Viral structure and functions

Definitions & Key takeaways

Viruses are a unique type of pathogen that lack cytoplasmic membrane, cytosol, or functional organelles and use the metabolic machinery of host cells to produce more viral molecules. They can exist extracellularly as a virion or intracellularly as nucleic acids that induce the host to synthesize viral components. Viruses come in many shapes and sizes, including helical, icosahedral, and complex. The viral genome can be DNA or RNA, single-stranded or double-stranded, and mutations in RNA viruses occur more frequently than in DNA viruses due to the likelihood of transcription errors by RNA polymerases.

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Viruses are a unique group of pathogens with a simple acellular organization and a distinct pattern of multiplication. Despite their simple structure they are a major cause of disease.
They have no cytoplasmic membrane, cytosol, or functional organelles, but they can infect all types of cells, and numerous viruses can also infect bacteria, which are called bacteriophages.
Viruses and bacteriophages are not capable of metabolic activity on their own, so instead, they invade other cells and use their metabolic machinery to produce more viral molecules, nucleic acid and proteins which then assemble into new viruses.
Viruses can exist either extracellularly or intracellularly. In the extracellular state, the virus is called a virion and isn’t capable of reproducing.
A virion consists of a protein coat, called a capsid, surrounding a nucleic acid core which contains the genetic material or the viral genome.
The nucleic acid and the capsid are collectively called a nucleocapsid. Some virions have a phospholipid membrane derived from the host cell, called an envelope which surrounds the nucleocapsid.
The viruses that have an envelope are called enveloped viruses and these include the herpesviruses and HIV, while the ones that lack the envelope, such as poliovirus, are called non enveloped or naked viruses.
Once inside the cell, the virus enters the intracellular state, where the capsid is removed and the virus becomes active.
In this state the virus exists solely as nucleic acids that induce the host to synthesize viral components from which virions are assembled and eventually released.
Now, the viruses are surrounded by an outer protein coating called the capsid, which protects the viral genome and aids in its transfer between host cells.
Also, according to their capsid symmetry the viruses can come in many shapes and sizes. There are three types of shapes: helical, icosahedral, and complex.
First, the helical viruses have a capsid with a central cavity or a hollow tube which is made by proteins arranged in a circular fashion, creating a disc like shape.
The disc shapes are attached helically, creating a tube with room for the nucleic acid in the middle. An example of a virus with helical symmetry is the tobacco mosaic virus which is the most studied example.
Moving on to the icosahedral viruses which are made up of equilateral triangles fused together in a spherical shape that fully encloses the genetic material.
These viruses are released into the environment when the cell dies, breaks down and lyses, thus releasing the virions. Some examples of icosahedral viruses include poliovirus, rhinovirus and adenovirus.
Finally, the complex viruses have a combination of icosahedral and helical shape and may have a complex outer wall or head-tail morphology.
The head-tail morphology is unique to viruses that infect bacteria, called bacteriophages, and they have an icosahedral shaped head and a helical shaped tail.
These viruses use their tail to attach to the bacterium and then they create a hole in the cell wall and insert their DNA into the cell using the tail as a channel.
Now, the nucleic acid core of the virus contains the genetic material which can be either DNA or RNA, and the total genetic content of the virus is called the viral genome.
The viral genomes are usually small and contain only a few genes that encode for proteins needed by the virus that’s not present in the host cell.
The genetic material can be single-stranded or double-stranded, linear or circular. In DNA viruses, the viral DNA enters the nucleus and uses the host’s DNA polymerase to create more copies of itself as well as viral mRNA that gets translated into viral protein.
With RNA viruses, many of them stay in the cytoplasm where the Viral RNA is used by the hosts ribosomes to create viral RNA polymerase proteins.
These will help replicate the viral genome so more viruses can be created. RNA polymerases are more likely to make copying errors than DNA polymerases, and therefore, often make mistakes during transcription.
This is why mutations in RNA viruses occur more frequently than in DNA viruses. ##Summary All right, as a quick recap.
Viruses are a unique group of infectious agents with a simple acellular organization and a distinct pattern of replication.
They can exist either extracellularly, where the virus is inactive and isn’t capable of reproducing, or intracellularly, where the capsid is removed and the virus becomes active.
A virus in the extracellular state is called a virion and consists of a protein coat called a capsid, which protects the viral genome and aids in its transfer between host cells.
The capsid is surrounding a nucleic acid core which contains the genetic material or the viral genome, which can be either DNA or RNA.
Some virions have a phospholipid membrane derived from the host cell, called an envelope which surrounds the nucleocapsid.
According to their capsid symmetry, the viruses can come in many shapes and sizes and they can have three types of shapes: helical, icosahedral, and complex.