Viral Genetics and Evolution

Question 1

Viral genomes are ______ changing

Answer 1

Continuously

Question 2

Segmented genomes

Answer 2

Transcribed to produce monocistronic mRNAs

• advantage: various proteins can be produced in different amounts, rather than in a constant ratio
• more control

Question 3

Non-segmented genomes

Answer 3

Produce polycistronic mRNA, which is translated to form a polyprotein, processed by proteolytic cleavage to form the mature gene products
- either make everything, or nothing

Question 4

_____ viruses are stronger than ____ viruses

Answer 4

DNA; RNA

- due to proofreading ability

Question 5

Spontaneous mutations

Answer 5

Arise naturally during viral replication

Question 6

Mutation rate

Answer 6

10^8 - 10^11 per base per cycle for DNA viruses

10^3 - 10^4 per base per cycle for RNA viruses

Question 7

1 progeny = 1 _______

Answer 7

Mistake

- not all mutations will have an effect due to redundancy of the genetic code

Question 8

What ensures that a mutation will survive?

Answer 8

If the mutation does not cripple essential viral function

Question 9

What allows most mutations to have no effect on viral protein or function?

Answer 9

The redundancy of the genetic code

Question 10

Point mutation

Answer 10

One base is replaced by another or insertion/deletion of a base

Question 11

Epitope

Answer 11

Smallest unit that can stimulate antibody binding

Question 12

Recombination

Answer 12

Exchange of genetic information between two genomes

- leads to creation of a new virus –> only happens between 2 viruses that are similar in structure

Question 13

Reassortment

Answer 13

If a virus has a segmented genome and if 2 variants of that virus infect a single cell

• only occurs in segemented RNA viruses
• ex; orthomyxoviridae, arenaviridae, birnaviridae, bunyaviridae, and reoviridae

Question 14

Definition

Answer 14

• segmented genomes: transcribed to produce monocistronic mRNA
• nonsegmented genomes: produce polycistronic mRNA, –> translated to form a polyprotein –> processed by proteolytic cleavage to form the mature gene products

Question 15

Multiplicity reactivation

Answer 15

Apply to a cell infected with 2 or more viruses of the same strain, each with a different mutated gene
- ex: dsDNA viruses are inactivated with UV and high multiplicity of infection, the virus may complement others = a new genotype or regenerating the wild type virus

Question 16

Defective viruses

Answer 16

Lack the full complement of genes necessary for a complete infectious cycle
- need another virus to provide the missing functions (helper virus)

Question 17

Examples of defective viruses

Answer 17

• retroviruses need related viruses as a helper

- hepatitis delta virus (RNA) needs hepatitis virus (DNA) as a helper

Question 18

Phenotype mixing

Answer 18

When 2 different viruses infect a cell, progeny viruses may contain coat components of both parents

• no alteration in genetic material
• only the outside coat has changed

Question 19

Pseudotype

Answer 19

Phenotype mixing is a life cycle of envelope-defective retroviruses

• has genome of the defective parental virus but the envelope glycoproteins of the helper virus
• new virus as a result of phenotype mixing

Question 20

Antigenic drift

Answer 20

Point mutations

Question 21

Antigenic shift

Answer 21

Genetic reassortment and generate a new subtype

- more dramatic change