Virus; characteristics, structure and life cycle

What is virus?

Typical illustration of virus


In 1852, the Russian botanist D.I Ivanovskyprepared an infectious extract from tobacco plants that were passed through afilter able to prevent the passage of bacteria, the filtered fluid was stillinfectious. In 1898 the Dutchman Beijerink coined the name ‘virus’ (Latin forpoison) to describe the infectious nature of certain filtered plant fluids.Although progress was made in isolating highly purified samples of viruses andin identifying them chemically as nucleoproteins (nucleic acids combined withproteins), the particles still proved elusive and mysterious because they weretoo small to be seen with the light microscope. As a result, they were amongthe first biological structures to be studied when the electron microscope wasdeveloped in the 1930s.

Characteristics of viruses.

Viruses have the following characteristics.

  • They are the smallest livingorganisms.
  • They do not have a cellularstructure.
  • They can only reproduce byinvading living cells. Therefore they are all parasitic. They are obligate endoparasites, meaning that they can only live parasitically insideother cells. Most cause disease.
  • They have a simple structure,consisting of a small piece of nucleic acid, either DNA or RNA, surrounded by aprotein or lipoprotein coat.
  • They are on the boundarybetween what we regard as living and non-living.
  • Each type of virus willrecognise and infect only certain types of cell. In other words, viruses arehighly specific to their hosts.

These characteristics will now be examinedin more detail.

Size of virus

Viruses are the smallest living organisms,ranging in size from about 20 – 300nm; on average they are about 50 timessmaller than bacteria. They cannot be seen with the light microscope and theypass through filters which retain bacteria.

Origin of viruses

The question is often posed, ‘Are viruses living?’. If, to be definedas living, a structure must possess genetic material (DNA or RNA), and becapable 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 arenot living. It should also be noted that viruses are not capable of reproducingoutside the host cell.

We can understand viruses much better if weunderstand their evolutionary origins. It is suspected,  though not proven, that viruses are pieces ofgenetic material that have ‘escaped’ from prokaryote and eukaryote cells andhave the potential to replicate themselves when they get back into a cellenvironment. A virus survives in a purely inert state outside cells, but hasthe set of instructions (genetic code) necessary to re-enter a particular typeof cell and instruct it to make many identical copies of itself. It istherefore reasonable to suppose that viruses must have evolved after cellsevolved.

Structure of virus.

structure of virus
virus structure

Viruses have a very simple structureconsisting of the following:

  • Core – the genetic material, either DNAor RNA may be single-stranded or double-stranded.
  • Capsid – a protective coat of proteinsurrounding the core.
  • Nucleocapsid – the combined structureformed by the core and capsid.
  • Envelope – a few viruses, such as theHIV and influenza viruses, have an additional lipoprotein layer around thecapsid derived from the cell surface membrane of the host cell.
  • Capsomeres – capsids are often built upof identical repeating subunits called capsomeres.

The overall form of capsid is highlysymmetrical and the virus can be crystallised, enabling information about itsstructure to be obtained by X-ray crystallography as well as electronmicroscopy. Once the subunits of a virus have been made by the host, they canself-assemble into a virus.

Certain types of symmetry are common amongcapsids, notably polyhedral and helical symmetry. A polyhedron is a many-sidedfigure. The most common polyhedral form in viruses is the icosahedron, whichhas 20 triangular faces with 12 corners and 30 edges. The herpes virus has 162capsomeres arranged into an icosahedron.

Helical symmetry is well illustrated by thetobacco mosaic viru (TMV), and RNA virus. Here the capsid is made up of 2130identical protein capsomeres. TMV was the first virus to be isolated in a purestate. It causes a mottled yellowing of leaves called leaf mosaic in tobacco,tomato and many other plants. The virus can spread extremely rapidly, eithermechanically if infected plants, or plant parts, come into contact with healthyplants, or even as airborne particles such as the smoke of cigarettes made fromcontaminated leaves.

Viruses that attack bacteria form a groupcalled bacteriophages, or simply phages. Some of these have a distincticosahedral head, with a tail showing helical symmetry.

