In 1852, the Russian botanist D.I Ivanovsky prepared an infectious extract from tobacco plants that were passed through a filter able to prevent the passage of bacteria, the filtered fluid was still infectious. In 1898 the Dutchman Beijerink coined the name ‘virus’ (Latin for poison) to describe the infectious nature of certain filtered plant fluids. Although progress was made in isolating highly purified samples of viruses and in identifying them chemically as nucleoproteins (nucleic acids combined with proteins), the particles still proved elusive and mysterious because they were too small to be seen with the light microscope. As a result, they were among the first biological structures to be studied when the electron microscope was developed in the 1930s.
Characteristics of viruses.
Viruses have the following characteristics.
- They are the smallest living organisms.
- They do not have a cellular structure.
- They can only reproduce by invading living cells. Therefore they are all parasitic. They are obligate endoparasites, meaning that they can only live parasitically inside other cells. Most cause disease.
- They have a simple structure, consisting of a small piece of nucleic acid, either DNA or RNA, surrounded by a protein or lipoprotein coat.
- They are on the boundary between what we regard as living and non-living.
- Each type of virus will recognise and infect only certain types of cell. In other words, viruses are highly specific to their hosts.
These characteristics will now be examined in more detail.
Size of virus
Viruses are the smallest living organisms, ranging in size from about 20 – 300nm; on average they are about 50 times smaller than bacteria. They cannot be seen with the light microscope and they pass through filters which retain bacteria.
Origin of viruses
The question is often posed, ‘Are viruses living?’. If, to be defined as living, a structure must possess genetic material (DNA or RNA), and be capable of reproducing itself, then the answer must be that viruses are living. If to be living demands a cellular structure then the answer is that they are not living. It should also be noted that viruses are not capable of reproducing outside the host cell.
We can understand viruses much better if we understand their evolutionary origins. It is suspected, though not proven, that viruses are pieces of genetic material that have ‘escaped’ from prokaryote and eukaryote cells and have the potential to replicate themselves when they get back into a cell environment. A virus survives in a purely inert state outside cells, but has the set of instructions (genetic code) necessary to re-enter a particular type of cell and instruct it to make many identical copies of itself. It is therefore reasonable to suppose that viruses must have evolved after cells evolved.
Structure of virus.
Viruses have a very simple structure consisting of the following:
- Core – the genetic material, either DNA or RNA may be single-stranded or double-stranded.
- Capsid – a protective coat of protein surrounding the core.
- Nucleocapsid – the combined structure formed by the core and capsid.
- Envelope – a few viruses, such as the HIV and influenza viruses, have an additional lipoprotein layer around the capsid derived from the cell surface membrane of the host cell.
- Capsomeres – capsids are often built up of identical repeating subunits called capsomeres.
The overall form of capsid is highly symmetrical and the virus can be crystallised, enabling information about its structure to be obtained by X-ray crystallography as well as electron microscopy. Once the subunits of a virus have been made by the host, they can self-assemble into a virus.
Certain types of symmetry are common among capsids, notably polyhedral and helical symmetry. A polyhedron is a many-sided figure. The most common polyhedral form in viruses is the icosahedron, which has 20 triangular faces with 12 corners and 30 edges. The herpes virus has 162 capsomeres arranged into an icosahedron.
Helical symmetry is well illustrated by the tobacco mosaic viru (TMV), and RNA virus. Here the capsid is made up of 2130 identical protein capsomeres. TMV was the first virus to be isolated in a pure state. It causes a mottled yellowing of leaves called leaf mosaic in tobacco, tomato and many other plants. The virus can spread extremely rapidly, either mechanically if infected plants, or plant parts, come into contact with healthy plants, or even as airborne particles such as the smoke of cigarettes made from contaminated leaves.
Viruses that attack bacteria form a group called bacteriophages, or simply phages. Some of these have a distinct icosahedral head, with a tail showing helical symmetry.
Life cycle of a bacteriophage
The life cycle of a typical bacteriophage is described as follows. E. coli is a typical host and can be attacked by at least seven strains of phage, known as T1 to T7.
