17. Viral genetics

Question 1

How can viruses modify their genome during replication

Answer 1

By mutation, recombination or gene reassortment( segmented genome viruses)

Question 2

What happens when changes are done to modify the viral genome during replication

Answer 2

Such changes affect the properties of the virus which therefore differs from the parental or wild-type strain

Question 3

What kind of mutants do we have

Answer 3

-Lethal mutants-can no longer replicate
-Deletion mutants-loss of a portion of the genome and associated functions
-Plaque mutants-altered plaque size or morphology
-Host spectrum mutants-altered tissue or host target of infection
-Attenuated mutants-decreased clinical symptoms
-Conditional mutants-replication occurs only under certain conditions(ex: low temperature)
-Drug-resistant mutants-replication occurs even in the presence of antiviral drugs effective against the virus

Question 4

Based on what properties are newly strains selected

Answer 4

Based on their ability to grow under certain conditions
-cell growth rate
-lack of the expression of specific cellular proteins necessary for the life cycle of the virus(host factors)
-presence of specific cellular proteins inhibiting the life cycle of the virus(restriction factors)
-Maximum replication temperature
-Innate/adaptive response evasion
-tolerance to antiviral drugs

Question 5

When do mutations occur

Answer 5

During the replication of a viral genome mistakes are made in incorporation of nucleotides

Question 6

Why do mutations happen

Answer 6

Because viruses have polymerases that are less faithful than eukaryotic cells
-cellular DNA polymerases proofreading activity–>high
-Viral DNA polymerases proofreading activity–>lower
-Viral RNA polymerases proofreading activity–>no

Question 7

Are errors randomly introduced in the genome

Answer 7

Yes, and they can determine aa variations or not(silent mutations)

Question 8

What happens with aa variations

Answer 8

AA variations can confer new properties to viruses(ex:drug resistance) or make viruses incapable of replicating(defective viruses)

Question 9

Why doesn’t the ability to mutate lead to loss of homogeneity of the viral population

Answer 9

Only few mutations are compatible with viral replication and there is strong elective pressure between different mutants

Question 10

What viruses are highly variable and respond rapidly to environmental changes

Answer 10

RNA

Question 11

What’s antigenic drift( use influenza virus as an example)

Answer 11

In influenza virus there is high mutation rate, and antigenic drift is frequent=> point mutations in NA and HA enveloped antigens result in change in antigenicity and recognition by neutralizing bodies but retain the ability to bind to cell receptors–>evasion of immune systems–> frequent influenza epidemics

Question 12

When does recombination occur

Answer 12

When the same cell is co-infected with two relative viruses and there is exchange of genetic material between two genomes

Question 13

What type of recombinations do DNA and RNA have

Answer 13

DNA viruses have classical recombination and RNA viruses have copy choice recombination

Question 14

What is classical recombination mediated by

Answer 14

Nuclear enzymes responsible for homologous recombination

Question 15

Does classical recombination occur in cytoplasmic replicating poxviruses

Answer 15

Yes, they encode their own recombination enzymes

Question 16

Where is classic recombination common

Answer 16

In DNA viruses and retroviruses during the nuclear stage

Question 17

What’s the process of classical recombination

Answer 17

There is breaking of covalent bonds, changing of genetic material and reformation of covalent bonds

Question 18

What happens during RNA synthesis with replicase

Answer 18

Replicase disassociates from its template and restarts copying from another template starting exactly from the point where it interrupted the synthesis on the original template
At the end of the process, produced neo-strand will contain information from the two RNA template molecules used

Question 19

What’s difference between classical recombination and copy choice recombination

Answer 19

In copy choice there is no enzyme involved

Question 20

What happens to polymerase during RNA synthesis

Answer 20

Polymerase jumps from one template to another

Question 21

Who has diploid genome

Answer 21

Retroviridae

Question 22

Can recombination occur between retroviridae

Answer 22

Yes

Question 23

Can cellular mRNAs be packed into capsids

Answer 23

Yes, because copy choice recombination can lead to insertion of cellular genes into viral genomes

Question 24

When does gene reassortment occurs

Answer 24

Only in case of co-infection of the same cell by two strain of the same segmented genome virus. Gene of the reassorted virus contains some gene segments from one strain and others from the other

Question 25

When is gene reassortment typical

Answer 25

Of type A influenza virus