Life cycle of a bacteriophage

The life cycle of a typical bacteriophageis described as follows. E. coli is atypical host and can be attacked by at least seven strains of phage, known asT1 to T7.

The life cycle is the same in principle forall 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 formany generations. Each time the host cell divides the phage DNA is copied withthe host cell DNA. This dormant stage of the phage is called the prophage.  Eventually it is activated again and completesits life cycle, causing death of the host cell in the usual way. Such phagesare described as lysogenic.

There are seven stages in the life cycle ofbacteriophages as outlined below:

  1. Phages approaches the bacteriumand tail fibres fit into receptor sites on bacterial cell surface.
  2. Tail fibres bend to anchor thepins and baseplate to the cell surface; tail sheath contracts, forcing hollowspike into cell; enzyme lysozyme in baseplate aids process; DNA thus injectedinto cell.
  3. Phage DNA codes for productionof phage enzymes, using protein-synthesising machinery (ribosomes etc.) ofhost.
  4. Phage inactivates host DNA andphage enzyme breaks it down; phage DNA takes over cell machinery.
  5. Phage DNA replicates itself andcodes or new coat proteins.
  6. New phage particles made byspontaneous assembly of protein coats around phage DNA; lysozyme is made byphage DNA.
  7. Cell lysis, i.e. bursting,assisted by action of lysozyme; about 200-1000 phages released; phages infectfurther bacteria.

1-7; time taken is 30 minutes; this is calledthe latent period.

Viruses as agents of disease

Viruses can also infect eukaryotic cellsand, as in prokaryotic cells, each has its own specific host. For example, TMVwill attack only tobacco plants. Between them, viruses cause a wide range ofdiseases among plants animals and fungi. Diseases of humans caused by virusesinclude measles, German measles (rubella), chickenpox, influenza, herpes andAIDS.

Viruses cause many different diseases inalmost 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 firstcases being reported in the United States in 1981. The virus which causes it isHIV, or human immunodeficiency virus. This is also of interest because itbelongs to group of RNA viruses known as retroviruses.This name comes the fact that these viruses can convert their RNA back intoa DNA copy using an enzyme known as reversetranscriptase. Normally a section of DNA (a gene) is copied to make RNA, aprocess called transcription. MakingDNA from RNA is therefore reverse transcription, and the enzyme controlling itis called reverse transcriptase. The enzyme has proved extremely useful ingenetic engineering.

The cone-shaped capsid of the retrovirus ismade of a helical spiral of capsomeres. It is cut open to reveal the two copiesof the RNA genetic code. Reverse transcriptase is an enzyme which convertssingle-stranded RNA into double-stranded DNA copies. The capsid is enclosed ina protein shell which is anchored in a lipid bilayer, or envelope, obtainedfrom the cell surface membrane of the previous host cell. This envelopecontains 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 cycleof HIV virus.

  1. Virus approaches a T4lymphocyte cell
  2. Virus glycoprotein attaches toa specific receptor protein in the cell surface membrane.
  3. Virus enters the cell byendocytosis.
  4. The viral RNA is released intothe cytoplasm of the host cell, together with the enzyme reverse transcriptase.
  5. A double-stranded DNA copy ofthe single-stranded virus RNA is made using reverse transcriptase.
  6. The DNA copy enters the nucleusand inserts itself into the host DNA. Whenever the cell divides, it also makesa copy of the viral DNA, increasing the number of infected cells.
  7. After a period of inactivityknown as the latency period, whichlasts on average of 5 years, the virus becomes active again. The stimulus forconverting a latent virus into an active virus is poorly understood.
  8. New RNA is produced(transcription) and viral proteins are made using the host’s proteinsynthesising machinery.
  9. New viral particles assemble.
  10. Virus particles bud off fromthe cell surface membrane of the host by exocytosis.
  11. The cell eventually dies as aresult of the infection.

The characteristics, structure and lifecycles of some common viruses have been discussed in this article; do ensure touse our search box should you have any question.

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