Introduction
Viruses are tiny parasites that are potential pathogens. They are widespread everywhere and can be detected in every cell. Like other parasitic organisms, they need a foreign organism to multiply.
Plants, animals or even humans can be used for this. If viruses attack a weak immune system or weak individuals, such as children, infection can occur. The viruses enter the body through openings such as the mouth, nose, eyes or through sexual intercourse. Nowadays one can protect oneself against some infectious diseases by means of vaccinations and suitable contraceptives. Known infectious diseases that are transmitted via viruses are AIDS (HI-Virus) or measles in children.
How is a virus structured?
A virus is a tiny organism that can only be observed with the help of an electron microscope. The size of viruses is in the nanometer range, but there are larger (Marburg virus with about 1. 000nm) and smaller viruses (poliovirus with a diameter of about 30nm).
Since viruses are obligatory parasites, they are not able to carry out a metabolism and therefore depend on a host cell. Viruses are composed of only a few components. Their genetic material consists of nucleic acid, as in other organisms.
Depending on the virus, they can be assigned to either DNA or RNA. Furthermore, this genetic material can be present in the virus in many different forms. For example, the genetic material can consist of either a single strand or a double strand and be straight or circular in shape.
In total, the nucleic acid can take up to 30% of the total weight of the virus. The genome of the virus is enveloped by structural proteins (capsomeres) that protect the genetic material from environmental influences. In their entirety, these structural proteins are called capsids because they form a kind of capsule around the DNA/RNA.
The complex of capsid and nucleic acid is called a nucleocapsid. Depending on the type of virus, a virus envelope is added. This includes a double envelope of fat (lipid envelope), which is derived from the envelope of the host cell.
If viruses have such a fat envelope, they are called enveloped viruses, the others are the naked viruses. Enveloped viruses are sensitive to fat-soluble substances. Thus, such viruses lose their infectivity when treated with fat-soluble chemicals.
For this reason, naked viruses are often more resistant than enveloped viruses. In addition, glycoproteins can be embedded in this fat cover, which are thus located on the surface of the virus. These are visible under the electron microscope as small projections and are called spikes.
They have the function of attaching themselves to the desired host cell and thus help the virus to penetrate. Some viruses also contain special enzymes. One example is the human immunodeficiency virus (HI virus), which belongs to the retroviruses and has a reverse transcriptase. This enzyme is able to transcribe an RNA into a DNA. The reverse transcriptase is also the site of attack of various substances that are administered as drugs against the infectious disease.