The life cycle is the same in principle for all phages. Some complete the life cycle without a break. Such life cycles are called lytic cycles. However, some phages, such as lamda phage, insert their DNA into the host DNA and remain dormant for many generations. Each time the host cell divides the phage DNA is copied with the host cell DNA. This dormant stage of the phage is called the prophage. Eventually it is activated again and completes its life cycle, causing death of the host cell in the usual way. Such phages are described as lysogenic.
There are seven stages in the life cycle of bacteriophages as outlined below:
- Phages approaches the bacterium and tail fibres fit into receptor sites on bacterial cell surface.
- Tail fibres bend to anchor the pins and baseplate to the cell surface; tail sheath contracts, forcing hollow spike into cell; enzyme lysozyme in baseplate aids process; DNA thus injected into cell.
- Phage DNA codes for production of phage enzymes, using protein-synthesising machinery (ribosomes etc.) of host.
- Phage inactivates host DNA and phage enzyme breaks it down; phage DNA takes over cell machinery.
- Phage DNA replicates itself and codes or new coat proteins.
- New phage particles made by spontaneous assembly of protein coats around phage DNA; lysozyme is made by phage DNA.
- Cell lysis, i.e. bursting, assisted by action of lysozyme; about 200-1000 phages released; phages infect further bacteria.
1-7; time taken is 30 minutes; this is called the latent period.
Viruses as agents of disease
Viruses can also infect eukaryotic cells and, as in prokaryotic cells, each has its own specific host. For example, TMV will attack only tobacco plants. Between them, viruses cause a wide range of diseases among plants animals and fungi. Diseases of humans caused by viruses include measles, German measles (rubella), chickenpox, influenza, herpes and AIDS.
Viruses cause many different diseases in almost every other kind of organism.
Structure and life cycle of a retrovirus, HIV
AIDS (acquired immune deficiency syndrome) is of particular interest because it is a relatively new disease, the first cases being reported in the United States in 1981. The virus which causes it is HIV, or human immunodeficiency virus. This is also of interest because it belongs to group of RNA viruses known as retroviruses. This name comes the fact that these viruses can convert their RNA back into a DNA copy using an enzyme known as reverse transcriptase. Normally a section of DNA (a gene) is copied to make RNA, a process called transcription. Making DNA from RNA is therefore reverse transcription, and the enzyme controlling it is called reverse transcriptase. The enzyme has proved extremely useful in genetic engineering.
The cone-shaped capsid of the retrovirus is made of a helical spiral of capsomeres. It is cut open to reveal the two copies of the RNA genetic code. Reverse transcriptase is an enzyme which converts single-stranded RNA into double-stranded DNA copies. The capsid is enclosed in a protein shell which is anchored in a lipid bilayer, or envelope, obtained from the cell surface membrane of the previous host cell. This envelope contains viral glycoproteins which bind specifically to helper T-cell receptors, enabling the virus to enter its host.
There are about 11 stages in the life cycle of HIV virus.
- Virus approaches a T4 lymphocyte cell
- Virus glycoprotein attaches to a specific receptor protein in the cell surface membrane.
- Virus enters the cell by endocytosis.
- The viral RNA is released into the cytoplasm of the host cell, together with the enzyme reverse transcriptase.
- A double-stranded DNA copy of the single-stranded virus RNA is made using reverse transcriptase.
- The DNA copy enters the nucleus and inserts itself into the host DNA. Whenever the cell divides, it also makes a copy of the viral DNA, increasing the number of infected cells.
- After a period of inactivity known as the latency period, which lasts on average of 5 years, the virus becomes active again. The stimulus for converting a latent virus into an active virus is poorly understood.
- New RNA is produced (transcription) and viral proteins are made using the host’s protein synthesising machinery.
- New viral particles assemble.
- Virus particles bud off from the cell surface membrane of the host by exocytosis.
- The cell eventually dies as a result of the infection.
The characteristics, structure and life cycles of some common viruses have been discussed in this article; do ensure to use our search box should you have any question.