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Structure of a virus

Viruses are tiny: most are between $$20$$ and $$300\text{ nm}$$ (nanometres) across.

This is about one thousandth the size of a eukaryotic cell, which is typically $$10-100\text{ }\mu\text{m}$$ (micrometres) across.

This virus causes disease in sheep. It is less than 100nm across.
This virus causes disease in sheep. It is less than 100nm across.

Viruses are very simple in structure and are made up of only two or three components:

  • Genetic material is the hereditary information of a virus. It is a collection of genes and carries all the information needed to allow the virus to replicate in a cell. The genome can be either RNA or DNA.
  • The capsid, or the protein coat, surrounds the genome. It consists of protein subunits called capsomeres linked to make a 3D structure.
  • The viral envelope surrounds some, but not all viruses. It is made up of a modified cell membrane obtained from the host cell as the virus leaves.

    The viral envelope is also known as the lipid envelope because it is formed of lipids.

The envelope and envelope proteins are not present in all viruses.
The envelope and envelope proteins are not present in all viruses.

The viral genome is the hereditary material of a virus.

As in prokaryotes and eukaryotes, the hereditary material is formed from nucleic acids.

Viruses have very diverse genomes. In other words, the differences between the genomes of different virus types are much greater than the differences observed in eukaryotes and prokaryotes.

Viral genomes can range from just two or three genes in length to over a thousand.

The viral genome is contained within a protein coat.
The viral genome is contained within a protein coat.

Viral genomes are either made of DNA or RNA, and can be single-stranded or double-stranded.

The genomes can also be linear, circular or segmented (in several parts).

Single stranded RNA genomes are classified as positive or negative.

  • Positive strands are translated directly into proteins.
  • Negative strands cannot be translated directly. A positive strand must be created through replication of the RNA before proteins can be synthesised.

The type of genome possessed by the virus influences its method of replication within a host cell.

Influenza virus with a segmented RNA genome
Influenza virus with a segmented RNA genome

The capsid is the outer shell of a virus. Its encloses and protects the genome.

The capsid is formed of protein subunits, and so is often called the protein coat. There are two main capsid structures:

  • Helical: the protein subunits are arranged in a spiral shape around the genetic material. This gives the virus a rod shape.

  • Polyhedral or icosahedral: the protein subunits have a regular 3D arrangement of faces. These viruses are often icosahedral, giving the capsid a roughly spherical shape.

Bacteriophages have a polyhedral head and a helical tail. Viruses with both protein coat structures and are called complex viruses.

The tobacco mosaic virus has a helical structure. Plant viruses tend to have helical capsids.
The tobacco mosaic virus has a helical structure. Plant viruses tend to have helical capsids.
Adenovirus has an icosahedral structure. Animal viruses tend to have polyhedral capsids.
Adenovirus has an icosahedral structure. Animal viruses tend to have polyhedral capsids.

The viral envelope is only present in some viruses, primarily those that infect animal cells.

The viral envelope surrounds the protein coat and is made of a portion of the host's cell membrane. Viral envelopes help the virus to enter the host cell.

The viral envelope is also known as the lipid envelope, because it contains lipids.

Glycoproteins (proteins that contain sugar chains) are present on the outside of the envelope. These proteins bind to receptor sites on the host's cell membrane and allow the virus to enter the cell.

The envelope and envelope proteins are not present in all viruses.
The envelope and envelope proteins are not present in all viruses.