Virus structure and life cycle

IMED2000

• prokaryotes - bacteria and archaea

• eukaryotes - eukaryota

• acellular infectious particles - viruses - needs electron microscope to be seen

Cellular microorganisms

• cannot carry out many life processes

• not made of cells

• cannot reproduce on their own

• dont grow or undergo division

• dont transform energy

• lack machinery for protein synthesis

why viruses are non living

• totally dependent on a host cell for replication

• isolated virus in unable to reproduce except infect host

• can only contain DNA or RNA not both

• must assemble into complete viruses (virions)

• it is package set of genes that move from one host to another

characteristics of viruses

• a complete virus particle that consists of an RNA or DNA core with a protein coat sometimes with external envelopes and that is the extracellular infectious form of a viru

virions

• nucleic acid, protein and sometimes lipids

• virus genome is nucleic acid often packed around proteins

• capsid - protective protein coat which surrounds nucleic acid

• capsomere - small circular structure of the capsid

• many animal viruses have a membrane outer layer made up of lipids and proteins which surround the capsid - envelope

what viruses contain

• more varied than that of cells

• classify viruses based on type of nucleic acid

• can be double or single strand DNA or double or single-stranded RNA

• linear genome with several segments or single circular genome

• much smaller than genomes of cells

genetic material for viruses

• protein coat - protection for viral nucleic acid and means for attachment to target host cells

• made of protein subunits - capsomeres

• some capsides composed of a single type of capsomere some have mutiple layers

• capsid and nucleic acid - nucleocapsid

viral capsid

• acquired from the host cell during viral release

• similar composition to host cell membrane

• virally encoded proteins/glycoproteins - spikes

• spikes important in the binding of a virion to the host cell

• virus which doesn’t have envelop - naked virus

viral envelope

• attachment - recognises and attach to the host cell

• entry - infect host cell

• replication - force the cell to manufacture virus components and spontaneously self-assemble of viral components

• release - new virions exit the host cell

viral life cycle

• viruses infect only a certain type of cell - cell tropism

• specificity is due to viral surface proteins for host cell surface molecules

• bacteriophages have proteins in their tail fibres that bind to bacterial cell surface molecules

• animal viruses have spikes that bind to specific proteins on the surface of animal cells

• recognition of the host cell may be so specific that they only infect one type of cell

how does a virus recognise and attach to host

• tail fibres attach to surface molecules

• phage capsid remains on the surface of the cell while the viral genome is injected into the cell

how bacteriophage enter the host

• naked viruses can bind to the cell curface and inject nucleic acid while capsid remains on cell’s surface

direct penetration

• some naked and enveloped viruses can induce the cell to take them up by endocytosis - once inside the capsid is removed

endocytosis

• some enveloped viruses,

• envelope can fuse with the host cell membrane and release the capsid inside the cell

• inside the capsid is broken down and viral DNA moves in the cytoplasm

fusion

• host cell proteins read viral genetic instructions

• manufacture raw materials needed to build many new virus particles

• viral components can assemble themselves

how viruses trick the host into manufacturing more viruses

• Bacteriophages released through cell lysis

• Endolysin enzyme is encoded by the viral genome of lytic phages

• endolysin attacks and breaks down bacteria’s cell wall peptidoglycan layer

• The infected bacterium is destroyed, and phage virions are released

how does new phages exit host bacterium

• naked viruses - released from cell through exocytosis or may cause lysis (breaking down cell membrane) and cell death

• enveloped viruses - released through a process called budding (looks like exocytosis)

• virus exits cell with part of cell’s plasma membrane

how naked viruses and enveloped viruses leave the cell

• horizontal transmission - direct contact (influenza)

• mechanical (animal bites or rabies)

• vertical transmission - mother to offspring

• transfusion products, needle stick, IV, transplantation

virus transmission

• extracellular spikes on a complete virus particle (virion) bind to specific molecules on the target cell surface

• intracellular systems - some cells have specific transcription factors that recognise viral promoter

• enhancer sequences on viral genome and gene of a virus may be more efficiently expressed in the target cell type

• skin (virus struggle to enter through) and mucous membranes (virus can enter more readily)

• can the virus survive in certain temp, pH, enzymes or other non-specific factors

• bile that may inactivate some viruses

what affects cell tropism of viruses

• process by which a viral infection leads to disease

• abnormal

• damage to the host and are subclinical (asymptomatic)

• consequences of viral infections depend on a number of viral and host factors

• influenced by number of infecting viral particles,

• speed of viral multiplication and spread

• effect of virus on cellular functions

• secondary responses of the host to cellular injury - inflammation

viral pathogenesis

viral reproduction that ends with the destruction (lysis) of the host cell.

lytic virus

a type of virus that integrates its genetic material into the host's genome, allowing it to hide and replicate silently as the cell divides without killing the host immediately.

lysogenic virus

• no apparent change, loss of cell function, cell death and cancer

• cell damage and death due to lysis, re-direction of cell energy, shutdown of important pathways, competition by viral nucleic acids for proteins in gene expression

• integration of the viral genome into the host DNA - mutation

• inflammation and host immune response

damage viruses do to cells

• rapid and clinically apparent disease short period of time

• releasing copies of the virus into the body (shedding) prior to symptoms

• complete recovery or recovery with side effects

• death and persistent infections can occur

acute infection

• many viruses can cause acute infections with symptoms in only a portion of infected patients

• asymptomatic

• influenza

subclinical infections

• occurs when host defences are either targeted by the virus or bypassed

• chronic - continuous viral replication and production with moderate or no symptoms

• latent infections - virus is generally hidden in a non-replicating form - virions not detectable, flare ups may occur

persistent infections

• known as oncogenic

• retroviruses (RNA virus causing reverse transcription) can inactivate genes responsible for suppressing tumour formation or activate genes involved in cell growth

• viruses can acquire oncogenes - coding for proteins involved in cell growth which can cause cancer

• papillomavirus and epstein-barr virus

cancer causing viruses

• oncogene is activated by the cell causing cancer and excessive cell division

• viruses can acquire this by integrating viral DNA into the host cells DNA,

• this gene is replicated using the viral gene and can infect other cells

oncogene

• The viral genome integrates (called a provirus) into the host cell genome.

• replication of the provirus, additional DNA from the host is transcribed and becomes part of the new viron genome.

• additional host DNA contains a gene that promotes cell growth, the next time the virus integrates into a host cell it will also carry/donate this gene DNA.

how does a viral oncogene get activated

• double strand DNA

• double strand RNA

• single strand RNA

• single strand DNA

different types of nucleic acids used as genetic material by viruses