Chapter 19 - genetics of viruses and bacteria

Question 1

what is a virus

Answer 1

• a small infectious particle that consists of nucleic acid enclosed in a protein coat
• some have a viral envelope derived from the plasma membrane of the host cell
• viroids are more primitive than viruses

Question 2

what are some characteristics of viruses that can vary

Answer 2

• host range
• structure
• genome composition

Question 3

host range

Answer 3

• the number of species and cell types that can be infected
• viruses can’t live on their own so they take up various types of host cells

Question 4

Viral structure

Answer 4

• all viruses have a capsid
• capsid can vary in shape and complexity
• some have viral envelopes
• different spike glycoproteins define how it interacts with the host

Question 5

capsid

Answer 5

• protein coat enclosing a virus’s genome

Question 6

viral envelope

Answer 6

• structure enclosing a viral capsid that consists of a membrane derived from the plasma membrane of the host cell
• embedded with virally encoded spike glycoproteins

Question 7

Viral genomic differences

Answer 7

• can have DNA or RNA
• can be single stranded or double stranded
• linear vs circular
• nucleotide length varies significantly

Question 8

are viruses alive?

Answer 8

• NO
• they are not cells or composed of cells
• cannot carry out metabolism on theirownb

Question 9

6 basic steps of the viral reproductive cycle

Answer 9

1. attachment
2. entry
3. integration
4. synthesis of viral components
5. viral assembly
6. release
   - looks different for different viruses

Question 10

Attachment in viral reproductive cycle

Answer 10

• first step of viral reproductive cycle
• usually specific to one kind of cell
  
  • due to binding of specific molecules on the cell surface

Question 11

Entry in the viral reproductive cycle

Answer 11

• second step
• bacteriophage inject only the DNA into bacteria
• viruses like HIV fuse with the plasma membrane to enter
• one or several viral genes are expressed immediately
  virus can either:
• synthesize viral components
• integrate into host chromosome

Question 12

integrase

Answer 12

• enzyme coded for by some viruses that catalyzes the integration of the viral genome into a host-cell chromosome

Question 13

integration in viral reproductive cycle

Answer 13

• third step
• uses integrase to insert viral genome into chromosomal DNA
• RNA viruses (like HIV) use reverse transcriptase to make it DNA

Question 14

prophage

Answer 14

• the DNA of a phage that has become integrated into a bacterial chromosome
• occurs during integration step

Question 15

reverse transcriptase

Answer 15

• enzyme that catalyzes the synthesis of viral DNA starting with viral RNA as a template
• used to integrate it into the chromosomal DNA

Question 16

retrovirus

Answer 16

• RNA virus that uses reverse transcription to produce viral DNA that can be integrated into a chromosome of a host cell

Question 17

provirus

Answer 17

• viral DNA that was integrated into a chromosome of the host cell

Question 18

Synthesis of viral components in viral reproductive cycle

Answer 18

• 4th step
• host cell enzymes make copies of the phage DNA
  
  • transcribes the genes in the DNA into mRNA
• in HIV, DNA isn’t excised, it’s transcribed in the nucleus
  
  • makes many copies of viral RNA
  • translated to make viral proteins
  • serves as genome for new viral particles

Question 19

viral assembly in the viral reproductive cycle

Answer 19

• 5th step
• some viruses self-assemble
• some are too complicated to self-assemble
  
  • require other proteins to either modify capsid proteins or serve as scaffolding to assemble the capsid

Question 20

release

Answer 20

• 6th step
• phages must lyse their host cell to escape
• enveloped viruses bud from the host cell

Question 21

latency in bacteriophage

Answer 21

• when viruses integrate into the genome of the host chromosome, it is latent (inactive)
• most viral genes are silenced
• may remain latent for a long time

Question 22

lysogenic cycle

Answer 22

• integration, replication, and excision
• prophage DNA gets copied as the cells divide

Question 23

Lytic cycle

Answer 23

• Synthesis, assemble, and release
• new phages are synthesized in the cell
• causes the host cell to lyse, releasing the new phages and allowing them to bind to another cells

Question 24

episomes

Answer 24

• genetic elements that replicate independently but occasionally integrate into host DNA

Question 25

two mechanisms of latency in human viruses

Answer 25

1. virus integrates into host genome and may remain dormant for long periods of time
   - ex: HIV
2. viruses exist as episomes

Question 26

temperate vs. virulent phages

Answer 26

• temperate phages can undergo either the lysogenic or lytic cycle
• virulent phages are only lytic