Viral genetics
Classed as ‘non-living’
- They are smaller and simpler structures than eukaryotes / prokaryotes and are not composed of cells.
- They do not carry out metabolism, reproduce, use energy or maintain homeostasis so they are unable to survive on their own and their only way to replicate is to take over a host cell.
Structure
- Virus are constantly mutating and changing so some can be more virulent then others.
- Genomic material can be, DNA or RNA and can be single stranded or double stranded with a circular or linear shape.
- The protein shell (capsid) can be composed of one or more types of proteins (capsomeres) and can be various shapes such as: rod, helical, spherical or polyhedral with the capsid protein being able to self-assemble.
- Envelope, many viruses have a lipid membranous envelop surrounding the capsid, which is derived from the infected cell’s membrane which helps to evade the immune response.
- Some virus have Glycoproteins which attach to receptors on host cells allowing them to gain entry into the cell.
Host range
- Is the number of host species the virus can infect, and this can be broad or narrow depending on the virus.
- Infection of multicellular eukaryotes is usually limited to particular tissue, as the virus must be able to recognise a specific binding site and bind through a lock and key fit method.
- Transmission route is how the virus spreads, this can be: respiratory, faecal-oral, sexual or blood.
- Positive strand RNA virus can inject their viral RNA into the cell and it can be directly translated into a protein by ribosomes.
- Negative strand RNA virus injected viral RNA cannot be directly translated into a polypeptide but uses its own RNA polymerase to encode for messenger RNA that can then be translated into a protein.
- RNA via DNA, requires Reverse transcriptase which converts the positive sense RNA into complementary DNA. RNA viruses can replicate through a DNA intermediate.
Replication cycle
- Via bacteria,
- Is made up of the lytic cycle, where the host cell lysis and dies and the Lysogenetic cycle where viral DNA is integrates into the bacterial chromosome becoming a prophage so when the cell reproduces it will also copy the prophage into the two daughter cells. The prophage can come back out of the bacterial chromosome to initiate viral replication (the lytic cycle) when needed.
- Via Eukaryotic cells,
- First there is Attachment via a receptor on the surface of the cell.
- Next is Entry and this can be by injection, by fusion if the virus has an envelope this can fuse with the plasma membrane and so the virus can release its viral content into the cell, or via endocytosis where the host cell could engulf the virus and bring it in to the cell.
- Then is Integration, some viruses have integrase which allows them to insert the host genome (DNA).
- There then Synthesis, which is replication pf the viral genome and synthesis of the protein coat.
- Self-assembly, which is the spontaneous assembly of the genome and capsomeres (protein capsid coat).
- And finally Release, the release of the new viruses either by lysis or budding, and sometimes taking the cell membrane with them if they are enveloped.
Vaccines
- Works with the body’s natural defences to build protection, by using a harmless variant of the pathogen such as an inactive version of the virus or a viral vector using a changed version of the virus which is no longer infective. Or a derivative of the pathogen, such as a protein or sugar subunit or newly mRNA translated into viral protein which can be recognised by the immune system and has been found to be highly effective. These then stimulates the immune system to mount a defence and can prevent certain viral infections.
- Polio virus main transmission route is faecal-oral.
- Baltimore Classification of viruses based on, genotype and mechanism of replication.