Detailed discussion on the general properties of viruses.

Have you ever suffered from chicken pox? Let's make it simpler: have you suffered from the common cold?

Most likely, you are suffering just now. Just kidding, but we all know how awful we felt during the common cold, with a sore throat, fever, runny nose, fatigue, ...etc. And the things, that are responsible for this are none other than viruses.

So, in this blog, we will discuss about viruses, its general properties, reproduction/replication, some basic classification,size,shape and how they invade our body.

1.General propertie of Viruses

• A virus is the smallest infectious particle that can only be seen through an electron microscope.
• Viruses are obligate intracellular organisms, which means they can only survive or grow inside the cell.
• It has no ribosomes present, which means they can't produce their own protein by themselves; they need some host or cell.

Structure:

Basic structure of virus 

In the picture, you can see the basic or fundamental structure of a virus.

Let me explain the diagram.

• On the outside, there is the envelope, which covers the virus as a sheath, and envelope proteins (called peplomers), which protrude to the outside.
• In the center, there is a capsid, which covers the nucleic acid, or genome. Together, we can say it is a nucleocapsid.
• Tegument, which is a protinaceous layer located between the viral envelope and the nucleocapsid.
• Mostly, viruses have two types of symmetry: isosahedral or helical symmetry. Some viruses can have complex symmetry, i.e., the Pox virus. All DNA viruses have isosahedral symmetry, and most of the RNA viruses have helical symmetry.

 

Symmetry of virus

Envelope:

The envelope of the virus is made up of lipoprotein. But this compound is not entirely produced by the virus; the lipid part comes from the host, and the proteinous part is of the virus.

This envelope is sensitive to ether and alcohol,so it can be eliminated by soap, alcohol, or a senitizer. Yes, alcohol, particularly high concentrations of ethanol or isopropyl alcohol, can effectively eliminate enveloped viruses by disrupting their lipid bilayer envelopes. This disrupts the structure of the virus, rendering it unable to infect host cells. Even if we eat them, they can be broken down by our bile salts.So, most of the enveloped viruses affect primarily the respiratory system or directly go into our circulatory system. That's why alcohol-based disinfectants are commonly used to sanitize surfaces and reduce the spread of enveloped viruses like influenza and coronaviruses.

2.Viral replication:

Firstly, viruses do not replicate by binary fission; they cannot divide into two by themselves. They lack the cellular machinery necessary for this process.

They divides by a process,which involve 5 stages:( They need a host cell for multiplication)

Stage 1:Attachment /Adsorption

This attachment stage in viral replication is the very first step where a virus attaches to a host cell. This attachment is mediated by specific interactions between viral surface proteins (such as glycoproteins or spikes) and receptors on the surface of the host cell. These interactions are often highly specific and determine the host range of the virus.

The attachment stage is crucial for viral infection because it facilitates the subsequent steps of viral entry into the host cell. Once attached, the virus can then penetrate the host cell membrane and deliver its genetic material into the host cell cytoplasm, initiating the process of viral replication.

Stage 2: Penetration/Viropexia

The penetration stage  involves the entry of the virus into the host cell. There are several mechanisms by which viruses can penetrate host cells:

1. Direct Fusion: Some viruses, such as enveloped viruses like HIV and influenza, fuse their viral envelope directly with the host cell membrane, releasing the viral genetic material into the host cell cytoplasm.

2. Endocytosis: Many viruses are taken up into the host cell via endocytosis, where the host cell membrane engulfs the virus particle and forms a vesicle called an endosome. The virus then either fuses its envelope with the endosomal membrane to release its genetic material or uses the acidic environment within the endosome to trigger conformational changes in viral proteins, facilitating penetration into the cytoplasm.

3. Receptor-Mediated Endocytosis:Some viruses specifically bind to cell surface receptors, triggering receptor-mediated endocytosis. This process ensures that the virus is taken up by the host cell and transported into the cytoplasm.

 Uncoating

The uncoating stage in viral replication is when the viral genome, whether it's DNA or RNA, is released from its protective protein coat (capsid) or envelope after the virus has penetrated the host cell. This step is crucial for the virus to begin hijacking the host cell's machinery for replication. One thing to note is that this stage is absent in bacteriophage because it directly releases its genetic material to the host cell.

Stage 3 : Biosynthesis 

The biosynthesis stage in viral replication involves the production of viral components, including viral proteins and nucleic acids, using the host cell's machinery. This stage typically follows the uncoating of the viral genome and involves several key steps:

1. Transcription: The viral genome is used as a template to produce viral mRNA molecules. These mRNA molecules are then translated into viral proteins by the host cell's ribosomes.

2. Translation: Viral mRNA molecules are translated by the host cell's ribosomes into viral proteins. These proteins include structural proteins (e.g., capsid proteins) and non-structural proteins (e.g., enzymes required for viral replication).

3. Replication:The viral genome is replicated to produce multiple copies of viral nucleic acids. This process may involve viral enzymes or utilize the host cell's DNA or RNA replication machinery, depending on the type of virus.

4. Post-translational Modifications:Viral proteins may undergo post-translational modifications, such as glycosylation or phosphorylation, to become functional or to aid in the assembly of new virus particles.

Stage 4: Assembly

The assembly stage in viral replication is when newly synthesized viral components, including viral proteins and nucleic acids, come together to form complete virus particles within the host cell. This process typically occurs in a coordinated manner and involves several key steps:

1. Nucleocapsid Formation: Viral nucleic acids, either DNA or RNA, associate with structural proteins to form the nucleocapsid, which serves as the core of the virus particle. The nucleocapsid protects the viral genome and provides structural integrity to the virus.

