA), which carries the information to the ribosomes for translation. During translation, the sequence carried by mRNA is translated into of amino acids in the ribosomes, which then assemble into proteins. These newly synthesized viral proteins are then packaged into virions, which are newly made virus particles that contain the structure of the “parent” virus before it had entered the host cell. This process is also referred to as viral maturation or assembly.

Unlike prokaryotic microorganisms, viruses cannot replicate through binary fission, which is an asexual reproduction process that creates two genetically identical daughter cells from one parent cell. Bacteria divide and replicate through binary fission.

Release of virions

Once the newly made virions are assembled, there are three main ways they can be released from the host cell:

1. Budding - Non-enveloped virions can "borrow" the host cell's membrane to create their envelope. This method is used by viruses like HIV and SARS-COV-2.
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, and examples include herpes simplex virus and poliovirus.
3. Exocytosis - Vesicles containing virions fuse with the cell membrane and releaseions. In this case, the host cell remains intact, as seen with varicella-zoster virus.

During viral infection, some viral proteins may end up being presented on the major histocompatibility complex (MHC), specifically MHC-I, on the infected cell’s surface. This phenomenon acts as a marker that indicates to T lymphocytes that a viral infection is happening, allowing T lymphocytes to initiate an anti-viral response. For more information about these cells, you can read our article on T Cell Immunity.

Lytic vs lysogenic viral replication of bacteriophages

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

During the lytic viral cycle, also known as a virulent infection, the bacteriophage takes over and destroys the host cell. The phage synthesizes viral proteins that break down the host cell's DNA. The bacteriophage can then control the cell and integrate its viral nucleic acid into the host genome, forcing the host to synthesize viral particles that are then released as new bacteriophages.

In contrast, the host cell does not die during the lysogenic cycle, which is a non-virulent infection. Instead, the phage integrates itself into the host cell's genome and replicates passively with the host. The host cell relatively, if the cell becomes stressed, theiophage can enter the lytic cycle and start destroying the host cell.

Differences between bacteria and viruses

Bacteria and viruses may cause similar symptoms, such as cough and fever, but they differ significantly in several ways:

1. Bacteria are living microorganisms that can exist inside another organism or in external environments while viruses are noniving microbes that require a host cell to survive.
2. Bacteria divide and replicate through binary fission, while viruses replicate by hijacking the host cell's protein synthesis organelles.
3. Antibiotics can treat bacterial infections, while viral infections are treated with antiviral drugs and vaccines. Often, the immune system clears the virus.

In terms of viral replication, it describes the formation of viruses that begins when they infect the host cell. Viruses are non-living microbes that require a host cell to survive. The viral replication process involves attachment, entry, replication, assembly, and release, in which the host cell's protein synthesis components are used to create viral proteins.

Bacteriophages, which infect bacterial cells, can have either a lytic (virulent infection) or lysogenic (non-virulent infection) viral replication cycle. A stressed bacteriophage can enter a lytic cycle.

Viral Replication

How do viruses replicate by binary fission?

Viruses do not replicate by binary fission. Instead, viruses hijack the host cell's protein synthesis components to create new virions. 

Is binary fission in a virus or bacteria?

Bacteria.

What is binary fission replication?

Binary fission is a type of asexual reproduction whereby two new genetically identical daughter cells are created from one parent cell. Prokaryotic microorganisms replicate by binary fission, like bacteria. 

What are two ways viruses replicate?

Lytic and lysogenic cycles.

 Do viruses divide by binary fission?

No, binary fission only occurs in prokaryotic microorganisms, like bacteria.

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