Viruses and their multiplication

Virus and what type of parasite

genetic element that can multiply only in a living host cell

• obligate intracellular parasite - needs host cell for energy, metabolic intermediates, and protein synthesis

  • has its own nucleic acid genome

Virion

extracellular form a virus - exist outside host and facilitates transmission from one host cell to another

• replication occurs only upon infection (entry into host cell)

Viral Structure

Capsid - protein shell that surround and protects the viral genome

• Self-assembly - capsid proteins spontaneously organize themself or host cell helps the proteins fold into correct shape

• Capsomer - individual protein unit that makes up capsid

  • Nucleocapsid - Nucleic acid + protein in enveloped viruses

  • 20 nanometers to 30 nanometers

  • bacteriophage - virus in bacteria

enveloped virus vs naked virus

enveloped virus - wrapped in an membrane stolen from its host cell

naked virus - lacks membrane and only has exposed capsid surround the nucleic acids

Virulent (lytic) infection

virus replicates inside the host cell and eventually causes the cell to burst (LYSIS), releasing new virus

Lysogenic infection

viral genome becomes part of the host’s DNA and replicates along with the host without immediately killing the cell

• passed to daughter cells

plaque

clear area on a lawn of host cells where viral infection has caused cell lysis

Viral titer and plaque assay

number of infectious virions per volume of fluid

• lab method to count the number of infectious viruses by measuring clear zones (plaques) where viruses have destroyed host cells

steps/phases of viral replication

1. attachment/absorption - virus attaches to specific receptors on the host cell

2. penetration/entry/injection - virus enters the host cell or injects its nucleic acid into host

3. synthesis - virus redirects the host cell’s metabolism to produce viral nucleic acids and proteins

4. assembly/maturation - Viral genome are packaged into capsids to form new virions

5. Release - New virions leave the host cell through cell lysis

Viral Growth Curve/one-step growth curve

Virion numbers increase when cells burst

• latent period, eclipse period, Maturation

• lytic pathway

Latent period

time between viral infection and the release of new virions

• includes eclipse and maturation

eclipse phase

when viral genomes are replicated and viral proteins are produced

• complete virions are not yet assembled

Maturation phase

packaging of viral nucleic acids into capsids to form complete virions

why is there a dip, spike, and plateau in the graph

dip - where viruses attach to host cell and inject genetic material

• decrease because viruses have entered the host cell

spike - new viruses are released from host cell

plateau at the end become the infected host cell finished releasing their new viruses

steps of lysogenic pathway

after entry

3. integration -Viral DNA incorporated with host chromosome and enters lytic cycle

4. replication - viral DNA replicates passing prophage to daughter cells

5. Induction - viral DNA can become active, leave host chromosome and enter the lytic cycle

   1. Lysis - new viruses are produced and released when host cell bursts

temperate phage

virus capable of both lysogenic and lytic pathways

Phophage

viral DNA integrated into a bacterial chromosome

lysogen

bacterial host cell carrying a temperate virus

Repressor protein

viral protein that keeps virus dormant

Induction

activation of Viral DNA causing cell to enter lytic cycle

Lysogenic conversion

when a harmless bacterium acquires new traits because it’s infected by a virus

5 outcomes of animal virus infection

Virulent infection (lysis) - virus replicates inside the host cell and burst to release new viruses

Latent infection - Viral DNA remains in the host cell’s genome as a provirus without producing new virions

• virus can become active later

Persistent infection- virus continuously leaves the host cell through budding without killing it, allowing cell to remain alive and produce viruses

Transformation - virus changes a normal host cell into a tumor cell

Cell fusion - infected cells becomes sticky & cells fuse together into multinucleated cells