25. Viruses

Question 1

What was the first discovered virus?

Answer 1

• 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)

Question 2

Explain viral genomes: classification, genetic molecule structure, gene size

Answer 2

Question 3

Explain viral capsids

Answer 3

• Capsid - protein shell that encloses the viral genome
• Capsids built from protein subunits capsomeres
• Variable capsid structures: helical / icosahedral

Question 4

Explain viral envelopes (how they obtained, their composition, glycoproteins)

Answer 4

• 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

Question 5

What is the structure of SARS-CoV-2?

Answer 5

Linear single stranded RNA molecule

Question 6

What are obligate parasites?

Answer 6

Obligate parasites use a host cell to reproduce (ex visruses)

Question 7

How do viruses replicate?

Answer 7

1. The viral genome enters the host cell
2. The host cell starts to synthesise viral proteins - virus makes use of host enzymes, ribosomes, tRNAs, am a, ATP and more
3. Viral nucleic acids and capsids (from capsomeres) spontaneously assemble into new viruses → released out of the host cell to infect other cells

Question 8

What are the two alternative mechanism of phage reproduction?

Answer 8

• Lytic cycle
• Lysogenic cycle

Question 9

Explain the lytic cycle of phage reproduction

Answer 9

• 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

Question 10

What are virulent phages?

Answer 10

Phages which reproduce only by the lytic cycle

Question 11

Explain the lysogenic cycle of phage reproduction

Answer 11

• 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

Question 12

What is prophage?

Answer 12

Viral DNA integrated into the host cell’s chromosome

Question 13

What are temperate phages?

Answer 13

Temperate phages use both lytic and lysogenic cycles to reproduce

Question 14

What is the bacterial defense mechanism against phages?

Answer 14

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

Question 15

Explain CRISPR-Cas system as bacterial defence

Answer 15

• 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

Question 16

What are the most common type of viruses that infect animals?

Answer 16

Question 17

What are retroviruses?

Answer 17

Question 18

Examples of DNA viruses

Answer 18

Question 19

What is the replication cycle of single stranded RNA negative sense genome (ssRNA (-)) viruses?

Answer 19

• Lipid membrane
• Negative sense → positive sense - template for production of negative sense genomic DNA

Question 20

Replication cycle of HIV

Answer 20

• 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

Question 21

Explain provirus and its difference from prophage

Answer 21

Question 22

What are formidable pathogens?

Answer 22

?? did not find a definition - look into cambell

Question 23

Quiz

Answer 23

Phage self-assembly

Question 24

Quiz

Answer 24

Uses viral RNA for DNA synthesis

Question 25

Quiz

Answer 25

SARS-CoV-2 DNA integrates into host chromosomes

Question 26

What are the pathways in which viruses cause disease?

Answer 26

Question 27

Define a vaccine

Answer 27

Question 28

What is the function of antiviral drugs?

Answer 28

Question 29

What are prions? What is their function?

Answer 29

very stable proteins (heat resistant)

Question 30

The machanism of prions in disease

Answer 30

Cause misfolding of healthy proteins → disease

Question 31

Quiz

Answer 31

Producing toxins

Question 32

Quiz

Answer 32

Prion

Question 33

Quiz

Answer 33

They only infect animals