How do viruses differ in their structure?
The many viruses can be divided into different groups according to their structure. An important criterion for classification is the type of nucleic acid. Some viruses encode their genetic genome using DNA, others use RNA for this purpose.
With regard to the genome, further classification criteria can be determined. A distinction is made between single-stranded and double-stranded nucleic acids. In addition, it can be straight (linear) or circular (circular) in the virus.
The genetic material of a virus does not necessarily have to be present as a whole, but can also be divided into fragments. In this case one speaks of a virus with a segmented nucleic acid. In addition to DNA and RNA viruses, there are viruses that use a reverse transcriptase.
This enzyme is so special that such viruses are again understood as a separate group. These viruses are able to transcribe their RNA into DNA and integrate it into the DNA of the host cell. The capsid, i.e. the envelope of the genome of structural proteins, can also be present in different forms.
These range from a helical shape to a cubic structure to an icosahedral envelope of the nucleic acid. A further and conspicuous distinguishing feature of viruses is the presence or absence of a fat envelope (lipid envelope). This surrounds the nucleocapsid, so that one speaks of enveloped or naked viruses.Known viruses that have a fat cover are for example the herpes virus and the HI virus.
The structure of known viruses
The HI virus (“HIV”, Human Immunodeficiency Virus) belongs to the retroviruses and can be classified as a lentivirus. The HI-Virus is about 100nm in size and thus belongs to the larger viruses. The genome of the HI-Virus consists of two single-stranded RNAs, which can be transcribed into DNA by reverse transcriptase.
Inside the capsid, there is intergrass as well as RNA, the reverse transcriptases. With the help of these enzymes, the genetic information transcribed into DNA can be incorporated into the DNA of the host cell and thus be distributed throughout the organism. In addition, proteases that are involved in the formation of new infectious viruses are found throughout the virus.
Due to the presence of a double lipid layer, it is an enveloped virus. Various surface proteins are embedded in this fat cover. In addition, under the electron microscope, approximately 10 to 15 projections per HI virus can be seen protruding from the fat envelope.
These so-called spikes are protein complexes that play an important role within the infection path. With the help of the spikes, the HI virus recognizes the target cells that form all CD4 receptors. The target cells of the HIV virus include in particular T-helper cells (part of the acquired immune system), which weakens the immune system of the infected patient.
The influenza virus can cause the “real” flu (influenza), which has much stronger symptoms than the common cold. There are several types of influenza virus, which differ in their structure in small details. However, all influenza viruses have the same basic structure.
The influenza virus
- Is about 100nm in size,
- Belongs to the RNA viruses,
- Has eight single-stranded RNA strands as genetic material, which are often present as fragments,
- Is surrounded by a fatty envelope, so that one speaks of an enveloped RNA virus and
- Contains several enzymes, such as the RNA polymerase complex (responsible for the amplification of the genetic material)
The measles virus is the trigger of the childhood disease measles. This pathogen only affects humans, so that the only source of infection is a sick person. The measles virus is a large virus with 100 to 250nm.
It belongs to the RNA viruses and has a fat (lipid envelope). This enveloped RNA virus can be assigned to the group of paramyxoviruses, which are all transmitted via droplet infection. This means that the pathogen is spread through the air, triggered for example by sneezing, coughing or simply speaking.
Infection with the virus almost always leads to an outbreak of measles. The simplest protection against this disease is offered by vaccination in childhood. This is often offered as a combination vaccination, so that one is protected against measles, rubella and mumps at the same time.
The hepatitis B virus is the trigger of hepatitis B, an inflammation of the liver. This infectious disease is the most common worldwide and can lead to liver cirrhosis or even hepatocellular carcinoma. The virus is an enveloped DNA virus, whereby the genetic material is partially double-stranded.
In addition, the hepatitis B virus, like the HI virus, has a reverse transcriptase. This enzyme transcribes RNA copies of the genetic material into DNA. This viral genetic information is then incorporated into the DNA of the host cell.
Thus, the virus is now located in the infected liver cells, which makes therapy more difficult. Today, vaccination against hepatitis B is possible and recommended for children. If the disease already exists, various substances can be used that are directed against the viruses (antivirals). This treatment, however, brings with it a wide variety of side effects.