Understanding Viruses: Structure, Replication, and Biology

Posted on May 6, 2026 in Biology

The Discovery of Viruses

Dmitri Ivanovski discovered that a disease in tobacco plants was spread by liquid extracted from infected plants.
Martinus Beijerinck named these tiny disease-causing particles “viruses,” the Latin word for poison.
Wendell Stanley showed that viruses can be crystallized, concluding they are not truly alive.
Viruses are defined as nonliving particles made of proteins, nucleic acids, and sometimes lipids.

Virus: A nonliving particle made of proteins, nucleic acids, and sometimes lipids that can reproduce only by infecting living cells.

Viruses vary greatly in size and structure, but most are visible only with an electron microscope.
The genetic material of a virus is protected by a surrounding protein coat.
Viruses use specific surface proteins to “trick” host cells into allowing them entry.
Because they must bind to specific receptors, most viruses only infect a specific range of host cells.

Lytic infection: The virus quickly replicates and causes the host cell to burst (lyse) to release new viruses.
Lysogenic infection: The virus inserts its DNA into the host’s DNA, where it can remain inactive for many generations.
Bacteriophages: Viruses that specifically target and infect bacteria.
External factors like heat or chemicals can trigger a lysogenic virus to enter the lytic cycle and begin destroying cells.

Bacteriophage: Viruses that infect bacteria.
Lytic infection: An infection where a virus enters a cell, makes copies of itself, and causes the cell to burst.
Lysogenic infection: An infection where the host cell is not immediately taken over; the viral nucleic acid is inserted into the host cell’s DNA.
Prophage: Bacteriophage DNA that becomes embedded in the bacterial host’s DNA.

Approximately 70% of viruses contain RNA as their genetic material instead of DNA.
Common cold viruses attack quickly by using the host’s ribosomes to translate viral RNA into proteins.
HIV is classified as a retrovirus because it reverses the usual genetic process by copying RNA into DNA.
Once HIV DNA is integrated into the host’s genome, it can stay dormant before eventually attacking the immune system.

Retrovirus: A group of RNA viruses whose genetic information is copied from RNA to DNA instead of DNA to RNA.

Viruses are considered parasites because they depend entirely on living organisms for existence and reproduction.
Despite having genetic material and the ability to evolve, viruses do not meet all the requirements for life, such as growth or energy use.
It is widely believed that viruses evolved from the genetic material of living cells rather than existing before them.
Viruses must infect living cells to utilize the nutrients and cellular machinery required to reproduce.