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Types of viruses

There are many types of viruses. They are categorised according to:

  • what type of organism they infect
  • which structural components they possess (e.g. presence of envelope and type of genome).

The table shows some typical characteristics of different types of viruses, although there are many exceptions:

Type Bacteriophage Animal virus Plant virus
Host Bacteria Animals Plants
Genome DNA/RNA RNA RNA
Shape Complex Sphere Rod
Envelope? None Present None
Examples T4 phage Tobacco mosaic H1N1 ('flu)

Phages tend to have a complex structure - they have both a polyhedral head and a hollow rod shaped tail. Some phages do not have this structure.

Viruses tend to only be highly specific. It is very unusual for a virus to be able to infect significantly different types of organism. Very few viruses are able to infect both plants and animals.

A bacteriophage or phage is any virus that can infect bacteria.

A typical bacteriophage has a double-stranded genome enclosed in a protein coat, and lacks a viral envelope.

They are between $$25$$ and $$200\text{ nm}$$ (nanometres) long.

The protein coat of a phage is made up of two distinct structures:

  • A polyhedral head which encloses the genetic material.
  • A helical tail which the genetic material travels down in order to be injected into the host cell.

Phages may also have tail fibres that allow them to bind to the host cell membrane.

One of the best-characterised phages is the T4 phage, which infects E. coli.

T4 bacteriophage
T4 bacteriophage

The tobacco mosaic virus is an RNA virus that infects the tobacco plant (N. tabacum) and closely related species. It is one of the best-studied plant viruses.

The virus causes patchy discolouration on infected leaves.

The tobacco virus is a single-stranded positive sense RNA virus and lacks a lipid envelope, like most plant viruses.

Its capsid (protein coat) is rod shaped and made up of repeating protein subunits that form a helical pattern around the genetic material.

Helical structure of the tobacco mosaic virus
Helical structure of the tobacco mosaic virus

The influenza virus is a disease-causing negative RNA virus.

Influenza has a spherical shape. It infects epithelial cells in the respiratory tract (cells on the surface of the nose, throat and lungs).

Influenza is unusual because it has a segmented genome.

The virus is encased by a lipid envelope. There are two important glycoproteins on the envelope:

  • Haemagglutinin (HA) mediates the binding and entry of the virus into the host cell.
  • Neuraminidase (NA) mediates the release of the replicated viral cells from the host cell.

Two factors account for symptoms of the influenza disease: damage to the respiratory tract due to the destruction of cells, and inflammation as the immune system tries to combat the virus.

Retroviruses are positive, single-stranded RNA viruses. They are characterised by their replication mechanism.

Rather than translating their genomes directly into proteins, retroviruses each use a reverse transcriptase enzyme, which converts the RNA genome into DNA.

The viral DNA is then inserted into the host genome, and replicates every time the host genome is replicated.

Retroviruses include the human immunodeficiency virus (HIV).

These HIV retroviruses are seen under a scanning electron microscope. Viruses are too small to be seen clearly, even under this powerful magnifying aid.
These HIV retroviruses are seen under a scanning electron microscope. Viruses are too small to be seen clearly, even under this powerful magnifying aid.

The human immunodeficiency virus (HIV) is a retrovirus.

It operates by the following mechanism:

  1. HIV binds to the CD4 receptor on the membrane of a T cell. This allows HIV to fuse to the cell and enter.
  2. Reverse transcriptase converts the single stranded RNA genome into a double stranded DNA genome.
  3. An enzyme called integrase inserts the HIV DNA into the host genome. The HIV replicates every time the cell duplicates. This type of lifecycle is called the lysogenic cycle.
  4. Under certain conditions, the HIV genome will be translated and transcribed, creating new viruses that can be released to infect new cells. When this happens, the virus is in a lytic cycle.
HIV is shown budding out of a human immune cell, which the virus infects and uses to replicate.
HIV is shown budding out of a human immune cell, which the virus infects and uses to replicate.