25. Viruses Flashcards
What was the first discovered virus?
- Tobacco mosaic disease gives tobacco leaves mosaic pattern → researchers hypothesised it was a small bacteria → further research did not find common bacterial features - probably somethis smaller than a cell (the filtering experiment) → crystallisation of the particles → tobacco mosaic virus (TMV)
Explain viral genomes: classification, genetic molecule structure, gene size
Explain viral capsids
- Capsid - protein shell that encloses the viral genome
- Capsids built from protein subunits capsomeres
- Variable capsid structures: helical / icosahedral
Explain viral envelopes (how they obtained, their composition, glycoproteins)
- Envelopes - accessory structure which helps infect hosts by surrounding the capsid
- The envelope is usually derived from the host cell’s membrane as the viral capsid exits the host → the envelope contains a mixture of virus and host cell molecules
- Viral glycoproteins on the envelope bind to receptor molecules on the host cell
What is the structure of SARS-CoV-2?
Linear single stranded RNA molecule
What are obligate parasites?
Obligate parasites use a host cell to reproduce (ex visruses)
How do viruses replicate?
- The viral genome enters the host cell
- The host cell starts to synthesise viral proteins - virus makes use of host enzymes, ribosomes, tRNAs, am a, ATP and more
- Viral nucleic acids and capsids (from capsomeres) spontaneously assemble into new viruses → released out of the host cell to infect other cells
What are the two alternative mechanism of phage reproduction?
- Lytic cycle
- Lysogenic cycle
Explain the lytic cycle of phage reproduction
- The lytic cycle - phage replicative cycle which culminates during the death of the cell
- The lytic cycle produces new phages and lyses the host cell’s wall releasing the progeny viruses
- Phage that reproduces only by the lytic cycle - virulent phage
What are virulent phages?
Phages which reproduce only by the lytic cycle
Explain the lysogenic cycle of phage reproduction
- In lysogenic cycle the phage genome is replicated without destroying the host cell → viral DNA molecule is incorporated into the host cell’s chromosome → the integrated viral DNA is known as prophage
- Every time the host divides - copies phage DNA and passes to daughter cells → an environmental signal triggers viral genome to exit the chromosome and switch to the lytic cycle - phages that use both lytic and lysogenic cycles - temperate phages
What is prophage?
Viral DNA integrated into the host cell’s chromosome
What are temperate phages?
Temperate phages use both lytic and lysogenic cycles to reproduce
What is the bacterial defense mechanism against phages?
Restriction modification system: innate immune response - foreign DNA identified and cut up by cellular enzymes - restriction enzymes - bacterium’s own genome protected against restriction enzymes by being methylated
Explain CRISPR-Cas system as bacterial defence
- Both bacteria and archaea protect themselves from viral infections using CRISPR-Cas system: based on sequences called clustered regularly interspaced short palindromic repeats (CRISPRs)
- When phage infects a cell - phage DNA integrated between two repeat sequences → each “spacer” sequence corresponds to DNA from a phage that has infected (creates a library of phage sequences that the cell was previosuly exposed to)
- Particular nucleases interact with CRISPR region → called CRISPR-associated (Cas) proteins
- Attempt of the phage to infect the cell triggers transcription of CRISPR region → RNAs cut into pieces and bound by Cas proteins - they use the related phage DNA to cut and destroy the phage DNA
- If the cell survives - can block the infection of the same phage
- Restriction system - innate while CRISPR-Cas - adaptive
What are the most common type of viruses that infect animals?
What are retroviruses?
Examples of DNA viruses
What is the replication cycle of single stranded RNA negative sense genome (ssRNA (-)) viruses?
- Lipid membrane
- Negative sense → positive sense - template for production of negative sense genomic DNA
Replication cycle of HIV
- Lipid membrane
- Two linear RNA molecules - reverse transcriptase → membrane fusion - viral DNA (RNA→DNA converted by reverse transcriptase) enters the nucleus - integrates into the host genome - remains latent as provirus → trigger → transcription of viral DNA into mRNA → mRNA will be the genomes for new viruses released from the cell
Explain provirus and its difference from prophage
What are formidable pathogens?
?? did not find a definition - look into cambell
Quiz
Phage self-assembly
Quiz
Uses viral RNA for DNA synthesis
