Virus Structure, Viral Replication Cycle & Prions Flashcards
What are the two types of length viruses (general)?
- Different viruses infect all organisms.
- Some contribute to microbiota
- Some infected cells can carry viral genomes lifelong (e.g. herpetic viruses)
▪ Once infected with those viruses, it is for life
What is the foundation knowledge on viruses?
*acellular infectious particles (10-400nm)
*carry genetic material (either DNA or RNA - never both)
*do not grow or undergo division outside a cell
*obligate intracellular parasites - can only replicate in host cells
-they lack necessary cell metabolism to produce and assemble viral progenies
What is a virus composed of?
*nucleic acid - DNA or RNA complexed with matrix proteins or enzymes
*capsid - protein coat to protect genome
*envelope - external occasional lipid bilayer membrane on some viruses, containing peripheral glycoproteins (e.g spikes) projecting outwards
What is in the covering of a virion?
*capsid
*envelope (not always)
What is in the central core of a virion?
*viral genome (DNA OR RNA)
*matrix proteins/ enzymes (virus specific)
What are the two types of envelope viruses?
*naked virus (envelope not present e.g rhinoviruses and adenoviruses)
*enveloped virus (SARS-CoV-2, HIV,Herpesviruses, influenza) = more susceptible to disinfectants
What is an envelope (virus)?
*external layer in enveloped viruses only
*a lipid bilayer derived from host cell membrane
*acquired by budding when viruses leave the infected cell
*contains viral glycoproteins (e.g spikes)
What are the details of enveloped glycoproteins?
*viral glycoproteins embedded in the envelope lipid bilayer
- synthesised through secretory pathway
- proteins destined to the plasma membrane
*facilitate attachment and fusion to cell receptors for entry
-determing tropism → specific extracellular receptors
*often form spike like structures protruding outwards (e.g hemagglutinin in influenza, gp120 in HIV)
What is a viral capsid + what it entails?
*protein shell that encases and protects the viral genome
*composed of repeating protein subunits called capsomers
*during infection, capsomers self assemble around viral genome
What are the capsid arrangements - shapes?
*helical (hollow, cylindrical)
*polyhedral (spherical)
*complex
What is entailed in the viral genome?
*contain the genetic information required for viral replication
*can be DNA or RNA
*low sized genomes contain few genes (4-100) encoding proteins needed for:
-viral structure (capsomers) genome replication, immune evasion, hijacking cellular metabolism
*replication occurs within host cell, often using the host’s machinery
*some viruses (e.g retroviruses) can integrate their genome into the host’s DNA
What are the classes of viral genomes?
1: dsDNA
2: gapped dsDNA
3: ssDNA
4: dsRNA
5: ss(+) RNA
6:ss (-) RNA
7: ss(+) RNA with DNA intermediate
What is viral tropism?
Refer to the specific host cells that a virus can infect. Determined by the presence of specific:
* host cell receptors (to mediate viral entry)
* required cell enzymes (to complete the viral cycle)
What is a bacteriophages or phages?
*viruses that infect only bacteria
→RBP receptors only in bacteria
What is step 1 of viral replication cycle?
*virus adheres to cell surface
- random collisions & electrostatics
- no specificity required
*attachment to specific receptors on cell surface by envelope glycoprotein or external capsid protein (naked virus)
What is step 2 of viral replication cycle? (2 ways virus is penetrated)
1: in enveloped viruses:
- fusion between viral envelope and host cell membrane
- thus, releasing capsid intracellularly
2: naked viruses
-by endocytosis which engulfs the virus into cellular vesicles
- receptor binding triggers the process of endocytosis
- cell membrane pinched off
-virus is engulfed
- a vesicle is formed
What is step 3 of viral replication?
*viral uncoating or transcapsidation
-process of capsid disassembly to release the viral genome into the cytoplasm
-viral genome migrated into the nucleus to start transcription of viral genes and genome replication
What is step 4 of viral replication?
*viruses hijack the host cell synthesis machinery and turn it into a virus factory to:
-replicate their genome
-synthesise viral proteins
-transcription of viral genes into mRNA in the nucleus
-translation of mRNA into proteins by cell ribosomes in the cytoplasm.
What is step 5 of viral replication?
*protein maturation
-in many viruses, the whole viral genome is transcribed and translated into large precursor polyproteins, which are unfunctional and immature
-This precursor is processed by viral & cell proteases to form individual, functional and mature viral proteins
*virion assembly
-capsomers assemble enclosing the replicated viral genome and some viral enzymes to form a capsid
-a single infected cell produces 10⁴/10⁶ new virions
What is step 6 to viral replication?
*viral release
a) budding/ exocytosis - enveloped
- assembled new capsid particles bind the membrane
- the membrane pinches off and viruses bud off carrying away at the envelope with their spikes
- infected cells do not die
b) lysis - naked viruses
- viruses induce cell death
- cell bursts and virions are released
What is a prion?
- Proteinaceous infectious particles – misfolded proteins
What are some details about prions?
*do not contain nucleic acids
* cause transmissible spongiform encephalopathies
*fatal neurogenerative diseases
e.g (mad cow disease BSE, Creutzfeldt-Jakob disease CJD, sheep scrapie)
How are prions transmissible?
*by ingestion of contaminated tissue (contaminated meat)
*contaminated surgical instruments
*generic inheritance - mutations in the PRNP group
What are spongiform encephalopathies + details?
*rapid and progressive neurodegenerative disorders
- cause progressive impairment of brain function
- symptoms: cognitive decline, involuntary movements, lack of coordination
* always fatal
*no cure
What is an important quality of prions?
*extremely resistant to standard sterilisation techniques:
- such as standard autoclave programs of 121℃ for 15 minutes (more than bacterial endospores)
*prion inactivation requires autoclave cycle at 134℃ for 18 minutes or more
