Virus Structure (HL) (HL IB Biology)

Virus structure

• Viruses are non-cellular infectious particles; they are not organisms as they are not considered to be alive
• Viruses possess none of the characteristic features used for classifying organisms so they sit outside of the three-domain classification system
• They are relatively simple in structure and much smaller than prokaryotic cells, with diameters between 20 and 300 nm
  • They can only be seen with an electron microscope
• They have no cellular structures and so no metabolism so they are considered to be acellular
• Structural features common to all viruses include
  • A small size
    • Viruses contain few molecules, so do not form large structures
  • A fixed size
    • Viruses do not grow
  • A nucleic acid core
    • Their genomes are made up of either DNA or RNA
    • Nucleic acids in viruses can be single or double-stranded
    • Nucleic acids can have a linear or circular structure
  • A protein coat called a ‘capsid’
    • Attachment proteins are present on the outer surface of a capsid that allow viruses to bind to and enter host cells
  • No cytoplasm
  • Very few, or no, enzymes
• Some viruses have an additional outer layer called a lipid envelope, formed usually from the membrane-phospholipids of a cell they were made in
  • Lipid envelope structures can be involved with cell recognition
• All viruses are parasitic in that they can only reproduce by infecting living cells and using their protein-building machinery (ribosomes) to produce new viral particles
  • The energy that viruses need for replication is released by the host cell; viruses do not respire

General virus structure diagram

Virus structure can vary, but all viruses have genetic material and a protein capsid with attachment proteins

Diversity of structure in viruses

• Although simple, there is huge variety and diversity of virus structure and shape:
  • Genetic material can be either RNA or DNA which can either be double or single stranded
  • Some viruses are enveloped, others are not
  • Viral shapes can be threadlike, polyhedral and spherical
• Each type of virus is able to attach to and infect a specific type of host cell; the host cell to which it can attach is determined by the attachment proteins, e.g.
  • HIV infects white blood cells
  • Hepatitis infects liver cells

Virus structure variety diagram

Virus structure varies widely

• Examples of viruses that have different structures are:
  • Bacteriophage lambda
  • Coronaviruses
  • HIV

Bacteriophage lambda

• This is a bacterial virus and it infects the bacterial species Escherichia coli (E.coli)
• It has a double stranded DNA genome contained within its capsid head
• The tail and fibrils enable it to attach itself to its host and insert its DNA into the cell
  • The tail consists of proteins that contract, allowing the virus to move the tail through the bacterial cell wall 
  • DNA from the virus is injected into the host cell through the tail

Bacteriophage lambda structure diagram

A bacteriophage virus

Coronaviruses

• Coronaviruses are a group of viruses that cause respiratory diseases in mammals and birds
  • They can be transmitted via respiratory fluids
• Their structure includes:
  • Single stranded RNA
  • A spherical shape
  • An envelope outside their capsid
  • Many glycoproteins that project from their surface, producing a "corona" 
• Examples include SARS-Cov-2 (COVID-19), Middle East Respiratory Syndrome (MERS), and Severe Acute Respiratory Syndrome (SARS)

Coronavirus structure diagram

A coronavirus

HIV

• The Human Immunodeficiency Virus is spread by intimate human contact and can only be transmitted by direct exchange of body fluids
• This means HIV can be transmitted in the following ways:
  • Sexual intercourse
  • Blood donation
  • Sharing of needles used by intravenous drug users
  • From mother to child across the placenta
  • Mixing of blood between mother and child during birth
  • From mother to child through breast milk
• HIV contains:
  • Two RNA strands
  • Proteins (including the enzyme reverse transcriptase)
    • Reverse transcriptase allows the production of DNA from the viral RNA; for this reason HIV is known as a retrovirus
  • A protein capsid
  • A viral envelope consisting of a lipid bilayer and glycoproteins that act as attachment proteins
    • The lipid bilayer is derived from the cell membrane of the host helper T cell that the particle escaped from

HIV structure diagram

A HIV particle