2. Envelope Acquisition: Enveloped viruses acquire their lipid bilayer envelope from the host cell membrane during the budding process. The nucleocapsid is embedded within the envelope, and viral glycoproteins may be incorporated into the envelope during budding.

3. Capsid Assembly: Non-enveloped viruses assemble their protein capsid around the nucleocapsid. This capsid provides protection to the viral genome and determines the overall shape and structure of the virus particle.

4. Maturation:In some viruses, newly assembled virus particles undergo maturation processes, such as cleavage of viral precursor proteins or structural rearrangements, to become fully infectious.

Stage 5 : Release

The release stage in viral replication is when newly assembled virus particles are released from the host cell to infect other cells and propagate the infection. The release process varies depending on the type of virus:

1. Enveloped Viruses: Enveloped viruses are typically released from the host cell by budding. During budding, the newly assembled virus particles acquire their lipid bilayer envelope from the host cell membrane. The virus particles then bud off from the host cell, taking a portion of the host cell membrane with them. This process is generally less destructive to the host cell compared to lysis.

2. Non-enveloped Viruses: Non-enveloped viruses are released from the host cell through processes such as lysis or exocytosis. During lysis, the host cell ruptures, releasing the virus particles into the extracellular environment. Exocytosis involves the packaging of virus particles into vesicles, which are then released from the host cell membrane.

3. Persistently Infected Cells: Some viruses establish persistent infections in host cells, where virus particles are continuously produced without causing cell lysis. In these cases, virus particles are released gradually over time without necessarily causing cell death.

Once released from the host cell, the newly formed virus particles can infect neighboring cells and continue the viral replication cycle, spreading the infection throughout the host organism or to new hosts.

Replication stages

Replication time differs from virus to virus, i.e., bacteriophage take 15–30 minutes to replicate, whereas animal viruses can take up to 15–30 hours. 

 

3.Classification: 

Viruses are basically divided into two categories, i.e., DNA viruses and RNA viruses, as per the presence of their genomes.

Under DNA virus:

• Hepadnaviridae: Hepatitis B virus
•  Herpesviridae: All class of Herpes virus
•  Adenoviridae: Adenoviridae
•  Poxviridae: Smallpox, Cowpox, Molluscum
•  Papovaviridae: HPV,BK, JC, Polyoma, SV40
•  Parvoviridae:Parvovirus
• Bacteriophage

Under RNA Virus:

• Picornaviridae (Polio)
• Paramyxoviridae
• Caliciviridae
• Togaviridae
• Orthomyxoviridae
• Bunyaviridae
• Flaviviridae
• Arenaviridae
• Coronaviridae
• Reoviridae
• Rhabdoviridae
• Retroviridae
• Filoviridae

There are 3 term, which you should know :

1. Virions:

•     Virions are complete virus particles that are capable of infecting host cells and replicating within them.
•    They consist of a nucleic acid genome (either DNA or RNA) surrounded by a protein coat called a capsid. Some viruses also have an additional lipid bilayer envelope surrounding the capsid.
•    Virions can infect a wide range of organisms, including animals, plants, bacteria, and archaea.

2. Viroids:

•    Viroids are small, circular RNA molecules that infect plants and cause diseases.
•    Unlike viruses, viroids do not have a protein coat (capsid) or an outer lipid envelope.
•   Viroids replicate and spread within plant cells by hijacking the host cell's RNA polymerase machinery for transcription and replication.
•   Viroid diseases in plants can lead to stunted growth, abnormal development, and reduced crop yields.

3. Prions:

•     Prions are infectious proteins that cause neurodegenerative diseases in animals, including humans.
•     Unlike viruses and viroids, prions do not contain nucleic acids (DNA or RNA). Instead, they consist solely of abnormal, misfolded proteins.
•    Prions propagate by inducing normal, properly folded proteins to adopt the abnormal conformation, leading to the formation of aggregates and neuronal damage.
•    Diseases caused by prions include Creutzfeldt-Jakob disease (CJD), variant Creutzfeldt-Jakob disease (vCJD), and bovine spongiform encephalopathy (BSE, or "mad cow disease").

While all three are infectious agents, they differ in their composition, replication mechanisms, and the types of organisms they infect.

Size of Viruses are so small , we can't see through normal microscope, it can only be determined by:

1. Electron microscopy
2. Ultra-filtration
3. Ultracentrifugation

 Largest virus is the Pox virus (300 nm) and smallest is the Parvovirus( 20 nm).

4.Genome :

The genome in viruses refers to the genetic material can either be DNA or RNA. 

1. DNA Viruses:

•     DNA viruses have genomes made of double-stranded DNA (dsDNA) or single-stranded DNA (ssDNA).
•     Examples include herpesviruses (dsDNA), adenoviruses (dsDNA), papillomaviruses (dsDNA), and parvoviruses (ssDNA).
•     DNA viruses typically replicate in the host cell nucleus, using the host cell's DNA replication machinery.

2. RNA Viruses:

•    RNA viruses have genomes made of single-stranded RNA (ssRNA) or double-stranded RNA (dsRNA).
•    Examples include influenza viruses (ssRNA), HIV (ssRNA), hepatitis C virus (ssRNA), and rotaviruses (dsRNA).
•    RNA viruses may replicate in the host cell cytoplasm or, in the case of retroviruses, undergo reverse transcription to produce DNA intermediates that integrate into the host genome.

In conclusion, viruses are a diverse and fascinating group of infectious agents with significant impacts on human health, so it is very important to know their structures, types, and processes.

That's it for viruses. I have tried to discuss as many possible topics related to viruses in this blog, but if you want to know more or discuss them,  please contact me on the Contact Us page.  

Thank you