viruses lecture 4 Flashcards
why are viruses considered non-living 2
- cannot replicate independently of host cell
2. cannot grow or metabolise
3 factors that affect the host range of viruses
- correct attachment: viral protein needs to bind to receptor on cell surface
- after entry appropriate cellular machinery must be available for virus replication
- needs mechanisms for release of virus particles to ensure infection can spread
2 components make up a virus
- genetic materila
- single/double stranded
- DNA/RNA
- linear/circular
- one/ more molecules - enclosed by a protective coat= capsid
5 basic structural forms of viruses
- naked icosahedral
- naked helical
- enveloped icosahedral
- enveloped helical
- complex
example of naked icosahedral virus 3
poliovirus, adenovirus, hepatitis A
example of naked helical virus 1
tobacco mosaic virus
example of enveloped icosahedral virus 3
herpes virus, yellow fever virus, rubella virus
example of enveloped helical virus 4
rabies virus, influenza virus, mumps virus, measles
example of complex virus 1
pox virus
virion
the complete, infective form of a virus outside of a host cell
viruses exist in 2 phases
- extracellular
2. intracellular
virus replication process 6 steps
- infection of host (attachment, entry and uncoating)
- transcription of viral genes
- replication- new nucleic acid generated
- translation-protein synthesis
- assembly of new virus particles
- release of mature virus
bacteriophages exhibit what 2 types of life cycles
- lytic
2. lysogenic
difference between lytic and lysogenic life cycles of bacteriophages
lytic cycle
-viral replication is followed by host cell lysis to release the virus particles
lysogenic
- viral genetic material is integrated into host’s genome for replication some time in the future
- infected bacteria (phage) is latent and called a prophage
lytic cycle (virulent phages) 5 stages
- attachment: capsid combine with bacterial cell wall receptors (‘lock and key’)
- penetration: viral enzyme digests part of cell wall & viral DNA is injected into cell
- biosynthesis: biosynthetic apparatus of host is taken over for viral DNA and coat protein production
- maturation: assembly of DNA & capsid protein to produce new phage particles
- release: lysozyme, encoded for by a viral gene, is produced and disrupts the cell wall, thereby releasing the phage particle resulting in host cell death
lysogenic life cycle of bacteriophages 5 stages
- attachment: capsid combine with bacterial cell wall receptors (‘lock and key’)
- penetration: viral enzyme digests part of cell wall & viral DNA is injected into cell
- viral DNA is integrated into the genome of the bacterial cell; the virus is called a prophage
- Prophage is replicated within host DNA and all bacterial daughter cells (lysogenic cells) carry a copy of viral DNA
- environmental factors such as UV radiation may induce prophages to leave host and induces a lytic cycle thus producing new virus particles
transduction
- host’s DNA may be packaged in the heads of prophages
- bacterial genes may then be transferred to next host via the bacteriophage
in lysogenic life cycle viral genes are expressed as if they are……
host’s genes
example of phage conversion
Vibrio cholera
- normally exists in aquatic environment as harmless form
- deadly, disease-causing bacteria due infection of the V.cholerae cell by a bacteriophage that introduces a gene coding for the cholera toxin
HIV 3 characteristics
human immunodeficiency virus
- retrovirus
- genome of 2 identical, positive, single stranded RNA molecules
- infects white blood cells presenting CD4+ receptors on their host cell membranes
5 stages in replication of HIV
- reverse transcriptase converts single-stranded RNA genome of virus to double-stranded cDNA (copy)
- integrase integrates cDNA into host’s genome
- transcription , HIV derived mRNA is produced
- translation of viral mRNA results in new virus production
- release does not involve destruction of host cell, occurs via budding
