infectious agents

Question 1

what is a viroid?

Answer 1

a small (200-400NT) circular RNA with a rod like secondary structure, they have no capsid or envelope

Question 2

what is a virusoid?

Answer 2

a satellite - viroid-like molecule that is larger than a viroid, but is dependent on the presence of virus replication for multiplication. they are packaged into virus capsids as passengers

Question 3

what are satellites?

Answer 3

a small RNA molecule that is dependent on the presence of another virus for replication

Question 4

what are the two classes of satellites?

Answer 4

• those that encode their own coat proteins

- those which use the coat protein of the helper virus

Question 5

how do satellites replicate?

Answer 5

in the cytoplasm using an RNA-dependent RNA polymerase

Question 6

what are the typical properties of satellites?

Answer 6

• genomes have approx.500-200 NTs of ssRNA
• little sequence similarity to helper virus
• cause distinct symptoms in plants
• replication interferes with replication of helpervirus

Question 7

what are some examples of satellites and their helper viruses?

Answer 7

barley yellow dwarf virus satellite RNA - helper is luteovirus
tobacco ringlet virus satellite - helper is nepovirus

Question 8

how is it suggested that viroids cause pathogenesis?

Answer 8

by interfering with host cell metabolism

Question 9

what is the key characteristic of viroids?

Answer 9

they all share a common conserved central region of the genome that is believed to be associated with replication

Question 10

what are the two types of viroids which are plant pathogens?

Answer 10

pospiviroidae: numerous, replicate in nucleus
avsunviroidae: few, chloroplast replication

Question 11

which three enzymes are required for replication of viroids?

Answer 11

DNA-dependent RNA polymerase, RNase, RNA ligase

Question 12

what is a prion?

Answer 12

an infectious agent without a nucleic acid genome

Question 13

what are the symptoms of CJD?

Answer 13

• loss of motor control
• dementia
• paralysis
• hypoplastic pneumonia

Question 14

what is meant by the ‘prion model’?

Answer 14

a disease of protein conformation in which there is a modified form of the protein which causes disease, and is resistant to proteases

Question 15

what evidence is there that prion diseases are caused by proteins?

Answer 15

• they are insensitive to radiation and nucleases that destroy DNA and RNA
• sensitive to proteases
• cannot purify a specific nucleic acid from prions

Question 16

what techniques have confirmed the conformational differences between native and prion forms of proteins?

Answer 16

2D NMR has found that the prion form contains B-sheets where a-helices should be

Question 17

what is meant by the term ‘PrPsc is transdominant’?

Answer 17

it converts the cellular form to disease form in an exponential fashion

Question 18

what is the significance of proline-leucine mutations for prion diseases?

Answer 18

provides susceptibility