Biological characteristics, classification, and hazards of viruses
What is virus?
A virus is a non-cellular life form that contains only one nucleic acid (DNA or RNA) and must be parasitic in living cells. Its replication, transcription and translation capabilities are carried out in the host cell, and when it enters the host cell, it completes its own life activities by using the materials and energy in the host cell, and replicates to produce a new generation of viruses similar to itself according to the genetic information contained in its own nucleic acid.
Characteristics of viruses
Parasitism: Viruses are parasitic microorganisms that must live inside the host cell to survive, viruses do not have the ability to reproduce on their own, and they use the metabolic and biological mechanisms of the host cell to replicate their own genetic material.
Lack of cell structure: Cells have cell structures such as cell membranes, cytoplasm, and nucleus, which can metabolize, grow, and reproduce independently, while viruses themselves do not have a cell structure and need to parasitize inside cells to replicate and survive.
Small particles: Viruses are very tiny particles, usually between 20 and 300 nanometers, and are very small in size, which makes them invisible under a light microscope and requires an electron microscope to see the morphology and details.
Infectious: Once the virus enters the host cell, it releases its genetic material (DNA or RNA) and other protein components, and the genetic material uses the host cell's metabolic mechanisms to replicate itself and synthesize a new virus.
Genetic variation: Due to the fact that their genetic material is prone to errors during replication and the high variability of the viral genome, viruses can produce different variants, which is one of the reasons why viruses can evade the immune system.
Virus structure
A virus is a tiny infectious microorganism with a relatively simple structure and usually consists of the following main parts:
Protein shell, protein shell is a shell composed of protein molecules, which encapsulates and protects the genetic material of the virus, it provides the structural support and stability of the virus, and protects the nucleic acid of the virus from external damage.
Nucleic acids, nucleic acids refer to the genetic material carried by viruses that are used to store the genetic information of the virus and direct its replication and infection of host cells, viral nucleic acids can be deoxynucleic acid (DNA) or ribonucleic acid (RNA).
Surface proteins, some viruses have surface proteins on the outer shell that can bind to the surface of the host cell and mediate the entry of the virus into the host cell, and the surface proteins have an infectious and immune escape effect on the virus.
Life cycle of virus
When a virus parasitizes a cell, it usually goes through four steps: adsorption and induction, uncoating, biosynthesis, assembly and expulsion.
Adsorption and cell entry: When the virus enters the alveoli through the respiratory tract, the virus binds specifically to the door locks of some cells, such as the ACE2 receptor, through the spike protein as a key, thus cleverly deceiving the cell, opening the portal, and at the same time quickly passing through the lipid bilayer through the lipid envelope and entering the host cell.
Compared to other organs, human respiratory cilia cells and alveolar epithelial cells have higher levels of ACE2 protein, which explains why the coronavirus chooses the lungs to attack first.
Uncoating: After the virus enters the cell, the virus will attract the cell factory pickets-lysosomes to inactivate it through its disguised coat, the protein nucleocapsid, which is called uncoating.
As a result, the virus's genetic material, RNA, is completely exposed inside the cell. Of course, it doesn't stop there.
Synthesis: When the genetic material RNA has invaded the synthesis workshop of the cell, the ribosome, the virus finally begins its wanton multiplication, a step in synthesis that focuses on reproducing the important accessories needed for the virus, and thus maps again to our previous narrative about the structure of the virus
First, unidirectional positive-stranded RNA directs the synthesis of a large number of virus-associated proteins, including nucleocapsid proteins. The unidirectional positive-strand RNA then transcribes itself as a template into negative-strand RNA, which in turn produces a new progeny positive-strand RNA. This process repeats itself to lead to a large amount of new RNA. Then, after the negative-strand RNA is selectively converted into a fragment of positive-stranded RNA, various small proteins are specifically synthesized.
Assembly & Cell: So now we have three main accessories and we are ready to complete the core assembly. Some theories suggest that the lipid envelope is mainly obtained by converting the lipid membrane components into cells when they pass through the endoplasmic reticulum or cell membrane.
Fig.1 Overview of the viral cleavage life cycle. (Munke Anna, 2020)
Classification of viruses
In terms of classification, there are classifications from genetic material: DNA viruses, RNA viruses, protein viruses. There are also classifications from host type: bacteriophages (bacterial viruses), plant viruses (such as tobacco mosaic virus), animal viruses (such as influenza virus, smallpox virus, HIV, etc.).
DNA viruses
DNA virus, also known as deoxynucleotide virus, that is, viral nucleic acid is a biological virus of DNA, which belongs to the first-order virus. DNA viruses are widely found in humans, vertebrates, insects, and a variety of passage cell lines, with each virus infecting only one animal (with a few exceptions) and only a few causing disease. Common DNA viruses are hepatitis B virus, smallpox virus, etc.
RNA viruses
RNA viruses are a class I virus. Their genetic material is RNA ribonucleic acid. The nucleic acids are usually single-stranded RNA (ssRNA) and double-stranded RNA (dsRNA).
RNA viruses have two replication methods: self-replication and reverse transcription, and in the process of viral RNA replication, the activity of enzymes in its error repair mechanism is very low, almost none, so its mutation is very fast. Vaccines are developed based on the fixed genes or proteins of the virus, so RNA virus vaccines are more difficult to develop. Compared with DNA viruses, RNA viruses are more likely to cause diseases, more lethal to hosts, and more prone to mutations, so there are more types, more difficult to develop effective vaccines, and difficult to prevent. RNA viruses are generally less resistant than DNA viruses and are easier to cure. However, there are exceptions, such as double-stranded RNA viruses that are very resistant, and it is extremely difficult to cure retroviruses. Common RNA viruses include HIV, SARS, all influenza viruses, and the novel coronavirus (2019-nCoV).
Prions
Prions, also known as prions, protein infecting factors, virions, or infectious proteins, are a class of hydrophobic proteins that can infect animals and are not immune in host cells. Prion is the old name for protein, prion means protein virus, prion virus is not strictly speaking virus, it is a class of infectious factors that do not contain nucleic acids and are composed only of proteins.
The harms of viruses
Viruses can enter the human body through airborne droplets (such as coughing, sneezing), food or water, blood or body fluids, insect bites, etc., parasitic in host cells, and then replicate and infect, directly destroying host cells, resulting in tissue and organ damage, and with the expansion of the scope of infection, it will cause a series of physical diseases. Viruses are the main cause of disease, and they can cause various infectious diseases such as respiratory tract infections, intestinal infections, skin infections, nervous system infections, leading to organ failure and even death.
Reference
- Munke, Anna. Small Particles with Big Impact: Structural Studies of Viruses and Toxicological Studies of Nanodiamonds. Diss. Acta Universitatis Upsaliensis 2020.
※ Please kindly note that our services are for research use only.
