Discovery Science: Earth – Microorganisms – The Virus

Earth Science: Microorganisms – The Virus

Viruses are tiny infectious parasites that lack their own metabolism. Therefore, viruses depend on host cells to reproduce and proliferate. Normally, viruses are strictly host-specific and affect either eukaryotes (organism with nucleus) or prokaryotes.

Several viruses cause dangerous diseases, many remaining without a cure.

Earth Science: Biology – Microorganisms – The Virus

Viruses are much smaller than bacteria, so they only become visible with an electron microscope. The smallest are only about 20 nanometers long, the largest are about 500 nanometers (one nanometer = 10^-9 meters).

Viruses occurring outside of cells are called virions and may differ considerably in appearance. Many virions are ball or rod shaped, while others look like small lunar modules (such as the bacteriophage T4). Genetic information is stored in the form of deoxyribonucleic acid (DNA) or ribonucleic acid (RNA). This trait is used for classifying the viruses into two groups: DNA viruses and RNA viruses.

The genome of a virus is enclosed by a coat of protein molecules (capsid), and sometimes additional membranes. Some phages not only have a head filled with nucleic acid, but also complex tails for attaching themselves to bacterial cells and for injecting their genome. Phages are extremely small and require an electron microscope for study. However, their relative sizes to each other varies considerably.


Reproduction of viruses can be divided into two cycles: the lytic cycle and the lysogenic cycle. The former can be found, for example, in T phages. Here, the genome of the virus is injected into the host cell. The protein coat stays behind on the surface of the cell. The viral nucleic acid then synthesizes its own proteins, which often block the activity of the bacterial DNA.

At the same time, the host cell is instructed to replicate viral nucleic acid and produce proteins for a protein coat to form a nucleocapsid. New viruses develop, causing the bacterial cell to burst (lysis) and release the virions, which can then attack new host cells.

The lysogenic cycle differs because the viral genome is temporarily built into the DNA of the host cell. The germs remain there in the form of so-called proviruses, or prophages, and replicate passively together with the cellular genome. The creation of active viruses and cell lyses does not happen until an activation event occurs. For example, a change in temperature could trigger the lytic behavior of viruses.


VIROIDS ARE RNA viruses without an envelope. The smallest self-reproducing molecules, they have only been found in plants such as potatoes and tomatoes