virus expression

Question 1

what is meant by gene expression?

Answer 1

the mechanisms by which cells express the information stored in genes

Question 2

what mechanisms do group I viruses use for replication?

Answer 2

rolling circle and rolling hairpin replication

Question 3

what mechanisms do group II viruses use for replication?

Answer 3

bi-directional replication
rolling circle
dsDNA strand displacement
replicative transposition

Question 4

when was bacteriophage lamda discovered?

Answer 4

by Esther lederberg in 1949

Question 5

what did Andre Lwoff’s 1950 experiments show?

Answer 5

• when irradiated, some strains of bacillus megasterium stopped growing and lysed
• they released bacteriophage particles

Question 6

what was the significance of Lwoff’s 1950 findings?

Answer 6

• some bacterial strains carried the bacteriophage in a dormant form, prophage
• the phage could be made to alternate between lysogenic and lytic growth cycles

Question 7

which class I viruses replicate in the nucleus?

Answer 7

adenoviruses, polyomaviruses and herpesviruses

Question 8

which class I viruses replicate in the cytoplasm?

Answer 8

poxviridae

Question 9

what are the characteristics of gene expression for polyomaviruses and papillomaviruses?

Answer 9

-they are heavily dependent on cellular machinery for replication and gene expression

Question 10

what are the characteristics of gene expression for adenoviruses?

Answer 10

-heavily dependent on the cellular apparatus for transcription, but possess various mechanisms to regulate viral gene expression

Question 11

what are the characteristics of herpesvirus gene expression?

Answer 11

• less reliant on cellular enzymes
• encode many enzymes involved in DNA metabolism
• encode trans-acting factors to regulate the temporal expression to control phases of replication

Question 12

what are the three classes of mRNAs produced by herpesviruses?

Answer 12

alpha: immediate-early mRNAs, encode trans-acting regulators of virus transcription
beta: delayed early mRNAs encode further non-structural regulatory proteins and some minor structural proteins
gamma: late mRNAs encode the major structural proteins

Question 13

which observations indicate that gene expression is tightly regulated in herpesviruses?

Answer 13

• if translation is blocked shortly after infection (e.g. by treatment with cycloheximide), the production of late mRNA is blocked
• synthesis of early gene products turn off the immediate-early products and initiates transcription
• some of the late structural proteins are produced independently of replication
• immediate-early and early proteins are required to initiate replication

Question 14

what is a dependovirus?

Answer 14

a virus with a very small genome that is replication defective and requires superinfection with parvovirus or adenovirus to replicate

Question 15

what observations suggest that helper viruses alter the cellular environment to permit replication?

Answer 15

treatment of cells with UV light, cyclohgeximide or carcinogens can replace the need for a helper virus

Question 16

what is a possible advantage of a segmented genome?

Answer 16

the various genome sequences can be transcribed and translated at different frequencies

Question 17

what do all class IV viruses do?

Answer 17

act as mRNA and are immediately translated after infection of the host cell

Question 18

what two groups can class IV viruses be separated into?

Answer 18

1) viruses which produce a poly protein that is later cleaved by proteases to produce precursor and mature polypeptides
2) viruses which produce subgenomic mRNAs resulting from two or more rounds of translation