Chapter 19 - Viruses

Question 1

viruses

Answer 1

infectious particles consisting of genes packaged in a protein coat (nucleic acid enclosed in protein coat and/or memranous envelope)

Question 2

bacteriophages

Answer 2

infect/genetically takeover bacteria (e. coli)

• most complex capsids
• elongated capsid head that encloses their DNA
• protein tail attaches to host and injects phage DNA

Question 3

viral envelopes

Answer 3

surround capsids of influenza viruses and other animal viruses

• contain combination of viral and host cell molecules
• viral glycoproteins bind to host cell surface receptors

Question 4

viral genomes

Answer 4

consists of single/double stranded DNA/RNA

• either linear or circular molecule of nucleic acid
• 3 to 1000+ genes contained

Question 5

viral replicative cycle (3.5 steps)

Answer 5

1. virus enters (uncoated), releasing viral DNA and capsid proteins
2. host enzymes replicate viral genome
3. 5. host enzymes transcribe viral genome into viral mRNA (host ribosomes to make more capsid proteins)
4. viral genomes and capsid proteins self-assemble into new viruses

Question 6

lytic cycle

Answer 6

lyses host cell, releasing progeny virus particles

• phage in lytic is a virulent phage
• virulent phages break open cell wall, killing the host

Question 7

lytic cycle steps (5)

Answer 7

1. attachment of protein tail to cell wall
2. entry of phage DNA and degradation of host DNA
3. synthesis of viral genomes and proteins
4. phage self-assembly (head, tail, tail fibres)
5. release of phages

Question 8

lysogenic cycle

Answer 8

replicates phase genome without destroying the host

• phage in lyso is prophage
• prophages incorporate into host cell’s chromosome (integrated viral DNA)
• environmental signal can trigger virus genome to exit and switch to lytic mode

Question 9

lysogenic with lytic cycle correlation (3 steps)

Answer 9

1. daughter cell with prophage can exit bacterial chromosome and initiate lytic cycle
2. phage DNA integrates into bacterial chromosome becoming a prophage
3. bacterium reproduces normally, copying prophage and transmitting it to daughter cells (many cell divisions produce bacteria infected with prophage)

Question 10

bacterial defences against phages

Answer 10

1. restriction enzymes degrade foreign (phage) DNA

2. CRISPR-cas system protects bacterial cells from phage infection

Question 11

retrovirus

Answer 11

use reverse transcriptase (RNA to DNA) to copy their RNA genome into DNA (HIV is retrovirus that causes AIDS)

Question 12

vaccines

Answer 12

pathogenic microbes that stimulate the immune system and mount defences against pathogen (prevent illness from virus)
-antiviral drugs can treat (not cure) viral infections, antibiotics can not

Question 13

emerging viruses

Answer 13

viral strains jump species (animal to human), exchange genetic information with other viruses to which human have no immunity

Question 14

CRISPR-cas system (4 steps)

Answer 14

1. phage infection triggers transcription of CRISPR region
2. RNA transcript is processed into strands and binds to cas protein
3. RNA binds to DNA from invading phage and cas protein cuts phage DNA
4. phage DNA is degraded and can no longer be used

Question 15

replicative cycle of enveloped RNA virus (7 steps)

Answer 15

1. glycoproteins of virus bind to receptors on host cell and enter
2. capsid is digested by cellular enzymes and releases viral genome
3. viral genome is a template for synthesis of RNA strands by a viral polymerase
4. new viral genome copies made using complementary RNA strands as templates
5. strands function as mRNA, translated into capsids proteins and glycoproteins (viral envelope)
6. vesicles take glycoproteins to plasma membrane while capsid assembles around viral genome molecules
7. new viruses bud from cell membrane where glycoproteins embed the viral membrane

Question 16

replicative cycle of retroviruses (9 steps)

Answer 16

1. glycoproteins of viral envelope and bind to certain white blood cells receptors
2. virus fuses with cell membrane, capsid proteins removed, release of viral proteins/RNA
3. reverse transcriptase catalyzes first and second synthesis of DNA to RNA
4. double-stranded DNA is incorporated as provirus into cell’s DNA
5. proviral genes are transcribed into RNA (genomes for progeny viruses and mRNAs for viral protein)
6. viral proteins include; capsid proteins, reverse transcriptase, and envelope glycoproteins
7. vesicles transport glycoproteins to membrane while capsids assemble
8. new viruses bud from host cell, addition of glycoproteins