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Basic Virology - A Preface

This virology primer is presented to help the beginner understand how viral infections work and how we survive them. Virology is a complicated subject as is immunology and the following is meant as a sort of a kindergarten of terms and events. We think it is helpful in understanding the biology of what happens in this type of infection.

WHAT IS A VIRUS?

Viruses are strange life forms. They have only the bare bones of biochemical equipment (barely enough to even qualify for the title "Life Form.")

• They have only one type of nucleic acid (either DNA or RNA but not both.)
• They reproduce solely from this nucleic acid, whichever type it is.
• They possess no metabolic enzymes.
• They are completely dependent on their host cell.

SOME DEFINITIONS

Viral Particle

One little old individual virus creature is called a "viral particle." Viral particles are so small (some are the size of a large protein molecule) that we cannot really call them "creatures;" they are more like particles, hence the term. The word “virus” is more correctly applied to a species or genus of virus rather than to the individual particles.

Capsid

The nucleic acids (the DNA or RNA) within the viral particles are surrounded by a protein coat, and sometimes an additional fatty envelope. The capsid consists of this protein coat and any fatty layer. The capsid proteins are crucial to the virus' ability to attach and infect a host cell.

Viruses that have fatty envelopes generally do not last long in the environment as the fat is easily disrupted; these viruses tend to require direct contact with fresh body secretions for transmission of infection.

DNA

DNA is the double stranded nucleic acid (deoxyribonucleic acid) that serves as the blueprint for all proteins a cell can make. It enables the cell to live and function within a body. It essentially amounts to instructions on how to make different proteins.

Messenger RNA

When it comes time to make a protein, a DNA segment unzips its double strands, allowing messenger RNA to enter and bind. The messenger RNA forms a "negative" image of the DNA segment it is “reading.” The messenger RNA then leaves the cell nucleus and travels out into the cell's main body, where its message is read by cell structures called ribosomes.

Transfer RNA is another type of RNA that floats around the cell. Each piece of transfer RNA has only three base codes (as opposed to the messenger RNA which has many base codes and is a long strand. The piece of transfer RNA carries with it an amino acid that its code corresponds to.

The ribosome looks like two round pieces attached together. The two pieces grab the strand of messenger RNA and sandwich it between them. The ribosome reads the first three codes on the messenger RNA and matches them to a piece of transfer RNA. It moves to the next three codes and matches them as well. With each match the transfer RNA leaves its amino acid for the ribosome to bind to the next amino acid so as the ribosome moves down the strand of messenger RNA, it creates a strand of amino acids based on the matching. As we know, a strand of amino acids forms a protein molecule. This is how our cells make proteins.

Transcription

The act of messenger RNA taking down the protein code from the original DNA segment is called "Transcription."

Want more detail on DNA Transcription? Watch this video:

(Video credit: ppornelubio via YouTube.com)

Translation

The act of producing a protein from the segment of messenger RNA using transfer RNA is called "Translation." It is performed by cell organs called "ribosomes."

Want more detail on Protein Translation? watch this video:

(Video Credit: DNA Learning Center via YouTube.com)

VIRAL INFECTIONS

The viral particle's goal is to attach to a host cell and somehow inject its nucleic acids (DNA or RNA) inside the cell. There are several techniques that viruses use to accomplish this but all involve the capsid. The capsid is very specific about how it binds to a host cell. Not only must the host species be right but the type of cell it binds to must be right. When the match is correct, the virus binds to the cell and injects its nucleic acids inside. Once the viral particle has injected its nucleic acids into the host cell, the next activity is to manufacture messenger RNA for the cell to translate into protein. The proteins which the viral nucleic acids make will shut down the cell’s normal function and convert the cell into a factory for viral particle production.

• Some viruses simply contain ready made messenger RNA and they just inject it into the host cell (Polio does this).
• Some viruses inject a "negative" of the messenger RNA they need and include with it an enzyme that will trick the host into making the usable messenger RNA from the negative (Rabies does this).
• Some viruses inject their DNA directly into the host cell. Messenger RNA is made from this DNA, just as it would be made from host DNA (Poxviruses do this).

Regardless of the tricks the virus uses to make messenger RNA, once the messenger RNA is made, the host cell is doomed. Ribosomes line up on the strand of messenger RNA reading it and using transfer RNA to mass produce the protein coded. This will be a viral protein and its action will be to shut down normal cell function and dedicate the cell to the production of viral capsid and viral DNA.

Soon the host cell is little more than a bag of virus. These viral particles either bud off the surface of the host cell or the host cell simply explodes leaving millions of new viral particles to seek new host cells. The only way to stop this process is for the immune system to recognize the infected cell early and destroy it before virus production becomes too advanced.

Want to see it happen? The following video follows a virus (in this case the HIV virus) from attachment to a host cell, injection of its contents (nucleic acids, two enzymes to facilitate sneaking into the host's protein-making machinery, and one enzyme to help make new viral protein), and conversion of the host cell into a virus factory.

(Video credit: Rufus Rajadurai via YouTube.com)

Watching a virus attack a cell may seem very discouraging but there is an entire immune system in the body designed to repel such invasion. Infected cells express viral proteins, shapes which the immune system can recognize. Within the body are several groups of cells all created to respond against a specific shape. For example, B lymphocytes which live in our lymph nodes, transform into antibody-producing plasma cells once they encounter their destined antigen. They produce Y-shaped antibodies which flood the circulation and bind the viral antigen, thus preventing virus from attaching to the host cell, clumping with other antibody-virus groupings to create a larger clump, which in turn attracts a cell called a "macrophage." The macrophage is the body's "pac man," swallowing and digesting debris.

Watch a depiction of a cytotoxic T cell recognize and destroy a virus infected cell.

(Video Credit: Andrew Lilja via YouTube.com)

Aside from antibodies, there are patroling T-lymphocytes which identify viral or other inappropriate shapes on the surfaces of host cells. The T-cell can thus recognize a virally infected cell or even a cancer cell, attach and destroy it before it causes harm.

Hopefully, between T-cell and antibody attack, the viral infection is removed from the body and status quo resumes.

Page last updated: 11/19/2024